Influence of auditory localization cues on neuronal activity in the auditory thalamus of the cat.

1. The response properties of auditory thalamic neurons to the two major localization cues characterizing the azimuth of sound sources in the horizontal plane were investigated in cats. Single-unit responses to auditory stimuli (white noise and tones) presented with interaural phase differences (IPD) or interaural intensity differences (IID) were studied. 2. The proportion of neurons in the medial geniculate body that were sensitive to the localization cues tested was 28% for IPD (n = 253) and 37% for IID (n = 65). Half of the IID-sensitive units were also sensitive to IPD, but when the range of IPDs and IIDs to which each unit responded was converted to the sound-source locations that would generate those ranges they did not always correspond to overlapping azimuth angles. 3. The changes in discharge rate in response to the two localization cues occurred over very broad IPD and IID ranges. If this activity is involved in the representation of acoustic space, then the responses of individual neurons do not provide fine spatial tuning. 4. Contralateral and ipsilateral ear leads were represented in a continuous manner by the maximum discharge rate of IPD-sensitive units. On the other hand, units that were sensitive to IIDs were activated over one of two delimited ranges of IIDs. The first corresponded to IID combinations in which the stimulus was presented at a higher intensity in one ear than in the other (for 15/17 units the contralateral one); these were the lateralized intensity response field units. The second are the centered intensity response field units, whose responses were maximal when the intensity was equal in both ears and decreased when IIDs were introduced.