Page 38 - Spring2020

Music Beyond Sound
reasonably be asked of music. It would seem that music per- ception, like speech perception, is possible on the basis of sensory input that does not involve any sound (see Figure 1). However, before considering how the brain manages to process music by touch, sight, and balance, it is useful to first consider music in its conventional form, as a sound-based temporal art form, but to do so from the perspective of audi- tory cognitive neuroscience.
Sound waves produced by voices or instruments are collected in the outer ear, mechanically amplified in the middle ear, and transduced by sensory hair cells in the cochlea within the inner ear to produce neuroelectric activity. This neuro- electric activity is then transmitted by the auditory nerve to the brainstem and onto the thalamus, which in turn proj- ects to the auditory cortex within the temporal lobe of the brain. The auditory cortex is the main cortical area involved in processing sensory information arising from music in nor- mal-hearing listeners. At the core of the auditory cortex lies a
“tonotopic map” wherein unique spatial positions correspond to unique frequency regions of sound input. Surrounding the core is a “belt area” and surrounding the belt lies the “para- belt.” In a normal-hearing adult, the core will be activated by sound alone, whereas the belt and parabelt can be activated by sound as well as other forms of sensory input. Figure 2 provides a macroperspective on the cortical modules and pathways beyond the auditory cortex that are believed to be involved in music perception (see article in Acoustics Today by Loui, 2019, for more on this subject).
A neuroimaging study by Zatorre and Belin (2001) revealed that responses to the local temporal features of sound (i.e., those with relevance for rhythm) were biased toward process- ing in the left hemisphere of the brain. In contrast, responses to the spectral features of sound (i.e., those with relevance for pitch and timbre) were biased toward processing in the right hemisphere. Consistent with these observations from neuroimaging studies are accounts from studies of patients experiencing unilateral brain damage. Patients with damage in the left hemisphere tend to show impairments in their abil- ity to discriminate rhythms relative to neurotypical controls (Peretz, 1990). In contrast, patients with damage to the right hemisphere tend to require larger pitch differences to make pitch discriminations (Milner, 1962), show a weaker ability to discriminate pitch direction (Johnsrude et al., 2000), and degraded sensitivity to the global pitch contour (i.e., whether a melody is rising or falling; Peretz, 1990).
The beat is an aspect of rhythm that is intuitive but challenging to define on the basis of physical features alone. Nevertheless, following the beat is considered to be essential for music pro- duction as well as perception. Almost all humans possess the ability to follow the beat, including many of those who are
“tone deaf” (Hyde and Peretz, 2003). This ability is thought to be primarily served by a dorsal pathway connecting sen- sory and motor areas of the cortex. This same dorsal pathway is thought to be involved in perceiving emotion in music (McGarry et al., 2015), learning a new piece of music by ear (Lahav et al., 2007), and in the type of feedback monitoring
Figure 2. A sagittal view of the human brain featuring modules and pathways that are believed to be involved in the perception of music. The front of the brain is to the left.
38 | Acoustics Today | Spring 2020