Sensitivity to Schroeder phase arising from sensitivity to temporal patterns across nerve fibers

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The flat-envelope harmonic complexes devised by Schroeder (1970) have been extensively used in psychophysical studies. The stimuli contain a linear frequency sweep, whose direction and speed depend on phase curvature C. For a given characteristic frequency, basilar membrane and nerve responses show only a subtle dependence on the sign and magnitude of C. The question arises whether CNS neurons are sensitive to the temporal pattern across nerve fibers. We studied octopus cells in the cochlear nucleus, which receive a convergent excitatory input from nerve fibers and have been hypothesized to be monaural coincidence detectors. We studied both their spike output and nerve input using multiple recording techniques in anesthetized chinchillas. Octopus cells showed marked sensitivity to C, which was broadly stable across SPL and fundamental frequency. They were tuned to a wide frequency range, but with discrete frequency “hotspots” that generate major and minor inputs. The sensitivity to C arises not from coincidence detection, but from a sensitivity to the temporal sequence of activation of inputs tuned to different frequencies. We conclude that marked sensitivity to Schroeder phase is already present at the first CNS processing stage, based on temporal sequence detection across frequency channels rather than on coincidence detection.