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Abstract
Objectives. 1. To determine which areas of the cerebral cortex are activated stimulating the left ear with pure tones, and what type of stimulation occurs (eg. excitatory or inhibitory) in these different areas. 2. To use this information as an initial step to develop a normal functional data base for future studies. 3. To try to determine if there is a biological substrate to the process of recalling previous auditory perceptions and if possible, suggest a locus for auditory memory.
Method. Brain perfusion single photon emission computerized tomography (SPECT) evaluation was conducted: 1-2) Using auditory stimulation with pure tones in 4 volunteers with normal hearing. 3) In a patient with bilateral profound hearing loss who had auditory perception of previous musical experiences; while injected with Tc99m HMPAO while she was having the sensation of hearing a well known melody.
Results. Both in the patient with auditory hallucinations and the normal controls -stimulated with pure tones- there was a statistically significant increase in perfusion in Brodmann's area 39, more intense on the right side (right to left p < 0.05). With a lesser intensity there was activation in the adjacent area 40 and there was intense activation also in the executive frontal cortex areas 6, 8, 9, and 10 of Brodmann. There was also activation of area 7 of Brodmann; an audio-visual association area; more marked on the right side in the patient and the normal stimulated controls. In the subcortical structures there was also marked activation in the patient with hallucinations in both lentiform nuclei, thalamus and caudate nuclei also more intense in the right hemisphere, 5, 4.7 and 4.2 S.D. above the mean respectively and 5, 3.3, and 3 S.D. above the normal mean in the left hemisphere respectively. Similar findings were observed in normal controls.
Conclusions. After auditory stimulation with pure tones in the left ear of normal female volunteers, there is bilateral activation of area 39 of Brodmann, more intense in the contralateral (right) side. There is activation of both frontal executive areas without lateralization. Simultaneously, while area 39 of Brodmann was being activated, the temporal lobe was being inhibited. This seemingly not previously reported functional observation is suggestive that also inhibitory and not only excitatory relays play a role in the auditory pathways. The central activity in our patient (without external auditory stimuli) -who was tested while having musical hallucinations- was a mirror image of that of our normal stimulated volunteers. It is suggested that the trigger role of the inner ear -if any- could conceivably be inhibitory, desinhibitory and not necessarily purely excitatory. Based on our observations the trigger effect in our patient, could occur via the left ear. Finally, our functional studies are suggestive that auditory memory for musical perceptions could be seemingly located in the right area 39 of Brodmann.
Key words. Auditory cortex, Auditory memory, Musical hallucinations, Musical perception, HMPAO, Brain SPECT.
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