MR: A Case Study of Amusia and Auditory Agnosia
This thesis extends a case study conducted by Beckwith (2003). MR is a 62 year old right handed male who was an avid music listener prior to a right temporal lobe stroke. He was admitted to hospital in May 2002 after experiencing left-arm tingling and clumsiness upon waking from sleep. MR's initial complaint after being discharged from hospital was that music no longer sounded like music, rather it sounded like broad spectrum noise. MR subsequently reported a loss of musical skills. His eagerness to participate in research aimed at understanding the nature of his musical deficits derived from the personal significance music once had for him.
Magnetic resonance imaging (MRI) of the brain in August 2003 indicated an infarction in the distribution of a branch of the right middle cerebral artery. The resultant cortical lesion was circumscribed and involved the posterior aspect of the right insula cortex, the lateral and superior aspects of the right temporal lobe, and the posterior and superior right frontal lobe.
The goal of the initial assessment of MR was to provide an overview of his general cognitive and musical abilities to gain an understanding of issues that required further exploration (Beckwith, 2003). MR exhibited no sign of impairment on tasks assessing verbal skills, executive functions, memory, visuo-spatial, or perceptuo-motor functions. Thus, his music-related impairments were unlikely to be the result of a general cognitive deficit.
MR was compared to five controls on a range of musical tasks. The controls were matched for age, sex, handedness, and musical background. Overall, his pattern of results showed that he was impaired on basic pitch tasks, implying a low level perceptual disturbance. Within the melodic domain he showed impairment for interval and tonal processing but unimpaired contour processing, although investigation of contour processing was limited. Within the temporal organisation domain, he showed impaired rhythmic processing despite preserved metre.
MR failed to recognise music and environmental sounds although these results were somewhat confounded by his involuntary experience of musical illusions while performing these tasks. He also showed impaired ability to access identifying information about musical works, such as 'The Magic Flute' by Mozart, that were once highly familiar to him. MR showed selectively spared identification of musical instruments, however, this inference was inconclusive due to a background variable - 'time spent listening to music' - on which the control group was significantly lower when compared with MR.
This thesis addresses a number of issues that remained unresolved by Beckwith (2003). MR's initial description of music as broad spectrum noise, together with his musical illusions pointed to a largely unexplored perceptual auditory deficit. While MR showed impaired performance on basic melodic discrimination tasks, these tasks assess the ability to form auditory representations and are based on the assumption that perceptual processing has already occurred. Therefore, a thorough perceptual analysis of MR's pitch perception and auditory scene analysis was required. The pattern of MR's deficits provided a rare opportunity to examine low-level perceptual problems that may underpin associative agnosia, not just in one domain, but across various domains.