How Sound Modulates Responses to Light with Different Time Intervals Between Stimuli of Different Modalities

Abstract

Extracellular recording of 34 neurons in the primary visual cortex of three conscious rabbits was performed in chronic experimental studies addressing the effects of sound (2000 Hz, 70 dB, 40 msec) on the discrimination of weak light intensities (0.3–1 cd/m2). Sound was delivered with different time intervals before and after substitution of one light intensity by another (using a total of 15 intervals in the range –750 to +150 msec). Factor analysis of variance (ANOVA) showed that the time interval between the sound and the light had statistically significant influences on neuron responses. Sound itself provoked no response. Neuron reactions consisted of responses to increases (on responses) and decreases (off responses) in light intensity. The most marked effect of sound was seen in the initial phase of the response (40–100 msec from the moment at which intensities were substituted). For every interval, neurons demonstrated both increases and decreases in reactions to complexes as compared with their responses to light. Wilcoxon’s T test was used to assess differences in sets of responses to light and to complexes. For the on responses of the whole group of neurons, the absolute values of responses to sound + light complexes were significantly (p < 0.05) different from responses to light (increased reactions) at intervals of –150, –40, and 0 msec. Two groups of neurons were identified, for which the effects of sound on responses to light were markedly different. Neurons of group 1 (n = 16) showed significant positive influences of light on on responses over a wide range of intervals (–150, –40, –20, 0, +20, +100 msec), along with a larger increase in the number of spikes due to sound (by 18–28%) as compared with responses to light. Neurons of group 2 (n = 18) had no significant intervals, i.e., at which reactions to complexes were not significantly different from responses to light. None of the study groups of neurons showed intervals with significant decreases in responses to complexes, though there was a tendency for reactions to complexes to shift towards weakening of responses at intervals of –750 and –80 msec for group 1 (p < 0.07) and at intervals of –500 and +20 sec for group 2 (p < 0.1). On responses were found to be more strongly affected by sound than off responses. The effects of sound on the second phases of responses to light (120–160 msec and later, n = 23) were also studied. Sound had markedly weaker effects on the second phase than the first. For the whole group of neurons with late phases, sound had significant influences on on responses with an interval of 0 msec and on off responses with intervals of +100 and +150 msec. Our study demonstrated similarity in the time intervals for modulation of reactions to light by sound in experiments on animals and psychophysical studies in humans. These data provide for more detailed studies of the integration of light and sound when used simultaneously.