Illusions as neuro-signs

Orientation is a dimension much studied in visual neuroscience. Many single cells in the visual cortex respond to lines or edges in specific orientations, and in monkey cortex these cells are specific for lines within ±20° of the preferred orientation. One class of illusion, involving perception of orientation, can direct us towards interpretations of neural function (in this series, referred to as neuro-signs). The horizontal parallel lines in the image below do not appear so — they look to be alternately inclined clockwise and anticlockwise. If the horizontal lines intersected the curved ones orthogonally, they would appear parallel. Thus, the illusion occurs when horizontal lines intersect nearhorizontals. This indicates that the perception of orientation is based not only on the orientation selectivity of a particular cell, but also on interactions with cells with similar orientation selectivities. The illusion could point to one of several neuro-signs: the range of active orientation-selective cells might be biased by the surrounding contours; the receptive field (the region of the retina that can change the activity of a group of nerve cells when light falls on it) properties of particular cells might be modified by their immediate context; or cells with similar receptive field orientations could inhibit one another. In addition to the simultaneous biasing of perceived orientation, there are tilt after-effects: observation of inclined lines for some seconds results in subsequently viewed vertical lines appearing tilted in the opposite direction. There are two distortions in the circular image above — tilt and displacement. The ‘Zöllner illusion’ is the apparent tilt of the fine vertical lines because of their intersection with the thicker inclined lines. Indeed, the vertical line segments do not even seem to be aligned, an effect that is called the ‘Poggendorff illusion’. Both illusions could be due to the same process: the interaction between orientation-selective cells results in acute angles appearing larger than they are. The short vertical line segments then look inclined anticlockwise, which in turn makes them appear misaligned. Usually, orientation illusions are based on interactions between contours of similar orientation. The coloured image below is composed of only vertical and horizontal lines, although the verticals do not appear so. Some squares of the