Neural correlates of pitch and roughness : toward the neural code for melody and harmony

Thesis (Ph. D.)--Harvard--Massachusetts Institute of Technology Division of Health Sciences and Technology, 2001.Vita.Includes bibliographical references (p. 127-138).The universality of many aspects of music, such as octave-based tuning systems and the use of dissonance and consonance to create harmonic tension and resolution, suggests that their perception may have fundamental neurophysiological bases. Thus, music provides a natural set of stimuli and associated percepts with which the auditory system can be studied. Here, we seek correlates of pitch, the essential element of melody, and roughness, a primary component of dissonance, in responses of single auditory neurons in anesthetized cats. Pitch, the perceived highness or lowness of sound, is generally thought to be based on a neurophysiological representation of frequency. Because neural responses (spikes) phaselock to low stimulus frequencies, interspike intervals (ISIs) reflect the stimulus period and can be used to estimate frequency. To rigorously test this potential code for pitch, we look for correlates of pitch under conditions where the percept deviates from a simple function of frequency. One such condition is the octave enlargement effect, listeners' preference for pure-tone octave ratios slightly greater than 2:1. Another is the pitch of a complex tone missing the fundamental frequency: the pitch matches that of the missing fundamental even when different harmonics are presented to opposite ears. We show that a correlate of the octave enlargement effect exists in ISIs of auditory nerve (AN) fibers and a correlate of the missing-fundamental pitch exists in ISIs of neurons in the inferior colliculus, the principal auditory nucleus of the midbrain. Results also reveal greater degradation of pitch representation at the midbrain compared to the periphery.(cont.) Roughness, the sensation of temporal envelope fluctuations in the range of 20-200 Hz, is often equated with sensory dissonance. Here we examine IC neural responses for correlates of sensory dissonance. We show that sensory dissonance correlates with discharge rate fluctuations of all IC neurons and with average rates of a subset of IC neurons which only respond at the onset of pure-tones. Results indicate that IC neurons are specifically important for the coding of the temporal envelope. Our findings illustrate the complexity and specificity of auditory neural processing in the brainstem and midbrain and show that percepts generally considered to be high order, such as the dissonance of musical intervals, have direct correlates in neural responses in the midbrain. More generally they show that the auditory system performs processing important for music at multiple time scales.by Martin Franciscus.Ph.D