Martin Heil

Slow negative brain potentials as reflections of specific modular resources of cognition.

The paper summarizes a series of experiments in which the topography and the amplitude of slow event-related brain potentials (slow ERPs) was studied in cognitive tasks which imposed different amounts of load on functionally distinct processing modules. The results suggest that the topography of slow ERPs reflects the relative activation/inactivation of distinct cortical cell assemblies while the absolute amplitude of the negative maximum seems to reflect how much a particular cell assembly is activated at a particular time. Translated into the terminology of cognitive resource theories one could say that tasks which evoke distinct slow wave patterns draw on independent resources. Likewise, the amplitude of a slow wave pattern could be related to the construct of resource allocation, i.e. the larger the amplitude of a slow wave pattern the more resources of a particular type are allocated to a task. These findings are discussed with respect to possible generator mechanism of slow waves and in relation to possible causes of capacity limitations of the human information processing system.

Distinct cortical activation patterns during long-term memory retrieval of verbal, spatial, and color information

Slow, DC-like event-related brain potentials (ERPs) were recorded from the scalp of 30 healthy young adults to test the hypothesis that distinct cortical areas are activated when different types of information are retrieved from long-term memory. Three groups of 10 subjects each were first trained with associations between either pictures and spatial positions (spatial condition), pictures and color patches (color condition), or nouns and nouns (verbal condition). All three experimental conditions were completely analogous with respect to the established associative structure, the learning procedure, the performance criterion, and the retrieval test that followed 1 day after learning. Slow event-related brain potentials being recorded during retrieval had a significantly distinct topography. The maximum of a DC-like negative wave was found in the verbal condition over the left frontal, in the spatial condition over the parietal, and in the color condition over the right occipital to temporal cortex. These results are consistent with the idea that memory representations are either down-loaded into or directly reactivated within those cortical processing modules in which the same material was handled during perception. Response times, on the other hand, revealed no difference between the three retrieval conditions. In each case RT increased monotonically, if more items had to be scanned. Thus, while the ERPs suggest the involvement of different cortical processors during memory search the response times suggest that a sequentially operating scanning mechanism applies to all of them.

Stimulus-induced gamma oscillations: harmonics of alpha activity?

Event-related changes of spectral power of the EEG were studied for each integer frequency between 5 and 100 Hz in three subjects during memory storage and retrieval. Spectra were calculated for successive, overlapping time epochs in seven channels. In one subject a stimulus-locked increase of power was observed at 12 Hz, while in the other two alpha power decreased at the individual peak frequencies of 9 and 11 Hz, respectively. In all subjects corresponding changes of power appeared at frequencies which were integer multiples of the individual dominant alpha frequencies. An analysis of the cross covariance of the alpha, beta and gamma activity revealed high coefficients for harmonic frequencies only while all other covariances were negligibly small. It is argued that event-related gamma activity may be an epiphenomenon of event-related changes within the alpha band.

Response preparation begins before mental rotation is finished : Evidence from event-related brain potentials

Behavioral data (response time (RT) and accuracy) and psychophysiological data (event-related brain potentials of ERPs, and lateralized readiness potential or LRP) were studied in an experiment in which rotated alphanumeric characters were presented normally or mirror-reversed. In half of the trials, character classification (letter versus digit) determined whether or not the response was to be executed (go versus nogo) and parity determined the responding hand. In the other half, classification determined the responding hand and parity determined go versus nogo. LRP data indicated that response preparation occurred before mental rotation was finished. These data contradict strictly sequential discrete models of information processing and suggest continuous flow of information.

Event-related Brain Potentials During Recognition of Ordinary and Bizarre Action Phrases Following Verbal and Subject-performed Encoding Conditions

Recognition performance of ordinary and bizarre action phrases was compared under two encoding conditions: In a verbal condition (V), participants learned the phrases by passively listening to them; in a subjectperformed condition (SP), participants learned the phrases by actively performing the denoted actions with imaginary objects. Recognition performance was better in condition SP than V. Moreover, recognition was better for bizarre than for ordinary phrases but only in condition V. Event-related brain potentials (ERPs) were recorded during the recognition test. In condition SP a larger fronto-central negativity was found. This confirms the hypothesis that the SP advantage is due to the reactivation of motor information. Additionally, the later part of the recognition memory ERP effect was analysed. This effect reflects the overall familiarity of items in a recognition test. The amplitude increases with decreasing familiarity. In the present study, an enlarged amplitude was found for bizarre phrases i...

Exploring memory functions by means of brain electrical topography: a review.

