Week 10: Neural Specializations for Tonal Processing; Zatorre

I found this chapter very interesting and therefore decided to do a slightly more in-depth summary that I would otherwise. The author’s abstract is good for a briefer summary.

Zatorre contends that music is a biological, not (solely) cultural, phenomenon; as such, he recognizes the importance of identifying dedicated (i.e. not shared) neural substrates for the processing of music. Investigating the neural map of music in the brain will provide much information about brain processing in general. Noting that language and music (though having some distinct neural substrates, most likely) are two ubiquitously human stimuli, Zatorre demonstrates the importance of understanding the biological foundations of both. Studies of brain-lesion patients (largely from cerebral-vascular brain injuries, such as those that may occur as a result of stroke) and studies of brain imaging techniques support the biological basis for music in humans.

This chapter looks specifically at ‘aspects of tonal processing and their neural substrates.’ The functional specialization of neural systems is accepts in other domains, but music remains somewhat vague in the research field of neural systems – particularly dedicated ones. Zatorre divides his paper by looking first at ‘low-level’ aspects of pitch processing, like pitch discrimination (under the heading ‘basic aspects of pitch processing’), then looks at higher-order aspects such as pitch patterns (under the heading ‘processing of pitch patterns’); ‘hemispheric differences in spectral processing’ and ‘anatomical considerations’ complete Zatorre’s presentation. Salient points from each section are presented below.

BASIC ASPECTS OF PITCH PROCESSING
• The midbrain or thalamus might be sufficient for pitch discrimination, as shown by studies of animals that have bilateral destruction of the auditory cortical areas yet retain the ability to process simple pitch discrimination.
• Left temporal lobe lesions can leave one quite unimpaired in regards to pitch discrimination and pitch-direction (higher and lower levels of processing, respectively)
• Right temporal lobe lesions that do NOT include Heschl’s gyrus also leave one quite unimpaired in regards to pitch discrimination and pitch-direction
• Right temporal lobe lesions that DO include Heschl’s gyrus leave one relatively unimpaired on pitch discrimination (lower level processing), but greatly impairs the ability to discriminate pitch-direction (higher level processing)
• This can be taken to mean that organizing sounds according to their pitch in some way requires the primary auditory area in Heschl’s gyrus in the right temporal lobe
• These findings are consistent with other research where patients with RTL lesions extending into HG had difficulty with missing fundamental pitch discrimination
• Brain imaging studies seem to be in agreement with these findings (based on cerebral blood flow to HG)

PROCESSING OF PITCH PATTERNS
• Here working memory is involved
• Zatorre notes that the hemispheric differences presented in this chapter are likely relative rather than absolute
• Duetsch has found that memory for tones is relatively specific, because the memory is not disrupted by other sounds but only other tones.
• Patients with excision of the right temporal lobe have more difficulty with tonal memory, as non-tone auditory interference worsens performance on tonal memory tests
• The right superior temporal gyrus, anterior to the primary auditory cortex, shows increased cerebral blood flow during active processing of novel melodies (more so than the left hemisphere), supporting the claim that neural networks within the right SECONDARY auditory cortices are critical for processing the perception of tonal information
• P. 240 ‘right interior lateral frontal areas are important for maintenance of tonal information, whereas dorsolateral frontal areas are required for higher-level functions such as monitoring the contents of working memory’
• Studies from musical imagery brain imaging support this behavioral evidence

HEMISPHERIC DIFFERENCES IN SPECTRAL PROCESSING
• ‘The hypothesis is that there may be a tradeoff in processing in temporal and spectral domains, and that auditory cortical systems in the two hemispheres have evolved a complementary specialization, with the left having better temporal resolution, and the right better spectral resolution’ (p. 241)

ANATOMICAL CONSIDERATIONS

• The anatomy in the left and right auditory cortical areas may be different: the left having more myelination (the sheath which quickens and protects information traveling along the neuron) and larger neuron composition, with more interconnectedness; the right may have thinner myelination and smaller neuronal composition with less interconnectedness. These findings would correspond with temporal processing as occurring primarily in the left auditory cortical area and spectral processing as occurring primarily in the right auditory cortical area for obvious reasons (less interconnectedness leads to greater differentiation in tone, more myelination leading to faster processing of temporal information, etc.)
• The information in the above bullet may represent an evolutionary adaptation originally geared toward improving the processing of speech