Illusory perception of gratings stimulating a small number of neurones.

We studied pattern perceptions caused by drifting gratings presented monocularly in the nasal and temporal visual fields at various suprathreshold contrasts. The grating and its surround and background were matched in luminance. Small grating produced illusions and reduced perceptions. When grating area or contrast increased from a subthreshold value, the gratings were first seen as mere flashes. Then each grating was sometimes perceived as a single small bright spot or point. Next each grating was seen as a single dark or bright line. Finally the stimuli were perceived as gratings consisting of several bars. Orientation or direction of movement were perceived correctly, but velocity, colour and number of bars were often perceived as illusions. Thus, in spite of the illusions, some features of the stimuli could have allowed correct discriminations. The area and contrast limits of illusory perception depended on eccentricity. Irrespective of retinal size, the stimuli were not perceived correctly as gratings at any eccentricities when the gratings were smaller than about 1 x 1 mm in their calculated cortical area and stimulated a small constant number of retinal ganglion cells. Relations between the results and retinal aliasing, cortical columns and phase locking of neuronal oscillations are discussed.