Kari L Hoffman

Kari L Hoffman
Vanderbilt University | Vander Bilt · Psychological Sciences and Biomedical Engineering

PhD

About

52
Publications
5,612
Reads
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2,424
Citations
Introduction
I am interested in the neural dynamics underlying changes in perception with learning. My lab uses multi-channel recording and stimulation techniques applied during behavioral tasks, along with time- and frequency-domain analysis techniques. The goal is to understand neural phenomena such as the cellular basis of oscillatory brain activity and then to determine the role these phenomena may play in adaptive behaviors such as exploration and memory-guided exploitation of the environment.
Additional affiliations
January 2007 - present
York University
Position
  • Professor (Associate)
July 2003 - January 2007
Max Planck Institute for Biological Cybernetics
Position
  • PostDoc Position
September 1997 - April 2003
The University of Arizona
Position
  • PhD Student

Publications

Publications (52)
Article
The hippocampus plays an important role in memory for events that are distinct in space and time. One of the strongest, most synchronous neural signals produced by the hippocampus is the sharp-wave ripple (SWR), observed in a variety of mammalian species during offline behaviors, such as slow-wave sleep [1-3] and quiescent waking and pauses in expl...
Article
Full-text available
Background Hippocampal sharp-wave ripples (SWRs) arising from synchronous bursting in CA3 pyramidal cells and propagating to CA1 are thought to facilitate memory consolidation. Stimulation of the CA3 axon collaterals comprising the hippocampal commissure in rats interrupts sharp-wave ripples and leads to memory impairment. In primates, however, th...
Article
Full-text available
Visual exploration in primates depends on saccadic eye movements (SEMs) that cause alternations of neural suppression and enhancement. This modulation extends beyond retinotopic areas, and is thought to facilitate perception; yet saccades may also influence brain regions critical for forming memories of these exploratory episodes. The hippocampus,...
Article
Full-text available
Conversion of new memories into a lasting form may involve the gradual refinement and linking together of neural representations stored widely throughout neocortex. This consolidation process may require coordinated reactivation of distributed components of memory traces while the cortex is “offline,” i.e., not engaged in processing external stimul...
Article
Full-text available
Episodic memory – composed of memory for unique spatiotemporal experiences – is known to decline with aging, and even more severely in Alzheimer ’s disease (AD). Memory for trial-unique objects in spatial scenes depends on the integrity of the hippocampus and interconnected structures that are among the first areas affected in AD. We reasoned that...
Article
Despite the critical link between visual exploration and memory, little is known about how neuronal activity in the human mesial temporal lobe (MTL) is modulated by saccades. Here, we characterize saccade-associated neuronal modulations, unit-by-unit, and contrast them to image onset and to occipital lobe neurons. We reveal evidence for a corollary...
Preprint
Full-text available
Nested hippocampal oscillations in the rodent gives rise to temporal coding that may underlie learning, memory, and decision making. Theta/gamma coupling in rodent CA1 occurs during exploration and sharp-wave ripples during quiescence. Whether these oscillatory regimes extend to primates is less clear. We therefore sought to identify correspondence...
Article
BACKGROUND Donepezil exerts pro-cognitive effects by non-selectively enhancing acetylcholine (ACh) across multiple brain systems. Two brain systems that mediate pro-cognitive effects of attentional control and cognitive flexibility are the prefrontal cortex and the anterior striatum which have different pharmacokinetic sensitivities to ACh modulati...
Preprint
Full-text available
BACKGROUND Donepezil exerts pro-cognitive effects by non-selectively enhancing acetylcholine (ACh) across multiple brain systems. The brain systems that mediate pro-cognitive effects of attentional control and cognitive flexibility are the prefrontal cortex and the anterior striatum which have different pharmacokinetic sensitivities to ACh modulati...
Article
Full-text available
Nonhuman primates (NHP’s) are self-motivated to perform cognitive tasks on touchscreens in their animal housing setting. To leverage this ability, fully integrated hardware and software solutions are needed that work within housing and husbandry routines while also spanning cognitive task constructs of the Research Domain Criteria (RDoC). Here, we...
Preprint
Full-text available
Despite the critical link between visual exploration and memory, little is known about how single-unit activity (SUA) in the human mesial temporal lobe (MTL) is modulated by saccadic eye movements (SEMs). Here we characterize SEM associated SUA modulations, unit-by-unit, and contrast them to image onset, and to occipital lobe SUA. We reveal evidenc...
Preprint
Full-text available
To understand the neural activity behind recollections of the distant past, we recorded from the hippocampus and retrosplenial cortex of macaques as they retrieved year-old and newly-acquired object-scene associations. Year-old memoranda preferentially evoked retrosplenial sigma oscillations (10-15 Hz), and phase-locking between RSC and eye movemen...
Preprint
Full-text available
Background: Nonhuman primates (NHPs) are self-motivated to perform cognitive tasks on touchscreens in their animal housing setting. To leverage this ability, fully integrated hardware and software solutions are needed, that work within housing and husbandry routines while also spanning cognitive task constructs of the Research Domain Criteria (RDoC...
