Tom C Freeman

Tom C Freeman
Cardiff University | CU · School of Psychology

About

88
Publications
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Introduction
Tom Freeman currently works at the School of Psychology, Cardiff University. Tom does research in vision and hearing, especially in the problems faced by active observers and listeners.

Publications

Publications (88)
Article
Purpose: Infantile nystagmus (IN) presents with continuous, predominantly horizontal eye oscillations. It remains controversial whether those with IN have normal horizontal pursuit, while vertical pursuit has rarely been studied. We examined whether there are pursuit deficits associated with IN by investigating the effect of target direction, velo...
Article
Full-text available
Purpose: Small moving targets are followed by pursuit eye movements, with success ubiquitously defined by gain. Gain quantifies accuracy, rather than precision, and only for eye movements along the target trajectory. Analogous to previous studies of fixation, we analyzed pursuit performance in two dimensions as a function of target direction, velo...
Article
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The dysconnection hypothesis of schizophrenia (SZ) proposes that psychosis is best understood in terms of aberrant connectivity. Specifically, it suggests that dysconnectivity arises through aberrant synaptic modulation associated with deficits in GABAergic inhibition, excitation-inhibition balance and disturbances of highfrequency oscillations. Us...
Article
To estimate object speed with respect to the self, retinal signals must be summed with extraretinal signals that encode the speed of eye and head movement. Prior work has shown that differences in perceptual estimates of object speed based on retinal and oculomotor signals lead to biased percepts such as the Aubert-Fleischl phenomenon (AF), in whic...
Article
Does the preference for visual speed extend to motion perception when the eye moves? Current evidence from psychophysics and neuroscience is limited to small patches of image motion and stationary fixation. Active observers, however, are more likely to use large patches of retinal flow and extraretinal signals accompanying eye movement to judge mot...
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According to Bayesian models, perception and cognition depend on the optimal combination of noisy incoming evidence with prior knowledge of the world. Individual differences in perception should therefore be jointly determined by a person’s sensitivity to incoming evidence and their prior expectations. Pellicano and Burr (2012) proposed that indivi...
Article
Hearing is confronted by a similar problem to vision when the observer moves. The image motion that is created remains ambiguous until the observer knows the velocity of eye and/or head. One way the visual system solves this problem is to use motor commands, proprioception and vestibular information. These ‘extra-retinal signals’ compensate for sel...
Article
To estimate object speed in the world, retinal motion must be summed with extra-retinal signals that tell about the speed of eye and head movement. Prior work has shown that differences in perceptual estimates of retinal and oculomotor (eye) speed lead to effects such as the Aubert-Fleischl phenomenon (AF) in which pursued targets are typically per...
Article
Suppression of 5-25 Hz oscillations have been observed in MT+ during pursuit eye movements, suggesting oscillations that play a role in oculomotor control and/or the integration of extraretinal signals during pursuit. The amplitude of these rhythms appears to covary with head-centred eye position, but an alternative is that they depend on a velocit...
Article
Hearing is confronted by a similar problem to vision as the observer moves. Movement of the sensors creates image motion that remains ambiguous until the observer knows the velocity of eye and head. The visual system solves this problem using motor commands, proprioception, and vestibular information (so-called “extra-retinal signals”), but the sol...
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Purpose: Infantile nystagmus (IN) is a pathological, involuntary oscillation of the eyes consisting of slow, drifting eye movements interspersed with rapid reorienting quick phases. The extent to which quick phases of IN are programmed similarly to saccadic eye movements remains unknown. We investigated whether IN quick phases exhibit 'saccadic inh...
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The natural viewing behavior of moving observers ideally requires target-selecting saccades to be coordinated with automatic gaze-stabilizing eye movements such as optokinetic nystagmus. However, it is unknown whether saccade plans can compensate for reflexive movement of the eye during the variable saccade latency period, and it is unclear whether...
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Evidence that the auditory system contains specialised motion detectors is mixed. Many psychophysical studies confound speed cues with distance and duration cues and present sound sources that do not appear to move in external space. Here we use the 'discrimination contours' technique to probe the probabilistic combination of speed, distance and du...
