Local field potentials recorded from macaque TE reveal posterior to anterior gradient in the extraction of diagnostic elements from natural scenes

Partial occlusion of objects is ubiquitous in nature. We have shown that Rhesus monkeys are able to identify partially occluded stimuli as long as diagnostic image parts remain visible. Based on these results, we constructed partially occluded variants for a number of natural scenes, so that the scenes’ diagnostic parts were either visible or occluded. Here we report how the local field potentials (LFPs) recorded in area TE depend on the diagnosticity of a visible image region. The LFP was recorded from a total of 214 sites in two Rhesus monkeys. We sampled a large extent of area TE ranging from AP 11 to 23 mm, and covering the lower bank of the STS and lateral TE. LFPs were characterized by computing the visual evoked potentials (VEPs). In both monkeys, we found sites at which diagnosticity was a significant determinant of VEP amplitude. Interestingly, these sites were distributed according to a posterior to anterior gradient, and were more common in anterior TE (χ2-test, p<.001). We also examined how diagnosticity modulated the raw LFP amplitudes. Again, the influence of diagnosticity on the LFP increased from posterior to anterior recording locations (correlation with recording location, p<.01). An analysis of single neuron responses recorded at the same sites revealed no significant differences along this posterior to anterior axis (correlation with recording location, p=.59). Our results reveal a posterior to anterior gradient in LFP amplitudes reflecting the extraction of diagnostic image parts. Such effects were not seen in single neuron responses collected at these sites. Because LFPs provide an estimate of local dendritic processing, our findings are consistent with the idea that diagnosticity is first encoded in posterior TE, but not in the preceding visual areas. These findings suggest that LFPs provide information about the distribution of function in the ventral pathway that is not available from single neuron activity