Article
Full-text available
Event-related potentials (ERPs) are a commonly used electrophysiological signature for studying mesial temporal lobe (MTL) function during visual memory tasks. The ERPs associated with the onset of visual stimuli (image-onset) and eye movements (saccades and fixations) provide insights into the mechanisms of their generation. We hypothesized that s...
Article
Full-text available
Eye movements during visual search change with prior experience for search stimuli. Previous studies measured these gaze effects shortly after initial viewing, typically during free viewing; it remains open whether the effects are preserved across long delays and for goal-directed search, and which memory system guides gaze. In Experiment 1, we ana...
Preprint
Full-text available
Wireless recordings in macaque neocortex and hippocampus showed stronger theta oscillations during early-stage sleep than during alert volitional movement including walking. In contrast, hippocampal beta and gamma oscillations were prominent during walking and other active behaviors. These relations between hippocampal rhythms and behavioral states...
Article
Full-text available
Sharp-wave ripples (SWRs) are spontaneous, synchronized neural population events in the hippocampus widely thought to play a role in memory consolidation and retrieval. They occur predominantly in sleep and quiet immobility, and in primates, they also appear during active visual exploration. Typical measures of SWRs in behaving rats include changes...
Preprint
Full-text available
The electrophysiological signatures of encoding and retrieval recorded from mesial temporal lobe (MTL) structures are observed as event related potentials (ERPs) during visual memory tasks. The waveforms of the ERPs associated with the onset of visual stimuli (image-onset) and eye movements (saccades and fixations) provide insights into the mechani...
Article
When neurons oscillate together in one brain area, they often synchronize to oscillations in other areas. By artificially entraining long range connections during beta oscillations, their strength is modified in the wake of that oscillation.
Article
Our brain's ability to flexibly control the communication between the eyes and the hand allows for our successful interaction with the objects located within our environment. This flexibility has been observed in the pattern of neural responses within key regions of the frontoparietal reach network. More specifically, our group has shown how single...
Article
Memory for scenes is supported by the hippocampus, among other interconnected structures, but the neural mechanisms related to this process are not well understood. To assess the role of the hippocampus in memory-guided scene search, we recorded local field potentials and multi-unit activity from the hippocampus of macaques as they performed goal-d...
Article
Full-text available
Visual search efficiency improves with repetition of a search display, yet the mechanisms behind these processing gains remain unclear. According to Scanpath Theory, memory retrieval is mediated by repetition of the pattern of eye movements or “scanpath” elicited during stimulus encoding. Using this framework, we tested the prediction that scanpath...
Preprint
Full-text available
We measured hippocampal sharp-wave ripples during goal-directed visual exploration in macaques. Exploratory sharp-wave ripples were more frequent on familiar trials, in the second half of search, and near the visual target (i.e., the goal location). These spatial and temporal properties may help SWRs coordinate hippocampal and extra-hippocampal fir...
Article
Full-text available
Hippocampal sharp-wave ripples (SWRs) are highly synchronous oscillatory field potentials that are thought to facilitate memory consolidation. SWRs typically occur during quiescent states, when neural activity reflecting recent experience is replayed. In rodents, SWRs also occur during brief locomotor pauses in maze exploration, where they appear t...
Article
Visual stimulus repetition has been shown to improve processing efficiency and performance on recognition memory tasks. According to scanpath theory, memory-based repetition effects are mediated by repetition of the pattern of fixations or "scanpath" elicited during stimulus encoding at subsequent retrieval. However, it remains unclear how scanpath...
Article
Full-text available
Eye-hand coordination is crucial for our ability to interact with the world around us. However, much of the visually guided reaches that we perform require a spatial decoupling between gaze direction and hand orientation. These complex decoupled reaching movements are in contrast to more standard coupled eye and hand reaching movements in which the...
Article
Full-text available
The aim of this research was to understand how the brain controls voluntary movement when not directly interacting with the object of interest. Here we examined the role of premotor cortex in this behaviour. The goal of this study was to characterize the oscillatory activity within the caudal and rostral subdivisions of PMd (PMdc and PMdr) when goi...
Article
Full-text available
Object perception and categorization can occur so rapidly that behavioral responses precede or co-occur with the firing rate changes in the object-selective neocortex. Phase coding could, in principle, support rapid representation of object categories, whereby the first spikes evoked by a stimulus would appear at different phases of an oscillation,...
Article
Full-text available
