THE EFFECT OF SPATIAL SEPARATION OF SIGNAL AND NOISE ON MASKING IN THE FREE-FIELD AS A FUNCTION OF SIGNAL FREQUENCY AND AGE IN THE MOUSE

Masking of low- (4 kHz) and high-frequency (25 kHz) signals by one-oct ave bandpass maskers either spatially coincident with the signal or co ntralateral to it was examined in mice, 4-6 and 20-22 months of age, i n the free field. Signals were presented 120 ms prior to a startle sti mulus and differences in their inhibition of the startle reflex, relat ive to startle stimulus alone trials, were used to measure the severit y of masking. Inhibition was reduced or eliminated by spatially coinci dent noise for weak but not for relatively intense signals, providing the type of ''loudness recruitment'' effect characteristic of human li steners in similar stimulus conditions. The spatial separation of the signal and its masker relieved this maskinglike effect for the high-fr equency pair in both young and old mice. In contrast there was no bene ficial effect of the shift in spatial location for the low-frequency p air at either age. This finding of masking release for high- but not l ow-frequency stimuli supports the hypothesis that the sound shadow pro vided by the head and pinna would yield a favorable signal-to-noise le vel difference for a contralateral masker and an ipsilateral signal on ly at very high frequencies in the mouse. The presence of masking rele ase in these old mice, a first generation hybrid strain with near-norm al high-frequency hearing in ABR measures, agrees with reports that th e masking release resulting from a similar manipulation in aged human listeners with minimal high-frequency hearing loss is the equal of tha t obtained in the young listener. (C) 1998 Acoustical Society of Ameri ca. [S0001-4966(98)07008-8]