Auditory evoked response

This dissertation examined the neural correlates of auditory perception, attention
and expectation in three experiments. Experiment 1 analyzed neural correlates of auditory
perception and expectation in an electroencephalography (EEG) experiment using a
temporally perturbed metronome to establish an expectation for auditory events, then
violate and reestablish that expectation. High frequency evoked (phase-locked) gamma
band activity (GBA) was observed to follow the onset of tones whereas induced (nonphase-
locked) GBA reached maximum power simultaneously with the occurrence oftone
onset. Moreover, the latency of induced GBA was perturbed after an expectancy violation
and relaxed back into synchrony as the expectation was reestablished.
Experiment 2 was a methodological study to compare two functional magnetic
resonance imaging (fMRJ) scanning techniques and assess their influence on auditory
processing. Subjects passively listened to isochronous tone sequences at three rates while
sparse or continuous scanning was employed. Sparse and continuous scanning was observed to yield comparable fMRI data, however, continuous scanner notse was
observed to perturb known EEG evoked response potentials. Moreover, high frequency
evoked activity, as identified by spectral analysis, was attenuated in the presence of
continuous fMRl noise.
Experiment 3 was conducted to study auditory expectancy and attention. First,
subjects were tested behaviorally to determine their ability to tap the beat of ten highly
syncopated patterns. Subjects were asked to return for one EEG and one fMRl session. In
these sessions, they were instructed to attend to a syncopated pattern, mentally rehearse
the pattern, and then reproduce the pattern. During the control condition, subjects heard
the auditory patterns, however, they were instructed to study a list of words, remember
the words during the retention interval, and then recall as many words as possible. Brain
activity was localized to frontal and auditory regions when attending to the patterns and
occipital-auditory areas when attending to the words. Evoked activity was shown to
reflect the subject's anticipation of the beat and was attenuated when ignoring the
auditory stimulus.
Taken together, these results suggest that GBA indexes auditory perception,
attention and expectation. The current results suggest that attention and task engagement
may elicit stronger neural phase locking.

The P300 component of the auditory event-related potential waveform was investigated in control children and two groups of HIV-infected children (asymptomatic and symptomatic), aged five to seven, under combinations of two interstimulus interval (ISI) rates (1 and 2 sec) and two target probability rates (.2 and.5). No group differences were found under the different combinations of ISI and target probability, for either P3a amplitude and latency, or P3b amplitude and latency. Although the present study was not able to distinguish between-groups by manipulating the target presentation rate, there were several within-group effects on P3a latency that were dependant upon the specific clinical or control group. The general findings for the study were that as the target presentation rates increased, P3a latencies increased.

The P300 component of the auditory event-related potential waveform was investigated in two groups of HIV positive children (symptomatic and asymptomatic) who were compared to HIV negative relatives. Results demonstrated the expected increased latencies of the P3b component in the symptomatic group, but no P3a latency differences. Amplitude measures of P3a and P3b showed no group differences. However, the symptomatic children had relatively larger P3a amplitudes whereas the asymptomatic children had a larger P3b. Difference measures (P3b minus P3a) revealed a significant difference across groups. Discussion focuses on three interrelated issues: (1) the cognitive mechanisms which could account for the current findings, (2) the relevance of a symptomatic/asymptomatic distinction and a P3a/P3b distinction for the purposes of clinical research, and (3) the clinical implications of these findings.

The experiments in this dissertation were designed to produce a systematic characterization of the neuroelectric and neuromagnetic correlates of isochronous tone stimulation and simple rhythmic movements over a broad range of rates. The goal was to determine how the cortical representation of rhythm changes with rate, which would provide insight into known rate-dependent differences in perceptual and coordinative abilities. Fundamental transitions in the composition of the auditory and motor responses were hypothesized to occur within the parameter ranges studied here, including the attenuation of major response components and a shift from discrete transient activity at low rates to continuous steady-state activity at high rates. The auditory responses were studied in separate electroencephalography (EEG) and magnetoencephalography (MEG) experiments with stimulation rates ranging from 0.5 to 8Hz. In both studies, a transition from a transient to a continuous steady-state representation of the tone sequence occurred near 2Hz. In addition, an N1m component of the transient responses disappeared at rates near 8Hz, which may indicate the border beyond which tones are no longer distinct since the response is known to be an index of novelty in the auditory environment. Moreover, in a result important for understanding how evoked activity interacts with activity already present in the cortex, the phase of ongoing 40Hz rhythms is shown to affect the amplitude of the auditory evoked 40Hz response. Rhythmic finger movement was studied using a continuation paradigm in two EEG and MEG experiments at movement rates from 0.5 to 2.5Hz. Major findings included the disappearance of activity associated with movement planning and initiation at rates above 1Hz, suggesting a transition into a steady-state motor response in which there is less direct control of individual movements by the cortex. In addition, the neural correlates of synchronization and continuation were compared, with the results showing a similar cortical organization of metronome-paced and self-paced movements. The attenuation of major response components and the development of continuous steady-state activity within the present parameter ranges indicate rate-dependent changes in the cortical representation of simple rhythms, which are proposed here to relate to known rate-dependent behavioral differences in more complex coordinative environments.