Regional variations in sensitivity to flicker

IT is generally stated that intermittent images formed in peripheral parts of the retina require a higher rate of alternation to abolish flicker than those formed at the fovea. This view is unreservedly supported for white light by observations of Aubert [1865], Exner [1870], Bellarminow [1889], Sherrington [1904], Lohmann [1908], and Woog [1919], who employed various methods for the production of flickering images. In no case does very intense illumination appear to have been used, and the objects probably all subtended more than 2° at the observer's eye. Many of those who frequent cinemas are aware that flicker, under the conditions there obtaining, is more pronounced in indirect than in direct vision. While Charpentier [1890] invariably obtained the same result with small objects, he could detect no difference between centre and periphery with objects whose images were much larger than the yellow spot. Moreover a second series of experiments by E x n e r [1886], using a much smaller object than in his earlier series, gave the same fusion frequency for centre and periphery with intense stimuli, although at lower frequencies of alternation flicker was coarser in indirect vision. With more feeble illumination, flicker was abolished for central vision at speeds insufficient to cause fusion in the periphery. RRupp [1869], however, is reported by Exner [1870] (who contradicts him) to have found discs with alternate black and white sectors, seen by reflected light, to cease flickering in indirect vision at slower speeds of rotation than when viewed directly. Braunstein [1903] too has reported that the centre of the retina is more sensitive to flicker than the periphery. Dow [1910] was only able to confirm the usual finding with weak illumination; at high illuminations he states that fine flicker may persist at the