Cynthia King

Central auditory system plasticity associated with speech discrimination training

A passively elicited cortical potential that reflects the brains discrimination of small acoustic contrasts was measured in response to two slightly different speech stimuli in adult human subjects. Behavioral training in the discrimination of those speech stimuli resulted in a significant change in the duration and magnitude of the cortical potential. The results demonstrate that listening training can change the neurophysiologic responses of the central auditory system to just-perceptible differences in speech.

Deficits in auditory brainstem pathway encoding of speech sounds in children with learning problems

Auditory brainstem responses were recorded in normal children (NL) and children clinically diagnosed with a learning problem (LP). These responses were recorded to both a click stimulus and the formant transition portion of a speech syllable /da/. While no latency differences between the NL and LP populations were seen in responses to the click stimuli, the syllable /da/ did elicit latency differences between these two groups. Deficits in cortical processing of signals in noise were seen for those LP subjects with delayed brainstem responses to the /da/, but not for LPs with normal brainstem measures. Preliminary findings indicate that training may be beneficial to LP subjects with brainstem processing delays.

Consequences of neural asynchrony: a case of auditory neuropathy.

AbstractAbstract The neural representation of sensory events depends upon neural synchrony. Auditory neuropathy, a disorder of stimulus-timing-related neural synchrony, provides a model for studying the role of synchrony in auditory perception. This article presents electrophysiological and behavioral data from a rare case of auditory neuropathy in a woman with normal hearing thresholds, making it possible to separate audibility from neuropathy. The experimental results, which encompass a wide range of auditory perceptual abilities and neurophysiologic responses to sound, provide new information linking neural synchrony with auditory perception. Findings illustrate that optimal eighth nerve and auditory brainstem synchrony do not appear to be essential for understanding speech in quiet listening situations. However, synchrony is critical for understanding speech in the presence of noise.

Discrimination of speech-like contrasts in the auditory thalamus and cortex

The neurophysiologic discrimination of acoustic contrasts was investigated as reflected by the mismatch negativity (MMN) response. Evoked responses were recorded from guinea pig thalamus (medial geniculate nucleus) and epidural surface in response to synthesized speech contrasts /ga/-/da/ and /ba/-/wa/. From the caudomedial portion of the medial geniculate nucleus, /ba/-/wa/ elicited a strong mismatch response, whereas /ga/-/da/ did not. Neither stimulus contrast elicited an MMN from the ventral, or primary, portion of medial geniculate. Both stimulus contrasts elicited an MMN from the midline surface. Neither contrast elicited an MMN from the surface over the temporal lobe. Results indicate a hierarchy of processing of the spectrotemporal changes which characterize formant transitions. Also, results indicate that the nonprimary portions of the auditory pathway contribute substantially to the MMN.

Long-term habituation of the speech-elicited mismatch negativity

A significant issue in the use of the mismatch negativity evoked potential (MMN) concerns its low signal-to-noise ratio (SNR). One can improve the noise level by increasing the number of samples included in the averaged response. However, improvement achieved in this way assumes that the signal, the MMN, remains stable for extended test times, an assumption which has not been tested. If the MMN is not stable, or exhibits habituation over the test session, then SNR would be adversely affected. MMN response magnitude was measured in 5-min intervals over the course of a test session in response to various speech syllable contrasts. Significant long-term habituation of MMN was observed for all three subject populations tested: young adults, school-age children, and guinea pigs. The time course of the habituation and the stimulus conditions under which it occurs have important implications for research and clinical applications of the MMN. Recording procedures that minimize habituation effects may be used to advantage to improve the signal-to-noise ratio of the MMN.

Acoustic elements of speechlike stimuli are reflected in surface recorded responses over the guinea pig temporal lobe.

