The role of parietal cortex in spatial processing such as saccade planning and execution and visuospatial attention has been well established. More recent studies have also identified strong non-spatial signals in the macaque lateral intraparietal area (LIP) which encode visual information related to both salient features (e.g. shape, motion direction, color) and learned associations (category membership, cognitive set). However, the relationship between spatial and non-spatial encoding in parietal cortex is poorly understood, and has important implications for determining the broader role of parietal cortex in sensory processing and decision-making. Here we present a series of experiments that begin to address the interaction, interdependence, and relative strength of spatial and non-spatial signals in both single neurons and neural ensembles of the posterior parietal cortex.
In the first experiment, monkeys were trained to perform a delayed match-to-category (DMC) task in which 360° of motion directions were divided into two categories by a learned category boundary. During the delay period of some trials, monkeys were cued to make a category decision-irrelevant saccade either toward or away from a neuron's receptive field (RF). We found that strong saccade-related responses minimally influence robustly encoded category signals in LIP, and that there was no apparent relationship saccade-related spatial selectivity and category selectivity. This underscores the importance of parietal cortex in non-spatial cognitive processing, and suggests that spatial and non-spatial information are encoded independently in LIP.
In the second experiment, we used expanded data from the first experiment with the same task design. We included an additional behavioral condition which presented the sample stimulus foveally (outside of the RF) to compare the strength of category selectivity with saccade-related and visuospatial signals. We also used population decoding to measure the relative strength, interdependence, compactness, and stability of category and spatial signals in neural ensembles. We found that in both single neurons and neural populations, category and spatial selectivity are encoded with similar strength in LIP, although the measured strength of spatial selectivity increases at the population level. We also found that category and spatial signals are independently and redundantly encoded in neural ensembles, and relatively stable in small fractions of the LIP population. This argues against the view that cognitive non-spatial processing is secondary to spatial encoding in LIP, and instead suggests that LIP is a flexible center for spatial and non-spatial processing.
Together, these results demonstrate that non-spatial processing related to visual motion categorization is at least as important as spatial processing in LIP. However, the vast majority of previous work in LIP has focused on its spatial properties, leading to popular hypotheses which propose that LIP is cardinally a spatial processor, and that non-spatial signals are secondary modulators of spatial encoding. The results presented here are fundamentally inconsistent with this view, and in context with other non-spatial studies, instead argue that LIP functions as a flexible associative nexus, integrating spatial and non-spatial factors for perceptual decision-making.
|Adviser||Nicholas G. Hatsopoulos|
|School||THE UNIVERSITY OF CHICAGO|
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