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Haptic Perception: From the Skin to the Brain

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From Reed C.L., Ziat M. Haptic Perception: From the Skin to the Brain. In Reference
Module in Neuroscience and Biobehavioral Psychology, Elsevier, 2018. ISBN 9780128093245
ISBN: 9780128093245
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Haptic Perception: From the Skin to the Brain
Catherine L Reed
and Mounia Ziat
Clarmont McKenna College, Claremont, CA, United States; and
Northern Michigan
University, Marquette, MI, United States
© 2018 Elsevier Inc. All rights reserved.
Active Touch: From Skin up to the Brain 2
Tactile Perception: Mechanoreceptor and Afferent Nerve Fiber Response 2
Labelled Line Theory 3
Pattern Theory 3
Haptic Perception 3
Proprioceptive and Kinesthetic Senses 3
Temperature 4
Afferent Neural Pathways and Thalamus 4
Medial Lemniscal Pathway 4
Spinothalamic Pathway 4
Thalamus 4
Somatosensory Cortices 4
Primary Somatosensory Cortex 5
Somatotopic Organization 5
Cortical Magnication 5
Multiple Body Representations 6
Function 6
Secondary Somatosensory Cortex 6
Somatotopic Organization 6
Function 7
Retroinsular Cortex and Somatosensory Insula 7
Somatosensory Areas of the Posterior Parietal Cortex 7
Summary 7
Primary and Complex Tactile Sensations 7
Primary Somatosensory Functions 7
Tactile Spatial Acuity 7
Light Touch 8
Vibration 8
Summary 8
Complex Somatosensory Functions 8
Texture 8
Complex Pattern and Shape 9
Object Recognition 9
Cortical Organization of the Somatosensory Cortex 10
Plasticity and Reorganization of Somatosensory Cortex 10
Changes in Sensory Input and Phantom Limbs 10
Effects of Training 10
Activity of Body Part 10
Dual Streams of Processing for Object Recognition and Localization in Somatosensory Cortices 11
The WhatPathway 11
The Where/HowPathway 11
Haptic Learning and Memory 11
Selective Attention in Touch 11
Hemispheric Specialization 12
Conclusions 12
Further Reading 12
Exploratory procedures Stereotypical hand movements used to extract distinctive object dimensions or qualities (e.g., pressure
is used to identify hardness).
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Haptics Active touch; typically used for object recognition by touch.
Homunculus Map of the body in somatosensory cortex; little man.
Secondary somatosensory cortex Cortical area located on the upper bank of the Sylvian ssure, ventrolaterally to primary
somatosensory area, that processes information about touch, pain, and temperature.
Somatosensory cortex Cortical strip located on the postcentral sulcus that receives input from the skin and is responsive to
somatosensory stimulation.
Somatosensory system Sensory system that includes the cutaneous senses, proprioception (the sense of limb position), and
kinesthesis (sense of limb movement).
Somatotopic map A map in somatosensory areas of the brain organized so neurons responsive to particular body parts are
located next to neurons representing adjacent body parts.
Our sense of touch connects us physically with the external world. Haptic perception, or somesthesis, refers to our ability to appre-
hend information through touch. Not only do objects in the world touch us but also we explore our environment actively with our
hands, ngers, and bodies. We use the motor capabilities of our hands to extract important characteristics necessary for identifying
and using objects. Thus, the haptic system is designed for processing the material properties of objects and surfaces via the medi-
ation of cutaneous and kinesthetic afferent subsystems. The passive aspect of haptic perception is often called tactile perception, and
it refers to sensations gleaned from being touched by items in the outside world. Mechanoreceptors and thermoreceptors in the skin
(e.g., cutaneous inputs) contribute largely to this tactile aspect of haptic perception. However, haptic perception also includes active
touch and the sensations that result from the stimulation of receptors in muscles, tendons, and joints (e.g., proprioceptive and
kinesthetic inputs). Our understanding of the neural bases of haptic perceptiondfrom the skin to the braindis based on the study
of perceptual and neurophysiological responses in animals and humans.
Active Touch: From Skin up to the Brain
Haptic perception begins with the mechanical stimulation of the skin. Tactile information travels from cutaneous receptors to the
afferent somatosensory pathways, the spinal cord and thalamus, and the brain. It is rst collected and grouped by peripheral recep-
tors in the skin, joints, and muscles. The hand is the primary organ for acquiring tactile information, although receptors are located
throughout the body in both glabrous (non-hairy) and non-glabrous (hairy) skin. This article focuses on the receptors located in the
glabrous skin of the hand that process non-noxious information.
To study the relationship between haptic perception and its neural mechanisms, psychophysical and physiological experiments
have used several stimulating techniques designed to distinguish the response properties of the mechanoreceptors and their afferent
nerve bers. These techniques include the cooling of the skin and the presentation of masking vibrations to decrease the sensitivity
of one type of tactile channel to study another one. Electrophysiological studies using microneurography record the activity of the
nerve bers innervating the hand. Vibration has been applied perpendicularly and tangentially to the skin by pins and probes. Peri-
odic and aperiodic gratings have been moved across the skin. Other stimuli include air puffs, embossed letters, steel wool, sand-
paper, and cloth.
How do the receptors and their nerve bers give rise to our perceptions of touch? Although tactile perception is correlated with
the properties of individual receptors and afferent bers, it results primarily from activity across multiple nerve bers. When we
interact with common objects, such as a cold wet cup, different combinations of neural bers tend to be activated. The brain
combines these temporal and spatial patterns of impulses from a large number of different types of receptors to create complex
tactile percepts.
Tactile Perception: Mechanoreceptor and Afferent Nerve Fiber Response
Cutaneous mechanoreceptors transduce mechanical energy into electrochemical energy to create the neural signal. In this section we
review the responses of these mechanoreceptors with respect to two different theories of processing. The established theory is called
labelled lineor specicity theory. In glabrous skin, four major mechanoreceptors have been identied to respond to specic types
of tactile information: Merkel disks, Meissner corpuscles, Pacinian corpuscles, and Rufni cylinders. Each receptor corresponds to
a specic type of afferent nerve ber that has differentiable response characteristics based on the size of the receptive eld and the
relative adaptation rate. The receptors and their associated nerve bers respond selectively to certain types of mechanical stimula-
tion, in that each has a lower threshold to a specic stimulus relative to the others. More recently researchers have proposed pattern
theoryin which each of the types of mechanoreceptors contributes slightly different information but it is the overall pattern of
responses across all the receptors that codes incoming tactile information.
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Labelled Line Theory
According to labelled line theory, mechanoreceptors can be classied by the type of ber, optimal stimulus, adaptation properties,
and size of receptive eld (Table 1). Slowly adapting (SA) bers signal the presence of stimulation and re steadily while the stim-
ulus is applied. Following a labelled line theory, Merkel discs and SA1 bers respond best to 0.3- to 3-Hz frequencies that are
perceived as pressure. They are slow to adapt and have small receptive eld sizes. SA1 ber activity is associated with the feeling
of texture or ne detail. Rufni cylinders and SA2 bers respond best to 15- to 400-Hz frequencies that are perceived as buzzing.
