Trials effects in single-trial ERP components and autonomic responses at very long ISIs
Early studies examining the Orienting Reflex (OR) utilised single-trial autonomic measures at long ISIs, permitting full resolution of responses. To investigate central correlates for the OR, ERP studies have often employed positional averaging of ERP responses across blocks to improve the signal/noise ratios. However, this maymask the rapid initial response characteristic of the OR. More recent ERP research employed shorter ISIs, often under a fewseconds. Paradigms incorporating long ISIs canproduce robust single-trial ERP waveforms, yielding central data concurrently with autonomic measures. Undergraduates (10 females and 6 males of mean age 24.2 years), as part of fulfilling course requirements, participated in a simple auditory dishabituation paradigm.We compared response patterns of single-trial ERPs, HR, and respiratory responses to the phasic skin conductance response (SCR) commonly used as the Orienting Reflex (OR) benchmark. Twelve 80 dB tones (1000/1500 Hz, 50 ms with 15 ms rise/fall times)were presented with randomISIs (45 to 70 s), with no task requirements. The first 10 standard trials were of one frequency, followed by a change trial at the other frequency (recovery), and subsequent dishabituation trial at the initial standard frequency. The evoked cardiac response (ECR), Respiratory Pause (RP), SCR, and singletrial ERPs from 19 sites, were collected. EOG-corrected ERP data were submitted to principal components analysis (PCA). SCR displayed decrement over trials, response recovery at the change trial, and dishabituation at the following standard. ECR showed no trial effects. RP decreased linearly over trials and displayed amarginal recovery at the change trial. Eight identifiable ERP components were extracted: P1, N1, Processing Negativity, P2, P3a, P3b, Novelty P3 and the SlowWave (SW); only SWshowed significant decrement over trials, but without recovery or dishabituation. Only the SCR fulfilled the formal definition of habituation required of the OR. The early phasic cardiac response, observed as a deceleration unaffected by trials, can be linked to processing of the stimulus onset transient. The respiratory pause decrement over trials suggests processing marking the novelty aspect of the stimulus, subsequent to that reflected in the cardiac response, but prior to the OR. Of the ERP PCA components, only the SW demonstrated an overall diminished response over trials, but without recovery or dishabituation, aligning it with processing prior to the OR. This study supports predictions of Preliminary Processing Theory demonstrating the fractionation of 3 autonomic and 8 ERP componentswith respect to the novelty determinant of the OR, and disconfirming the unitary nature of the OR.