Article
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As a potential exemplar for understanding how volitional actions emerged from reflexes, we studied the relationship between an ancient reflexive gaze stabilization mechanism (optokinetic nystagmus [OKN]) and purposeful eye movements (saccades) that target an object. Traditionally, these have been considered distinct (except in the kinematics of the...
Article
Extra-striate regions are thought to receive non-retinal signals from the pursuit system to maintain perceptual stability during eye movements. Here, we used magnetoencephalography (MEG) to study changes in oscillatory power related to smooth pursuit in extra-striate visual areas under three conditions: 'pursuit' of a small target, 'retinal motion'...
Conference Paper
Background / Purpose: As an active observer progressing through the visual world, saccades intended to foveate targets of interest must co-occur with eye movements required to stabilize the retinal image (e.g. the vestibular-ocular reflex or optokinetic nystagmus [OKN]). However, saccades and OKN are generally treated as fundamentally different t...
Article
Full-text available
Previous research has shown that vection can be enhanced by adding horizontal simulated viewpoint oscillation to radial flow. Adding a horizontally oscillating fixation target to purely radial flow induces a superficially similar illusion of self-motion, where the observer's perceived heading oscillates left and right as their eyes pursue the movin...
Article
Accurate estimation of object movement is difficult for moving observers, since it is unclear whether to attribute retinal motion to movement of the object or observer. Recently, we have proposed a purely visual neural mechanism (‘flow parsing’) which aids in this process by attempting to globally filter retinal motion due to observer movement. Her...
Article
Full-text available
Pursuit eye movements give rise to retinal motion. To judge stimulus motion relative to the head, the visual system must correct for the eye movement by using an extraretinal, eye-velocity signal. Such correction is important in a variety of motion estimation tasks including judgments of object motion relative to the head and judgments of self-moti...
Article
Introduction Adaptation to smooth pursuit produces a compelling motion aftereffect, such that a stationary test stimulus is perceived to move in the opposite direction to that of the adapting eye movement. It has been proposed that the illusory motion results from adaptation of pursuit-sensitive neurons in MT+ that receive extra-retinal eye movemen...
Article
Simultaneously adapting to retinal motion and non-collinear pursuit eye movement produces a motion aftereffect (MAE) that moves in a different direction to either of the individual adapting motions. Mack, Hill and Kahn (1989, Perception, 18, 649-655) suggested that the MAE was determined by the perceived motion experienced during adaptation. We tes...
Article
Age is known to affect sensitivity to retinal motion. However, little is known about how age might affect sensitivity to motion during pursuit. We therefore investigated direction discrimination and speed discrimination when moving stimuli were either fixated or pursued. Our experiments showed: (1) age influences direction discrimination at slow sp...
Article
Large moving patterns give rise to nystagmus eye movements. We find that following a prolonged period of nystagmus a pronounced motion aftereffect (MAE) is experienced. Naive observers viewed two vertically translating striped patterns (16.5×25 deg) separated horizontally by 10 deg. Stripes consisted of dimly illuminated dots shown on a black backg...
Article
Following prolonged unidirectional smooth pursuit, a stationary, fixated test appears to move in the opposite direction to the adapting eye movement. The site of the adaptation is almost certainly extra-retinal and thought to be a consequence of the suppression of pursuit afternystagmus (Chaudhuri, 1991, Vis. Res. 31, 1639-1645). We investigated th...
Article
Motion gradients are important cues for depth perception but can be modified by eye-movements. For example, a translating corrugated surface can be specified by combining relative motion (shear) and translation, both of which are needed to determine depth order correctly. During pursuit, however, the eye movement cancels retinal translation so an e...
Article
Perceived visual speed has been reported to be reduced during walking. This reduction has been attributed to a partial subtraction of walking speed from visual speed (F. H. Durgin & K. Gigone, 2007; F. H. Durgin, K. Gigone, & R. Scott, 2005). We tested whether observers still have access to the retinal flow before subtraction takes place. Observers...
