Kevin Allan

Dissociation of the neural correlates of implicit and explicit memory

One presentation of a word to a subject is enough to change the way in which the word is processed subsequently, even when there is no conscious (explicit) memory of the original presentation. This phenomenon is known as implicit memory. The neural correlates of implicit memory have been studied previously, but they have never been compared with the correlates of explicit memory while holding task conditions constant or while using a procedure that ensured that the neural correlates were not ‘contaminated’ by explicit memory. Here we use scalp-recorded event-related brain potentials to identify neural activity associated with implicit and explicit memory during the performance of a recognition memory task. Relative to new words, recently studied words produced activity in three neuroanatomically and functionally dissociable neural populations. One of these populations was activated whether or not the word was consciously recognized, and its activity therefore represents a neural correlate of implicit memory. Thus, when task and memory contamination effects are eliminated, the neural correlates of explicit and implicit memory differ qualitatively.

Electrophysiological evidence for dissociable processes contributing to recollection

This paper reviews a number of studies in which we have employed event-related potentials (ERPs) to investigate the cognitive processes which contribute to conscious recollection. Across a range of tasks (including recognition memory, source memory, associative recall and word-stem cued recall) there is evidence for the proposal that recollection involves processes which are both functionally and neurologically dissociable. This evidence takes the form of temporally and topographically dissociable ERP effects, which attain their maximum amplitude when elicited by items that satisfy operational definitions for having been recollected. The ERP effects are interpreted as reflecting retrieval and post-retrieval processes which, we argue, constitute two separate components of recollection as defined within the process dissociation framework of Jacoby and colleagues. The ERP findings suggest that post-retrieval processing is particularly sensitive to task variables, implying that recollection may be neither functionally nor neurologically homogeneous.

Electrophysiological Evidence for the Modulation of Retrieval Orientation by Depth of Study Processing

Event-related potentials (ERPs) were employed to investigate whether brain activity elicited by retrieval cues in a memory test varies according to the encoding task undertaken at study. Two recognition memory test blocks were administered, preceded, in one case, by a shallow study task (alphabetic judgement) and, in the other case, by a deep task (sentence generation). ERPs elicited by the new words in each test block differed, the ERPs elicited in the block following the shallow study task exhibiting the more positive-going waveforms. This finding was taken as evidence that subjects adopt different retrieval sets when attempting to retrieve items that had been encoded in terms of alphabetic versus semantic attributes. Differences between the ERPs elicited by correctly classified old and new words (old/new effects) also varied with encoding task. The effects for deeply studied words resembled those found in previous ERP studies of recognition memory, whereas old/new effects for shallowly studied words were confined to a late-onsetting, right frontal positivity. Together, the findings indicate that the depth of study processing influences two kinds of memory-related neural activity, associated with memory search operations, and the processing of retrieved information, respectively.

Say it to my face: Examining the effects of socially encountered misinformation

Objectives. Errors in eyewitness accounts can occur when a witness comes into contact with post-event ‘misinformation’. A common way to encounter misinformation is through face-to-face interaction, in particular, via conversation with other individuals who also witnessed the crime. The current research compares this kind of misinformation with the non-social post-event narrative method typically employed in laboratory studies. Method. Young (17–33 years) and older (58–80 years) adults viewed a simulated crime event on video and were later exposed to four items of misinformation about it. The misinformation items were either introduced as part of a discussion about the event with a confederate or were embedded within a written narrative about the event that participants were asked to read. A questionnaire containing 20 items about the event was given to participants before and after the experimental manipulation. Results. Participants were less accurate than controls on questionnaire items after encountering misinformation. More importantly, misinformation encountered socially was significantly more misleading than misinformation from a non-social source. This was true for both young and older adults. Conclusion. Misinformation encountered socially produced more errors than misinformation from a non-social source. This finding has implications both for applied (forensic) and theoretical understanding of eyewitness memory.

Neural Correlates of Memory Retrieval during Recognition Memory and Cued Recall

Regional brain activity, measured by H215O PET, was investigated during recognition memory and word-stem cued recall of words in order to compare the neural correlates of two components of memory retrieval-effort and success-as a function of task. For each task there was a baseline and two retrieval conditions. In one retrieval condition (zero density), none of the test items corresponded to words encoded in a preceding study phase. Differences in activity between this condition and the baseline were employed to characterize the neural correlates of retrieval effort in each task. In the other retrieval condition (high density), 80% of the test items had been studied previously. Differences in brain activity between this condition and the zero-density condition were taken to represent the neural correlates of successful retrieval. The principal findings concern the right anterior prefrontal cortex, a region demonstrated previously to be active during episodic retrieval. Relative to baseline, this region showed no evidence of activation in the zero-density condition of the recognition task, but did show enhanced activity in the equivalent condition of the cued-recall task. In contrast, relative to the zero-density condition, the high-density condition was associated with increased right prefrontal activity during recognition, but reduced activity during cued recall. It is proposed that the right prefrontal cortex supports cognitive processes that operate on information retrieved in response to a test item and that these processes contribute to the evaluation of whether the information represents an appropriate prior episode.

The role of the right anterior prefrontal cortex in episodic retrieval.