They are slow to adapt and have large receptive elds. SA2 bers respond to the stretching of the skin or movements of the joints.
Rapidly adapting (RA) bers signal stimulus onset by ring strongly and briey when a stimulus is applied and when it is
removed. Meissner corpuscles and RA1 bers respond best to 3- to 40-Hz vibrations or taps on the skin that are perceived as utter.
They adapt rapidly and have small receptive elds. RA1 ber activity is associated with rapid changes of pressure that occur when the
hand feels a textured surface. Last, Pacinian corpuscles and RA2 bers respond best to 10- to 500-Hz frequencies that are perceived as
vibration. They adapt rapidly and have large receptive elds. RA2 bers are specialized to respond to changes in stimulation.
Pattern Theory
More recently researchers have proposed a more dynamic theorydcalled Pattern Theorydabout how the brain interprets incoming
information from the somatosensory system. Pattern Theory suggests that the ultimate perceived sensation is encoded across the
pattern of peripheral nerve activation before being decoded by the brain. This theory, opposing labelled line theory, hypothesizes
that there are no unique or specic circuits for any distinct sensation, but rather all sensations are encoded in the particular activa-
tion patterns of sensory neurons and when decoded, results in the designated sensation that is perceived. It was rst proposed for
pain and itch perception, but has recently been extended to other cutaneous responses as well.
Pattern theory holds that pain signals are sent to the brain only when stimuli sum together to produce a specic combination or
pattern. The theory does not posit specialized receptors for pain nor does it see the brain as having control over the perception of
pain. Rather, the brain is merely viewed as a message recipient.
These theories set the stage for the inuential gate control theorythat combines concepts from both of them to explain why
pain perception can be inuenced by both cortical inuences and by non-painful tactile stimuli (e.g., pain is decreased by rubbing
the skin). Gate control theory posits that small nerve bers carry pain information from nociceptors and large nerve bers carry
tactile information into the spinal cord. A gate control circuit in the dorsal horn of the spinal cord includes substantia gelatinosa
(SG) and transmission (T) cells. The gate mechanism compares the relative activity of the two inputs as well as contributions from
cortical control areas. In this system, pain is perceived if the activation from the small nerve bers (SGþ) opens the pain gate and is
greater than the tactile and cortical inputs that close the gate (SG) and activates the T cells.
Haptic Perception
Haptic perception involves more than static touch. It arises from active exploration of objects and their surfaces by the hands and
body. Thus, in addition to cutaneous information (tactile perception), proprioceptive, kinesthetic, and thermal information
contribute to haptic perception.
Proprioceptive and Kinesthetic Senses
Proprioceptive and kinesthetic information are essential to tactile perception as they contribute to the perception and control of
limb movement. Proprioception and kinesthesia are often used interchangeably, but the two senses present some differences.
The literature is often divergent, with some considering proprioception as an inclusive sense containing both the kinesthesis and
vestibular systems, while others consider them as two separate senses with proprioception as part of the balance sense (along
with vision and the vestibular system) and the kinesthetic sense as related to movements. What is clearly convergent is that propri-
oception informs us about where our body is positioned in space and kinesthesia provides useful information on how we move in
space. Another important distinction is that proprioception is more related to our awareness of our body and therefore is often used
to describe the cognitive component of the sense; while kinesthesia is related to the behavioral component of the sense.
Both share similar receptors and there is evidence that there are additional receptors for the kinesthesis sense. Proprioceptors
include muscle spindles, Golgi tendon organs, and brous capsules found in muscles, tendons, and joints respectively. Kinesthesis
Table 1 Properties of mechanoreceptors and afferent nerve bers
Mechanoreceptor Afferent nerve fiber Optimal stimulus Adaptation rate Receptive field size
Merkel receptor SA-I Pressure Slow Small
Meissner corpuscle RA-I Taps on skin Rapid Small
Rufni cylinder SA-II Skin stretch, joint movement Slow Large
Pacinian corpuscle PC Rapid vibration Rapid Large
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is also said to include vestibular inputs. Keep in mind that this separation is only related to the function of these senses since the
cutaneous sense contributes to proprioception as well and it is often very hard to draw a sharp line between the two when describing
the mechanisms.
Our perception of temperature also contributes to haptic object recognition. Temperature can be characteristic of a haptically
perceived object (e.g., metal objects have different ambient temperatures than ceramic, paper or wooden objects). Changes in
temperature can indicate slippage in held objects. The perception of temperature is produced by the heating or cooling of the
skin. Thermoreceptors respond to specic temperatures and changes in temperature but do not respond to mechanical stimulation.
There are separate thermoreceptors for warm and cold, but also multimodal thermoreceptors that activate both.
Afferent Neural Pathways and Thalamus
Haptic signals from the body travel through the peripheral nerves to the dorsal root ganglia in the spinal cord. Once they enter the
spinal cord, the nerve bers go up the spinal cord in one of two major pathways: the medial lemniscal pathway and the anterolateral
system known as the spinothalamic tract that carries nociceptor and thermoreceptor information. Neural information is divided
into two adjacent pathways: anterior and lateral. On the way to the thalamus, nerve bers in these pathways cross over to the other
side of the body so that signals originating from the left side of the body are represented in the right hemisphere of the brain and
signals from the right side of the body are represented in the left hemisphere.
Medial Lemniscal Pathway
The medial lemniscal pathway consists of large bers that convey information about aspects of touch and proprioception used to
identify objects and surfaces (e.g., form, position, and temporal change). The neural signals travel via the medial lemniscal pathway
to synapse in the ventroposterolateral thalamic nuclei. From the thalamus, signals travel to the post-central gyrus [somatosensory
cortex (SI)], secondary somatosensory cortex (SII), and other higher order areas.
Spinothalamic Pathway
The spinothalamic pathway consists of smaller bers that convey information primarily about pain, temperature, and crude touch.
The lateral spinothalamic pathway carries exclusively information related to pain and temperature, while the anterior spinothalamic
pathway relays crude (non-discriminative) touch sensations such as pressure, tickling, and itching. The neural signals travel via the
spinothalamic pathway to the reticular formation, the intrinsic thalamic nuclei, and the SII.
All somatosensory signals are processed by specic thalamic nuclei. The ventroposteromedial nucleus receives cutaneous inputs
from the face and the ventroposterolateral nucleus receives inputs from the rest of the body. These nuclei have precise somatotopy:
the area of the body corresponds to a point on the thalamic nuclei as well as primary somatosensory cortex (postcentral gyrus) to
maintain the stimuluslocation on the body. The ventroposteroinferior (VPI) and the medial posterior nucleus (Pom) may also
have functional importance for the processing of tactile information in that they have cutaneous receptive elds and project to
cortical somatosensory areas.
