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source characteristic and is directly related to the rate of vibra- tion of the vocal folds; in music it is dictated by such things as string length. We recognize that the percept of pitch is not sole- ly conveyed by the fundamental frequency, but pitch is our shorthand for the repeating period of the signal. We define har- monics as the overtones of the fundamental. We recognize that harmonics arise from the same source as the f0 and also con- tribute to the percept of pitch. But in music, their relative ampli- tudes contribute to the identity of the instrument, and in speech, the identity of the particular vowel or consonant that is being spoken. These properties, not their shared origin with the fun- damental, put harmonics into a different camp from pitch in our model. Timing refers to the major acoustic landmarks in the temporal envelope of the signal, in speech, arising from the alternating opening and closing of the articulators and from the interplay between laryngeal and supralaryngeal gestures, and in music the rhythmic structure of the phrase. In speech, timing also includes spectrotemporal features of speech such as the changing of formants over time. The three components of our model, pitch, timing and harmonics, as defined here, have direct and separable parallels in the speech- and music-evoked brain- stem responses.
b How do we know that what we are recording does not arise from structures more central to the auditory midbrain? We acknowl- edge that our non-invasive (scalp electrode) technique prohibits certainty of source. We believe that the low-pass characteristic of the auditory system minimizes the possibility that the highly-fil- tered activity we measure is cortical afferent activity.110 Moreover, the complex auditory brainstem response (cABR), especially its frequency-following response (FFR) component has been wide- ly studied, and several converging lines of evidence point to a subcortical source. The FFR appears in response to tone pips that are shorter than the time required for cortical propaga- tion.138 The time delay of the individual FFR cycles, with respect to the evoking stimulus, is around six milliseconds, which is too
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ilarity of latencies of FFRs recorded from cat scalp and brain- stem inferior colliculus, and second, from the abolition of sur- face-recorded responses with cryogenic cooling of inferior col- liculus. Additionally, Galbraith141;142 demonstrated that record- ings from the scalp reflect a response of central brainstem origin. However, due to the length of our cABR stimuli—100 millisec- onds and up, cortical influence can not be completely ruled out. More probable, is that the responses are a mix of afferent brain- stem activity and cortically modulated efferent effects on brain- stem function. It also bears mentioning that responses from putative deep-brain sources are less topographically variable than responses from more superficial cortical areas. Much insight on voltage sources is gained by a full topographical array of electrodes in the investigation of cortical responses. However, due to their long travel in propagation to the scalp, speech-ABRs lose site-specificity; hence, little is to be gained by studying their topographic distribution. A single vertex electrode is suffi- cient.143
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Animal work
of evidence of a subcortical origin for FFRs: first, based on sim-
early for cortical involvement.
added two lines
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