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| Paper IPM / Cognitive / 14085 |
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Modulation of the presaccadic activity of extrastriate neurons has been suggested to underlie the representational enhancement of saccadic targets. In order to understand how changes in firing rate, correlated activity (noise correlation) and response variability (Fano factor) of extrastriate neurons contribute to presaccadic representational enhancement, we used 16 channel linear array electrodes to simultaneously record the activity of multiple neurons in the middle temporal (MT) cortex of macaque monkey. Two monkeys were trained to maintain fixation while a target stimulus was presented for 1s within the neurons' receptive field (RF). At the end of the trial, animals moved their eyes either toward or away from the stimulus, as instructed by a visual cue. Similar to previous studies of presaccadic responses in V4, we found an increase in firing rate and a reduction in Fano factor in the responses of MT neurons prior to saccades toward a target within their RF. Moreover, we found that the correlation in trial-to-trial variability between pairs of MT neurons drops prior to a saccade into their RF. The increased magnitude and reduced variability of presaccadic responses resulted in an enhanced representation of visual targets, as quantified by the performance of a support vector machine classifier designed to discriminate the orientation of the RF stimulus based on neuronal responses. In order to understand the relative contributions of response magnitude and variability to this representational enhancement, we developed an artificial spike generator to control for the effects of rate as well as correlated and independent variability (noise correlation and Fano factor, respectively). Our results indicate that changes in the response magnitude play a prominent role in driving the presaccadic enhancement of visual representations. These findings contrast with previous reports that changes in correlated variability, rather than firing rate, are the primary drivers of representational enhancement during covert attention, potentially indicating different mechanisms of representational enhancement during covert and overt attentional deployment.
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