The fusion frequency of flicker in the central and peripheral field with photopic levels of surround luminance

Flicker-fusion frequencies have been measured against a photopic level of surround luminance in the four quadrants of the visual field and the critical fusion frequences (c.f.f.s.) taken in direct vision compared with those determined at 15° and 35° in the periphery. The central c.f.f.s. of 47 subjects exceeded the peripheral values only in just over half of all the quadrants tested, although smaller differential thresholds were given almost without exception in the centre than in the periphery by the same observers and under similar conditions on a test of brightness discrimination. Likewise higher central than peripheral c.f.f.s. were recorded in the great majority of all quadrants when determinations were made against a dark surround. The contrasting types of flicker response observed against the photopic ground were shown to be relatively reproducible, and the same pattern was as a rule found in at least three quadrants of any one visual field. It was ascertained that the different patterns might result from a reciprocal shift of fusion frequencies either in central or peripheral vision.

Interpretations of the higher peripheral c.f.f.s which adduce the effect of accentuated pupillary constriction with central stimulation mere contra-indicated since such patterns of flicker perception survive after the photopupillary reflex has been abolished. Some evidence was, however, forthcoming for the view that peripheral fusion frequencies tend to exceed central values when the test object is larger than about 1·5° in angular size and more intense than the surround. It was suggested that the contrasting types of flicker response might be related to individual differences in the effect that stimulus size has on the flicker pattern.