Nash, Allan J.

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.

This research investigated whether choice reaction time (RT) measures and the P300 component of the event-related brain potential (ERP) could be employed to index the attentional resources associated with performing two near-simultaneous tasks. Specifically, this study investigated the effects of auditory tones on the ERPs and RTs associated with a visual stimulus when the stimuli in both modalities were presented in close temporal proximity. The hypothesis that the ERPs and RTs elicited by the deviant visual stimulus would index the processing demands associated with the auditory modality was confirmed. In general, greater P300 amplitude associated with one task indicates diminished P300 and poorer performance on a second task. The results indicate that P300 may be a sensitive indicator of shared processing resources when two tasks are performed near simultaneously.

A delayed response paradigm was used to investigate the cerebral electroencephalographic (EEG) signal preceding bimanual finger flexions of continuously increasing and decreasing movement rates. The Bereitschaftspotential displayed larger amplitudes at faster required response rates as did two spectral frequency modes, which also showed magnitude reversals depending on the initiating finger. Furthermore, at these specific frequency modes, the averaged relative phase between electrode locations C3 and C4, as well as the variance in this measure was found to correspond closely to the variance in inter-response times derived from the subjects' movements. The results suggest the existence of possible signatures on the neurophysiological level which may yield information regarding the efficacy and parametric properties of the impending movement.

The effect of alternating primary task stimulus modality on event-related brain potentials and secondary signal detection performance was examined in a dual-task paradigm. An odd-ball paradigm with alternating pairs of auditory tones and visual lights was employed as the primary counting task. Primary task stimuli were classified by previous trial modality (same vs different), previous trial stimulus type (standard vs deviant), and current trial stimulus type. A signal detection task in which an auditory tone followed the primary task stimuli by 120 ms on 50 percent of the trials served as the secondary task. For all but one condition, deviant stimuli produced large P300 and decreased signal detection performance. When a deviant stimulus preceded a deviant in the same modality, however, signal detection performance was better than on standard trials preceded by a deviant. These results were interpreted using a two-parameter model of stimulus probability and subjective value of the stimulus sequence. Also examined were other effects of alternating modality of the primary task stimulus and the sequences defined by the modality and stimulus type of the current and previous trial on the event-related potentials elicited by the primary task stimuli.

Sokolov's (1963) hypothesis of selective extinction of the
orienting response accounts for the phenomenon of dishabituation by a
discrepancy between immediate sensory input and a stored template of a
habituated stimulus. The "oddball" stimulus procedure often used to
elicit the P300 evoked potential waveform bears resemblance to a
habituation-dishabituation paradigm. In the present experiment
subjects were habituated to a 70 dB tone burst which was then occasionally
replaced by 50, 60, 80 or 90 dB stimuli. According to a selective
extinction model of the P300, equal absolute amounts of stimulus change
should evoke equal amplitude waveforms. While the decreased intensity
stimuli did evoke a P300, the largest P300s were associated with an
intensity increase. The N200 component was seen to be largest with
intensity decrease. It is suggested that the N200 is the evoked
potential correlate of discrepancy detection which can be obscured by
an intensity-driven P300.

The P300 (P3) component of the auditory Event-Related Potential (ERP) waveform is a sensitive and clinically useful marker of central nervous system impairment. The amplitude (in muV) and latency (in ms) of the P3 component of the auditory ERP were used in the present study to investigate differences between HIV-asymptomatic, HIV-symptomatic, and HIV-negative control adult (18--35 years old) women under three different task conditions. Results indicated that P3 amplitude measures were consistently greater in controls than in the asymptomatic and symptomatic groups for all task conditions and at all electrode locations, suggesting that the available level of attentional resources are reduced similarly in both symptomatics and asymptomatics. P3 latency measures revealed that processing speed by asymptomatics is more similar to controls than it is to symptomatics, indicating that speed of processing is compromised in symptomatics, and possibly, to a lesser degree in asymptomatics.