Article
We've known for over a century that estimates of pursuit speed are typically lower than estimates of retinal speed. The benchmark is the Aubert-Fleischl phenomenon, the name given to the perceived slowing of moving objects when they are pursued. Many other pursuit-related phenomena can be accounted for in a similar way: stationary objects appear to...
Article
There is little direct psychophysical evidence that the visual system contains mechanisms tuned to head-centered velocity when observers make a smooth pursuit eye movement. Much of the evidence is implicit, relying on measurements of bias (e.g., matching and nulling). We therefore measured discrimination contours in a space dimensioned by pursuit t...
Article
One means of judging velocity during pursuit is to compare signals encoding retinal motion with information about the eye movement. Some authors argue the latter is based exclusively on extra-retinal signals originating from the motor-control system. Others argue for a compound reference signal comprising extra-retinal and retinal components. Recen...
Article
Adapting to retinal motion yields a motion aftereffect (MAE). An ‘extra-retinal’ MAE can be obtained when adapting to repetitive pursuit. When the adaptation simultaneously combines retinal motion and pursuit, the resulting direction of the MAE suggests the two types of aftereffect combine vectorially. An alternative is that the direction of the af...
Article
During smooth pursuit, object motion is often judged by combining estimates of retinal motion with extra-retinal estimates of eye velocity. The latter are thought to be based on motor commands emanating from the eye movement system. Retinal motion sensitivity is known to decline in older observers but little is known about the precision of extra-re...
Article
The accuracy of pursuit and optokinetic nystagmus is known to decline with age. However, only a few studies have compared these two types of ocular following in the same observers and those that do confound stimulus size with type of eye movement (small for pursuit, large for nystagmus). Moreover, measures of precision have not been reported, despi...
Article
Full-text available
During smooth pursuit eye movement, observers often misperceive velocity. Pursued stimuli appear slower (Aubert-Fleishl phenomenon [1, 2]), stationary objects appear to move (Filehne illusion [3]), the perceived direction of moving objects is distorted (trajectory misperception [4]), and self-motion veers away from its true path (e.g., the slalom i...
Article
Repetitive eye movement gives rise to motion aftereffect (MAE). Most examples use stimulus configurations producing significant retinal motion during adaptation and so emphasise the contribution of retinal motion sensors to the effect. However, pursuit to a target undergoing sawtooth motion in the dark also produces MAE, which suggests extra-retina...
Article
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Neuronal orientation selectivity has been shown in animal models to require corticocortical network cooperation and to be dependent on the presence of GABAergic inhibition. However, it is not known whether variability in these fundamental neurophysiological parameters leads to variability in behavioral performance. Here, using a combination of magn...
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Previous work on ocular-following emphasises the accuracy of tracking eye movements. However, a more complete understanding of oculomotor control should account for variable error as well. We identify two forms of precision: 'shake', occurring over shorter timescales; 'drift', occurring over longer timescales. We show how these can be computed acro...
Article
Full-text available
One way the visual system estimates object motion during pursuit is to combine estimates of eye velocity and retinal motion. This questions whether observers need direct access to retinal motion during pursuit. We tested this idea by varying the correlation between retinal motion and objective motion in a two-interval speed discrimination task. Res...
Article
Full-text available
Smooth pursuit eye movements add motion to the retinal image. To compensate, the visual system can combine estimates of pursuit velocity and retinal motion to recover motion with respect to the head. Little attention has been paid to the temporal characteristics of this compensation process. Here, we describe how the latency difference between the...
Article
A number of models of motion perception include estimates of eye velocity to help compensate for the incidental retinal motion produced by smooth pursuit. The 'classical' model uses extra-retinal motor command signals to obtain the estimate. More recent 'reference-signal' models use retinal motion information to enhance the extra-retinal signal. Th...
Article
Recent evidence suggests that a key visual motion centre in the brain ignores extra-retinal motor information concerning reflexive eye movement. Instead it seems that neurons sensitive to oculomotor actions in this area fire at will.
