Centre-Surround Tuning of Orientation Detectors in Human Vision

Abstract

We present a new psychophysical method to study the tuning of orientation detectors in the human visual system. The stimulus consists of a sequence of sinusoidal gratings of random orientations and spatial phases (but of fixed spatial frequency) shown at a high presentation rate (30 Hz) in 60 s long trials. The gratings are seen through a circular aperture. The subject's task is to report, as fast as possible, by pressing a key, when the presence of a horizontal grating is seen embedded in the stimulus sequence. The data are analysed by calculating the distribution of orientations present in the stimulus sequence at different times [before] the key was pressed. Similar experiments can be done by asking the subject to detect vertical and oblique orientations. In these experiments we used 100% contrast, a 3 deg diameter circular aperture, and spatial frequencies ranging from 1 to 4 cycles deg−1. The resulting orientation-tuning profiles have a ‘centre—surround’ (or Mexican hat) shape in the orientation domain. These findings are consistent with the idea of fast ‘lateral inhibition’ between orientation detectors. The centre—surround profiles may explain systematic errors in visual angle judgments, such as the perceptual expansion of acute angles and the contraction of obtuse angles, the tilt aftereffect, and the effects observed in the Zöllner, Hering, Ponzo, and Poggendorff illusions.