Abstract
The prefrontal cortex (PFC) plays an important role in working memory. In order to understand how it mediates this function, we tested how training to perform a working memory task modifies the properties of different types of neurons. We performed neurophysiological recordings, collecting data from all neurons isolated from our electrodes. We evaluated spatial selectivity of neuronal responses to visual stimuli appearing on a 10-degree, 3x3 grid, and feature selectivity to eight geometric shapes. We used a Selectivity Index defined as (max−min)/(max+min) to quantify neurons‘ selectivity. Then we classified neurons as Regular Spiking (putative pyramidal neurons) and Fast Spiking (putative interneurons) based on the characteristics of the action-potential waveform and baseline firing rate (Constantinidis and Goldman-Rakic, 2002). Prior to training, we found 191 RS neurons and 9 FS neurons that responded to the visual stimuli. After training in a working memory task that required monkeys to remember the locations and features of stimuli, we recorded from 150 RS and 10 FS neurons. Approximately equal number of neurons exhibited delay period activity, before and after training (26% of RS and 33% of FS neurons had delay period activity prior to training and 22% of RS and 40% of FS neurons after training). Fast Spiking neurons had broader selectivity for the stimuli than RS neurons, indicated by lower Selectivity Index values (0.65 vs 0.75 for spatial, 0.30 vs. 0.52 for feature stimuli, which was significantly different, t-test, p[[lt]]0.01). Average Selectivity Index values did not change appreciably after training for either the FS or the RS neurons. Our results suggest that both pyramidal neurons and interneurons in the prefrontal cortex respond to visual stimuli and exhibit delay-period activity before and after training and training appears to have the same effect on both types of neurons.
Supported by: NIH grant EY017077