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To make vision possible, the visual nervous system must represent the most informative features in the light pattern captured by the eye. Here we use Gaussian scale-space theory to derive a multiscale model for edge analysis and we test it in perceptual experiments. At all scales there are two stages of spatial filtering. An odd-symmetric, Gaussian...
Article
Full-text available
Repetitive eye movement produces a compelling motion aftereffect (MAE). One mechanism thought to contribute to the illusory movement is an extra-retinal motion signal generated after adaptation. However, extra-retinal signals are also generated during pursuit. They modulate activity within cortical motion-processing area MST, helping transform reti...
Article
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Pursuit eye movements alter retinal motion cues to depth. For instance, the sinusoidal retinal velocity profile produced by a translating, corrugated surface resembles a sinusoidal shear during pursuit. One way to recover the correct spatial phase of the corrugation's profile (i.e. which part is near and which part is far) is to combine estimates o...
Article
Sensing the movements of the world and the objects within it appears to be a fundamental job for our visual system. In rare cases of brain damage, we find that individuals lacking motion perception live in a very different world of frozen images, where simple tasks like filling a kettle or crossing the road take on alarming difficulties. That task...
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The aim of this study was to examine the effect of eye-movements on subjective and psychophysiological measures of arousal and distress associated with positive and negative autobiographical memories. These memories were ‘brought-to-mind’ whilst engaging in eye-movement or eyes-stationary conditions in a counterbalanced within subjects design, with...
Article
Theoretical models implicating the orienting reflex as an explanatory mechanism in the eye-movement desensitization and reprocessing (EMDR) treatment protocol are contrasted and tested empirically. We also test whether EMDR effects are due to a distraction effect. A repeated measure design is used in two experiments. The first experiment employed t...
Article
Extra-retinal information about eye velocity is thought to play an important role in compensating the retinal motion experienced during an eye movement. Evidently this compensation process is prone to error, since stimulus properties such as contrast and spatial frequency have marked effect on perceived motion with respect to the head. Here we inve...
Article
Repetitive eye movements are known to produce motion aftereffect (MAE) when made to track a moving stimulus. Explanations typically centre on the retinal motion created in the peripheral visual field by the eye movement. This retinal motion is thought to induce perceived motion in the central test, either through the interaction between peripheral...
Article
Full-text available
Observers can recover motion with respect to the head during an eye movement by comparing signals encoding retinal motion and the velocity of pursuit. Evidently there is a mismatch between these signals because perceived head-centred motion is not always veridical. One example is the Filehne illusion, in which a stationary object appears to move in...
Article
Full-text available
Stationary objects appear to move in the opposite direction to a pursuit eye movement (Filehne illusion) and moving objects appear slower when pursued (Aubert-Fleischl phenomenon). Both illusions imply that extra-retinal, eye-velocity signals lead to lower estimates of speed than corresponding retinal motion signals. Intriguingly, the velocity (i.e...
Article
Full-text available
By adding retinal and pursuit eye-movement velocity one can determine the motion of an object with respect to the head. It would seem likely that the visual system carries out a similar computation by summing extra-retinal, eye-velocity signals with retinal motion signals. Perceived head-centred motion may therefore be determined by differences in...
Article
Full-text available
Pursuit eye movements give rise to retinal motion. To judge stimulus motion relative to the head, the visual system must correct for the eye movement by using an extraretinal, eye-velocity signal. Such correction is important in a variety of motion estimation tasks including judgments of object motion relative to the head and judgments of self-moti...
Article
Eye movements introduce retinal motion to the image and so affect motion cues to depth. For instance, the slant of a plane moving at right-angles to the observer is specified by translation and a component of relative motion such as shear. To a close approximation, the translation disappears from the image when the eye tracks the surface accurately...
Article
Pursuit eye movements introduce retinal motion that complicates the recovery of self-motion from retinal flow. An extra-retinal, eye-velocity signal could be used to aid estimation of the observer's path, perhaps by converting retino-centric into head-centric motion. This conversion is apparently not precise because we often misperceive head-centri...