Somatosensory Cortices
Much of our knowledge regarding the anatomy and function of cortical areas comes from research on animals and brain-injured
humans. However, recently noninvasive neuroimaging techniques have permitted the investigation of the relationship between
stimulus characteristics (e.g., frequency and amplitude) and physiological responses in the brains of neurologically intact humans.
These techniques include magnetoencephalography/electroencephography, positron emission tomography, and functional
magnetic resonance imaging. Brain activity is indexed by changes in cerebral blood ow, blood oxygenation levels, electrical activity,
and magnetic eld generation.
When the skin is touched, tactile information is sent to somatosensory areas of the brain (Fig. 1). If neurons in a particular
cortical area have predominant or exclusive responses to somatosensory stimuli, the area is considered to be involved in tactile
perception. In monkey and human brains, the major cortical areas considered to be somatosensory cortices are primary somato-
sensory cortex (SI), secondary somatosensory cortex (SII), parietal areas (areas 5 and 7b), the retroinsular cortex (Ri), and the
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Primary Somatosensory Cortex
Primary somatosensory cortex, or SI, is a region of postcentral cortex activated by tactile somatic stimuli. It includes Brodmann areas
1, 2, 3a, and 3b. These areas of cortex are activated almost exclusively from the contralateral side of the body. The neurons in SI have
preserved somatotopy and are arranged in columns so that neurons with receptive elds processing information from like areas of
the body are located together.
Somatotopic Organization
Each of the four subdivisions of SI has a complete representation of the body, or a homunculus (Fig. 2). The homunculus has
a medial-to-lateral organization, with the representations of the inferior limbs at the vertex, the upper limbs and mouth at the
convexity, and the tongue at the bottom of gyrus. In the homunculus, adjacent body parts are represented primarily next to
each other, but the cortical organization does not exactly match the body surface. For example, the representation of the hand
lies between the back of head and the face.
Cortical Magnification
There is a relative magnication of the amount of cortical area devoted to body parts with the highest tactile acuity and sensory
specialization (Fig. 2). For example, the lips, ngers, and hands are the primary organs for extracting tactile information and
they have proportionately larger representations. Cortical magnication means that there are many cortical cells for each afferent
nerve ber. This increase in the number of cells processing the same sensory input suggests that incoming information may be elab-
orated and recoded. Higher level processes, such as object identication, may require information in a different format from which
it was encoded.
Figure 1 Schematic diagram of ventrolateral (top) and dorsomedial (bottom) somatosensory areas in the human brain. Vertical lines indicate SI,
and small dots indicate SII, parietal operculum, and posterior insula. Checkerboards indicate SSA, and horizontal lines indicate SMA. From Caselli,
R.J., 1997. Tactile agnosia and disorders of tactile perception. In: Feinberg, T.E., Farah, M.J. (Eds.), Behavioral Neurology and Neuropsychology.
McGraw-Hill, New York, pp. 277288. by permission of Mayo Foundation.
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Multiple Body Representations
The different body representations are not just duplications of each other; areas 1 and 3b have two mirror-reversed body maps of
cutaneous receptors. Areas 2 and 3a respond primarily to proprioception or deep stimulation of the joints and muscles, although
area 2 does receive cutaneous inputs. One reason for multiple body representations is that different areas within the somatosensory
cortex have different functions.
To determine the function of SI, monkeys with damage to postcentral cortex have been studied. Lesions in area 3b affect all tactile
discrimination, with the exception of crude touch. Lesions to area 1 impair texture discrimination. Lesions to area 2 impairthe discrim-
ination of shape and size as well as nger coordination. Supporting these functions, single-cell recording research has located orien-
tation- and direction-selective neurons in area 2. Other single-cell recording research has shown that area 3a lacks the precise
somatotopy of tactile input areas and its connections to the cerebellum suggest it is important for proprioceptive feedback and move-
ment stabilization. In humans, damage to SI is rarely restricted to one area. Clinical data from extensive postcentral lesions suggest that
damage to SI produces lasting impairments of pressure sensitivity, two-point discrimination, point localization, and the discrimination
of object shape, size, and texture. In contrast, cortical lesions rarely affect the detection of touch, pain, and temperature.
Secondary Somatosensory Cortex
SI is bounded ventrolaterally by secondary somatosensory cortex (SII) or the parietal operculum (PO). In monkeys SII is located in
the parietal bank of the Sylvian sulcus behind the insula. In humans, SII includes the PO, and it extends more posteriorally to the
inferior parietal cortex. SII receives its peripheral inputs from SI and from the VPI thalamus. There are reciprocal connections
between SI and SII. It has many direct connections with all somatosensory areas with the exception of area 5. Unlike SI, in which
inputs are almost entirely contralateral, some proportion of SII neurons receive input from both sides of the body. Thus, SII can be
activated bilaterally with unilateral stimulation.
Somatotopic Organization
SII also has somatotopic organization. Although the entire body is systematically represented in SII, the digits of hand and the face
representations occupy most of the total area. According to cortical stimulation studies, human SII cortex has the face area in the
most supercial part, the foot area in the most medial part, and the hand area in between.
Figure 2 The homunculus on somatosensory cortex. Body parts with the highest tactile sensitivity occupy larger areas of cortex. From Open Stax
College, Illustration from Anatomy & Physiology, Connexions Website., June 19, 2013.
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The role of SII in tactile perception is not certain, but evidence suggests that SII is part of a higher-order association center for tactile
object recognition and learning. In monkeys, damage to SII produces severe impairments of shape discrimination, impairments of
learning, and altered size and roughness discrimination. In humans, lesions of SII impair texture and shape discrimination that
require active haptic exploration. Another role of SII is to integrate tactile and motor actions. In humans, neural activity in SII
can be modulated by concurrent somatosensory stimulation and nger movement. This modulation may reect an improved anal-
ysis of tactile signals during movements. fMRI and whole-head MEG studies show activation of SII during haptic object recognition
and tactile pattern discrimination.
Retroinsular Cortex and Somatosensory Insula
Retroinsular cortex (Ri) and the posterior insula also have robust somatosensory properties. They are contiguous with SI and are
located within the lateral Sylvian sulcus. Ri is adjacent to SII. In addition to SII and posterior SI, many neurons in Ri have bilateral
receptive elds. Neurons in these areas respond primarily to contralateral tactile stimuli and somatotopic representation is only
rough. In primates, the majority of neurons in the insula (Ig) also have bilateral receptive elds. The insula may be involved in
tactile learning and its role may be similar to the role of area TE in inferior temporal cortex for visual learning. In contrast to TE,
which is purely visual, the insula is multimodal. In humans, lesions involving area 40 (putative area SII) and posterior insula
can cause disorders of tactile object recognition.
