Gary C. Galbraith

EEG correlates of visual-motor practice in man ☆

Special analysis of EEG signals was performed for 15 subjects engaged in three motor tasks of differing difficulty. A measure of average weighted coherence (C) was computed between the six possible combinations of four scalp areas: Oz, C3, C4 and Fz. In all subjects, regardless of task, scalp recordings over cortical areas known to have relatively dense fiber connections had significantly greater C values. However, the effects of task difficulty and practice were superimposed upon this basic pattern. Thus, the most difficult task (pursuit-rotor tracking) resulted in the highest coherence levels, while the least difficult task (visual tracking only of the pursuit-rotor disk) resulted in the lowest coherence levels. Practice, on the other hand, was associated with a significant decrease in overall level of coherence. This decrease is consistent with an interpretation of reduced task difficulty due to visual-motor learning. The results of the present study suggest that patterns of scalp EEG coherence may reflect some aspects of the underlying pattern of anatomical pathways, as well as the more dynamic properties of task difficulty and visual--motor practice.

Intelligible speech encoded in the human brain stem frequency-following response

The human brain stem frequency-following response (FFR) registers phase-locked neural activity to cyclical auditory stimuli. We show that the FFR can be elicited by word stimuli, and when speech-evoked FFTs are reproduced as auditory stimuli they are heard as intelligible speech. Stimuli were 10 high- and 10 low-probability words drawn from normative verbal responses of university students. Horizontal and vertical dipole FFRs based on 1000 repetitions of each word were recorded from two different participants. Speech-evoked FFRs were evaluated by 80 listeners. The results showed significant effects of FFR participant, word probability, and whether or not words were presented with category cues. Depending on such subject and experimental variables, FFRs were correctly perceived from 5% to 92% of the time.

Blindness and auditory impairment caused by loss of the sodium bicarbonate cotransporter NBC3

Normal sensory transduction requires the efficient disposal of acid (H+) generated by neuronal and sensory receptor activity. Multiple highly sensitive transport mechanisms have evolved in prokaryotic and eukaryotic organisms to maintain acidity within strict limits. It is currently assumed that the multiplicity of these processes provides a biological robustness. Here we report that the visual and auditory systems have a specific requirement for H+ disposal mediated by the sodium bicarbonate cotransporter NBC3 (refs. 7,8). Mice lacking NBC3 develop blindness and auditory impairment because of degeneration of sensory receptors in the eye and inner ear as in Usher syndrome. Our results indicate that in certain sensory organs, in which the requirement to transduce specific environmental signals with speed, sensitivity and reliability is paramount, the choice of the H+ disposal mechanism used is limited.

Brain stem frequency-following response to dichotic vowels during attention

FREQUENCY-FOLLOWING responses (FFRs) were elicited by English long vowels (female /a/ and male /e/) in a dichotic listening task. Stimuli were simultaneous and of equal duration, but differing spectra permitted unique identification of vowel components in the compound FFR. Horizontal and vertical montage FFRs were recorded with putative origins in the acoustic nerve and central brain stem, respectively. FFRs obtained during attention to each vowel showed significant effects for the voice fundamental frequency, f0, which is perceptually salient and conveys paralinguistic information such as the sex of the speaker. Amplitudes of f0 were larger when vowels were attended than when ignored. These findings provide evidence of short-latency attention effects in humans and suggest that linguistic attention may initially filter inputs based on salient paralinguistic cues.

Brain stem evoked response to forward and reversed speech in humans

Speech stimuli played in reverse are perceived as unfamiliar and alien-sounding, even though phoneme duration and fundamental voicing frequency are preserved. Although language perception ultimately resides in the neocortex, the brain stem plays a vital role in processing auditory information, including speech. The present study measured brain stem frequency-following responses (FFR) evoked by forward and reverse speech stimuli recorded from electrodes oriented horizontally and vertically to measure signals with putative origins in auditory nerve and rostral brain stem, respectively. The vertical FFR showed increased amplitude due to forward speech. It is concluded that familiar phonological and prosodic properties of forward speech selectively activate central brain stem neurons.

Putative measure of peripheral and brainstem frequency-following in humans

The human brainstem frequency-following response (FFR) registers phase-locked neural activity to periodic auditory stimuli. FFR waveforms were extracted from the electroencephalogram by averaging responses to repeated auditory stimulation. Two channels of data were simultaneously recorded from horizontally (electrodes placed in ear canals) and vertically (vertex scalp referenced to midline) oriented electrode configurations. Eight participants each received a total of 2000 tone repetitions for each of ten stimulus frequencies ranging from 133 to 950 Hz. FFRs were quantified by fast-Fourier spectral analysis. The largest spectral intensities at the stimulus frequency were recorded in the horizontal FFR, which also followed higher frequencies and showed better signal-to-noise ratios then did the vertical FFR. The horizontal FFR pattern suggests an acoustic nerve origin, while the vertical FFR pattern suggests a central brainstem origin.

