Activation of the auditory cortex by cochlear stimulation in a deaf patient

Neural prostheses and brain plasticity

The success of modern neural prostheses is dependent on a complex interplay between the devices' hardware and software and the dynamic environment in which the devices operate: the patient's body or 'wetware'. Over 120 000 severe/profoundly deaf individuals presently receive information enabling auditory awareness and speech perception from cochlear implants. The cochlear implant therefore provides a useful case study for a review of the complex interactions between hardware, software and wetware, and of the important role of the dynamic nature of wetware. In the case of neural prostheses, the most critical component of that wetware is the central nervous system. This paper will examine the evidence of changes in the central auditory system that contribute to changes in performance with a cochlear implant, and discuss how these changes relate to electrophysiological and functional imaging studies in humans. The relationship between the human data and evidence from animals of the remarkable capacity for plastic change of the central auditory system, even into adulthood, will then be examined. Finally, we will discuss the role of brain plasticity in neural prostheses in general.

Cochlear implants and brain plasticity

Cochlear implants have been implanted in over 110,000 deaf adults and children worldwide and provide these patients with important auditory cues necessary for auditory awareness and speech perception via electrical stimulation of the auditory nerve (AN). In 1942, Woolsey and Walzl presented the first report of cortical responses to localised electrical stimulation of different sectors of the AN in normal hearing cats, and established the cochleotopic organization of the projections to primary auditory cortex. Subsequently, individual cortical neurons in normal hearing animals have been shown to have well characterized input-output functions for electrical stimulation and decreasing response latencies with increasing stimulus strength. However, the central auditory system is not immutable, and has a remarkable capacity for plastic change, even into adulthood, as a result of changes in afferent input. This capacity for change is likely to contribute to the ongoing clinical improvements observed in speech perception for cochlear implant users. This review examines the evidence for changes of the response properties of neurons in, and consequently the functional organization of, the central auditory system produced by chronic, behaviourally relevant, electrical stimulation of the AN in profoundly deaf humans and animals.

Cortical Activation Shortly after Cochlear Implantation

We evaluated the cortical activations in postlingually deaf cochlear implant (CI) users in the early period (0-2 months) of CI usage. The subjects were 8 early CI users and 8 normal subjects. With tone burst stimuli (1 kHz) delivered to the right side, strong and broad activation of the ipsilateral (right) primary auditory cortex with 2 peaks and weaker activation of the contralateral (left) temporal lobe were observed in early CI users, in a clear contrast with the normal subjects in whom activation was observed in a small area of the contralateral (left) primary cortex. With word stimuli, activation of the superior frontomedian cortex presumably including the supplementary motor area and the neighboring cingulate gyri was observed in early CI users, which was absent in normal subjects. The activation in the immediate association cortices near the primary area was lower in early CI users, while the periphery of the association cortex seemed to be more mobilized.

Cochlear implants and brain stem implants

This chapter describes the development of two implantable prosthetic neurostimulators which, in the last 20 years, have revolutionised the management of severe-to-profound sensorineural deafness. We have witnessed their rapid evolution from the realms of esoteric laboratory abstraction, with many critics and little perceived clinical use, to a routine treatment which is safe, effective and, indeed, cost effective. It is one of the great triumphs of biomedical and surgical collaboration, and is without any doubt the greatest ever advance in the treatment of deafness.

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).

Auditory pathway plasticity in adult humans after unilateral idiopathic sudden sensorineural hearing loss

We recorded auditory evoked magnetic fields from 8 patients with unilateral, idiopathic, sudden, sensorineural hearing loss and from 8 healthy controls, using a 122-channel whole-scalp neuromagnetometer. The stimuli were 50-ms l-kHz tone bursts, delivered to the healthy ear at interstimulus intervals (ISIs) of 1, 2, 4, 8, and 16 s. On average, as in normal-hearing controls, the dipole moments and the latencies of N100m, the 100-ms response, increased as a function of ISI over both hemispheres to left- and right-ear stimulation. Four patients had shorter response latencies and 4 had stronger dipole moments over the hemisphere ipsilateral to the stimulation. In 3 patients, one additional source was observed over the anterolateral right hemisphere and another near head midline. These findings suggest that unilateral sensorineural hearing loss may modify information processing in the central auditory pathways.

Auditory cortical responses in humans with congenital unilateral conductive hearing loss

We recorded auditory evoked magnetic fields from 6 patients with congenital unilateral conductive hearing disorder with a 122-channel whole-head neuromagnetometer. The stimuli were 50-ms 1-kHz tones delivered to the better ear at interstimulus intervals (ISIs) of 2 and 8 s at two different intensities (50 and 70 dB HL). As in normal-hearing subjects, the amplitudes of N100m, the 100-ms response, were larger in 5 patients and the latencies were shorter in 3 patients over the hemisphere contralateral to stimulation. However, in one patient N100m peaked already at 61 ms over the contralateral hemisphere and amplitudes were larger over the ipsilateral hemisphere, possibly reflecting reorganization of the auditory pathways. In 3 patients the latencies were shorter over the ipsilateral hemisphere. The effects of ISI and intensity were similar over both hemispheres and did not differ from those in controls. It seems that congenital unilateral conductive hearing loss does not necessarily lead to any gross disturbances in the human auditory cortex.