Somatosensory Areas of the Posterior Parietal Cortex
In humans, SI is bounded dorsomedially by another somatosensory association cortex called supplementary sensory area (SSA)
encompassing Brodmanns area 5 (inferior parietal cortex) and area 7b (supramarginal gyrus). The stimulation of supplementary
motor area (SMA) during brain surgery sometimes elicits tactile sensations, suggesting that SMA may be a dorsomedial somatosen-
sory association cortex. Posterior parietal cortex is believed to integrate sensory and motor processing and to combine tactile and
proprioceptive information with other sensory modalities. In monkeys, damage to area 5 affects proprioceptive inputs and impairs
the non-visual guidance of arm movements. In humans, damage to the right superior parietal lobe can produce similar impair-
ments. Area 7b may have a role in higher order integration within the somatosensory system. The majority of neurons in area
7b have bilateral receptive elds. Furthermore, the area has multiple intra- and interhemispheric connections. They are activated
by non-painful tactile stimuli and, to a lesser extent, by visual and painful stimuli.
The multiple somatosensory cortical areas form a complex network. Somatosensory receiving areas are functionally involved with
cortical areas subserving motor and spatial functions. The existence of such a complex system in the human brain is important for
intentional, spatially guided motor movements that allow us to interact with tactile stimuli. The organization of the somatosensory
system emphasizes the fact that perception is an active process.
Primary and Complex Tactile Sensations
What parts of the neural system are involved in processing haptic information and our perceptions of touch? Primary tactile sensa-
tions refer to percepts arising from stimuli that vary along a single dimension. Often, primary somatosensory sensations can be eli-
cited through passive stimulation. Complex tactile sensations arise from stimuli that vary along multiple dimensions. Typically, in
haptic perception, complex tactile sensations are enhanced by active exploration of the stimuli.
Primary Somatosensory Functions
To study the relations between haptic perception and its neural mechanisms, psychophysical and physiological experiments have
used several stimulating techniques designed to distinguish the response properties of the mechanoreceptors and their afferent
nerve bers. These techniques include the cooling of the skin and the presentation of masking vibrations to decrease the sensitivity
of one type of tactile channel to study another one. Electrophysiological studies using microneurography record the activity of the
nerve bers innervating the hand. Vibration has been applied perpendicularly and tangentially to the skin by pins and probes. Peri-
odic and aperiodic gratings have been moved across the skin. Other stimuli include air puffs, embossed letters, steel wool, sand-
paper, and cloth.
Tactile Spatial Acuity
The sensitivity of tactile perception depends on the relation between mechanoreceptors and somatosensory cortex. Tactile acuity, or
sensitivity, is typically dened as the ability to distinguish between two points of stimulation. Different parts of the body have
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greater sensitivity to tactile stimulation than others. For example, the ngertips and the lips are more sensitive than the back. Tactile
acuity can be correlated with the properties of the afferent nerve bers and of neurons in SI. At the afferent nerve ber level, tactile
acuity corresponds to parts of the body that have the greatest density of nerve bers with both small and large receptive elds.
However, better two-point discrimination is found for nerve bers with small receptive elds (SA1 and RA1 channels): Two points
of stimulation are more likely to fall in two separate receptive elds and be distinguished from each other. Thus, there are more
small receptive eld nerve bers innervating the sensitive ngertips than the less sensitive palm. A similar relationship exists at
the cortical level. Larger regions of the SI are devoted to sensitive parts of the body. The cortical area of the hand is relatively greater
than that of the back, presumably to provide additional neural processing necessary for the perception of ne detail.
Although two-point threshold is still widely used among clinicians, this classic method of measuring tactile sensitivity has been
criticized for not measuring the true tactile acuity of the skin. Previously, researchers who used two-point threshold designs found
participants claimed to be able to distinguish two points of stimulation on the nger pad, even when there was no separation
between the two points. It is believed that the presence of non-spatial cues during the task allows smaller discrimination even
when the points of separation are set at zero. Therefore, many experimenters question the validity of two-point discrimination
and have moved to other methods of measuring tactile acuity: the grating threshold and the two-point orientation discrimination
threshold. Although both of these measures have their own limitations, they provide researchers and clinicians with a truer measure-
ment of tactile acuity. The grating threshold task uses different sizes of grooves and ridges. Easily manipulated, this technique has
allowed researchers to investigate tactile acuity on different parts of the body using a vertical/horizontal test. Usually, grooved
stimuli are presented with ridges oriented either horizontally or vertically, and the participant indicates the perceived orientation
of the stimulus. The two-point orientation discrimination (TPOD) task is a relatively novel measure of spatial acuity that combines
the orientation technique existing in grating stimuli tasks with the discrimination technique in the two-point threshold task. This
task relied on perception rather a response magnitude cue by stimulating participants with two contact points in two consecutive
trials and asking them to determine which orientation comes rst (vertical or horizontal).
Light Touch
Light touch refers to the sensation of near-threshold tactile stimulation. Nerve bers carrying light touch information are part of
both the medial lemnescal system and the spinothalamic system. The exact contribution of different cortical areas to these sensory
functions is not established, but 3- to 5-Hz pressure stimulation on various parts of the body produces contralateral activation in SI
and bilateral activation in SII.
Vibration activates most of the mechanoreceptors. Furthermore, it is easily manipulated experimentally and produces robust
changes in cortical activity. Typically, neuroimaging experiments compare simple vibratory stimulation with no stimulation (i.e.,
a resting state). Stimulating either the hand or the foot with vibration produces bilateral activation in SII, PO, and the insula.
However, activation in SI is greater in the contralateral hemisphere. Vibration also activates contralateral motor cortex, supplemen-
tary motor area, and anterior parietal areas.
Primary somatosensory functions are largely processed by SI and, to some extent, by SII. Neuropsychological data support this
conclusion. In patients who underwent surgery to relieve epileptic symptoms, the most severe disorders of tactile sensitivity,
two-point discrimination, point localization, and position sense were produced by lesions in the contralateral, postcentral gyrus.
The most severe defects occurred in patients whose lesions encroached the hand area of SI. Furthermore, unilateral cortical lesions
of SI produced clear bilateral sensory defects.
Complex Somatosensory Functions
Primary tactile perception varies along a single attribute and requires primarily passive touch. In contrast, most tactile experiences
involve three-dimensional objects that vary on multiple attributes. The perception of complex objects is enhanced by active touch,
or haptics, to help apprehend characteristic features that distinguish one object from another. When we explore objects, we use
stereotypical hand movements called exploratory procedures. People use exploratory procedures to obtain particular types of infor-
mation from objects. Lateral motion or rubbing is used to extract texture, pressure is used to extract hardness, and contour following
is used for detailed shape. Enclosure, or a grasp, is often used to extract global shape and size. In addition, exploratory procedures
are often executed simultaneously to extract multiple object features. Thus, the achievement of tactile object recognition results from
the integration of information from cutaneous, motor, and cognitive systems.
