A comment: Mackay Rays shimmer due to accommodation changes

Abstract

Mackay (1957) reported visual shimmer and chrysanthemum-like patterns seen when looking at a display of lines converging to a central point: 'Mackay Rays’. Mackay attributed the shimmer to some central brain effect, related to the high information redundancy of the figure. The answer seems more prosaic. The chrysanthemum patterns show every sign of being moire patterns, given by displaced afterimages, as they are very similar to the moire patterns produced by overlaying the Ray figure with its slightly shifted transparency. A simple observation suggests an optical origin for the shimmer. When the Ray or other repeated-line patterns (including Leviant’s ‘traffic illusion’) are viewed through a pin-hole the shimmer disappears. As the pin-hole increases the depth of field of the optics of the eye, so the usual small continual fluctuations in accommodation (focusing) of the lens have small or no effect. This strongly suggests that the dynamic shimmer seen in these static figures has an optical cause: changes of size of the retinal image, due to the usual rapid ‘hunting’ of accommodation (Alpern 1962) giving retinal motion signals. As measured with a high-speed infrared optometer (Campbell et al. 1959), the oscillatory changes of focus of the lens of the eye have large individual differences and are affected by various conditions, but generally there is a maximum amplitude around 0.5 Hz and another peak around 2 Hz. The amplitude can be nearly half a dioptre.