Differences in peripheral and foveal effects observed in stabilized vision.

Foveal images fade much faster than peripheral images, at light-on as well as at light-off. An opposite result can be obtained, however, in case of unsatisfactory stabilization. The amount and extension of the brightness spreading before the fading depend on the stimulus brightness and its location on the retina. The intensity threshold and the maximal obtainable sharpness of the image decrease towards the periphery, the image becomes wider and sharper on increasing the stimulus intensity. In the foveal area brightness or darkness can be observed (even from stimuli below the foveal threshold) by a filling-in from the periphery, which has a lower threshold. On increasing the stimulus intensity very small pinpoints of light, tiny dots are observable in the foveal area. Each dot measures less than 1 min arc in diameter and is visible for a few hundred milliseconds only. The final sharply contoured foveal percept of a narrow line, observed at higher intensities, is made up of a crowding of these tiny dots. When lines wider than about 10 min arc are used as stimuli these small foveal dots are only seen along the inside contours but not on the (diffuse) interior of the line. In the periphery no tiny dots but only spots of larger size can be observed. At each location of the visual field the final percept seems to be built up from the co-operation of a number of perceptive elements of different sizes. Small elements fade faster, have higher thresholds, habituate more easily on repeated stimulation and become easier activated by small-amplitude stimulus displacements than larger elements. The results described, contradicting some literature date, depend heavily on the quality of the stabilization.