SPET assessment of auditory cortex stimulation and tonotopic spatial distribution in auditory brainstem implant

Activation of the auditory cortex by multifrequency acoustic stimuli has been evaluated using Single Photon Emission Tomography in a case of auditory brainstem implant after activation of 6 and 11 electrodes. Before implantation, no activation of the auditory cortex has been observed after acoustic stimulation. Following auditory brainstem implant, the stimulation of 11 electrodes showed an activation value, in terms of blood flow increase, of the contralateral temporal cortex similar to that obtained with 6 electrodes (47.70 vs. 43.76%), but a significantly stronger activation was present in the contralateral parietal region (29.59 vs. 14.73%), in the homolateral temporal area (22.02 vs. 10.46%) and, especially, in the homolateral parietal zone (16.6 vs. 4.33%). The strongest activation in the contralateral temporal cortex was detected in the sagittal tomogram at 26.25 mm from the midline, that is in the areas where high frequencies are projected, both with 6 and 11 active electrodes. The medio-lateral auditory cortex, where the middle and lower frequencies are projected, showed an overall lower activation which was however significantly lower with 6-electrode stimulation. Stimulation of the surface of cochlear nuclei determines mainly an activation of the high frequency domain, independently of the electrodes number. This finding may explain the better results of cochlear implants in comparison with auditory brainstem implant and could justify the use of needle electrodes in auditory brainstem implant. In conclusion, Single Photon Emission Tomography can be considered useful in evaluating auditory brainstem implant placement and function. It is also able to define the effectiveness of acoustic stimulation, the degree and tonotopic spatial distribution of auditory cortex activation.