Thomas M. Nelson

A Further Study of Pulse-to-Cycle Fraction and Critical Flicker Frequency. A Decisive Theoretical Test*

Various curves relating CFF to log I cross each other when these curves represent different PCF’s (pulse-to-cycle fractions). This was interpreted by Bartley to mean that PCF and conditions of flicker and fusion are not simply related, as has been generally taken for granted. He supposed that, whereas short pulses in a given repetitive cycle would produce flicker and longer pulses would produce fusion, still longer ones would reintroduce flicker, and finally the longest ones would produce fusion. An electronic fusion apparatus producing square waves and varying PCF from 0.02 to 0.98 was used to test this directly. PCF was varied over this range while CFF (and thus cycle length) and intensity were held constant. The supposition was readily confirmed. In addition, data were collected suggesting that the very shortest pulses would produce fusion rather than flicker at some intensity levels.

The Talbot-plateau law and the brightness of restricted numbers of photic repetitions at CFF

Abstract Measurements of the brightness and CFF resulting from brief sequences ofintermittent stimulation are reported. Talbots law does not predict the brightness resulting. It must be restricted to sequences occupying longer periods. Thealternation of response theory is pointed to for explaining the present results; that is to say, the explanation is based upon neurophysiological discoveries showing how the optic pathway operates in relation to temporal and quantitative features of input.

Equivalence of various pulse-to-cycle fractions in producing critical flicker frequency.

An earlier investigation of the senior author suggested very strongly that various quite different PCF’s (pulse-to-cycle fractions) were, under some conditions, equivalent in producing CFF. This was contrary to common expectation, and was not fully confirmed by the scant amount of data then obtained. The present study consisted of testing the earlier suggestions by the use of seven observers using one method, and two observers using a second method of collecting data. The CFF’s for each of the following PCF’s, 1/30, 1/8, 1/4, 1/2, 3/4, 7/8, and 29/30, were obtained as the photic intensity was varied. In Part I, the range was from 1.28 c/ft2 to 1164 c/ft2. In Part II, the range was from 0.014 c/ft2 to 1400 c/ft2. The data produced families of curves, one curve for each PCF. Some pairs of these curves crossed each other, the points of crossing indicating that, under those conditions, two PCF’s were equivalent.

A theoretical study of shape and proportion

SummaryA white disk lacking visible texture was placed in several types of visibly structured surrounds. S responded by drawing the shape of the target and by representing the proportions. The results indicate that shape and proportion responses are distinct from each other. Changes in the nature of the area external to the area seen bounded by the edge affect shape responses more than proportion responses.