Biomedical subjects

David J Freedman

Publications and source records attributed to David J Freedman.

8 recordsLinked to original sources

PubMed2006

Experience-dependent representation of visual categories in parietal cortex.

Categorization is a process by which the brain assigns meaning to sensory stimuli. Through experience, we learn to group stimuli into categories, such as 'chair', 'table' and 'vehicle', which are critical for rapidly and appropriately selecting behavioural responses. Although much is known about the neural representation of simple visual stimulus features (for example, orientation, direction and colour), relatively little is known about how the brain learns and encodes the meaning of stimuli. We trained monkeys to classify 360 degrees of visual motion directions into two discrete categories, and compared neuronal activity in the lateral intraparietal (LIP) and middle temporal (MT) areas, two interconnected brain regions known to be involved in visual motion processing. Here we show that neurons in LIP--an area known to be centrally involved in visuo-spatial attention, motor planning and decision-making-robustly reflect the category of motion direction as a result of learning. The activity of LIP neurons encoded directions of motion according to their category membership, and that encoding shifted after the monkeys were retrained to group the same stimuli into two new categories. In contrast, neurons in area MT were strongly direction selective but carried little, if any, explicit category information. This indicates that LIP might be an important nexus for the transformation of visual direction selectivity to more abstract representations that encode the behavioural relevance, or meaning, of stimuli.

Animals↗

PubMed2005

Experience-dependent sharpening of visual shape selectivity in inferior temporal cortex.

Whereas much is known about the visual shape selectivity of neurons in the inferior temporal cortex (ITC), less is known about the role of visual learning in the development and refinement of ITC shape selectivity. To address this, we trained monkeys to perform a visual categorization task with a parametric set of highly familiar stimuli. During training, the stimuli were always presented at the same orientation. In this experiment, we recorded from ITC neurons while monkeys viewed the trained stimuli in addition to image-plane rotated versions of those stimuli. We found that, concomitant with the monkeys' behavioral performance, neuronal stimulus selectivity was stronger for stimuli presented at the trained orientation than for rotated versions of the same stimuli. We also recorded from ITC neurons while monkeys viewed sets of novel and familiar (but not explicitly trained) randomly chosen complex stimuli. We again found that ITC stimulus selectivity was sharper for familiar than novel stimuli, suggesting that enhanced shape tuning in ITC can arise for both passively experienced and explicitly trained stimuli.

Algorithms↗

PubMed2004

Posterior parietal cortex: Space--and beyond.

How do we decide how to react to a stimulus or event? To do so requires recognition of the stimulus itself as well as an appreciation of the context within which that stimulus is encountered. In this issue of Neuron, Stoet and Snyder report that neurons in the parietal cortex of monkeys can carry contextual information related to the rules that are relevant for solving a visual discrimination task.

Animals↗

PubMed2003

A comparison of primate prefrontal and inferior temporal cortices during visual categorization.

Previous studies have suggested that both the prefrontal cortex (PFC) and inferior temporal cortex (ITC) are involved in high-level visual processing and categorization, but their respective roles are not known. To address this, we trained monkeys to categorize a continuous set of visual stimuli into two categories, "cats" and "dogs." The stimuli were parametrically generated using a computer graphics morphing system (Sheltonelton, 2000) that allowed precise control over stimulus shape. After training, we recorded neural activity from the PFC and the ITC of monkeys while they performed a category-matching task. We found that the PFC and the ITC play distinct roles in category-based behaviors: the ITC seems more involved in the analysis of currently viewed shapes, whereas the PFC showed stronger category signals, memory effects, and a greater tendency to encode information in terms of its behavioral meaning.

Analysis of Variance↗

PubMed2003

Neural correlates of categories and concepts.

The ability to readily adapt to novel situations requires something beyond storing specific stimulus-response associations. Instead, many animals can detect basic characteristics of events and store them as generalized classes. Because these representations are abstracted beyond specific details of sensory inputs and motor outputs, they can be easily generalized and adapted to new circumstances. Explorations of neural mechanisms of sensory processing and motor output have progressed to the point where studies can begin to address the neural basis of abstract, categorical representations. Recent studies have revealed their neural correlates in various cortical areas of the non-human primate brain.

Animals↗

PubMed2002

Representation of the quantity of visual items in the primate prefrontal cortex.

Deriving the quantity of items is an abstract form of categorization. To explore it, monkeys were trained to judge whether successive visual displays contained the same quantity of items. Many neurons in the lateral prefrontal cortex were tuned for quantity irrespective of the exact physical appearance of the displays. Their tuning curves formed overlapping filters, which may explain why behavioral discrimination improves with increasing numerical distance and why discrimination of two quantities with equal numerical distance worsens as their numerical size increases. A mechanism that extracts the quantity of visual field items could contribute to general numerical ability.

Animals↗

PubMed2002

The prefrontal cortex: categories, concepts and cognition.

The ability to generalize behaviour-guiding principles and concepts from experience is key to intelligent, goal-directed behaviour. It allows us to deal efficiently with a complex world and to adapt readily to novel situations. We review evidence that the prefrontal cortex-the cortical area that reaches its greatest elaboration in primates-plays a central part in acquiring and representing this information. The prefrontal cortex receives highly processed information from all major forebrain systems, and neurophysiological studies suggest that it synthesizes this into representations of learned task contingencies, concepts and task rules. In short, the prefrontal cortex seems to underlie our internal representations of the 'rules of the game'. This may provide the necessary foundation for the complex behaviour of primates, in whom this structure is most elaborate.

Animals↗

PubMed2002

Visual categorization and the primate prefrontal cortex: neurophysiology and behavior.

The ability to group stimuli into meaningful categories is a fundamental cognitive process. To explore its neuronal basis, we trained monkeys to categorize computer-generated stimuli as "cats" and "dogs." A morphing system was used to systematically vary stimulus shape and precisely define a category boundary. Psychophysical testing and analysis of eye movements suggest that the monkeys categorized the stimuli by attending to multiple stimulus features. Neuronal activity in the lateral prefrontal cortex reflected the category of visual stimuli and changed with learning when a monkey was retrained with the same stimuli assigned to new categories. Further, many neurons showed activity that appeared to reflect the monkey's decision about whether two stimuli were from the same category or not. These results suggest that the lateral prefrontal cortex is an important part of the neuronal circuitry underlying category learning and category-based behaviors.

Action Potentials↗