Neural bases of complex sound perception: Insights from an avian speech mimic

Abstract

Many complex sounds including speech contain periodic fluctuations in signal level known as amplitude modulation (AM). Auditory-nerve fibers encode AM primarily through response synchrony to signal modulation. In contrast, neurons of the midbrain encode AM through both response synchrony and substantial changes in average discharge rate. Specifically, many midbrain neurons with band-pass modulation tuning show enhanced discharge activity in response to a limited range of modulation frequencies. Here, we describe several recent studies on the relative importance of envelope synchrony vs. average rate coding in the midbrain for AM perception. Behavioral and neurophysiological experiments were conducted in the budgerigar, an avian species with the unusual capacity to mimic speech. Budgerigars were found to exhibit human-like behavioral sensitivity to a variety of complex sounds including AM stimuli and synthetic vowels with triangular and natural spectral envelopes. Most neurons in the budgerigar midbrain showed band-pass modulation tuning. Whereas either response synchrony or average discharge rate was sufficient to account for behavioral performance in some cases, only response synchrony could explain behavioral thresholds for (1) detection of low modulation frequencies and (2) formant-frequency discrimination in noise. These results highlight the significance of midbrain envelope synchrony for perception of complex sounds. [R00-DC013792 and R01-DC001641.]