The hand, with its mechanoreceptors and motoric abilities, is specialized to process information about an objects material
substance. This information includes texture (roughness), hardness, and temperature. Sometimes, it is referred to as microgeometric
information because it does not change the global shape of an object. Compared to vision, touch is superior for the discrimination
of surface texture. When multiple object properties within an object are equated for perceptual discriminability, people prefer to
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categorize objects by touch on the basis of texture rather than shape. Furthermore, blind and sighted people are equally good at
discriminating texture.
The perception of surface texture includes attributes such as roughness, hardness, softness, elasticity, and viscosity. The most
widely examined texture dimension is roughness. To investigate roughness, studies use surfaces with regularly spaced ridges (grat-
ings) and surfaces with raised dots to systematically and independently manipulate the relevant physical features. The spacing
between the ridges (groove width) and the amount of force applied to the stimulus correlate highly with perceived roughness.
The speed at which stimuli are moved across the skin, actively or passively, has little effect if the forces are equal. These results
suggest that mechanoreceptors provide the signals for roughness perception independently of kinesthetic input.
However, roughness perception is not a simple function of any particular mechanoreceptor and its afferent nerves response rate.
Instead, roughness perception is related to spatial variation in responses between nerve bers. Specically, it is the between-ber
spatial variation in ring responses of the SA bers that corresponds with perceived roughness. For example, in the ngertip,
between-ber spatial variation is the difference in the discharge rate over a distance of 1 or 2 mm.
At a cortical level, roughness perception can be associated with groove width and, to some extent, with force and velocity. In
monkeys, strong responses in areas 1 and 3b of SI are produced from the active stroking of the ngertips over gratings with constant
ridge and varying groove. In these areas, neurons respond differentially for rough stimuli, smooth stimuli, and combinations of the
roughness, the force applied, and the velocity of stroking. In SII, neurons respond to the beginning, middle, and end of the strokes.
Passive and active touching of the gratings activates different proportions of neurons. In humans, neuroimaging studies of passive
roughness perception demonstrate corresponding SI and SII activation.
Complex Pattern and Shape
Shape and complex patterns can be considered part of an objects macrogeometry. In contrast to texture, the hand is less well suited
for the extraction of these global properties: The tactile perception of pattern and shape involves the integration of input from
multiple receptors in the skin and sometimes the integration of contour information over space and time. It appears that somato-
sensory cortex has specialized regions for processing different types of tactile information.
The identication of complex forms, such as Braille and alphabet letters, requires the recruitment of information from a number
of afferent nerve bers. Pattern perception appears to rely on the pattern of SA1 nerve ber responses over a region of skin. The
pattern of SA1 responses, often illustrated using spatial event plots, is similar to the stimulus pattern, indicating that they are impor-
tant for the tactile perception of detail. Confusion in the perception of two patterns correlates with the similarity between their
spatial event plots. For example, Cs are confused with Os.
Pattern and shape information is processed by different areas of cortex from texture and hardness information. In monkeys, the
discrimination of hardness, texture, and shape can be differentially impaired depending on which part of SI is ablated. Lesions of
area 2 impair shape and contour discriminations but spare texture and hardness discriminations. The opposite pattern is found for
lesions of area 1. Excisions of area 3b produce impairments in all aspects of tactile discrimination learning.
Neuroimaging studies in humans provide converging evidence that different areas of the somatosensory cortices are involved in
the tactile perception of shape and size from texture and hardness. Contralateral activation of SI is produced by the discrimination
of texture, shape, and hardness compared to a grasp. Although both shape and texture activate contralateral inferior temporal
regions, texture differentially activates the parietal operculum. Shape and length discrimination activates the contralateral supramar-
ginal gyrus, contralateral premotor cortex, and bilateral angular gyri. Thus, specic parts of the somatosensory cortices beyond SI are
responsible for the perception of particular tactile properties. Global or macrogeometric features (shape and size) require additional
integration of somatosensory information over space and time as well as more extensive somatosensory processing.
Object Recognition
The recognition of everyday, common objects through touch can be fast and accurate, often requiring less than 2 s. Haptic object
recognition plays a frequent role in our lives. We use it every time we reach into our pockets for keys or coins. Real objects vary along
multiple object dimensions and typically have characteristic features to help identify them. To determine the cortical areas involved
in tactile object recognition, I turn to studies of patients with selective brain lesions and neuroimaging studies of normal
Tactile object recognition can be defective if primary sensory perception is impaired. Astereognosis is dened as the general inability
to recognize objects by touch in the absence of vision. Although damage to higher level somatosensory regions can produce haptic
object recognition decits, astereognosis refers most commonly to haptic object recognition decits resulting from severe primary
somatosensory imperception. It reects damage to any level of the somatosensory system from the peripheral nerves, spinal cord,
brain stem, and thalamus to SI. Patients with astereognosis typically have difculty perceiving light touch, vibratory sensation,
proprioception, supercial pain, temperature, two-point discrimination, weight discrimination, texture, substance, double simul-
taneous stimulation, and shape. The impairment is usually restricted to one hand. SI is considered to be the cortical substrate of
Furthermore, when trying to identify objects by touch, patients with astereognosis do not tactually explore the object. This lack of
appropriate exploration suggests that primary tactile functions are not adequately perceived or integrated with motor information
during tactile object processing. Although sensorimotor integration and proprioception are important for tactile object recognition,
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it should be noted that tactile exploration is not necessary. Paralysis by itself does not produce defective tactile object recognition. In
addition, objects can often be identied by touch if they are passively moved over a persons hand.
Tactile Agnosia
Impaired haptic object recognition can arise from decits in higher order tactile processing. In contrast to astereognosis, tactile
agnosia is the inability to recognize objects by touch despite adequate primary somatosensory functions, intellectual ability, atten-
tional capacity, and linguistic skill. Although patients with tactile agnosia cannot recognize objects by touch, they can often draw an
accurate picture of an unrecognized tactually perceived object or match it to another object. The parietotemporal cortices, possibly
involving SII, are considered to be cortical substrates for tactile agnosia. One patient with unilateral tactile agnosia had a left inferior
parietal lesion involving the SII complex. She was able to discriminate and categorize objects on the basis of stimulus properties, but
she was unable to recognize objects with the contralateral hand. It appeared that she was impaired in her ability to integrate infor-
mation about the various object attributes to create a representation of the object as a whole.
Evidence from neuroimaging studies with non-brain-damaged individuals has also implicated SII as being important for tactile
object recognition. When activation from exploration is subtracted, SII is activated bilaterally, with stronger activation on the contra-
lateral side. Superior parietal and medial temporal cortices are also involved. Together, these cortical regions suggest that informa-
tion about tactile features, spatial properties, and object identity is being integrated during tactile object recognition.
Tactile agnosia may also be produced from bilateral lesions in the subcortical part of the angular gyrus. The subcortical lesions
presumably disconnect somatosensory association cortex from semantic memory stores located in the inferior temporal lobe.