Article
When presented with random-dot displays with little depth information, observers cannot determine their direction of self-motion accurately in the presence of rotational flow without appropriate extra-retinal information (Royden CS et al. Vis Res 1994;34:3197–214.). On theoretical grounds, one might expect improved performance when depth informatio...
Article
When we make a smooth eye movement to track a moving object, the visual system must take the eye's movement into account in order to estimate the object's velocity relative to the head. This can be done by using extra-retinal signals to estimate eye velocity and then subtracting expected from observed retinal motion. Two familiar illusions of perce...
Article
Observers used a cursor to mark the location and polarity of all the bar and edge features seen in compound (f + 3f) gratings of moderate frequency and contrast. They almost always reported six bars and six edges per cycle of the fundamental frequency (f = 0.4 c/deg, contrast 32%), for all phases of the third harmonic (3f = 1.2 c/deg, contrast 10.7...
Article
Purpose: During smooth pursuit eye movements, head-centric object velocity is estimated from motion measured retinally and an c: tra-rctinal eye-velocity signal. Previous studies have concluded that errors in judging head-centric velocity are caused by errors m the extra-retinal signal onlv. Many stimulus manipulations such as altering spatial freq...
Article
Purpose: To estimate head-centric velocity during eye movements, an observer must take eye velocity into account. In previous studies of this estimation ability, several authors have concluded that errors in judging head-centric velocity are due to errors in an extra-retinal eye-velocity signal; in particular, that the gain of this signal is < 1. W...
Article
A growing body of evidence suggests that optic flow is processed by specialised 2-D motion mechanisms. We asked whether the visual system has parallel, or rapid serial, access to representations of optic flow components in a spatial 4AFC task. Random-dot kinematograms (144 dots per interval) depicting expansion, rotation, deformation (horizontal sh...
Article
Many visual experiments call for visual displays in which dots are plotted with very fine positional accuracy. Spatial hyperacuities and motion displacement thresholds can be as low as 5 sec arc. On computer graphics displays small angular displacements of a pixel can be obtained only with long viewing distances which impose a small field of view....
Article
Purpose: A number of investigators have reported accurate heading judgments during pursuit eye movements. However, when observers pursue a horizontally-oscillating target during simulated linear translation across a ground plane, perceived heading oscillates somewhat. This suggests that the mechanism that uses eye-velocity information to cancel the...
Article
A compelling impression of surface slant is produced by random dot displays depicting deformation and translation alone. A simple model of slant estimation based upon deformation is shown to capture quantitatively both the perceived slant in this situation and the distortion in perceived slant produced when constant deformation is added to random d...
Article
Motion parallax provides cues to the three-dimensional layout of a viewed scene and, in particular, to surface tilt and slant. For example, as a textured surface, inclined around a horizontal axis, translates horizontally relative to an observer's view point, then, in the absence of head and eye movements, the observer's retinal flow will contain a...
Article
Above threshold, two superimposed sinusoidal gratings of the same spatial frequency (eg 1 cycle deg-1) and equal contrasts, and with orientations balanced around vertical, usually look like a compound structure containing vertical and horizontal edges. However, at large plaid angles (ie large differences between component orientations) and low plai...
Article
Full-text available
Estimates of temporal proximity (sometimes called time-to-collision) from random-dot flow patterns are shown to be based upon retinal speed, rather than upon changes in dot density. Neither the spatial nor the temporal gradient of motion is essential to the task, but estimates can be made from either alone. Performance is unaffected by the addition...
Article
Previous work has demonstrated a difference in human sensitivity to compressive and shearing speed gradients. This raises the possibility that the ability to estimate the slant of a surface may vary with its direction of tilt. No such variance was found here, which may indicate that slant estimation depends upon deformation rather than upon compres...
Article
A comparison of sensitivity to expanding, rotating, translating and random motion suggests the existence of specialised mechanism for the detection of expansion and rotation. Complementary masking shows that the detection of expansion is unaffected by the presence of rotation, and vice versa. These results are interpreted in terms of a Relative Mot...