Sound direction influences the frequency-tuning characteristics of neurons in the frog inferior colliculus

Abstract

1. We investigated the influence of sound direction on the frequency-tuning characteristics of neurons in the frog inferior colliculus, the torus semicircularis. For each neuron, we used tone bursts to determine the frequency-threshold curves (FTCs) for three to seven loudspeaker azimuths. The loudspeaker was mounted on a rotatable arc and could be swung through the frontal field between positions opposite the ear that was contralateral to the recording site (C90 degrees) and the ear that was ipsilateral to the recording site (I90 degrees). 2. Frequency-tuning data from 83 units showed that the characteristic frequency (CF) shift through a 180 degrees change in loudspeaker azimuth was typically small, i.e., 85% of neurons showed maximum absolute changes in CF that were < 0.4 octaves. Paired comparisons of CFs for each neuron when the loudspeaker was located at C90 degrees, and the frontal midline position (0 degree) revealed no significant differences (P > 0.2) between azimuths. The magnitude of CF shift between different sound directions showed no systematic pattern. 3. In contrast to the CF, midbrain neurons showed distinct changes in the minimum threshold (MT) at CF across 180 degrees of azimuth. The maximum absolute change in MT ranged from 0 to 38 dB, with a mean of 10.9 dB. A pair-wise comparison of MTs, for each neuron, derived with the speakers at C90 degrees, 0 degree, and I90 degrees, revealed that the MT typically increased when the loudspeaker was rotated toward I90 degrees (P < 0.0001). 4. The most prominent effect of rotating the loudspeaker from C90 degrees to I90 degrees was a narrowing of the FTC. Sharpness of tuning in simple V-shaped FTCs was most directly shown by changes in the Q factors at 10, 20, and 30 dB above MT as a function of sound direction. A pair-wise comparison for individual neurons showed that all Q factors were significantly larger (sharper tuning) for I90 degrees compared with C90 degrees (P < 0.02). The Q20dB values were also significantly larger for I90 degrees compared with 0 degree (P < 0.05). For the majority of units, the maximum Q10dB and Q20 dB values were displayed when the loudspeaker was positioned at I90 degrees; however, the maximum Q30dB was distributed nearly equally for the three azimuths. These results suggest that a change in sound direction has the most robust effect on tuning properties of the FTCs nearest the MT.(ABSTRACT TRUNCATED AT 400 WORDS)