Cortical Organization of the Somatosensory Cortex
Plasticity and Reorganization of Somatosensory Cortex
Haptic perception can be inuenced by changes occurring in somatosensory cortex as a result of trauma or experience. Several
factors can inuence the relative proportion of somatosensory cortex devoted to a particular body part or parts: the lack of sensory
input due to the loss of a limb, the amount of stimulation or practice on a particular region of skin, and the active nature of the
tactile task.
Changes in Sensory Input and Phantom Limbs
The somatosensory cortical maps can change when peripheral input changes or when signals from a specic part of the body no
longer reach cortex. Signals can be prevented from reaching cortex due to the amputation of a body part or damage to the peripheral
nerves conducting the signal from the skin. In monkeys, cutting nerves from the ngers eliminates responses in the corresponding SI
region, but over time, stimulation of adjacent ngers may activate cortical cells in the lost digits cortical area. The area of cortex
devoted to that body part reduces over time.
The presence of phantom limbs provides good evidence for the plasticity of the human somatosensory cortex and how experi-
ence can change the functional organization of the brain. In humans, amputation often produces a phenomenon called the
phantom limb; amputees experience sensations as if the limb existed. The plasticity of the synaptic connectivity of neurons in SI
is one explanation for the cause of phantom limbs. For example, phantom sensation is related to changes in SI. Some studies
support a remapping of somatosensory cortex in which the portion of SI devoted to the face (an active area) encroaches or is remap-
ped into the adjacent area devoted to the absent limb (an inactive area). Some amputees can relieve an itch in their phantom limb
by scratching the body region that is located in the homunculus next to the cortical area of the amputated limb. Neuroimaging
experiments of patients with phantom upper limbs conrm an enlargement of the primary sensory eld of the face.
Effects of Training
Somatotopic maps can be altered by increasing the stimulation to a particular body part location. Stimulating a specic region of
skin can produce an expansion of the cortical area receiving its input. Furthermore, practice in the use of a particular ngertip can
increase the cortical area representing that ngertip.
Evidence that training can effectively modify cortical representations comes from neuroimaging studies comparing string players
and non-musicians. Musicians had stronger cortical responses than non-musicians when the ngers of their left hands were stim-
ulated. Furthermore, the effect was strongest in musicians who began their careers early in life. It can also be demonstrated that the
somatotopic cortical representations of the nger areas were altered in blind Braille readers who used three ngers of both hands to
read. Instead of the typical homuncular pattern observed in the neural responses of nger stimulation in sighted subjects, Braille
readers had distorted organization in one or both hands. This disorganization had perceptual correlates. The same Braille readers
also had difculty identifying which nger was touched.
Activity of Body Part
The active nature of perception appears to inuence cortical representation. Before surgery, patients with webbed hands had
decreased cortical representations for the ngers and a reduced hand area in SI. After surgery, when the ngers functioned indepen-
dently, the cortical hand area expanded and a greater distance was found between the representations of the thumb and little nger.
Thus, somatosensory cortical maps are not static.
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Dual Streams of Processing for Object Recognition and Localization in Somatosensory Cortices
When we recognize objects, not only do we have to identify the objects but also we must localize them in space. In visual cortical
areas, two separate processing streams have been identied. The dorsal stream, leading from visual cortex to the parietal lobes,
conveys information about an objects spatial location (where) and how to interact with it. The ventral stream, leading from visual
cortex to the temporal lobes, conveys information about object identity (what). A similar dual stream of processing is proposed
for the somatosensory system. Physiologic studies and behavioral studies suggest a functional distinction between ventrolateral
(what) and dorsomedial (where) somatosensory association cortex (Fig. 1). Ventrolateral somatosensory association cortex
includes SII, the parietal operculum, and the posterior insula. Dorsomedial somatosensory association cortex involves SSA
and SMA.
The “What” Pathway
The ventrolateral stream processes somatosensory information regarding object recognition, tactual learning, and memory. Lesions
of this pathway result in tactile agnosia, independent of other tactile or spatial abnormalities. Specic evidence for a distinction
between somatosensory whatand wheresystems in humans comes from a patient with unilateral tactile agnosia. Her selective
impairment of tactile object recognition arose from a unilateral lesion involving the left inferior parietal lobe, thought to be part of
the ventral pathway. She was unable to recognize objects with her right hand despite normal exploration procedures. In contrast, she
was completely normal in her ability to perform visual and tactile spatial tasks. Thus, she was selectively impaired in whatbut not
The “Where/How” Pathway
The dorsomedial stream is concerned with sensorimotor integration and tactile spatiotemporal functions. In particular, an apraxia
astereognosis syndrome has been identied in patients with damage to dorsomedial somatosensory association areas. Patients with
extensive damage to SMA and SSA have moderate to severe impairment of primary and complex tactile functions. They have severe
limb apraxia with extremely disordered tactile search strategies. In addition to the faulty spatial and temporal control of movement,
they have an analogous spatiotemporal defect of tactile perception: They have difculty localizing a stimulus within a limb or deter-
mining whether they have been touched once or twice. Other patients with damage to the superior parietal lobe have profound
impairments in shape discrimination and spatial orientation without tactile sensory impairments. From these patients, it is sug-
gested that there may be a non-tactual, supramodal factor in tactile impairments. In summary, patient data suggest that the somato-
sensory wheresystem can be differentially impaired.
Haptic Learning and Memory
Once haptic information is perceived, what parts of the brain are involved with its storage and retrieval? The hands feel object
surfaces to sample and discriminate particular object properties. This information is sent to the brain so that it can be retrieved later.
Most of our knowledge about haptic memory comes from experimental lesion studies of learning in the monkey, but recent func-
tional neuroimaging studies have provided supporting evidence in humans. Haptic learning and recognition appear to be mediated
by a circuit going from SI to SII, the insular granular cortex, the amygdala, hippocampus, and perirhinal cortex. This pathway is
organized in a similar fashion as the visual learning pathway that accesses the limbic structures of the temporal lobe through inferior
temporal cortex.
The anatomical organization of haptic learning in monkeys and in humans appears to be similar. Haptic learning and recogni-
tion activate areas involved in motor function (premotor areas, SMA, and the cerebellum) and in haptic perception (SI, SII, SSA, and
superior parietal lobule). In addition, the tasks activate limbic and paralimbic structures (hippocampus, amygdala, insular cortex,
orbitofrontal cortex, and cingulate gyrus), prefrontal areas, and the striatum. The major difference between haptic learning and
recognition tasks is the relatively higher activation of the striatum and cerebellum during learning.
Selective Attention in Touch
Selective attention has been shown to inuence haptic perception. It not only affects the ability to discriminate among stimuli, but
also appears to modulate the properties of somatosensory cortical neurons by either enhancing or suppressing their activity. In
monkeys, the ring of neurons in SI and SII is dependent on whether the monkey is paying attention to a stimulus. Attention
can increase the neurons response to a stimulus. Relatively larger responses are found in somatosensory neurons when the animal
is attending to the tactile stimulus than when it is not. Similarly, neuroimaging studies in humans demonstrate that attention to
vibratory stimulation can enhance signal processing in SI.