Regional brain activity was measured with H(2) (15)O PET while participants attempted to complete word-stem and word-fragment retrieval cues with previously studied words. The retrieval cue manipulation was employed to gain control over the monitoring operations associated with evaluating the episodic status of alternative cue completions. These operations were more constrained for fragments, which had fewer possible completions than each corresponding stem. In one condition (zero target), during the scanning interval none of the cues could be completed with studied items, whereas in another condition (high target), 80% of cues belonged to studied items. Relative to baseline tasks, right anterior prefrontal activity was greater for stems than for fragments in the zero target condition. The target density manipulation did not modulate right anterior prefrontal activity, but was associated with increased activity in right dorsolateral prefrontal cortex. These findings are consistent with the proposal that the right anterior prefrontal cortex supports monitoring operations during episodic retrieval tasks. In addition, the findings add to evidence suggesting that the dorsolateral and anterior right prefrontal cortex make functionally distinct contributions to episodic retrieval.

A motor signal and "visual" size perception.

Recent models of the visual system in primates suggest that the mechanisms underlying visual perception and visuomotor control are implemented in separate functional streams in the cerebral cortex. However, a little-studied perceptual illusion demonstrates that a motor-related signal representing arm position can contribute to the visual perception of size. The illusion consists of an illusory size change in an afterimage of the hand when the hand is moved towards or away from the subject. The motor signal necessary for the illusion could be specified by feedforward and/or feedback sources (i.e. efference copy and/or proprioception/kinesthesis). We investigated the nature of this signal by measuring the illusions magnitude when subjects moved their own arm (active condition, feedforward and feedback information available), and when arm movement was under the control of the experimenter (passive condition, feedback information available). Active and passive movements produced equivalent illusory size changes in the afterimages. However, the illusion was not obtained when an afterimage of subjects hand was obtained prior to movement of the other hand from a very similar location in space. This evidence shows that proprioceptive/kinesthetic feedback was sufficient to drive the illusion and suggests that a specific three-dimensional registration of proprioceptive input and the initial afterimage is necessary for the illusion to occur.

A Modulatory Effect of Male Voice Pitch on Long-Term Memory in Women: Evidence of Adaptation for Mate Choice?

From a functionalist perspective, human memory should be attuned to information of adaptive value for one’s survival and reproductive fitness. While evidence of sensitivity to survival-related information is growing, specific links between memory and information that could impact upon reproductive fitness have remained elusive. Here, in two experiments, we showed that memory in women is sensitive to male voice pitch, a sexually dimorphic cue important for mate choice because it not only serves as an indicator of genetic quality, but may also signal behavioural traits undesirable in a long-term partner. In Experiment 1, we report that women’s visual object memory is significantly enhanced when an object’s name is spoken during encoding in a masculinised (i.e., lower-pitch) versus feminised (i.e., higher-pitch) male voice, but that no analogous effect occurs when women listen to other women’s voices. Experiment 2 replicated this pattern of results, additionally showing that lowering and raising male voice pitch enhanced and impaired women’s memory, respectively, relative to a baseline (i.e., unmanipulated) voice condition. The modulatory effect of sexual dimorphism cues in the male voice may reveal a mate-choice adaptation within women’s memory, sculpted by evolution in response to the dilemma posed by the double-edged qualities of male masculinity.

The amount of retrieval support modulates age effects on episodic memory: Evidence from event-related potentials

This experiment was designed to explore the impact of age and amount of retrieval support on episodic memory and its electrophysiological correlates. Event-related potentials (ERPs) were recorded while young and older participants performed a word-stem cued-recall task in a low-support condition (LSC) in which the stem was composed of three letters, and a high-support condition (HSC) in which the cue consisted of four letters. Behavioral analyses showed that recall in the older group was less accurate than in the young group in the LSC, but no age differences were observed in the HSC. In the LSC, old/new ERP effects at frontal and parietal sites were later and less sustained for the older adults. Furthermore, the parietal old/new effect was symmetrically distributed for older adults, whereas it was predominant over the left hemisphere for their younger counterparts. In addition, young participants demonstrated early and long-lasting frontal and parietal effects in the HSC but with predominance over the right hemisphere, whereas the older adults exhibited a frontal effect and an early and long-lasting parietal effect becoming predominant over the left hemisphere. No age differences in the time course of the parietal old/new effect were observed in this more supportive condition. In addition, in the last period, the left parietal effect was greater for the older group. This study suggests that episodic memory performance and ERP correlates of recall processes are more similar between young and older adults when increased support is provided at retrieval.

Is the N400 effect a neurophysiological index of associative relationships

The N400 is one of the most widely studied ERP components and has come to be viewed as an index of the semantic processing that relates distinct stimuli. In this study, we examine whether the N400 is sensitive to the associative relationship between distinct stimuli, and not the degree to which the stimuli share semantic features. We used previously established norms to parametrically vary the strength of linguistic association between words within word-pairs, while holding constant their degree of semantic congruency. This manipulation allowed us to compare N400s elicited by unrelated prime-target word-pairs (e.g. mirror-thumb) with N400s generated by related prime-target word-pairs of either moderate (e.g. camera-lens) or high (e.g. cherry-tree) degrees of association. We observed that larger N400 effects occurred for highly associated versus moderately associated pairs despite the fact that no differences in terms of semantic congruency existed between pairs belonging to the highly and moderately associated conditions. These findings demonstrate that the N400 can be modulated by associative relationships quite independently of semantics, and suggest that the N400 effect reflects processes sensitive to the contiguity of distinct elements within ones past experience and not their semantic properties per se.