Binocular fusion as summation of monocular luminances is the essential feature of the previously developed model of binocular summation, as opposed to other models which offer a common rule for binocular combinations of monocular contrasts. Recent results suppport the fusion hypothesis, but not exclusively. The quadratic rule of contrast summation which explicitly prohibits luminance summation, could also account for a large part of the results. The main purpose of the present, psychophysical progam is to resolve the dilemma between luminance-summation and contrast-summation. We shall compare the predictions of the fusion model and the quadratic summation model in three paradigms. (i) We shall measure monocular (M) and binocular (B) increment thresholds, and modulation thresholds, all on binocularly fused backgrounds of luminance I and 2I (1 log unit photopic range). On full luminance summation (S), we shall always have S-(M/I)/(B/2I)=1. (ii) Two-pulse increment thresholds will be measured as a function of interval, and polarity of the pulses. The pulses will be given to the same eye (M), or to opposite eyes (B). Temporal summation at threshold will be similar for M and B, if the hypothetical fused path exhibits the same bi-phasic impulse response, as a monocular path. [On quadratic summation, the B thresholds at zero delay must be independent of polarity.] (iii) Binocular sensitivity to modulation will be determined as a function of interocular phase angle and modulation frequency. The change of sensitivity with phase would be sinusoidal at all frequencies, if a constant rule sums monocular signals. We shall also determine whether selective adaptation of the fused channel can spuriously reduce the binocular advantage at threshold (so that M/B = 2, instead of a larger value, e.g., M/B=2, on full summation). Continuous modulation, as opposed to pulsed increments, should adapt the monocular channels and the fused channel to a similar degree. We should find that the binocular advantage is larger with modulated, than with pulsed stimuli. The long-range goal of the program is to develop a model of normal binocular functionining that accounts both for luminance summation and higher binocular contrast sensitivity.
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