Spiking and oscillatory correlates of visual short-term memory for multiple items

Abstract

The richness of visual experience far exceeds our ability to remember what we have seen. However, it is unclear what neural mechanisms give rise to these limits to visual short-term memory capacity. Here, we measured neural activity in a change localization task, in which monkeys viewed two displays of multiple colored squares separated by a brief delay, and made a saccade to the square that changed color between displays. In chapter 2, we examine local field potentials in the lateral intraparietal area (LIP), frontal eye field, and lateral prefrontal cortex (PFC). At stimulus encoding, lower frequency oscillations decreased in power in proportion to the total number of stimuli presented, while higher frequency oscillations increased in power in proportion to the number of stimuli contralateral to the recording site. During the delay, lower frequency power instead increased with the number of contralateral stimuli, while higher frequency power was not modulated. We interpret these findings in terms of roles for low- and high-frequency oscillations in changing and maintaining cognitive state. In chapter 3, we compare spiking activity between LIP, PFC, and inferotemporal cortex (IT). Although the task required that the animal remember stimulus colors, activity in LIP and PFC primarily reflected the stimulus positions, while activity in IT primarily reflected color. In PFC, color information increased with the number of stimuli presented, while in IT, color information remained constant or decreased. Thus, IT was more strongly capacity-limited than PFC. Color selectivity during the delay was weak in all regions. However, in IT, activity at test stimulus presentation reflected the difference in square colors between the sample and test displays, while in PFC, activity primarily reflected the location of the changed square. Selectivity to these attributes was stronger on correct trials than incorrect trials. Our findings suggest a possible role for passive processes in IT in visual short-term memory.