Scientific Understanding of Consciousness
Integration of What and Where in Prefrontal Cortex
Science 2 May 1997: Vol. 276. no. 5313, pp. 821 - 824
Integration of What and Where in the Primate Prefrontal Cortex
S. Chenchal Rao, Gregor Rainer, Earl K. Miller
Department of Brain and Cognitive Sciences and The Center for Learning and Memory, Massachusetts Institute of Technology, Cambridge, MA 02139, USA.
The visual system separates processing of an object's form and color ("what") from its spatial location ("where"). In order to direct action to objects, the identity and location of those objects must somehow be integrated. To examine whether this process occurs within the prefrontal (PF) cortex, the activity of 195 PF neurons was recorded during a task that engaged both what and where working memory. Some neurons showed either object-tuned (what) or location-tuned (where) delay activity. However, over half (52 percent, or 64/123) of the PF neurons with delay activity showed both what and where tuning. These neurons may contribute to the linking of object information with the spatial information needed to guide behavior.
Anatomical segregation of processing is an important principle of neural organization. Even within a modality, largely separate pathways process different attributes of the same stimulus. Perhaps the best explored example of segregation is in the visual system, where the analysis of visual scenes is carried out by at least two pathways. A "ventral pathway" through inferior temporal (IT) cortex processes information about features that identify objects, such as shape and color (object, or "what" information), and a "dorsal pathway" through posterior parietal (PP) cortex processes information about location and spatial relations among objects (spatial, or "where" information). This example raises the question of where and how information about object identity is integrated with information about object location. One region that may play a role in integration is the prefrontal (PF) cortex, which receives inputs from virtually all of the brain's sensory systems and has long been thought to be an area where diverse signals are integrated to serve higher order cognitive functions.
A major contribution of the PF cortex to cognition is the active maintenance of behaviorally relevant information "online," a process known as working memory. Working memory is typically studied in tasks in which an animal must remember a cue stimulus over a delay period and then make a behavioral response based on the cue. Physiological studies in monkeys have revealed that many PF neurons are highly active during the delay of such tasks. The activity is often cue-specific, suggesting that this "delay activity" is the neural correlate of the working memory trace. Given its central role in cognition, PF neurons that contribute to working memory are obvious candidates for integrating diverse signals. However, the extent to which different types of information, such as what and where, are integrated within the PF cortex is not well understood. Highly processed spatial information from the PP cortex and object information from the IT cortex are received by separate regions of the PF cortex, the dorsolateral (areas 46 and 9) and the ventrolateral (area 12) PF cortex, respectively, but there are interconnections between these regions that could bring what and where together.
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