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Fig.1: Simulated neural responses (Patterson et al., 1995) to a random noise (A, C) and a Regular-Interval (RI) sound with a pitch of 83.3 Hz (F, H). The RI sound was constructed by (1) delaying a copy of a random noise by 12 ms (1/83.3 Hz), (2) adding it back to the original noise, and (3) repeating the process 16 times (Yost, 1996; see Methods for details). The ordinate in panels (A-C) and (F-H) is the tonotopic dimension of hearing, that is, ‘place’ along the basilar membrane. Panels (A) and (F) show the detailed temporal structure of the neural response at the output of the cochlea, which is similar for the two sounds. Panels (B) and (G) show that the average neural activity over time is very similar for the two sounds; there are no harmonically related peaks in (G) to identify the pitch. Panels (D) and (J) show that the average activity over time is also very similar; there are no regularly repeating features to identify the pitch of the RI sound. The temporal regularity that distinguishes the RI sound is in the time-interval information of the neural patterns (A and F). When the time-intervals between peaks are calculated in the individual channels of panel F and summarized in the corresponding time-interval interval histograms of panel H, a concentration of activity appears at the RI delay (12 ms) (Patterson et al., 1996). In contrast, the time-intervals in the neural pattern of noise (A) are randomly distributed in the time-interval histogram (C) (the concentration at 0 ms simply indicates the presence of activity in the channel). The position and height of peaks in the average interval histogram (K) are used to evaluate quantitative models of the pitch of RI sounds (e.g. Pressnitzer et al., 2001; Krumbholz et al., 2000).
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