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Defining a role for prefrontal cortex in memory-guided sensory decision-making
Dr. Tatiana Pasternak
Monday, March 23, 2015, 12:00pm - 07:00pm
University of Rochester, Departments of Neurobiology & Anatomy, Brain & Cognitive Science, Biomedical Engineering, and C
Comparing two visual stimuli that occur at different times demands the coordination of bottom-up and top-down processes. Such tasks require processing and storage of sensory stimuli, followed by their retrieval and comparison leading to perceptual decision. It is widely accepted that the lateral prefrontal cortex (LPFC) is an area that integrates the various components of such tasks in its neural activity. It is reciprocally connected with neurons processing sensory information and is thought to be an important source of top-down influences it exerts on these neurons.
When such tasks involve comparisons of motion directions separated by a delay, prefrontal neurons show direction selective responses that are reminiscent of responses in the motion processing area MT, suggesting that LPFC responses represent bottom-up signals arriving directly from MT of the same hemisphere. These neurons are also active during the memory delay and during the comparison, reflecting the remembered direction. Furthermore, at the time of perceptual decision their activity is highly predictive of the animals’ behavioral report. Thus, the activity recorded in the LPFC during memory-guided comparisons of visual motion suggests a role for this region in the memory for motion tasks.
Despite being strongly implicated in sensory working memory, the LPFC contribution to this important process is the topic of active debate. For years it has been argued that prefrontal cortex is the site of sensory storage. However, there is accumulating evidence that its role is not in the storage of relevant sensory stimuli, but in the allocation of visual attention to such stimuli and in providing top-down signals to sensory neurons where such signals are likely to be stored. We used unilateral LPFC lesions to examine its role in working memory by measuring coherence thresholds for remembering motion direction at short and long memory delays. We found that the motion thresholds, intact at shortest delays, were impaired at longer delays but only when stimuli appeared in the hemifield contralateral to the lesion, the effect particularly pronounced when the task required rapid reallocation of spatial attention. The contralesional nature of the deficits highlights the importance of the interactions between the LPFC and MT during motion comparisons. Because memory impairment was equally pronounced when the remembered stimuli were at threshold and when they were coherent, it is unlikely that the LPFC is the site of storage of stimuli giving rise to the remembered direction. It is more likely that the role of the LPFC in working memory is in accessing this information during the delay and allowing its utilization in the comparison process.