Regional cerebral activation during auditory stimulation in patients with cochlear implants

Auditory cortical activation and speech perception in cochlear implant users

Cochlear implantation is generally accepted as a successful means of restoring auditory sensation to profoundly deaf individuals. Although most patients can expect a satisfactory outcome following implantation, some have poor speech perception outcomes. This investigation used [18F]-fluorodeoxyglucose positron emission tomography to measure cortical activity resulting from auditory stimulation in seven ‘good’ and four ‘poor’ cochlear implant recipients. Activations were significantly greater in both the primary and association cortices in the good compared with the poor implant users. We suggest that the ability to access the more specialised speech processing abilities of the auditory association cortices helps determine outcome following cochlear implantation.

Restored speech comprehension linked to activity in left inferior prefrontal and right temporal cortices in postlingual deafness

The left inferior prefrontal cortex (LIPC) is involved in speech comprehension by people who hear normally. In contrast, functional brain mapping has not revealed incremental activity in this region when users of cochlear implants comprehend speech without silent repetition. Functional brain maps identify significant changes of activity by comparing an active brain state with a presumed baseline condition. It is possible that cochlear implant users recruited alternative neuronal resources to the task in previous studies, but, in principle, it is also possible that an aberrant baseline condition masked the functional increase. To distinguish between the two possibilities, we tested the hypothesis that activity in the LIPC characterizes high speech comprehension in postlingually deaf CI users. We measured cerebral blood flow changes with positron emission tomography (PET) in CI users who listened passively to a range of speech and non-speech stimuli. The pattern of activation varied with the stimulus in users with high speech comprehension, unlike users with low speech comprehension. The high-comprehension group increased the activity in prefrontal and temporal regions of the cerebral cortex and in the right cerebellum. In these subjects, single words and speech raised activity in the LIPC, as well as in left and right temporal regions, both anterior and posterior, known to be activated in speech recognition and complex phoneme analysis in normal hearing. In subjects with low speech comprehension, sites of increased activity were observed only in the temporal lobes. We conclude that increased activity in areas of the LIPC and right temporal lobe is involved in speech comprehension after cochlear implantation.

Functional MRI of congenital hyposmia: brain activation to odors and imagination of odors and tastes

PURPOSE

Our goal was to use functional MRI (fMRI) to define brain activation in response to odors and imagination ("memory") of odors and tastes in patients who never recognized odors (congenital hyposmia).

METHOD

Functional MR brain scans were obtained in nine patients with congenital hyposmia using multislice echo planar imaging (EPI) in response to odors of amyl acetate, menthone, and pyridine and to imagination ("memory") of banana and peppermint odors and to salt and sweet tastes. Functional MR brain scans were compared with those in normal subjects and patients with acquired hyposmia. Activation images were derived using correlation analysis, and ratios of areas of brain activated to total and hemispheric brain areas were calculated. Total and hemispheric activated pixel counts were used to quantitate regional brain activation.

RESULTS

Brain activation in response to odors was present in patients with congenital hyposmia. Activation was significantly lower than in normal subjects and patients with acquired hyposmia and did not demonstrate differential vapor pressure-dependent detection responsiveness or odor response lateralization. Regional activation localization was in anterior frontal and temporal cortex similar to that in normal subjects and patients with acquired hyposmia. Activation in response to presented odors was diverse, with a larger group exhibiting little or no activation with localization only in anterior frontal and temporal cortex and a smaller group exhibiting greater activation with localization extending to more complex olfactory integration sites. "Memory" of odors and tastes elicited activation in the same central nervous system (CNS) regions in which activation in response to presented odors occurred, but responses were significantly lower than in normal subjects and patients with acquired hyposmia and did not lateralize.

CONCLUSION

Odors induced CNS activation in patients with congenital hyposmia, which distinguishes olfaction from vision and audition since neither light nor acoustic stimuli induce CNS activation. Odor activation localized to anterior frontal and temporal cortex, consistent with the hypothesis that olfactory pathways are hard-wired into the CNS and that further pathways are undeveloped with primary olfactory system CNS connections but lack of secondary connections. However, some patients exhibited greater odor activation with response localization extending to cingulate and opercular cortex, indicating some olfactory signals impinge on and maintain secondary connections consistent with similar functions in vision and audition. Activation localization of taste "memory" to anterior frontal and temporal cortex is consistent with CNS plasticity and cross-modal CNS reorganization as described for vision and audition. Thus, there are differences and similarities between olfaction, vision, and audition, the differences dependent on unique qualities of olfaction, perhaps due to its diffuse, primitive, fundamental role in survival. Response heterogeneity to odors may reflect heterogeneous genetic abnormalities, independent of anatomic or hormonal changes but dependent on molecular abnormalities in growth factor function interfering with growth factor/stem cell interactions. Patients with congenital hyposmia offer an unique model system not previously explored in which congenital smell lack as measured by fMRI is reflective of congenital dysfunction of a major sensory system.

Neuro-functional imaging and profound deafness

This paper deals with the subject of the place of neuro-functional imaging in cochlear implantation. Neuro-functional imaging is a means to assess the functioning of the brain in some normal or pathological conditions. Deafness is a model of sensorineural deprivation. Cochlear implantation offers to clinicians and neuroscientists a model for understanding influence of deafness on cerebral connectivity, and to assess mechanisms of plasticity when the auditory inputs are given again to a deaf cochlea. This paper proposes a brief review of the literature on cochlear implantation and neuro-functional imaging. Patients can be scanned pre- or post-operatively, thus different questions can be investigated about neurobiology of the consequences of deafness and auditory rehabilitation by cochlear implantation, and about the possible role of neuro-functional imaging in cochlear implant candidates to predict the future benefit of the implantation.

Single unit activity in the inferior colliculus of the rat elicited by electrical stimulation of the cochlea

The activity of single neurons (n = 182) of the central nucleus of the inferior colliculus (CIC) of the rat was recorded in response to unilateral electrical stimulation of the left cochlea and/or acoustical stimulation of the right ear. The probability of response to both modes of stimulation was comparable (90 per cent for contralateral and 60 per cent for ipsilateral presentation). Response patterns consisted predominantly of onset excitations. Response latencies to electrical stimuli ranged from 3 to 21 ms, with an average value of 9.7 ms (SD = 3.5 ms) in the ipsilateral CIC and 6.6 ms (SD = 3.4 ms) in the contralateral CIC. With respect to binaural inputs, the majority of units were excited by stimulation of either ear (EE; about 60 per cent) while about one third were influenced by one ear only (EO). Units excited by one ear and inhibited by the other (EI) were rare. The main difference between the present implanted rats and normal animals was the virtual absence here of inhibitory effects for both types of stimuli when they were delivered to the ipsilateral ear (very few EI units).