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Zusammenfassung:
Both the extrastriate area V4 and the dorsolateral prefrontal cortex (PFC) play an important role in the processing of visual information. Their role can be best understood by examining single unit responses as well as local field potentials (LFPs) that are indicators of integrative dendro-somatic events. Here, we examine tuning properties of LFPs simultaneously in both areas during a visual memory task. We first focus on the visual evoked potential (VEP). We employed a set of natural images as stimuli that were shown in chromatic and achromatic conditions at different degradation levels.
A majority of recorded sites (110/151 in V4, 91/140 in PF in two monkeys) showed significant deflections in stimulus-locked averaged LFP waveforms or VEPs in both areas. Across sites, the VEP in V4 tended to exhibit two peaks with approximate latencies of 80 and 200 ms. In PFC the VEP was characterized by a unimodal peak at a latency of about 200ms, which corresponded in terms of its onset dynamics very closely to the late peak in V4 (200 msec). This suggests that the arrival of sensory information is reflected in an early peak in V4, and that both areas process information jointly during a period characterized by a late VEP peak.
VEPs in both regions exhibited tuning for stimulus identity and color. In V4 the amplitude of both peaks carried information about stimulus identity (p<0.05, two-way ANOVA main factor stimulus for 30% and 45% of sites for peak 1 and 2 respectively) and color of the images (p<0.05 for 17% and 18%). We also found stimulus selectivity at the majority of sites in PF cortex (p<0.05 for 65% of sites) as well as color modulated responses in 18% of sites (p<0.05). However, tuning for stimulus degradation with highest responses for non-degraded stimuli was observed in PFC (linear regression, F=5.26,p<0.05) but not in V4 (F=1.65,p>0.05) VEPs. In other words, stimulus degradation has a more systematic influence on neuronal population responses as measured by the LFP in PFC than in V4.
All in all, our findings suggest that processing in V4 and PFC during visual perception is bidirectional rather than feed-forward.
