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1 Depepartment of Neurobiology, Duke University Medical Center, Durham, NC, USA
* To whom correspondence should be addressed. E-mail: dean{at}neuro.duke.edu.
Previous neurophysiological studies have reported that neurons in posterior cingulate cortex (PCC) respond following eye movements, and that these responses may vary with ambient illumination. In monkeys, PCC neurons also respond following the illumination of large visual patterns but not after the illumination of small visual targets on either reflexive saccade tasks or peripheral attention tasks. These observations suggest that neuronal activity in PCC is modulated by behavioral context, which varies with the timing and spatial distribution of visual and oculomotor events. To test this hypothesis, we measured the spatial and temporal response properties of single PCC neurons in monkeys performing saccades in which target location and movement timing varied unpredictably. Specifically, an unsignalled delay between target onset and movement onset permitted us to temporally dissociate changes in PCC activity associated with either event. Response fields constructed from these data demonstrated that many PCC neurons were activated following the illumination of small contralateral visual targets, as well as following the onset of contraversive saccades guided by those targets. In addition, the PCC population maintained selectivity for small contralateral targets during delays of up to 600 ms. Overall, PCC activation was highly variable trial to trial and selective for a broad range of directions and amplitudes. Planar functions described response fields nearly as well as broadly-tuned 2-dimensional gaussian functions. Additionally, the overall responsiveness of PCC neurons decreased during delays when both a fixation stimulus and a saccade target were visible, suggesting a modulation by divided attention. Finally, the strength of the neuronal response following target onset was correlated with saccade accuracy on delayed saccade trials. Thus, PCC neurons may signal salient visual and oculomotor events, consistent with a role in visual orienting and attention.
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