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.