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Temporal coding of the periodicity of monaural and binaural complex tones in the guinea pig auditory brainstem
Humans report a strong pitch percept in response to a complex tone – the sum of a series of harmonics – presented to either a single ear (‘monaurally’) or both ears (‘diotically’). Interspike interval histograms of responses of neurons in the auditory system to monaural complex tones show a peak at the period of the pitch reported by humans – a ‘neural correlate of pitch’. However, the same pitch percept can be generated by presenting complexes with harmonics distributed across both ears (‘dichotically’). This requires combination of the neural signals underlying pitch from both sides of the auditory system, termed ‘binaural fusion’. Temporal coding generally deteriorates along the auditory pathway; binaural fusion should occur at a relatively early stage. One of the prime candidates is in the superior olivary complex (SOC).
Although the guinea pig auditory system has been extensively studied, this work is the first in vivo investigation of the guinea pig SOC. Cells of the lateral superior olive (LSO) show sensitivity to interaural level differences; medial superior olive (MSO) cells show sensitivity to interaural time differences. Additionally, cells with responses similar to the medial nucleus of the trapezoid body (MNTB) and superior paraolivary nucleus (SPN) of other species were found in the guinea pig SOC. Presumed MNTB cells showed a three-component spike waveform shape; presumed SPN cells responded at the offset of contralaterally-presented stimuli.
MSO and LSO cells respond to the overall pitch of complex tones, even if the monaural waveforms presented to each ear differ; this is consistent with the perception of humans. In contrast, cells of the ventral cochlear nucleus, which provide the main input to MSO and LSO cells, do not show evidence of a binaural pitch response. In conclusion, SOC cells are able to encode the pitch of binaural complex tones in their spike timing patterns.MRC Studentshi
Effect of Context on the Contribution of Individual Harmonics to Residue Pitch.
There is evidence that the contribution of a given harmonic in a complex tone to residue pitch is influenced by the accuracy with which the frequency of that harmonic is encoded. The present study investigated whether listeners adjust the weights assigned to individual harmonics based on acquired knowledge of the reliability of the frequency estimates of those harmonics. In a two-interval forced-choice task, seven listeners indicated which of two 12-harmonic complex tones had the higher overall pitch. In context trials (60 % of all trials), the fundamental frequency (F0) was 200 Hz in one interval and 200 + ΔF0 Hz in the other. In different (blocked) conditions, either the 3rd or the 4th harmonic (plus the 7th, 9th, and 12th harmonics), were replaced by narrowband noises that were identical in the two intervals. Feedback was provided. In randomly interspersed test trials (40 % of all trials), the fundamental frequency was 200 + ΔF0/2 Hz in both intervals; in the second interval, either the third or the fourth harmonic was shifted slightly up or down in frequency with equal probability. There were no narrowband noises. Feedback was not provided. The results showed that substitution of a harmonic by noise in context trials reduced the contribution of that harmonic to pitch judgements in the test trials by a small but significant amount. This is consistent with the notion that listeners give smaller weight to a harmonic or frequency region when they have learned that this frequency region does not provide reliable information for a given task