Attention can also change the signal-to-noise ratio in neuronal response. Attention may be a mechanism for reducing distrac-
tions through the suppression of signals from non-cued channels. This produces a selective increase of signal magnitude in an atten-
tionally cued channel. In monkeys, attending to one of several simultaneous vibrotactile stimuli suppressed ring rates in
populations of neurons in SII and area 7b prior to the presentation of the target stimulus and enhanced activity during and after
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the target. A similar nding can be demonstrated in humans anticipating a stimulus. Behaviorally, individuals respond more quickly
to a site when a vibratory stimulus is expected. Neurophysiologically, anticipation of either a focal touch or painful shock produces
decreases in activation for regions of SI located outside of the representation for that skin area of the expected stimulus. Although the
mechanisms underlying increases and decreases of activation of somatosensory cortical areas are not fully understood, it is clear that
attention modulates the activity of the neurons.
Hemispheric Specialization
As noted previously, haptic perception is largely lateralized. Do both hemispheres equally process haptic information? To
demonstrate hemispheric function and interhemispheric communication, two types of patients have been studied: patients
with unilateral hemispheric lesions and split-brainpatients who have undergone surgery dividing the cerebral commissures.
Most interhemispheric transfer occurs via the corpus callosum (a large collection of nerve bers that carry information from
one side to the other).
In intact humans, it remains unclear whether hemispheric specialization exists for primary somatosensory functions (e.g.,
pressure sensitivity, vibration sensitivity, two-point discrimination, or point localization). However, unilateral SI lesions
frequently result in severe and long-lasting defects in the contralateral hand. There is growing evidence from brain-damaged
patients that the two brain hemispheres are specialized for certain higher haptic functions requiring the spatial exploration
of objects or ne temporal analysis. The left handright hemisphere combination is better able to perform tasks with a spatial
component, such as reading Braille or tactually determining the orientation of rods. The right handleft hemisphere combina-
tion can perform these tasks if familiar stimuli are used, only a few objects are presented, or when linguistic processing is
Given the lateralization of the haptic system, what information perceived by one hand is available to other if the two hemi-
spheres cannot share information? The lack of interhemispheric transfer of haptic information (touch, pressure, and proprio-
ception) can be demonstrated in the performance of split-brain patients. In a haptic task in which a set of objects are rst felt
using one hand and then retrieved from a larger set of objects, split-brain patients can accurately retrieve the objects with the
same hand but not the other hand. Furthermore, these patients can name and describe objects explored tactually by the right
hand but not by the left hand. In a tactile task in which the patient is touched on the ngertip of one hand and has to touch
that ngertip with his or her thumb, the split-brain patient can perform the task with the same hands thumb but not with the
other hands thumb. Last, in a tactile task in which the split-brain patients hand is pressed into a posture, the patient can
mimic the posture with the same hand but not with the other hand. In summary, haptic perception and proprioception are
largely lateralized.
The understanding of haptic perception requires an evaluation of haptic object recognition in addition to an evaluation of
basic tactile responses. Most haptic object recognition tasks contain sensory, spatial, proprioceptive, constructive, and motor
components. As a result, haptic perception involves functional interconnections between SI, SII, premotor regions, and
more posterior portions of the parietal cortex. There is still much to be learned about the neural bases of haptic perception.
In particular, we need to know more about the roles of somatosensory cortices for the processing of complex multidimensional
stimuli, such as real objects. Recent evidence suggests that the functional organization of the neural pathways is similar to that
of vision.
Further Reading
Caselli, R.J., 1997. Tactile agnosia and disorders of tactile perception. In: Feinberg, T.E., Farah, M.J. (Eds.), Behavioral Neurology and Neuropsychology. McGraw-Hill, New York,
pp. 277288.
Dijkerman, H.C., De Haan, E., 2007. Somatosensory processes subserving perception and action. Behav. Brain Sci. 30, 189201.
Galace, A., Spence, C., 2010. Touch and the body: the role of the somatosensory cortex in tactile awareness. Psyche 16, 3167.
Grunwald, M., 2008. Human Haptic Perception: Basics and Applications. Birkhauser, Basel.
Heller, M.A., Schiff, W., 1991. The Psychology of Touch. Erlbaum, Hillsdale, NJ.
James, T.W., Kim, S., Fisher, J.S., 2007. The neural basis of haptic object processing. Can. J. Exp. Psychol. 61, 219229.
Klatzky, R.L., Lederman, S.J., 2011. Haptic object perception: spatial dimensionality and relation to vision. Philos. Trans. R. Soc. Lond B Biol. 366, 30973105.
Lederman, S.J., Klatzky, R.L., 2009. Haptic perception: a tutorial. Atten. Percept. Psychophys. 71, 14391459.
Reed, C.L., Caselli, R.J., Farah, M.J., 1996. Tactile agnosia. Underlying impairment and implications for normal tactile object recognition. Brain 119 (Pt 3), 875888.
Saal H.P., Bensmaia S.J., 2014. Bensmaia Touch is a team effort: interplay of submodalities in cutaneous sensibility. Trends Neurosci. 37(12), 689697.
Sathian, K., 2016. Analysis of haptic information in the cerebral cortex. J. Neruophysiol. 116, 17951806.
Weisenberger, J.M., 2001. Cutaneous perception Blackwell. In: Goldstein, E.B., Humphreys, G.W., Shiffrar, M., Yost, W. (Eds.), Handbook of Perception 2001. Wiley-Blackwell.
Change History: May 2017. Authors Reed and Ziat added Abstract and updated the text and further readings to this entire article, and added new Figure 2.
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... The human foot is highly sensitive to touch stimulation ((Dim and Ren, 2017)) and the sole of the foot contains similar mechanoreceptors that are found in the human palm (Strzalkowski et al., 2017). Because the feet serve different functions (i.e., gait control, maintaining posture, body orientation, and walking) than the hands, the distribution of afferent receptors and their frequency responses rely on population firing rather than individual neuron firing as it seems to be the case for the hand (Strzalkowski et al., 2017;Reed and Ziat, 2018;de Grosbois et al., 2020). Haptic feedback on the feet have been used for multiple purposes such as robotic telepresence (Jones et al., 2020), illusory self-motion (Riecke and Schulte-Pelkum, 2013), gait control in elderly (Galica et al., 2009;Lipsitz et al., 2015), and improved situational awareness in blind people (Velázquez et al., 2012). ...
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... Haptic perception is divided into two dependent sensory sub-modalities (Reed & Ziat, 2018): ...
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... Haptic perception is divided into two dependent sensory sub-modalities (Reed & Ziat, 2018): ...