Event Related Potential Assessment of Sensory and Cognitive Deficits in the Mentally Retarded

The application of event related potential (ERP) techniques to the mentally retarded presents a somewhat different challenge to the electrophysiologist than does the application of ERPs to other clinical populations. In particular, while differential diagnosis is of major interest in the investigation of such problems as minimal brain damage (MBD), schizophrenia and dementia, diagnosis is of less importance in mental retardation. What is important is to differentiate among the different types of information processing deficits occurring in the retarded so that educational remediation may be designed on an individual basis to compensate for sensory and intellectual inadequacies.

Slc4a11 gene disruption in mice: Cellular targets of sensorineuronal abnormalities

NaBC1 (the SLC4A11 gene) belongs to the SLC4 family of sodium-coupled bicarbonate (carbonate) transporter proteins and functions as an electrogenic sodium borate cotransporter. Mutations in SLC4A11 cause either corneal abnormalities (corneal hereditary dystrophy type 2) or a combined auditory and visual impairment (Harboyan syndrome). The role of NaBC1 in sensory systems is poorly understood, given the difficulty of studying patients with NaBC1 mutations. We report our findings in Slc4a11−/− mice generated to investigate the role of NaBC1 in sensorineural systems. In wild-type mice, specific NaBC1 immunoreactivity was detected in fibrocytes of the spiral ligament, from the basal to the apical portion of the cochlea. NaBC1 immunoreactivity was present in the vestibular labyrinth, in stromal cells underneath the non-immunoreactive sensory epithelia of the macula utricle, sacule, and crista ampullaris, and the membranous vestibular labyrinth was collapsed. Both auditory brain response and vestibular evoked potential waveforms were significantly abnormal in Slc4a11−/− mice. In the cornea, NaBC1 was highly expressed in the endothelial cell layer with less staining in epithelial cells. However, unlike humans, the corneal phenotype was mild with a normal slit lamp evaluation. Corneal endothelial cells were morphologically normal; however, both the absolute height of the corneal basal epithelial cells and the relative basal epithelial cell/total corneal thickness were significantly increased in Slc4a11−/− mice. Our results demonstrate for the first time the importance of NaBC1 in the audio-vestibular system and provide support for the hypothesis that SLC4A11 should be considered a potential candidate gene in patients with isolated sensorineural vestibular hearing abnormalities.

Interhemispheric Transfer in Normals and Acallosals: Latency Adjusted Evoked Potential Averaging

Interhemispheric transfer time (IHTT) can be estimated from visual evoked potentials (EPs). Latency adjusted averaging (LAA) produces EPs which have enhanced components. LAA also provides estimates of EP latency variance and signal-to-noise ratio (S/N). LAA was tested in analysis of EP-IHTT in normal and acallosal subjects. It was hypothesized that in normals S/N and latency variance would reveal signal degradation resulting from interhemispheric transfer. LAA in normals replicated IHTT findings for both P1 and N1 latency. Latency variance did not increase for cross-callosal measures, whereas the S/N measure showed significant EP degradation due to callosal transfer. EPs from five subjects with callosal absence (two commissurotomy; two complete and one partial callosal agenesis) showed significantly larger than normal latency variability, as well as decreased S/N ratios, for cross-hemisphere visual EPs. Results support the value of LAA in EP research on adequacy of hemispheric interactions in clinical populations.

Speech-evoked brainstem frequency-following responses during verbal transformations due to word repetition.

Speech-evoked brainstem frequency-following responses (FFRs) were recorded to repeated presentations of the same stimulus word. Word repetition results in illusory verbal transformations (VTs) in which word perceptions can differ markedly from the actual stimulus. Previous behavioral studies support an explanation of VTs based on changes in arousal or attention. Horizontal and vertical dipole FFRs were recorded to assess responses with putative origins in the auditory nerve and central brainstem, respectively. FFRs were recorded from 18 subjects when they correctly heard the stimulus and when they reported VTs. Although horizontal and vertical dipole FFRs showed different frequency response patterns, dipoles did not differentiate between perceptual conditions. However, when subjects were divided into low- and high-VT groups (based on percentage of VT trials), a significant Condition x Group interaction resulted. This interaction showed the largest difference in FFR amplitudes during VT trials, with the low-VT group showing increased amplitudes, and the high-VT group showing decreased amplitudes, relative to trials in which the stimulus was correctly perceived. These results demonstrate measurable subject differences in the early processing of complex signals, due to possible effects of attention on the brainstem FFR. The present research shows that the FFR is useful in understanding human language as it is coded and processed in the brainstem auditory pathway.