Neuroanatomical and perceptual deficits in auditory agnosia : a study of an auditory agnosia patient with inferior colliculus damage

• Main Author: Poliva, Oren
• Published: Bangor University 2014
• Subjects: 150
• Online Access: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.665578

Auditory agnosia is a rare disorder in which individuals lose the ability to understand sounds. In this thesis, I examine an auditory agnosia patient with brainstem damage, but intact cortex. The patient was severely impaired when instructed to type the names of sounds. The patient, however, was only mildly impaired when instructed to choose the correct sound out of four written alternatives, which implies partial auditory perception. In two fMRI scans, conducted a year apart, passive listening to sounds resulted with a unique activation pattern in her auditory cortices. In particular, her anterior primary and associative auditory fields were much less responsive to sounds than more posterior primary and associative auditory fields. The functional dissociation between these regions suggests connections between the anterior primary and associative regions, and between the posterior primary and associative regions. Hitherto, these connections were only reported in monkeys. An EEG study that examined mismatch negativity for frequency, duration, and intensity of sounds, demonstrated that the patient’s ability of detecting changes to frequency and duration of sounds is bilaterally impaired, whereas the detection of changes to sound’s intensity is impaired in the left hemisphere but intact in the right hemisphere. Behavioral studies also show that the patient’s auditory perceptual deficit is partially due to impaired perception of the duration of sounds. For instance, when the patient heard two subsequent clicks, she was impaired at discriminating these sounds by the duration of their intervening interval. In a spoken word discrimination task, she was also impaired at discriminating words that could only be distinguished by their temporal properties (voice onset-time). Based on these findings, I argue that the patient experiences auditory agnosia be- cause the brain stem injury prevents the transmission of critical auditory information to the auditory cortex. As a result of this absence, the auditory fields responsible for sound recognition, the anterior auditory fields, are not recruited. In a dichotic listening task, the patient extinguished sounds presented to the right ear, and in a sound localization task she perceived sounds as emerging from the left auditory hemi-field. Given cumulative evidence that associates the posterior auditory cortex with sound localization and phonological-acoustic analysis of verbal material from the contra-lateral hemi-field, the patient’s performance in these tasks suggest that her spared auditory abilities is due to processing in her right posterior auditory cortex. This role of the patient’s right posterior auditory cortex is consistent with both the fMRI study, in which the right posterior auditory cortex was consistently responsive to sounds, and the EEG study, in which detection of changes to sound intensity was restricted to the right hemisphere.