Karl-Heinz T. Bäuml

Prestimulus oscillations predict visual perception performance between and within subjects

In the present study, the electrophysiological correlates of perceiving shortly presented visual stimuli are examined. In particular, we investigated the differences in the prestimulus EEG between subjects who were able to discriminate between four shortly presented stimuli (Perceivers) and subjects who were not (Non-Perceivers). Additionally, we investigated the differences between the subjects perceived and unperceived trials. The results show that Perceivers exhibited lower prestimulus alpha power than Non-Perceivers. Analysis of the prestimulus EEG between perceived and unperceived trials revealed that the perception of a stimulus is related to low phase coupling in the alpha frequency range (8-12 Hz) and high phase coupling in the beta and gamma frequency range (20-45 Hz). Single trial analyses showed that perception performance can be predicted by phase coupling in the alpha, beta and gamma frequency range. The findings indicate that synchronous oscillations in the alpha frequency band inhibit the perception of shortly presented stimuli whereas synchrony in higher frequency ranges (>20 Hz) enhances visual perception. We conclude that alpha, beta and gamma oscillations indicate the attentional state of a subject and thus are able to predict perception performance on a single trial basis.

Semantic Generation Can Cause Episodic Forgetting

The repeated retrieval of a subset of previously learned items can cause forgetting of the nonretrieved items. The study reported here investigated whether retrieval-induced forgetting generalizes to a situation in which the retrieved and nonretrieved items are not part of the same experiential episode and task. Subjects learned an item list that they had to recall later in the experiment. In a separate intermediate phase, they repeatedly generated related items from semantic memory, or were presented the same items intact for study. Only the semantic generation of items, and not their presentation for study, induced forgetting of the initially learned items. This result indicates that, first, semantic generation can cause recall-specific episodic forgetting and, second, retrieval-induced forgetting can occur even if the retrieved and nonretrieved items belong to different experiential episodes and tasks. Connections of the present results to other memory phenomena, such as part-set cuing and the generation effect, social cognition, and eyewitness memory, are discussed.

Brain Oscillations Dissociate between Semantic and Nonsemantic Encoding of Episodic Memories

Prior studies, mostly using intentional learning, suggest that power increases in theta and gamma oscillations and power decreases in alpha and beta oscillations are positively related to later remembering. Using incidental learning, this study investigated whether these brain oscillatory subsequent memory effects can be differentiated by encoding task. One group of subjects studied material performing a semantic (deep) encoding task, whereas the other group studied the same material performing a nonsemantic (shallow) encoding task. Successful encoding in the semantic task was related to power decreases in the alpha (8-12 Hz) and beta (12-20 Hz) frequency band, and a power increase in the gamma band (55-70 Hz). In the shallow task, successful encoding was related to a power decrease in the alpha band and a power increase in the theta frequency band (4-7 Hz). A direct comparison of results between the 2 encoding tasks revealed that semantic subsequent memory effects were specifically reflected by power decreases in the beta (0.5-1.5 s) and the alpha frequency band (0.5-1.0 s), whereas nonsemantic subsequent memory effects were specifically reflected by a power increase in the theta frequency band (0.5-1.0 s).

Remembering Can Cause Forgetting—but Not in Negative Moods:

Repeated retrieval of a subset of previously observed events can cause forgetting of the nonretrieved events. We examined how affective states experienced during retrieval modulate such retrieval-induced forgetting by inducing positive, negative, and neutral moods in subjects immediately before they attempted to retrieve studied items. On the basis of recent work, we hypothesized that positive moods encourage relational processing, which should increase interference from related events and thus enhance retrieval-induced forgetting. By contrast, negative moods should encourage item-specific processing, which should reduce interference and thus reduce such forgetting. Our results are consistent with these predictions. When subjects were in negative moods, repeated retrieval did not cause forgetting of the nonretrieved material, whereas when subjects were in positive and neutral moods, they showed reliable retrieval-induced forgetting. Our findings suggest that the emotions involved during interrogation of a witness can affect the result of repeated interrogations.

Simultaneous color constancy: how surface color perception varies with the illuminant.

In two experiments simultaneous color constancy was measured using simulations of illuminated surfaces presented on a CRT monitor. Subjects saw two identical Mondrians side-by-side: one Mondrian rendered under a standard illuminant, the other rendered under one of several test illuminants. The matching field was adjusted under the test illuminant so that it (a) had the same hue, saturation, and brightness (appearance match) or (b) looked as if it were cut from the same piece of paper (surface match) as a test surface under the standard illuminant. Matches were set for three different surface collections. The surface matches showed a much higher level of constancy than the appearance matches. The adjustment in the surface matches was nearly complete in the L and M cone data, and deviations from perfect constancy were mainly due to failures in the adjustment of the S cone signals. Besides this difference in amount of adjustment, the appearance and surface matches showed two major similarities. First, both types of matches were well described by simple parametric models. In particular, a model based on the notion of von Kries adjustment provided a good, although not perfect, description of the data. Second, for both types of matches the illuminant adjustment was largely independent of the surface collection in the image. The two types of matches thus differed only quantitatively, there was no qualitative difference between them.

