Masking of Cortical Responses in Middle Ectosylvian Auditory Area of the Dog

¹ From the Department of Anatomy, University of Oregon Medical School, Portland, Oregon

The masking of cortical responses in the middle ectosylvian (MES) auditory area of the dog was studied by oscillographic recording and by using noise, pure tones, the onset of pure tones and the ‘p’ pulse for acoustic stimuli. Local applications of strychnine to the cortex were used to enhance the responses and thus obtain information from the strychninized portion of the cortex. The characteristic of the masking curve was a low frequency slope which was less steep than the high frequency slope. The lowest threshold for masking occurred when the frequency difference of the masking tone and a masked tone was small. The curves were wider from the low frequency end of the area than from the high frequency end. Contralateral masking was absent although a refractory period could be demonstrated by two consecutive ‘p’ pulses. For a fixed frequency difference the intensity level of the masking tone or noise was a linear function of intensity level of the masked onset signal with a slope of one. A masking tone would cause the localized cortical response to an electrical shock delivered to a specific portion of the cochlea to vanish. Critical band width measurements with the onset of a tone for the masked signal and thermal noise for the masking signal were of the same general order of magnitude as the critical bands for human hearing. Certain similarities between the masking of the cortical responses and the general phenomenon of masking in human subjects were pointed out.