Apparent Rate of Flicker at Various Retinal Loci and Number of Perceived Flashes per Unit Time: A Paradox

Abstract

Short trains of 25 flashes per second were presented at various retinal eccentricities. Although the count rate, or number of flashes perceived and counted per unit of real time remains relatively constant regardless of retinal locus upon which the flashes fall, the rate at which the perceived flashes appear to occur does not remain constant but, paradoxically, decreases markedly with retinal displacement. To resolve the paradox, it was postulated that subjective flash rate is a resultant of the perceived flashes divided by perceived time between them where perceived time under conditions of this experiment is not constant, but increases with stimulus eccentricity. The implications of this postulate in terms of visual “scanning” mechanisms was discussed. The idea was introduced that several function-specific “scan” rates, rather than a single generalized one, are operant in the visual system. In a secondary phase of the study, it was shown that, although slope decreased with retinal displacement, the psychophysical function relating physical to subjective flash rates for sub-fusional frequencies has a form which is similar at four peripheral observation loci to that at the fovea.