Neuro-functional imaging and profound deafness

Functional near-infrared spectroscopy for neuroimaging in cochlear implant recipients

Functional neuroimaging can provide insight into the neurobiological factors that contribute to the variations in individual hearing outcomes following cochlear implantation. To date, measuring neural activity within the auditory cortex of cochlear implant (CI) recipients has been challenging, primarily because the use of traditional neuroimaging techniques is limited in people with CIs. Functional near-infrared spectroscopy (fNIRS) is an emerging technology that offers benefits in this population because it is non-invasive, compatible with CI devices, and not subject to electrical artifacts. However, there are important considerations to be made when using fNIRS to maximize the signal to noise ratio and to best identify meaningful cortical responses. This review considers these issues, the current data, and future directions for using fNIRS as a clinical application in individuals with CIs. This article is part of a Special Issue entitled <Annual Reviews 2016>.

Cortical reorganization in children with cochlear implants

Congenital deafness leads to atypical organization of the auditory nervous system. However, the extent to which auditory pathways reorganize during deafness is not well understood. We recorded cortical auditory evoked potentials in normal hearing children and in congenitally deaf children fitted with cochlear implants. High-density EEG and source modeling revealed principal activity from auditory cortex in normal hearing and early implanted children. However, children implanted after a critical period of seven years revealed activity from parietotemporal cortex in response to auditory stimulation, demonstrating reorganized cortical pathways. Reorganization of central auditory pathways is limited by the age at which implantation occurs, and may help explain the benefits and limitations of implantation in congenitally deaf children.

Functional imaging of the central auditory system using PET

In the last few decades functional neuroimaging tools have emerged to study the function of the human brain in vivo. These techniques have increased the knowledge of how the brain processes stimuli of different sensory modalities, including auditory processing. Positron emission tomography (PET) has been used for nearly 20 years to study changes in cerebral blood flow associated with auditory stimulation in normal and hearing impaired subjects. PET studies gave insight into the neural base of processing basic sound features such as frequency and intensity, but complex stimuli such as speech and music have also been investigated extensively. Knowledge of the normal auditory function of the brain helps us to understand the neural base of hearing deficits and provides ideas for possible treatments. Although functional magnetic resonance imaging (fMRI) is replacing PET in many neuroimaging studies nowadays, PET still holds unique advantages and can give us valuable knowledge about the auditory cortex and auditory perception.

Use of time differences in normal hearing – cortical processing of promontorial stimuli

To test the hypothesis that ability to discriminate small duration differences is positively correlated with activity in the right temporal lobe, we used positron emission tomography in six normally hearing subjects, stimulated via the promontory in a procedure that mimics the auditory nerve stimulation with a cochlear implant. Stimulus consisted of electrical bursts, and tasks included gap detection and temporal difference limen (TDL). TDL is a measure of discriminatory processing of sound duration in cochlear implant candidates, demonstrated to predict outcome. Good speech perception after cochlear implantation is associated with activity in right temporal areas. Although perceived variably by the subjects, the stimulus itself activated bilateral secondary somatosensory cortex, suggesting differential stimulation of multiple sensory modalities. Only TDL raised blood flow in both posterior middle temporal gyri (MTG) and the right prefrontal cortex. As the right posterior MTG is known to be active during duration discrimination of different modalities and in the perception of words containing manipulated phonemes, we conclude that recruitment of this part of the right hemisphere is important to the comprehension of speech containing mostly temporal cues. The study shows that stimulus-induced activation reflects the goal of the task rather than the nature of the stimulus.

Developmental hemispheric asymmetry of interregional metabolic correlation of the auditory cortex in deaf subjects

The functional connectivity of the auditory cortex might be altered in deaf subjects due to the loss of auditory input. We studied the developmental changes of functional connectivity of the primary auditory cortex (A1) in deaf children, deaf adults, and normal hearing adults by examining interregional metabolic correlation with (18)F-FDG PET. The mean activity of FDG uptake in the cytoarchitectonically defined A1 region served as a covariate in the interregional and interhemispheric correlation analysis. A1 metabolic rate was correlated with that of the ipsilateral superior temporal lobe in both normal and deaf subjects. This correlated area was larger in deaf children than in deaf or normal hearing adults. Concerning the functional connectivity of A1, a hemispheric asymmetry was found in that the extent of interregional correlation was clearly larger in the right than in the left hemisphere. This asymmetry was particularly pronounced in the younger deaf children. Both extent and asymmetry of the functional connectivity of A1 subsided with age. Contrary to this, a correlation between the left and the right primary auditory cortices was absent in younger deaf children but became apparent as they grew older.

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.