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Driving a motorcycle relies on the feedback provided by several human sensory systems, on the one hand, and anticipation of the consequences of control actions, on the other hand. Driving simulators aim to create the illusion of driving by stimulating the driver’s sensory systems. However, a significant number of drivers experience simulator sickness, which hinders the usefulness of driving simulators in their applications, such as driving behavior research or training / re-training. Simulator sickness occurrence is often attributed to sensory conflict. In this work, we propose an approach to understanding simulator sickness by considering the need for coherence between the complexity of the vehicle model and the complexity of the simulator from a hardware point-of-view, which constrains the fidelity of the reproduced sensory stimuli. We then describe the design of a proof-of-concept system that considers the particular issue of haptic feedback for the handlebars of a motorcycle-riding simulator. We will use this system in further experiments to demonstrate the impact of the coherence or mismatch of those two aspects on controllability and simulator sickness occurrence.
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This tutorial focuses on the sense of touch within the context of a fully active human observer. It is intended for graduate students and researchers outside the discipline who seek an introduction to the rapidly evolving field of human haptics. The tutorial begins with a review of peripheral sensory receptors in skin, muscles, tendons, and joints. We then describe an extensive body of research on "what" and "where" channels, the former dealing with haptic perception of objects, surfaces, and their properties, and the latter with perception of spatial layout on the skin and in external space relative to the perceiver. We conclude with a brief discussion of other significant issues in the field, including vision-touch interactions, affective touch, neural plasticity, and applications.
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In a series of experimental investigations of a subject with a unilateral impairment of tactile object recognition without impaired tactile sensation, several issues were addressed. First, is tactile agnosia secondary to a general impairment of spatial cognition? On tests of spatial ability, including those directed at the same spatial integration process assumed to be taxed by tactile object recognition, the subject performed well, implying a more specific impairment of high level, modality specific tactile perception. Secondly, within the realm of high level tactile perception, is there a distinction between the ability to derive shape ('what') and spatial ('where') information? Our testing showed an impairment confined to shape perception. Thirdly, what aspects of shape perception are impaired in tactile agnosia? Our results indicate that despite accurate encoding of metric length and normal manual exploration strategies, the ability tactually to perceive objects with the impaired hand, deteriorated as the complexity of shape increased. In addition, asymmetrical performance was not found for other body surfaces (e.g. her feet). Our results suggest that tactile shape perception can be disrupted independent of general spatial ability, tactile spatial ability, manual shape exploration, or even the precise perception of metric length in the tactile modality.
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We review the organization of the neural networks that underlie haptic object processing and compare that organization with the visual system. Haptic object processing is separated into at least two neural pathways, one for geometric properties or shape, and one for material properties, including texture. Like vision, haptic processing pathways are organized into a hierarchy of processing stages, with different stages represented by different brain areas. In addition, the haptic pathway for shape processing may be further subdivided into different streams for action and perception. These streams may be analogous to the action and perception streams of the visual system and represent two points of neural convergence for vision and haptics.
Haptic sensing of objects acquires information about a number of properties. This review summarizes current understanding about how these properties are processed in the cerebral cortex of macaques and humans. Non-noxious somatosensory inputs, after initial processing in primary somatosensory cortex, are partially segregated into different pathways. A ventrally directed pathway carries information about surface texture into parietal opercular cortex and thence to medial occipital cortex. A dorsally directed pathway transmits information regarding the location of features on objects to the intraparietal sulcus and frontal eye fields. Shape processing occurs mainly in the intraparietal sulcus and lateral occipital complex, while orientation processing is distributed across primary somatosensory cortex, the parietal operculum, anterior intraparietal sulcus and a parieto-occipital region. For each of these properties, the respective areas outside primary somatosensory cortex also process corresponding visual information and are thus multisensory. Consistent with the distributed neural processing of haptic object properties, tactile spatial acuity depends on interaction between bottom-up tactile inputs and top-down attentional signals in a distributed neural network. Future work should clarify the roles of the various brain regions and how they interact at the network level.
Traditionally, different classes of cutaneous mechanoreceptive afferents are ascribed different and largely non-overlapping functional roles (for example texture or motion) stemming from their different response properties. This functional segregation is thought to be reflected in cortex, where each neuron receives input from a single submodality. We summarize work that challenges this notion. First, while it is possible to design artificial stimuli that preferentially excite a single afferent class, most natural stimuli excite all afferents and most tactile percepts are shaped by multiple submodalities. Second, closer inspection of cortical responses reveals that most neurons receive convergent input from multiple afferent classes. We argue that cortical neurons should be grouped based on their function rather than on their submodality composition.
Comprehensive textbook combining research and practical applications Edited by Dr. Martin Grunwald, director of the Haptic-Research Laboratory at the Paul-Flechsig Institut for Brain Research, University of Leipzig, GermanyChapters written by leading scientists in the fieldInternational authorship from academia and industryActive touch perception - also known as haptic perception - is of primary importance for the planning, direction and execution of everyday actions. This most complex of human sensory systems is gaining ever more importance for various scientific disciplines as well as practical industrial applications.In this book an international team of 80 authors presents a comprehensive collection of writings on both aspects of research on human haptic perception.After a theoretical and historical introduction, the chapters are dedicated to neurophysiological basics as well as the psychological, clinical and neuropsychological aspects of haptic perception. Results of studies into human haptic perception in the fields of virtual haptics and robotics are also included. In the final section, contributions from the applied and industrial sectors illustrate the practical uses of knowledge about the human sense of touch.This easily accessible textbook gives not only students, scientists and those with prior knowledge, but also interested laypersons insights into a fascinating area of study that is constantly discovering new challenges and presenting innovative solutions.
The functions of the somatosensory system are multiple. We use tactile input to localize and experience the various qualities of touch, and proprioceptive information to determine the position of different parts of the body with respect to each other, which provides fundamental information for action. Further, tactile exploration of the characteristics of external objects can result in conscious perceptual experience and stimulus or object recognition. Neuroanatomical studies suggest parallel processing as well as serial processing within the cerebral somatosensory system that reflect these separate functions, with one processing stream terminating in the posterior parietal cortex (PPC), and the other terminating in the insula. We suggest that, analogously to the organisation of the visual system, somatosensory processing for the guidance of action can be dissociated from the processing that leads to perception and memory. In addition, we find a second division between tactile information processing about external targets in service of object recognition and tactile information processing related to the body itself. We suggest the posterior parietal cortex subserves both perception and action, whereas the insula principally subserves perceptual recognition and learning.
Tactile agnosia and disorders of tactile perception
  • R J Caselli
Caselli, R.J., 1997. Tactile agnosia and disorders of tactile perception. In: Feinberg, T.E., Farah, M.J. (Eds.), Behavioral Neurology and Neuropsychology. McGraw-Hill, New York, pp. 277-288.
Touch and the body: the role of the somatosensory cortex in tactile awareness
  • A Galace
  • C Spence
Galace, A., Spence, C., 2010. Touch and the body: the role of the somatosensory cortex in tactile awareness. Psyche 16, 31-67.