PHYSIOLOGICAL DETERMINATION OF THE ARRANGEMENT OF THE AFFERENT CONNECTIONS TO THE MIDDLE ECTOSYLVIAN AUDITORY AREA IN THE DOG

Abstract

By the method of strychninizing small patches of the cortex and testing with tones, the arrangement of the afferent connections to the middle ectosylvian area of the auditory cortex was detd. The fibers for each frequency terminate in a band no wider than 0.2 mm. and 5-7 mm. in length extending transversely across the gyrus. In the anterior-posterior direction the bands are arranged as a series of parallel strips with a spacing of 2 mm./octave over the frequency range between 250 and 8000 cps., the high frequencies being located in the anterior part of the gyrus and the low frequencies posteriorly. For each patch a response curve was obtained which represented the intensity-frequency range that caused induced strychnine spikes to appear. The response curve showed that each patch was most sensitive to 1 frequency, but responded to increasingly wider bands of frequencies as the intensity level was increased. The high-frequency slope of the curves measured 30 db/octave in the posterior or low frequency end of the gyrus and as much as 90 db per 1/10 octave in the anterior regions. The slope was a straight line and was subject to little variation. The low-frequency slope was less steep and several factors were found to contribute to its variable shape. These factors were: Loudspeaker characteristics; time constant of the onset of the tone; region of the cortex from which obtained; geometry of the strychninized patch; and harmonics generated by the sound equipment. No evidence of even harmonics was found. This was verified by using a continuous tone for masking of the sensitive frequency. It is concluded that a typical response curve for a patch of the middle-frequency range consists of a low threshold (0-10 db) for its sensitive frequency, with an attenuation of 90 db per 1/10 1/4 octave for the hf slope and an attenuation of 90 db in the 1st octave and 10 db in each succeeding octave for the low-frequency slope. The sensitivity curve of the middle ectosylvian area is very near the threshold curve of the best human hearing.