Auditory evoked potentials measured from the guinea pig temporal lobe surface reflect acoustic elements of synthesized speech syllables. Eliciting stimuli included a four formant anchor stimulus /ba/, with a 40-ms formant transition duration. The other stimuli differed from /ba/ along simple acoustic dimensions. The /pa/ stimuli differed on a VOT continuum; /da/ stimuli had a higher frequency F2 onset; /wa/ had a longer (80 ms) formant transition duration; and /bi/ differed in three vowel formant frequencies. The /ba/ and /da/ onset response latencies decreased systematically with increasing F2 onset frequency. The response to the /pa/ voicing increased in latency with increasing VOT and showed a physiologic discontinuity at VOT of 15-20 ms. Responses to /ba/ and /wa/ showed similar onset morphology but significant amplitude differences at latencies corresponding to vowel onset. Significant amplitude differences in /ba/ and /bi/ responses corresponded in latency to both consonant and vowel portions of the syllables. Similar to previous reports in the awake monkey for VOT, these results demonstrate in the anesthetized guinea pig that acoustic elements essential to speech perception are reflected in aggregate response of ensembles of cortical neurons.

Thalamic asymmetry is related to acoustic signal complexity.

Hemispheric asymmetries in response to speech sounds are well documented. However, it is not known if these asymmetries reflect only cortical hemispheric specialization to language or whether they also reflect pre-conscious encoding of signals at lower levels of the auditory pathway. This study examined differences in neural representations of signals with acoustic properties inherent to speech in the left versus right side of the thalamus. Specifically, 2000 Hz tone bursts, clicks and synthesized forms of the phoneme /da/ were presented to anesthetized guinea pigs. Evoked responses were recorded simultaneously from aggregate cell groups in the left and right medial geniculate bodies. Results showed an asymmetric response to complex auditory stimuli between the left versus right auditory thalamus, but not to the simple tonal signal. Moreover, asymmetries differed in male versus female animals.

Improving the reliability of the auditory middle latency response by monitoring EEG delta activity.

ABSTRACT The auditory middle latency response (MLR], a useful tool in the assessment of low frequency auditory sensitivity, can be consistently recorded in young children during wakefulness, stages 1 and 2, and REM sleep. Responses are often absent or questionable during stage 4. An on-line measure indicating favorable periods for recording MLR during sleep is important for interpretation of absent potentials. Here, for children 5 to 7 years old, the reliability and detectability of MLR was compared to sleep state and the dominance of delta activity (0-3 Hz] in the EEG frequency spectrum. Dominance of delta activity, a characteristic of stage 4, was expressed in a “delta ratio,” a measure of relative EEG energy in the 0 to 3 Hz frequency spectrum. A fixed delta ratio (DR=9) allowed the differentiation of periods favorable for MLR. MLR wave Pa amplitude and latency also varied with delta ratio. Results indicate that on-line monitoring of the delta ratio will allow reliable testing of MLR in clinical situations.

Developmental change in unilateral olfactory habituation is mediated by anterior commissure maturation

Habituation of an orienting response in rat pups was used to study the development of the anterior commissure (AC) and its role in olfactory memory. Six- and 12-day-old pups received odor presentations to one side of their olfactory system and were tested later for habituation to additional presentations made to either the trained or untrained side. Six-day-old pups remembered only on the trained side. Twelve-day-old pups remembered when tested on either side. Transection of the AC in 12-day-old pups before, but not after, training prevented the transfer of memory. Thus, between 6 and 12 days of age, olfactory cross-projections carried in the AC mature and provide a functional substrate for transfer, at the time of training, of a form of olfactory learning.

Neural representation and perception of speech sounds in children with learning problems

An inability to process auditory information, especially speech, characterizes many children with learning and attention problems. These speech–sound perception problems may arise, at least in some cases, from faulty representation of the speech signal in central auditory centers. Our working hypothesis is that acoustic–phonetic disorders are abnormalities in preconscious neurophysiologic representation of sound structure by central auditory pathway neurons and is reflected by subcortical and cortical aggregate neural responses. Brainstem and cortical potentials (ABR, FFR, cortical P1/N1/N2, MMN) reflect activity from different anatomic sources and represent different aspects of auditory function. An obvious characteristic distinguishing these responses is their development time course. Results indicate that while normal and learning impaired children do not differ in auditory system development per se, they develop different listening strategies which affect the neural representation of sound structure a...