Daniel Javitt and Robert Sweet recently published a review article in the journal Nature Reviews Neuroscience on the interesting case of altered auditory processing in individuals with schizophrenia.
Schizophrenia is a psychiatric disorder with a range of symptoms that usually lead to severe difficulties in social or occupational functioning. The most commonly known symptoms include perceptual hallucinations, paranoia and false beliefs, usually coined under the term psychosis. The most debilitating symptoms may however be the cognitive symptoms common in schizophrenia: difficulties in attention and working memory, which influence functioning in a large number of everyday situations.
It seems that in addition to these functions, basic sensory processing, is also affected in schizophrenia. In the review, Javitt and Sweet summarize findings from studies showing that individuals with schizophrenia have difficulties in very basic auditory functions. For instance, individuals with schizophrenia have been found to perform poorly in tone matching tasks. In these tasks, the subject is asked to listen to a tone, and then after a brief period another tone is presented. The subject has to then indicate whether the two tones are the same.
In addition to behavioral tasks, studies using neurophysiological measures have also shown abnormal auditory function in schizophrenia. One particular brain response, measured with EEG, has become popular in investigating how the brain processes changes in sound. This response is called Mismatch Negativity (MMN). It is typically measured by presenting a stream of repeating sounds with an occasional deviant sound that differs from the others, and observing the resulting changes in the EEG signal. The magnitude of the MMN indicates how precisely the brain is able to detect small differences in the stream of sounds. Individuals with schizophrenia have been found to demonstrate attenuated MMN responses – their brains are poorer at detecting these small differences.
Schizophrenia is connected to dysfunction in two neurotransmitters: glutamate (GABA) and N-methyl-D-aspartate (NMDA). Interestingly, the injection of NMDA antagonists (ketamine) that disrupt the neurotransmitter’s function in healthy adults also produces reductions of the MMN response similar to what is observed in schizophrenia. Therefore, it is possible that the dysregulation of NMDA in schizophrenic patients might lead to the inability to detect small differences between sounds. Structural brain imaging studies provide further support for understanding the auditory deficits in schizophrenia: studies have found reduced grey matter volume in the superior temporal gyrus, including a structure called Heschl’s gyrus. These areas are important for auditory processing.
What can these kinds of deficits in basic auditory processing lead to? Not surprisingly, the music perception abilities of individual with schizophrenia are poor. Understanding subtle auditory cues is also important in a wide range of social situations. Individuals communicate emotion and intent by modulating speech prosody – the melody of speech, and the differences in pitch. This means that individuals with schizophrenia may have a hard time understanding small nuances of verbal communication like sarcasm and changes in the emotional state of the speaker. This will lead to increased misunderstandings and perhaps increased social isolation, which may cause considerable personal distress.
In summary, in addition to the hallucinations and deterioration of executive functions, schizophrenia also entails disruption of basic auditory processing. However, basic sensory processing is not typically a target for rehabilitation in schizophrenia. Interventions targeting basic auditory skills could however significantly improve functioning and quality of life of individuals with schizophrenia, especially in social situations. Music therapy has been successfully used as a supplement to standard care in this population. An intriguing question is whether and how music could be used as a targeted intervention for rehabilitation of auditory processing in schizophrenia.
written by ketki karanam
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Javitt, D. C., & Sweet, R. A. (2015). Auditory dysfunction in schizophrenia: integrating clinical and basic features. Nat Rev Neurosci, 16(9), 535–550. doi:10.1038/nrn4002
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