Auditory magnetic fields in cochlear-implant patients

Neuromagnetic recordings were used to check if electrical stimulation of the auditory nerve in cochlear-implant patients activates cortical auditory areas in a similar manner as acoustic stimulation in normal hearing subjects or whether the excitation processes are different. The waveform of the evoked magnetic field complex elicited by electrical stimulation of the auditory nerve as well as the distribution of the magnetic responses over the head resemble that of auditory-evoked fields elicited by acoustic stimulation of normal subjects. The location and direction of the equivalent dipole are consistent with activation of the auditory cortex. Neuromagnetic recordings could be used in the future (i) to check activation of the auditory cortices in cochlear-implant patients; (ii) to gather information about the functional properties of this activity, and (iii) to study the integrity of central auditory pathways in totally deaf patients.

Bilateral electrical stimulation of a congenitally-deaf ear and of an acquired-deaf ear

Two identical multichannel intracochlear prostheses were implanted in the same patient. The first prosthesis, implanted in the congenitally-deaf right ear, elicited clear sound perception but no speech recognition. After 2 years, a second prosthesis, implanted in the acquired-deaf left ear, enabled the patient to understand speech without lip-reading. Brainstem and middle-latency evoked potentials were similar with electrical stimulation of both ears and resembled those evoked by acoustic stimuli in subjects with normal hearing. Cortical electric and magnetic responses differed for right- and left-sided electrical stimulation suggesting that stimulation of the congenitally-deaf ear elicited an abnormal activation of the auditory cortex. These results suggest that only cortical responses were affected by the different histories of deafness of the ears.

Binaural interaction in a cochlear implant patient

Binaural interaction was demonstrated in electrically evoked brainstem responses (EBRs) of a bilaterally implanted patient. A clear binaural difference waveform (BD), consisting of a negative peak near 3.6 ms followed by a positive peak near 4.4 ms, was found by subtracting the recordings with diotic stimulation from the sum of the recordings with monotic stimulation. These results are consistent with those reported for normal subjects and suggest that neural processing in this patient might resemble those ordinarily used in binaural hearing. They strengthen the argument that EBRs in cochlear implant patients do result from activity in auditory brainstem neurons and suggest a method for aligning the positions of the intracochlear electrodes.

Recording and interpretation of cerebral magnetic fields

Contemporary brain research progresses along two main lines: the microlevel approach explores single neurons and subcellular elements, while macrolevel studies focus on more complex cerebral functions, including behavior. This review presents results obtained mainly in our laboratory by means of an intermediate method, magnetoencephalography (MEG), which reflects cortical activity of neuronal populations at the level fo cytoarchitectonic areas. Because it is completely noninvasive, MEG can be used to study brain functions that are characteristically human.

Auditory evoked potentials and magnetic fields in patients with lesions of the auditory cortex

Auditory evoked potentials and magnetic fields of 40-200 ms latencies were measured from 4 patients with temporal lobe infarcts and compared with results from healthy subjects. Magnetic fields over the diseased hemisphere were abnormal in all 4; the responses were missing in 2. In one patient the responses were of abnormally high amplitude, and in one parts of the response sequence were missing. Evoked potentials were also abnormal in all 4 patients and the results were in accordance with the findings in the magnetic field measurements.

Magnetic fields from the auditory cortex of a deaf human individual occurring spontaneously or evoked by stimulation through a cochlear prosthesis

In a postlingually deaf individual, the magnetic field evoked by stimulation through a cochlear prosthesis (extracochlear electrodes) as well as of the spontaneous magnetoencephalogram was measured over the hemisphere contralateral to the prosthesis (CP), and the results were compared with those obtained from normal-hearing subjects. The latency of the 2 best developed waves M100 and M200 turned out to be prolonged in the CP patient by approximately 40 ms. The amplitude of wave M100 was significantly diminished, while wave M200 was only poorly developed. Location and direction of the equivalent current dipole (ECD) calculated for wave M100 was in good agreement with normal data, whereas the dipole moment was only about one third of the average dipole moment found in normals. Furthermore, evidence was obtained for another magnetic field wave, preceding the delayed auditory wave M100, which exhibits the same latency, ECD location and direction as reported in the literature for the somatosensory evoked magnetic field. This wave probably results from stimulation, through the intratympanic electrodes, of somatosensory nerves innervating the tympanic cavity. A potential clinical application of neuromagnetic measurements is discussed: The calculation of the ECD moment from the auditory cortical magnetic field evoked by electrical stimulation at the promontory would allow to estimate, prior to CP implantation, the number of persisting, excitable nerve fibres.