0.7 Music, biological evolution, and the brain  (Page 15/30)

The three groups showed no significant differences in any measures soon after stroke onset. However, at three and/or six months, significant differences emerged between groups on two cognitive tests and two mood measures. On the cognitive tests, verbal memory and focused attention were superior in the music group compared to the other two groups. On the mood measures, the music group showed significantly less depression and confusion than the control group. For other cognitive and mood measures, the groups showed comparable performance at three and six months, and in no case did the music group perform worse than the other groups.

These findings are striking because they suggest lasting positive effects of passive music listening on neural recovery after stroke. What physiological mechanisms might underlie these effects? Prior research with healthy individuals indicates that pleasurable music listening is associated with activation of reward areas of the brain (e.g., the ventral tegmental area) that project dopamine to wide regions of the cerebral cortex (Menon and Levitin, 2005). The authors thus speculate that the activation of the dopaminergic mesocorticolimbic system by music may have led to enhanced general arousal and mood and suggest that this in turn influenced performance on cognitive tasks. In support of this idea, they point to prior research with healthy individuals that finds links between music-induced positive arousal/mood and performance on nonmusical cognitive tasks (e.g., Thompson et al., 2001).

Two problems with this account, however, are that the prior research was concerned with transient effects of music on immediately administered cognitive tasks (i.e., effects lasting minutes, not days or weeks), and that the dopamine-arousal hypothesis cannot explain why the music group showed improvement on only the verbal memory and focused attention cognitive tasks. It is interesting to consider the possible role of hormones in the current findings because of their long-lasting effects on brain physiology. For example, the glucocorticoid hormone cortisol is secreted by the adrenal glands in response to stress, as a result of neuroendocrine signals from the brain. A major stroke is a life-changing event that seems likely to result in greatly elevated stress in the months following the stroke, due to loss of one’s normal physical and mental abilities. This may in turn result in chronically elevated cortisol levels. Sustained high cortisol levels have deleterious structural effects on neurons in the hippocampus, a brain region rich in glucocorticoid receptors (Sapolsky, 2000) and involved in verbal memory in older adults (Zimmerman et al., 2008).

How does music enter this picture? Cortisol production is regulated by signals from the hypothalamus, a brain structure that is influenced by projections from the limbic system (brain structures involved in regulating emotion). The limbic system in turn is influenced by music (Peretz, 2010; Koelsch, 2010). The mechanisms underlying this influence remain unclear and may involve sensitivity of the limbic system to voice-like acoustic cues to affect, cues that occur in exaggerated form in music (cf. Juslin and Laukka, 2003). Interestingly, experiments with healthy individuals show that music listening immediately after a stressful event transiently reduces cortisol levels (Khalfa et al., 2003). Thus, one can hypothesize that regular music listening after stroke helps lower average cortisol levels, and these reduced levels facilitate hippocampal function. This could help account for the superior verbal memory of the stroke patients in the music-listening group. The superior performance of this group on sustained attention tasks remains to be explained, however. Neuroimaging research has shown that attentive listening to music recruits domain-general attentional networks (Janata et al., 2002), but it is not clear why or how regular activation of these networks by music would facilitate their operation during nonmusical tasks.