Starratt, Gerene K.

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.

Mismatch Negativity (MMN) is a component of the event-related potential (ERP) that is associated with the detection of novel stimuli in one's environment. Naatanen has suggested that a neural template theory provides the best explanation of the mechanism that underlies this psychophysiological phenomenon. The purpose of the present project was to propose that a Hebbian model of cell-assemblies (Hebb, 1949) provides a plausible competing theory of MMN. A Hebbian model is consistent with the evidence provided by imaging studies that demonstrate increased neural efficiency in learning cognitive (as opposed to skilled motor) tasks and with recent animal studies in an analogous model. This model suggests three hypotheses which were addressed by the present study. First, it is proposed that the method that is traditionally used to calculate MMN may not be the ideal. Specifically, it is proposed that the baseline measure used in the calculation does not yield optimal MMN data and the present experiment investigated a new method of gathering baseline data. Second, it was hypothesized that an investigation of sequence effects related to standard and deviant stimuli in an oddball paradigm would provide further support for a Hebbian reinterpretation of MMN. Finally, the argument is made that a Hebbian model of MMN garners additional support in terms of parsimony and ecological validity in addition to being consistent with our current understanding of the physiological underpinnings of learning phenomena. Results indicate that the new method of calculating MMN does, indeed, provide a more robust measure of MMN. Furthermore, sequence effects were demonstrated for both the standard and deviant stimuli, however the sequence effects observed in standard stimuli were not in the expected direction. Both practical and theoretical implications are discussed.