ASA 130th Meeting - St. Louis, MO - 1995 Nov 27 .. Dec 01

4aPP6. An evoked potential study of directional sensitivity in the inferior colliculus of the laboratory mouse (Mus domesticus).

David M. Cain

Philip H.-S. Jen

Div. of Biological Sciences, Univ. of Missouri, Columbia, MO 65211

Directional hearing of the laboratory mouse was studied by recording 30 evoked potential responses from the inferior colliculus to best frequency (BF) sounds delivered from within the frontal auditory field. Audiograms determined with maximal response amplitude (N=6) and minimum threshold (N=8) showed maximal auditory sensitivity occurring between 10--15 kHz (M(plus or minus)s.d.=11.2 kHz(plus or minus)2.34). For five evoked potentials, amplitude-intensity functions were obtained for seven selected azimuthal angles at 0(degrees) elevation. All 35 functions were nonmonotonic but their dynamic range was affected by sound direction. The azimuthal and elevational angles of maximal auditory sensitivity (the response centers) of 25 evoked potentials responses, were always located in the upper, contralateral quadrant (M(plus or minus)s.d.=up 25(degrees)(plus or minus)19.97, contralateral 39(degrees)(plus or minus)11.73). Spatial response areas measured at either 3 or 5 dB above the minimum threshold decreased with stimulus frequency (3-dB area: r=0.53, p=0.0118, and 5-dB area: r=0.44, p=0.0338). Spatial response areas associated with higher BFs were more concentric and smaller than those of lower BFs, which were larger and often expanded irregularly beyond the tested angles of the frontal auditory field. The results reflect the directionality of the sound-pressure transformation at the pinna of the mouse as demonstrated in our recent study (Chen et al., in press).