Dissociation

Definitions

In neuropsychology, operationalization of the term dissociation has been broad ranging. The most straightforward example of dissociation in relation to music occurs in the area of brain lesion studies. Brain injury can affect music processing but leave language processing intact, suggesting that music and language are handled in distinct parts of the brain. Subsystems of music may also be dissociated. Brain injury may leave melodic processing impaired but rhythm processing intact and vice versa. Dissociation can also be demonstrated in other ways: two brain functions may exhibit different developmental trajectories; some task may interact with one function (e.g., cause a distraction effect) but not another function, and brain stimulation (transcranial magnetic stimulation or transcranial direct-current stimulation) may be used to temporarily inhibit or excite one function while not affecting another function.

A logical problem with such evidence, however, is that apparent dissociations appear when the tasks used to evaluate two functions are not matched in overall difficulty. Brain injury may lead to impairment in the ability to detect small changes in pitch but intact ability to judge emotion in music. Does this mean melody perception and emotion processing are handled in distinct areas of the brain? Not necessarily: There are many redundant attributes of music that permit emotional decoding, which make it a relatively easy task, but even a modest impairment in pitch discrimination will lead to impaired melodic processing.

To overcome this challenge, researchers often seek evidence for a double dissociation. A double dissociation describes two individuals with opposite patterns of dissociation. Two functions (such as rhythm and melodic contour) are doubly dissociated if one individual exhibits significantly reduced capacity for one function (e.g., rhythm) but not another function (e.g., melodic contour) and the other individual shows the opposite pattern of deficit. Double dissociations are considered the strongest possible evidence that two functions recruit distinct circuitry in the brain because differences in task difficulty cannot simultaneously account for both cases.

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