Limulus psychophysics: Increment threshold

Abstract

Prior temporal summation work had indicated that the sensory code for certain behaviors (in both Limulus and humans) can be understood if one suggests that the central nervous system analyzes the integral of the photoreceptor potential. An independent test of this suggestion is available because (in Limulus) the physiological increment threshold function obtained from the receptor potential integral is inflected, whereas that obtained from the initial transient peak of the receptor potential is not. The behavioral increment threshold function was measured in Limulus and found to be inflected. Fechner's scaling assumption (that equally detectable stimulus increments are mediated by sensory signals of equal size) was supported by the fact that a theoretical function, which was calculated from the receptor integral intensity-response function by using Fechner's scaling assumption, was able to fit the behavioral increment threshold function quite well. Furthermore, the variability of the behavioral data was proportional to the receptor integral variability. A seasonal effect was observed: Fall animals were more sensitive and had a higher criterion than winter animals. These findings permit the specification of the rerceptor potential that mediates this particular behavior at threshold: It has a steady-state amplitude of 7 mY in winter and 16 mY in fall. Taken in conjunction with the results of earlier temporal summation work indicating that the threshold receptor potential is 4.5 sec long, this specification implies that at least some behaviors are mediated by large, long sensory signals, which have properties very different from those of small, short signals, particularly with regard to both linearity and their relative dependence on time vs. energy.

Daily and seasonal variations inlimulusvisual behavior

“…This would cause a daily variation in visual behavior. Hypothesis 2 is therefore consistent with the data from the present experiment; however, Hypothesis 2 is still subject to the same inability of Hypothesis 1 to account completely for the increment threshold data reported in Wasserman (1981).…”

“…For example, Wasserman and Patton (1970) found that Winter animals almost never responded to a light of a fixed intensity while Spring animals almost always responded to a light of the same intensity. In a more recent study, Wasserman (1981) found that Winter animals were 0.5 log units less sensitive than Fall animals. The only difference between the present study and these earlier studies is that the present one covered all seasons of a year whereas the prior studies permit only a comparison between two adjacent seasons.…”

“…An optical hypothesis would also have to deal with the results of an earlier increment threshold experiment (Wasserman, 1981) to be described more fully below. That experiment yielded opposite effects of season on responsiveness to the test stimulus versus responsiveness to the background stimulus.…”

“…Wasserman, 1976) and the spectrum of the stimulus (cf. Wasserman, 1981), the incident quantal flux at the highest intensity can be estimated as approximately 5 x 10 u quanta/sec/receptor. After allowing for photopigment density (Wasserman, 1976), approximately 2 quanta were absorbed by each receptor every millisecond at the lowest intensity.…”

Cochlear implant codes and speech perception in the profoundly deaf

“…These studies have compared intracellular recordings taken from single receptors with the behavior of intact subjects. Problems studied have included: the problem of temporal summation, or the tradeoff between stimulus duration and intensity (Kong & Wasserman, 1978a, 1978bWasserman, 1978;Wasserman & Kong, 1975); the problem of the psychophysical function, relating sensation quantity to stimulus intensity (Wasserman, Felsten, & Easland, 1978, 1979Wasserman, 1981); and the problem of perceptual masking, or the effect that one stimulus has on the perception of a second stimulus , 1979a, 1979b, 1981Wasserman, Lo, & Easland, 1976).…”