engleski

Spatial Profiles of Correlation in Spike Timing to Broadband Noise Across Auditory Nerve Fibers

The cat auditory nerve (AN) contains ~50000 neurons innervating ~2500 inner hair cells across the cochlear basilar membrane. We are interested in the extent to which these neurons carry correlated temporal patterns, which depends on both the acoustic stimuli and intrinsic properties of the auditory periphery. Due to cochlear filtering, even Gaussian broadband noise – for which adjacent frequencies have random phase – can produce correlated responses in fibers originating from nearby cochlear positions. We studied the spatial profile and extent over which correlated responses to broadband noise are found in the AN. Neural responses to repeated presentations of a single token of broadband noise presented at 50 dB and 70 dB SPL were obtained from all fibers encountered in a given nerve. Spike trains from each fiber (“reference fiber”) were compared with spike trains of all other fibers (“test fibers”) in order to obtain normalized same-stimulus cross-correlograms (SCC). Cochlear positions of fibers were estimated from their characteristic frequency (CF) of the neural tuning curve. As reported earlier (Joris et al., 2006), such SCCs show maxima at delays that depend on the distance between cochlear positions of the two fibers. We measured correlation in two ways: a) as the height of the largest peak of the SCC and b) as the SCC value at zero delay. The spatial correlational profiles were quantified either as the full-width at the half maximum (FWHM or “half-width”) or as the correlation area under the spatial profile. The observed spatial profiles show broad regions of correlation of 2-3 millimeters centered on the reference fiber, at all CFs. The half-width of the magnitude of correlation is surprisingly invariant with cochlear position, particularly when measured at a delay of 0 ms. The correlation area, which provides a more global measure of level of correlation (“correlation energy”), clearly decreases towards the base, reflecting the gradual decline of temporal coding with increasing CF. The results indicate that 1) spatial profiles are narrower in shape when the range of delays is restricted to 0 ms, compared to when it is unconstrained ; and 2) a 20 dB increase in stimulus level is not accompanied by a broadening in extent of across-fiber correlations, despite the known broadening in cochlear filter shape. These findings suggest that if the central nervous system has monaural delays available to compare timing across fibers of different CFs at different delays, the spatial bandwidth of correlation is broader than when such delays are not available. Supported by fellowships to DK (Marie-Curie GA221755 & NSF Croatia) and PM (IWT SB-81346), and grants FWO (G.0633.07 & G.0714.09) and BOF (OT/05/57).

Auditory nerve; cross-correlation; broadband noise

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