Evidence for a neural mechanism that encodes angles.

We measured the discrimination threshold (delta theta)Th for angle theta, where theta was either the angle of a Vee composed of two straight lines contained within the frontoparallel or the angle intersection of two straight lines contained within the frontoparallel plane. The two-line pattern was rotated bodily through a random angle between trials with the aim of eliminating the absolute orientation of one or the other line as a reliable cue to the task. We report evidence that this aim was achieved. Our main conclusion is that the ability to discriminate a change in angle theta cannot entirely be explained in terms of the ability to discriminate changes in the orientations of the individual lines that comprise the Vee. We propose that the human visual pathway contains a neural mechanism that encodes the difference in the orientations of two simultaneously-presented straight lines. Discrimination threshold for angle (delta theta)Th is roughly twice orientation discrimination threshold for an isolated line. When subjects cannot use the orientation of one or another line as a cue to the task, the plot of (delta theta)Th vs theta is approximately flat between the delta = 20 and 160 deg.