The pattern of visual fixations on an image depends not only on the image content but also on the viewer's disposition and on the function (or pathology) of underlying neural circuitry. For example, human viewers display changes in viewing patterns toward face images that differ in gaze direction or in the viewer's familiarity with the face. Macaqu...
Article
Full-text available
Saccadic eye movements (SEMs) are the primary means of gating visual information in primates and strongly influence visual perception. The active exploration of the visual environment ("active vision") via SEMs produces suppression during saccades and enhancement afterward (i.e., during fixation) in occipital visual areas. In lateral temporal lobe...
Article
Full-text available
Tests of recognition memory in macaques typically assay memory for objects or isolated images, over time spans of seconds to hours from stimulus presentation, and/or require extensive training. Here, we propose a new application of the flicker change detection task that could measure object-in-scene memory days after single-trial exposures. In thre...
Article
Face perception in humans differs from perception of most objects: faces are recognized at the individual or subordinate level (e.g. Madonna, Collie), whereas objects are recognized at the basic level (e.g. face, dog). Additionally, faces are perceived holistically, i.e. features are not functionally independent. To date, these criteria have yet to...
Article
Faces are processed in similar ways across cultures, and, in many instances, across primate species. Consistent and discrete brain regions are active for processing faces across human subjects, some of which appear to have homologous structures in the nonhuman primate. These behavioral and neural similarities across species suggest that face proces...
Chapter
This chapter reviews the neural mechanisms underlying cognitive processes including emotion, reward, memory, social behavior, numerosity, and executive control. The first section identifies the main structures that comprise the emotional circuits of the primate brain. The second section describes what is known about the neural basis of emotional pr...
Article
Full-text available
Learning about the world through our senses constrains our ability to recognise our surroundings. Experience shapes perception. What is the neural basis for object recognition and how are learning-induced changes in recognition manifested in neural populations? We consider first the location of neurons that appear to be critical for object recognit...
Article
A new study has found that, when a macaque monkey chooses where to look, activity in parietal cortex reflects anticipated reward value, whether from fluid to drink or from social information such as dominance.
Article
Full-text available
Despite considerable evidence that neural activity in monkeys reflects various aspects of face perception, relatively little is known about monkeys' face processing abilities. Two characteristics of face processing observed in humans are a subordinate-level entry point, here, the default recognition of faces at the subordinate, rather than basic, l...
Data
An example scanpath for an original aligned trial
Data
The proportion of fixation durations for the upper background region to total fixation duration. For each condition (aligned or misaligned) data were pooled across sessions and trials (Adaptation and Novel). Fixations in the top half of the misaligned condition (t(166)=−2.37; p<0.01).
Data
An example scanpath after a new bottom half is presented
Data
An example scanpath after a new bottom half is presented
Data
An example scanpath for an original misaligned trial
Article
The social behavior of both human and nonhuman primates relies on specializations for the recognition of individuals, their facial expressions, and their direction of gaze. A broad network of cortical and subcortical structures has been implicated in face processing, yet it is unclear whether co-occurring dimensions of face stimuli, such as express...
Article
Full-text available
Auditory and visual signals often occur together, and the two sensory channels are known to influence each other to facilitate perception. The neural basis of this integration is not well understood, although other forms of multisensory influences have been shown to occur at surprisingly early stages of processing in cortex. Primary visual cortex n...
Article
Full-text available
In the social world, multiple sensory channels are used concurrently to facilitate communication. Among human and nonhuman primates, faces and voices are the primary means of transmitting social signals (Adolphs, 2003; Ghazanfar and Santos, 2004). Primates recognize the correspondence between species-specific facial and vocal expressions (Massaro,...
Article
After a spatial behavioral experience, hippocampal CA1 pyramidal cells express the activity-regulated, immediate early gene Arc in an environment-specific manner, and in similar proportions (˜40%) to cells exhibiting electrophysiologically recorded place fields under similar conditions. Theoretical accounts of the function of the fascia dentata sug...
Article
Sleep can facilitate memory formation, but its role in cortical plasticity is poorly understood. A recent study found that sleep, following monocular deprivation (MD), facilitated cortical changes in ocular dominance. The magnitude of plasticity was similar to that observed after continued MD, and larger than that seen after sleep deprivation in da...
Article
Full-text available
In rats shuttling between a variably placed landmark of origin and a fixed goal, place fields of hippocampal CA1 cells encode location in two spatial reference frames. On the initial part of the outbound journey, place fields encode location with respect to the origin while on the final segment, place fields are aligned with the goal (Gothard et al...

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