Functional studies of the human auditory pathway after monaural stimulation with pure tones. Establishing a normal database

Conclusions. Bilateral activation of Brodmann area 39 provides functional support for the concept of bilateral cortical activation with auditory stimulation. The observation that when Brodmann area 39 is activated, area 38 of the temporal lobe is simultaneously inhibited ( a functional observation that has seemingly not been previously reported) supports the concept that inhibitory as well as excitatory relays play a role in the auditory pathways. In addition, this report helps to establish a functional database for future studies. Objectives. (i) To determine which areas of the cerebral cortex and basal ganglia are activated when the left and right ears are monaurally stimulated with pure tones, and what type of response ( e. g. excitatory or inhibitory) occurs in these different areas. (ii) To determine the degree of ipsi- and/or contralateral cortical activation and/ or inhibition depending on the ear that is stimulated. (iii) To use this information as an initial step to develop a normal functional database for future studies. Material and methods. A brain perfusion single-photon emission CT ( SPECT) evaluation was conducted using auditory monaural stimulation with pure tones in 10 normal volunteers. Both ears were tested independently ( 20 examinations in total). Tc-99m hexamethyl-propylenamine-oxime was injected while pure tones were delivered, and the SPECT imaging was done 1 h later. Results. After delivering pure tones there was bilateral cortical activation in Brodmann areas 39 and 40, regardless of which ear was stimulated. Despite marked hyperperfusion in both areas 39, a more intense response was seen on the left compared to the right side, although this was not statistically significant. There was also activation in the executive frontal cortex areas 9 and 10 as well as in the temporal Brodmann areas 21 and 22. Simultaneously, area 38 of the temporal lobe was deactivated. In the subcortical structures there was also marked activation in both thalami and deactivation of the caudate nuclei without lateralization.