Organization of auditory cortex in the albino rat: binaural response properties

1. The binaural response properties of neurons in the auditory cortex of the albino rat were examined using microelectrode mapping techniques. Characteristic frequencies, binaural response classes, and interaural intensity differences for binaural interaction were determined for multiple electrode penetrations across the cortical surface. The location of electrode penetrations was determined by reference to the cortical vascular pattern in individual animals. 2. When examined over a wide range of interaural intensities binaural responses could be classified as one of the following types: summation, i.e., excited by stimulation of either ear alone and facilitated by stimulation of both ears together (35.3%); suppression, i.e., excited by contralateral stimulation, unaffected by ipsilateral stimulation alone, but inhibited under binaural stimulus conditions (42.2%); mixed, i.e., facilitated by binaural stimulation at near threshold levels, but strongly inhibited by increased sound pressure levels in the ipsilateral ear (18.5%); or other, i.e., responses that could not be classified as any other type (4%). 3. Neurons of the summation and suppression class often exhibited binaural interaction when the intensities at both ears were approximately equal. The modal interaural intensity difference for both response types was between 0 and +5 dB. Neurons of the mixed interaction class were facilitate at near equal dichotic intensity but suppressed when the intensity in the ipsilateral ear was increased. The modal value was between 0 and +5 dB for summation and +20 dB for suppression. 4. Summation, suppression, and mixed binaural response types were found over a wide range of sound frequencies from 1 to 40 kHz. There was some tendency for summation responses to prevail at lower frequencies and suppression responses to prevail at higher frequencies but the differences were not large. Generally, responses from each of the three binaural classes were well represented over the rat's hearing range. 5. Cells of the same binaural response type were grouped together to form aggregates of summation, suppression, or mixed interaction patterns. Cortical areas with similar binaural response properties appeared in some cases to extend across isofrequency contours.