Sound Improves the Discrimination of Low-Intensity Light in the Visual Cortex of Rabbits

Abstract

Experiments were performed on four European rabbits (Oryctolagus cuniculus). Rabbits underwent implantation of silver-plated electrodes into the bone over the primary visual cortex to record evoked potentials. Evoked potentials to substitution of threshold visual stimuli of intensity 0.28 and 0.31 cd/m2 were initially recorded, after which these stimuli were supplemented by sounds (2000 Hz, 84 dB, 40 msec). Sounds presented alone did not induce responses. Measurement of the amplitude of the N1 wave (85–110 msec) showed that the magnitudes of the response to substitution of sound + light complexes were significantly greater, by a factor of 1.6, than responses to substitution of light stimuli. Paired substitutions of light stimuli of eight intensities over the range 0.28–20.2 cd/m2 were then performed, and factor analysis was used to reconstruct the intensity sensory space. This was compared with a similarly obtained sensory space constructed with light stimuli accompanied by sounds. Analysis of the distances between the ends of the vectors representing the stimuli showed that addition of sounds led to enlargement of the space occupied by low-intensity light (0.28, 1.02, 3.05, and 6.35 cd/m2) by an average factor of 1.4 and ordered the positions of light in the space – strictly from the less to the more intense. At the same time, sounds shrank the space delimited by stimuli of greater intensities (8.48, 13.7, 16.8, 20.2 cd/m2) by a mean factor of 1.33. It is suggested that addition of sounds leads to improvements in the discrimination of lowintensity lights and leads to some degree of restriction to the discrimination of high intensities. The sensory spaces constructed from these substitutions of complex sound + light stimuli, as in the case of light stimuli, were two-dimensional, providing evidence supporting the suggestion that these stimuli are integrated into the single complexes when delivered simultaneously.