Figure 4.

Phase sensitivity experiment in pitch-selective neurons. A, B, Tuning curves of two pitch-selective neurons in response to harmonic complex tones varying in fundamental frequency. COS (blue) and ALT (red) complex tones were used. For these two neurons, the Bf₀ for ALT stimulus is ∼1 octave below that of COS stimulus. C, An example of a pitch-selective neuron (unit M36N-534.1) that is sensitive to changes in the phase of harmonics in a complex tone. The Bf₀ of this neuron is 152 Hz. Four types of stimuli were tested in four harmonic compositions (1–9, 2–10, 4–12, and 8–16): COS stimulus at Bf₀ (solid blue curve), COS stimulus at Bf₀/2 (dashed blue curve), ALT stimulus at Bf₀ (solid red curve), and ALT stimulus at Bf₀/2 (dashed red curve). The dashed line indicates the minimum firing rate criterion for a significant response (2 SDs above the mean spontaneous rate). D, An example of a pitch-selective neuron (unit M32Q-44.1) that is less sensitive to changes in the phase of harmonics in a complex tone. The Bf₀ of this neuron is 1.45 kHz. Four types of stimuli were tested in seven harmonic compositions. Format is the same as in A. E, The PSI is plotted against Bf₀ for a population of pitch-selective neurons. The PSI was calculated for complex tones with low-order harmonics (containing the fundamental) and for complex tones with higher-order harmonics (harmonic 4 or higher). Vertical dashed line indicates fundamental frequency of 450 Hz. The two horizontal lines indicate a PSI of 0 and 1, respectively. F, Mean PSI as a function of the lowest harmonic for pitch-selective neurons. Only for Bf₀ < 450 Hz were PSIs significantly different from 0 for harmonic compositions, with the lowest harmonics being the 4, 6, 8, or 12 (signed-rank test, p < 0.05, Bonferroni's corrected) when tested at a the Bf₀ of a neuron (solid green curve). The dashed horizontal lines indicate a PSI of 1 and −1, respectively.