Michael C. Doyle

Event-Related Potentials and Stimulus Repetition in Direct and Indirect Tests of Memory

In this chapter we give an overview of some of the studies in our laboratory in which event-related brain potentials (ERPs) have been recorded in tasks involving the repetition of stimuli such as words, nonwords, and pictures, and outline our current interpretation of the findings that have emerged. A major aim of this work has been to determine the conditions under which ERPs are sensitive to stimulus repetition when it is incidental to the experimental task, and hence when no discriminative response is required on the basis of whether an item is repeated or unrepeated.

Word and nonword repetition within-and across-modality: An event-related potential study

The effects on event-related potentials (ERPs) of within- and across-modality repetition of words and nonwords were investigated. In Experiment 1, subjects detected occasional animal names embedded in a series of words. AU items were equally likely to be presented auditorily or visually. Some words were repetitions, either within- or across-modality, of words presented six items previously. Visual-visual repetition evoked a sustained positive shift, which onset around 250 msec and comprised two topographically and temporally distinct components. Auditory-visual repetition modulated only the later of these two components. For auditory EMS, within- and across-modality repetition evoked effects with similar onset latencies. The within-modality effect was initially the larger, but only at posterior sites. In Experiment 2, critical items were auditory and visual nonwords, and target items were auditory words and visual pseudohomophones. Visual-visual nonword repetition effects onset around 450 msec, and demonstrated a more anterior scalp distribution than those evoked by auditory-visual repetition. Visual-auditory repetition evoked only a small, late-onsetting effect, whereas auditory-auditory repetition evoked an effect that, at parietal sites only, was almost equivalent to that from the analogous condition of Experiment 1. These findings indicate that, as indexed by ERFs, repetition effects both within- and across-modality are influenced by lexical status. Possible parallels with the effects of word and nonword repetition on behavioral variables are discussed.

Cognitive brain potentials in a three-stimulus auditory oddball task after closed head injury

Event-related potentials (ERPs) were recorded in a three stimulus oddball task from 16 patients who had sustained a severe closed head injury at least 6 months before testing, and from 16 control subjects. The stimuli comprised a random sequence of frequent non-target tones (P = 0.70), rare target tones (P = 0.15), and rare novel sounds (P = 0.15). The task requirement was to respond promptly to each target tone. From a latency of 200 msec onwards, the ERPs evoked by frequent non-targets were substantially more negative-going in the head-injured than in the control group. When this difference in the ERPs to the frequent tones was taken into account, there was no evidence to suggest that either the latency or the amplitude of the target-evoked N2 and P3b components differed between the groups. The novel stimuli evoked a prominent P3a component. The amplitude and scalp distribution of this component differed little between the groups, but its peak latency was reliably longer in the head-injured subjects. The findings in respect of the N2 and P3b components suggest that impairments in early processing of task-relevant stimuli are not an invariant feature of closed head injury. The findings regarding P3a suggest that, in the majority of patients, head injury has only a limited effect on the neural systems underlying involuntary shifts of attention.

An event-related potential study of the effects of within- and across-modality word repetition

Abstract Subjects performed a nonword detection task, in which they responded to occasional nonwords embedded in a series of words. The stimuli were equally likely to be presented in the visual or auditory modality. Some of the words were repetitions of items that had occurred six items previously. Repetitions were in either the same or in the alternative modality. Compared to event-related potentials (ERPs) evoked by unrepeated, visually presented words, visual-visual repetitions gave rise to a sustained positive shift, which onset around 250 msec. Auditory-visual repetition also gave rise to a positive shift. This onset some 100 msec later than that associated with within-modality repetition. For auditory ERPs, within- and across-modality repetition gave rise to almost identical effects, consisting of a sustained positive-going shift onsetting around 400 msec. The findings were interpreted as reflecting the different representations generated by visually and auditorily presented words. Whereas visually ...

An event-related potential study of retroactive interference in memory

NEURAL activity associated with overcoming retroactive interference in episodic memory was investigated with event-related brain potentials (ERPs). ERPs were recorded during a combined recognition/associative recall memory test in which subjects were required to identify previous studied words and recall their study associates. A retroactive interference condition (A–B, A–C, A–?) was compared with two control conditions A–B, C–D, A–? and A–B, A–B, A–?). No ERP effect specific to the interference condition was identified. ERPs to all three classes of old word differed however from those to new words. These differences involved two effects, one with a left parietal maximum that has been described before, and a left frontal effect of earlier onset which has not been reported previously. This latter effect may indicate a role for the left prefrontal cortex in associative retrieval.

Word repetition within and across visual fields: an event-related potential study

A divided visual field (DVF) procedure was used to investigate the scalp distribution of the event-related potential (ERP) repetition effect. ERPs were recorded from 27 scalp sites whilst subjects (n = 20) discriminated between words and non-words presented to either the left (LVF) or the right (RVF) visual field. A proportion of the words were repeated on the trial immediately following their first presentation. In two within-field repetition conditions the two encounters with a word occurred in the same visual field (LVF or RVF). In two across-field repetition conditions, the two encounters with a word occurred in different visual fields. For both words and non-words, task performance was better for RVF presentations than for LVF presentations. In each repetition condition there was a positive-going shift in the ERP elicited by repeated words compared to that elicited by words on their first presentation. This ERP repetition effect was equivalent in magnitude and lateralised to the right hemisphere to an equivalent degree in all four repetition conditions. It is suggested that the ERP effects largely reflect the processing of visual form thought to occur predominately in the right hemisphere.