“School of Cognitive Sciences”
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| Paper IPM / Cognitive Sciences / 14532 |
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Psychophysical experiments have shown that remembering a location enhances visual discrimination performance at that location. Findings in our lab indicate that neurons in extrastriate visual areas receive a strong topÂdown signal from prefrontal cortex during the maintenance of spatial information. Although delayÂperiod activity is very weak or nonÂexistent in these visual areas, their sensitivity to incoming visual signals is enhanced during spatial memory maintenance. One possible explanation for this increase in sensitivity to visual input without an increase in baseline firing rate is that the topÂdown signal provides a subÂthreshold modulation.
Since the local field potentials (LFPs) are believed to reflect the level of input to an area, we sought to determine whether, and in what frequencies, extrastriate LFPs are modulated during spatial memory maintenance. For this purpose, we recorded neuronal spiking and LFP signals in the middle temporal (MT) area of rhesus monkeys performing a memoryÂguided saccade task. In this task, the target was presented for one second; the monkey had to remember the target location during a 1.5Â2 second blank delay, and then saccade to the remembered location when the fixation point disappeared. Linear array electrodes were used to simultaneously record the activity of multiple MT neurons and LFPs. We measured the power density function (PSD) of the LFPs at each recording site prior to target presentation, during target presentation, and during the memory period. The PSDs were normalized based on the PSD prior to target presentation.The beta (15Â30 Hz) power significantly increased during the memory period. Evaluating the temporal relationship between spikes and LFP signals revealed a significant increase in the locking between spikes and the phase of ongoing beta oscillations during the memory period. We also found that the beta power during the memory period predicted the upcoming saccade accuracy and the reaction time: trials with higher beta power had more precise and faster saccadic responses. These findings provide a mechanistic insight into how the maintenance of spatial information contributes to enhanced sensory processing, potentially underlying the behavioral effects of spatial working memory.
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