Spatial and Temporal Integration of Signals in Foveal Line Orientation

Abstract

Westheimer, Gerald and Eric J. Ley. Spatial and temporal integration of signals in foveal line orientation. J. Neurophysiol. 77: 2677–2684, 1997. The discrimination of the orientation of a line improves with line length, reaching an optimum when a foveal line is ∼0.5° long. We studied the effect of eliminating sections of the line, of displacing them out of alignment, and of delaying them. Orientation discrimination thresholds are only a little elevated when a 25-arcmin line is replaced by three equally spaced collinear 5-arcmin segments. Two collinear 5-arcmin segments show better thresholds than a single one when they are separated by as much as 20 arcmin. But thresholds are impaired by bringing line segments out of collinearity by as little as 1 arcmin. Asynchrony of up to 50 ms can be tolerated, but when the middle segment of a three-line pattern is delayed by ∼100 ms there is active inhibition, thresholds being now higher than when the middle segment is absent. It is concluded that for signals to address the orientation discrimination mechanism optimally, they have to be contained inside a narrow spatial corridor and be presented within a time window of ∼50 ms, but that some spatial summation can take place over a length of ≥0.5° in the fovea. Because short lines made of black and white collinear segments do not have good orientation thresholds, whereas longer and interrupted lines do, it is concluded that what is involved is potentiating interaction between collinearly arranged neurons with identical orientation selectivity rather than summation of signals within the receptive fields of single neurons.