Determination of visual and auditory thresholds was carried out under a response duration schedule of tone and/or light-dipper presentation. This schedule enabled an exact assessment of visual and auditory sensitivities, since the direction and distance from the rat to the sound or light sources were constant, and the rat could thus acquire the conditioned behavior in a short time. The rat has to put its nose into the hole and hold it there until a conditioned stimulus (tone and/or light) was presented. The animal could get water-reinforcement by taking its nose out from the hole during the period of conditioned stimulus. To determine the visual threshold, the intensity of light was reduced, from 100 Lx (starting point) by 10 Lx steps in each trial, until the subject made 10 non-response trials (i.e., no dipper approach within 1.0 sec after the light onset) among 10 trials. In terms of auditory threshold, the intensity of 3, 7 and 10 KHz pure tone was reduced, from 90 dB by 10 or 2 dB steps in each trial, until the subject made 10 non-response trials among 10 trials. The light or tone intensity that prolonged the response latency 2 times compared to normal rats was taken as the threshold. The visual sensitivity in the rats suffering from alkaline-burn on its corneas decreased about 14 Lx. The auditory thresholds in rats were 19.5 ± 1.3, 37.5 ±1.8 and 54.6 ±1.3dB at 10, 7 and 3 KHz, respectively. The auditory sensitivity in the rats with cotton-stuffed ears and pierced eardrums decreased about 6 and 12 dB, respectively.
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