Functional micro-organization of primary visual cortex: Receptive field analysis of nearby neurons

It is well established that multiple stimulus dimensions (e.g., orientation and spatial frequency) are mapped onto the surface of striate cortex. Howe ver, the detailed organization of neurons within a local region of striate cortex remains unclear. Within a vertical column, do all neurons have the s ame response selectivities? And if not, how do they most commonly differ an d why? To address these questions, we recorded from nearby pairs of simple cells and made detailed spatiotemporal maps of their receptive fields. From these maps, we extracted and analyzed a variety of response metrics. Our r esults provide new insights into the local organization of striate cortex. First, we show that nearby neurons seldom have very similar receptive field s, when these fields are characterized in space and time. Thus, there may b e less redundancy within a column than previously thought. Moreover, we sho w that correlated discharge increases with receptive field similarity; thus , the local dissimilarity between neurons may allow for noise reduction by response pooling. Second, we show that several response variables are clust ered within striate cortex, including some that have not received much atte ntion such as response latency and temporal frequency. We also demonstrate that other parameters are not clustered, including the spatial phase (or sy mmetry) of the receptive field. Third, we show that spatial phase is the si ngle parameter that accounts for most of the difference between receptive f ields of nearby neurons. We consider the implications of this local diversi ty of spatial phase for population coding and construction of higher-order receptive fields.