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Abstract

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.