Orientation detection and discrimination domains in the primary visual cortex

Abstract

Orderly maps of orientation preference are a hallmark of the primary visual cortex in many species. This systematic arrangement is thought to facilitate averaging within groups of similarly tuned neurons, minimize connection length, and ensure uninterrupted functionality over visual space. The preferred orientation of a neuron, i.e., the location of the peak of its orientation tuning curve, is the orientation best detected by the neuron. But the location of the steepest gradient of the tuning curve determines the orientations best discriminated by the neuron. Since primates excel at both discrimination and detection, we asked whether the organization of sensory information for both tasks follows the same principles. We combined electrophysiology with optical imaging of intrinsic cortical signals to study the primary visual cortex of anesthetized, paralyzed bush babies for fine orientation discrimination. Sinusoidal gratings at behaviourally optimal spatial and temporal frequencies were presented at different orientations. Using receiver operating characteristic (ROC) analysis, we computed neurometric curves and best discriminable orientations for single neurons as well as discrimination probability maps over large patches of V1 from the intrinsic signals. For single neurons, preferred orientation alone was not a consistent predictor of the best discriminable orientations since the latter depended on the peak as well as the width of the orientation tuning curve. But discrimination probabilities were organized in orderly maps for different axes of discrimination across the cortical surface, indicating that orientation tuned neurons in V1 are grouped together by the location of the steepest gradient of their tuning curves in addition to the location of the peaks. Pinwheels in detection and discrimination maps were coincident and edges of ocular dominance domains intersected both detection and discrimination domains at steep angles. We suggest that sensory information is similarly and equitably organized across V1 for both detection and discrimination of orientation.

Footnotes

 Supported by EY01778, EY007135, MH064913 and core grants EY08126 and HD 155052.