SummaryA series of experiments is reviewed which explored whether the functional brain state of long-term memory retrieval is correlated with specific changes in slow, DC-like event-related brain potentials. The main results are: (1) Retrieving associations from long-term memory is accompanied by a slow negative shift of 5–10 μV which prevails about as long as the retrieval process lasts, i.e., in our experiments, for a period of several seconds. (2) When different types of representations have to be reactivated in memory the slow negative wave shows a clearly distinct topography. The maximum was found in a verbal condition over the left frontal, in a spatial condition over the parietal, and in a color condition over the right occipital to temporal cortex. All these conditions were completely equivalent with respect to the established associative structure, the learning procedure, and the performance criterion. (3) The amplitude of the topographic maximum increases with the number of representations which have to be reactivated. This effect is not due to a non-specific increase of effort but specifically related to the number of activated episodic memory contents which had been experimentally established. In contrast, the reactivation of a priori given semantic association did not become manifest in a specific slow wave effect. These findings are compatible with the idea that memory retrieval implies a reactivation of those cortical cell assemblies in the cortex in which the constituting features of a mnestic entity had originally been processed during perception and learning. The results are also discussed with respect to the possible advantages of EEG and MEG recordings for a cognitive psychophysiology in comparison to other brain imaging techniques as PET or fMRI.

Topographically distinct cortical activation in episodic long-term memory: The retrieval of spatial versus verbal information

Two experiments are reported to study slow potentials in the EEG during reactivation of spatial and verbal information. Subjects had to learn associations between drawings and one, two, or three mediators (locations in Experiment 1, nouns in Experiment 2). During recall, subjects had to decide whether or not two drawings were linked to each other by a common mediator. EEG was recorded during learning and recall. Both experiments were completely equivalent. Irrespective of the quality of the mediators, response time proved to be a linear function of the numbers of mediators to be recalled. Negative slow potentials that accompanied the reactivation of information during anticipation learning and cued recall had a material-specific topography: The maximum was found over the parietal cortex for spatial information and over the left frontal cortex for verbal information. Moreover, the amplitude at these scalp locations varied with the amount of the to-be-retrieved information. The results support the claim of topographically distinct cell assemblies specialized for storage and retrieval of distinct kinds of information.

On separating processes of event categorization, task preparation, and mental rotation proper in a handedness recognition task.

We investigated the nature of event-related potential (ERP) effects in a handedness recognition task requiring mental rotation. Thirty subjects were tested with rotated and sometimes reflected alphanumeric characters while ERPs were recorded from 18 electrodes. On each trial, a cue provided valid information about the angular displacement of the following probe. This design allowed a distinction between three processing episodes: evaluation of the difficulty of the forthcoming task, preparation for the task, and the mental rotation task itself. The three episodes were accompanied by distinct ERP effects having distinct polarities, a different rank order of amplitudes for different probe orientations, and a different topography. These data confirm previous findings showing that mental rotation is accompanied by a parietal negativity. However, they also suggest that the rotation-related negativity found after tilted stimuli in standard mental rotation tasks is most likely overlapping with another, simultaneously triggered ERP effect functionally related to an evaluation of task difficulty.

Topography of brain electrical activity dissociates the retrieval of spatial versus verbal information from episodic long-term memory in humans

Topography and amplitude of slow event-related potentials (ERPs) of the electroencephalogram (EEG) were studied during acquisition and recall of spatial and verbal associations. Subjects learned associations between line drawings and two types of mediators. The latter were either positions in a grid or concrete nouns. In a cued recall test subjects had to decide whether two drawings were linked to each other or not via an associated position or noun. The topography of slow ERPs 1-4 s after stimulus presentation obtained from 18 scalp electrodes dissociated the memory processes: The maximum potential was found over the parietal cortex with spatial and over the left frontal cortex with verbal information. The same topographic pattern emerged during both anticipation learning and cued recall. Moreover, the amplitude at the topographic maximum increased when more associations had to be retrieved. These results are compatible with the idea that memory representations are reactivated in localized cortical cell assemblies specialized for particular codes.

Mentale Rotation und Rotationsnacheffekt: Nachweis des kontinuierlichen Bewegungscharakters mentaler Rotation

Prasentiert man Probanden Buchstaben in unterschiedlichen Orientierungen und last sie entscheiden, ob es sich unabhangig von der Orientierung um einen normalen oder einen spiegelbildlichen Buchstaben handelt, so dauert diese Entscheidung umso langer, je mehr der Buchstabe von seiner aufrechten Position abweicht (Cooper & Shepard, 1973). Aufgrund dieses Befundes wurde angenommen, das die Reprasentation des Buchstabens zuerst mental in die aufrechte Position gedreht wird, bevor die Spiegelentscheidung getroffen werden kann. Trotz etlicher kritischer Einwande (siehe z.B. Intons-Peterson, 1983; Pylyshyn, 1981; eine gute Ubersicht gibt Tye, 1991) hat sich aber allgemein eine Betrachtung dieser kognitiven Leistung durchgesetzt, die davon ausgeht, das es sich um einen “analogen” Prozes handelt, der kontinuierlich ablauft. D.h., die vorgestellte Rotation einer Objektreprasentation vollzieht sich isomorph (strukturerhaltend) einer physikalischen Drehung eines realen Objektes.