Working Memory Related Neuronal Activity and Theta Oscillations in Extrastriate V4

Working memory is used to store information for brief periods. The mechanism of working memory is studied in both humans and monkeys. While in monkeys working memory is thought to depend on elevated ring activity in prefrontal cortex during the memory period of a cognitive task, EEG studies in humans have linked oscillations to working memory not only in the frontal but also in the occipital cortex. It is not known, whether these two ndings -the delay activity and the oscillatory phenomena- interact with each other and how they may support working memory. We addressed this issue by recording simultaneously single unit activity (SUA) and local eld potentials (LFP) from an occipital brain region -area V4- in monkeys. The monkeys had to perform a delayed-matching-to-sample task. The task started with a 1000 ms xation period, followed by a 300 ms presentation of a sample object. After a delay of 1000 ms a probe object was presented and the monkeys had to decide whether the probe matched the sample by releasing a lever. The objects were natural images of the size 10 f x10 f . The sample stimuli had contrast levels between 5 and 100, while the probe objects were always at 100 contrast. The monkeys' performance was almost at ceiling for contrast levels higher than 25 and at chance for low contrast levels. We compared the delay period to the xation period by assessing power in the theta band of the LFP during the last 800 ms of the delay and the xation respectively. Compared to the xation period we observed increased theta oscillations for high contrast stimuli at many recording sites during the delay. Almost all sites showed reduced theta oscillations at 5 contrast level, when the monkeys were performing at chance. Unlike in prefrontal cortex there was no elevated neuronal activity during the delay period. SUA during the delay was similar to values during the xation. But examining at the relationship between theta phase and SUA, we found that many neurons red preferentially at a particular phase of the theta cycle. These ndings suggest that theta oscillations increase during the delay period in visual cortex, and that these oscillations serve to structure neuronal activity