Abstract
Humans can accurately localize sounds even in unfavourable signal-to-noise conditions. To investigate the neural mechanisms underlying this, we studied the effect of background wide-band noise on neural sensitivity to variations in interaural level difference (ILD), the predominant cue for sound localization in azimuth for high-frequency sounds, at the characteristic frequency of cells in rat inferior colliculus (IC). Binaural noise at high levels generally resulted in suppression of responses (55.8%), but at lower levels resulted in enhancement (34.8%) as well as suppression (30.3%). When recording conditions permitted, we then examined if any binaural noise effects were related to selective noise effects at each of the two ears, which we interpreted in light of well-known differences in input type (excitation and inhibition) from each ear shaping particular forms of ILD sensitivity in the IC. At high signal-to-noise ratios (SNR), in most ILD functions (41%), the effect of background noise appeared to be due to effects on inputs from both ears, while for a large percentage (35.8%) appeared to be accounted for by effects on excitatory input. However, as SNR decreased, change in excitation became the dominant contributor to the change due to binaural background noise (63.6%). These novel findings shed light on the IC neural mechanisms for sound localization in the presence of continuous background noise. They also suggest that some effects of background noise on encoding of sound location reported to be emergent in upstream auditory areas can also be observed at the level of the midbrain.
Highlights
Sound localization can be impaired by background noise, but it is often forgotten that we maintain relatively good sound localization performance across many signal-to-noise ratios (SNRs) (Good & Gilkey, 1996; Good et al, 1997; Lorenzi et al, 1999; Andeol et al, 2011; Kerber & Seeber, 2012)
We describe the potential correlation between the effect of background noise on the excitatory and inhibitory inputs received from individual ears and the effect of binaural background noise on spatial coding
We recorded from 67 cells, confirmed by their response characteristics and the progression of characteristic frequency (CF) in successively encountered neurons to be in the central nucleus of inferior colliculus (IC)
Summary
Sound localization can be impaired by background noise, but it is often forgotten that we maintain relatively good sound localization performance across many signal-to-noise ratios (SNRs) (Good & Gilkey, 1996; Good et al, 1997; Lorenzi et al, 1999; Andeol et al, 2011; Kerber & Seeber, 2012). Some limited data are available on the neural bases of the effect of background noise on coding of cues for sound location or on the spatial tuning in auditory cortex and superior colliculus (SC) (Brugge et al, 1998; Furukawa & Middlebrooks, 2001; Martin et al, 2010). With respect to response strength, the effects in SC (Martin et al, 2010) and A1 (Brugge et al, 1998) seem to differ. It is the site of de novo synthesis
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