Strong items get suppressed, weak items do not: The role of item strength in output interference

An experiment is reported that examines the role of item strength in output interference. Subjects studied two types of categorized item lists: lists in which each category consisted of strong and moderate items, and lists in which each category consisted of weak and moderate items. Different degrees of item strength were accomplished by varying the items’ taxonomic frequency within a category. The subjects either recalled a category’s strong and weak items before its moderate items, or vice versa. The prior recall of the moderate items impaired the later recall of the strong items, but did not impair the later recall of the weak items. This effect of item strength indicates that output interference is caused by a process of retrieval suppression. It additionally suggests that, in order to minimize output-interference effects in recall, a list’s strong items should be recalled before its weak items.

Retrieval During Learning Facilitates Subsequent Memory Encoding

In multiple-list learning, retrieval during learning has been suggested to improve recall of the single lists by enhancing list discrimination and, at test, reducing interference. Using electrophysiological, oscillatory measures of brain activity, we examined to what extent retrieval during learning facilitates list encoding. Subjects studied 5 lists of items in anticipation of a final cumulative recall test and did either a retrieval or a no-retrieval task between study of the lists. Retrieval was from episodic memory (recall of the previous list), semantic memory (generation of exemplars from an unrelated category), or short-term memory (2-back task). Behaviorally, all 3 forms of retrieval enhanced recall of both previously and subsequently studied lists. Physiologically, the results showed an increase of alpha power (8-14 Hz) from List 1 to List 5 encoding when no retrieval activities were interpolated but no such increase when any of the 3 retrieval activities occurred. Brain-behavior correlations showed that alpha-power dynamics from List 1 to List 5 encoding predicted subsequent recall performance. The results suggest that, without intermittent retrieval, encoding becomes ineffective across lists. In contrast, with intermittent retrieval, there is a reset of the encoding process for each single list that makes encoding of later lists as effective as encoding of early lists.

The Relationship between Brain Oscillations and BOLD Signal during Memory Formation: A Combined EEG–fMRI Study

Previous studies demonstrated that increases in the theta frequency band with concomitant decreases in the alpha/beta frequency band indicate successful memory formation. However, little is known about the brain regions and the cognitive processes that underlie these encoding-related oscillatory memory effects. We investigated this relationship using simultaneous EEG–fMRI recordings in humans during long-term memory encoding. In line with prior studies, we demonstrate that a decrease in beta power and an increase in theta power positively predict subsequent recall. In fMRI, stronger activity in the left inferior prefrontal cortex and the right parahippocampal gyrus correlated with successful memory formation. EEG source localization revealed that the subsequent memory effect in the beta band was localized in the left inferior prefrontal cortex, whereas the effect in the theta band was localized in medial temporal lobe regions. Trial-by-trial correlations between EEG and BOLD activity showed that beta power correlated negatively with left inferior prefrontal cortex activity. This correlation was more pronounced for items that could later be successfully recalled compared to items later forgotten. Based on these findings, we suggest that beta oscillations in the left inferior prefrontal cortex indicate semantic encoding processes, whereas theta oscillations in the medial temporal lobe reflect the binding of an item to its spatiotemporal context.

Part-list cuing as instructed retrieval inhibition

The reexposure of a subset of learned material as a retrieval cue can impair recall of the remaining material. Like part-list relearning—the reexposure of learned material for additional learning—this part-list cuing is often assumed to be the result of output order biases at test, caused by the increased strength of the reexposed material. We directly compared the effects of cuing and relearning when controlling for output order biases. In addition, we compared the two forms of reexposure with the effect of part-list retrieval. Both part-list cuing and part-list retrieval reduced recall performance for the remaining material. By contrast, part-list relearning had no such detrimental effect. These results indicate that the effect of reexposure depends on whether material is reexposed as a cue or for relearning, suggesting that part-list cuing reflects an instructional effect. Evidence is provided that part-list cuing leads to instructed covert retrieval of cue items and causes retrieval inhibition of noncue items, similar to how overt retrieval inhibits nonretrieved items.

On the role of item similarity in retrieval-induced forgetting

We report on two experiments designed to examine how the similarity of retrieval-practised and not-retrieval-practised items influences the amount of retrieval-induced forgetting. Participants studied categorised item lists with each category consisting of exemplars from two different semantic subcategories. Using both the retrieval practice paradigm (Experiment 1) and the output interference paradigm (Experiment 2) we found that the retrieval of a subset of the studied items impaired the subsequent recall of the nonpractised items if the two types of items were fairly dissimilar to each other (same category but different subcategory) but did not induce impairment if the two types of items were highly similar (same category and subcategory). These results indicate that a high degree of similarity of practised and nonpractised items can eliminate retrieval-induced forgetting. They also suggest that forgetting in the retrieval practice paradigm and forgetting in the output interference paradigm are mediated by the same mechanisms. The relation of the present results to other very recent findings about the role of item similarity in retrieval-induced forgetting in discussed.