  A main challenge for the development of cortical visual prostheses is to
spatially localize individual spots of light, called phosphenes, by assigning
appropriate stimulating parameters to implanted electrodes. Imitating the
natural responses to phosphene-like stimuli at different positions can help in
designing a systematic procedure to determine these parameters. The key
characteristic of such a system is the ability to discriminate between
responses to different positions in the visual field. While most previous
prosthetic devices have targeted the primary visual cortex, the extrastriate
cortex has the advantage of covering a large part of the visual field with a
smaller amount of cortical tissue, providing the possibility of a more compact
implant. Here, we studied how well ensembles of Multiunit activity (MUA) and
Local Field Potentials (LFPs) responses from extrastriate cortical visual area
V4 of a behaving macaque monkey can discriminate between two-dimensional
spatial positions. We found that despite the large receptive field sizes in V4,
the combined responses from multiple sites, whether MUA or LFP, has the
capability for fine and coarse discrimination of positions. We identified a
selection procedure that could significantly increase the discrimination
performance while reducing the required number of electrodes. Analysis of noise
correlation in MUA and LFP responses showed that noise correlations in LFP
responses carry more information about the spatial positions. Overall, these
findings suggest that spatial positions could be localized with patterned
stimulation in extrastriate area V4.
