Multiple visuospatial working memory buffers: evidence from spatiotemporal patterns of brain activity.

Numerous studies have shown that the visual system involves different cortical pathways in the perception of object (ventral visual pathway) and spatial (dorsal visual pathway) information. The present study was concerned with whether human visuospatial working memory divides along similar lines. We used event-related brain potentials (ERPs) recorded from scalp of normal humans to show the existence of different buffering systems for the retention of object and spatial visual information. Subjects were presented with object or spatial stimuli to be retained for a 3.6-sec interval. The ERPs isolated brain activity associated with retention from earlier storage and later retrieval processes. The ERP scalp topographies indicated that the underlying patterns of brain activation were different during retention of object and spatial information.

Distinctions and similarities among working memory processes: an event-related potential study.

Working memory has been conceptualized as consisting of a number of components, such as an articulatory loop for rehearsing verbal material, a visuo-spatial sketch pad for maintaining visual images and a central executive that controls which information is made available for conscious processing. We recorded event-related brain potentials (ERPs) from normal human subjects while they maintained either visuo-spatial or phonological material in short-term memory for a 5-s interval. The results indicated that specialized brain systems for short-term storage of phonological and visuo-spatial information could be identified on the basis of marked differences between the topographies and morphologies of the ERP components elicited during these two types of short-term memory. The differences emerged during early encoding stages and continued through later retention stages.

Modality-specific processing streams in verbal working memory: evidence from spatio-temporal patterns of brain activity.

The present study was concerned with whether there are separate, modality-specific processing "streams" in verbal working memory for information that is heard or read. We used event-related brain potentials (ERPs) recorded from scalp of normal humans to show between-modality differences in spatio-temporal patterns of brain activity during retention in working memory of aurally or visually presented verbal information. The ERP patterns suggested that a sustained, automatically maintained auditory store was activated by auditory presentation and a transient, visual-verbal store was activated by visual presentation. In addition to these modality-specific differences, the ERPs indicated that the phonological loop was activated in both modalities and further suggested that the onset of phonological loop activation was earlier for auditory presentation.

Human versus computer interviewing.

Responses to counterbalanced human and computer interviews were examined for Gordon's Survey of Interpersonal Values to determine whether such responses would differ. Of an initial 72 male and female subjects, 52 completed both interviews scheduled one week apart. A 3-way MANOVA revealed significant response differences between interviewer type. However, neither significant sex or interviewer order differences, nor significant interactions were found. The results suggest that the interpretive validity of responses obtained in a computer interview may be threatened. The authors additionally suggest that the observed differences may be due to an interaction between interviewer type and test anxiety level.

Short-term memory storage and retention: an event-related brain potential study.

This experiment was concerned with event-related brain potential (ERP) activity related to short-term storage and retention of information in working memory. Our approach was to record the ERPs elicited by a stimulus that had to be memorized while varying the number of items (1, 3 or 6) in the task stimulus. In order to distinguish between ERP effects associated with perceptual complexity and retention of information, there was a second condition in which subjects were required to search the task stimulus for a match with a previously presented item. Thus, in the search condition subjects only had to remember one item (match or mismatch). ERP activity was recorded for 2450 msec after task stimulus offset. Two long-duration components varied as a function of task and memory load: a posterior positive wave and a frontal negative wave. Posterior positive wave amplitude was directly related to information load in the memory task but was negligible in the search task. Following the posterior positive wave was a frontal negative wave which occurred at the highest load level in the memory task but was totally absent in the search task. A P3b was elicited in both tasks. P3b was sensitive to information acquisition processes, but it did not distinguish between memory retention and visual search processes. While P3b amplitude did not vary with task or load, its latency increased with load in both tasks.

Event-related potentials during arithmetic and mental rotation.

In separate studies of arithmetic and mental rotation, similar posterior negative slow waves have been found. This similarity was surprising given the difference in cognitive processing required by these tasks. Furthermore, delayed responses were employed in these studies, so that it was not possible to determine the extent to which the slow wave activity was too late to be associated with processing that was specific to performance of the tasks. This experiment was intended to clarify the task-specific and non-specific nature of the slow wave activity. Subjects performed either an arithmetic or mental rotation task at two levels of difficulty on a random, trial-to-trial basis and gave an immediate response. There were a number of late posterior negativities, each with a different timing and topography, which were sensitive to type of task and/or task difficulty. Some components were associated with early task processing that was synchronized to the stimulus while others, revealed by response-synchronized averaging, were associated with later stages of task processing. There also was post-task activity that was sensitive to the difficulty level of the prior operations. In both tasks, there was a pre-frontal positive wave that persisted over most of the pre-response epoch, evidently related to a process that was active throughout the task. There also was centro-frontal phasic negativity, with a large peak at 380 msec in mental rotation, and a smaller, longer latency peak in arithmetic, apparently related to the complexity of the stimulus. Thus we conclude that arithmetic and mental rotation each elicit task-specific slow wave activity.

Working memory and preparation elicit different patterns of slow wave event-related brain potentials.

Some event-related brain potential (ERP) studies of working memory have used delayed match-to-sample designs in which a stimulus (S1) is held in memory for comparison with a subsequent stimulus (S2). During the S1-S2 interval, ERP slow negatives varied with both the type and amount of material held in working memory. One interpretation is that these slow waves index working memory operations. An alternative explanation is that they only reflect general preparatory processing for the response to S2. To decide between these explanations, we used two visual processing tasks that required similar preparation for S2. In one task, visual memory rehearsal operations were required. During the S1-S2 interval, there were clear differences between the amplitudes, topographies, and the effect of information load on the slow waves in two tasks, thus ruling out preparation only as an explanation.

Event-related brain potential evidence for a verbal working memory deficit in multiple sclerosis.

Memory deficits are frequently identified in patients with multiple sclerosis. Both working (short-term) and long-term memory appear affected, particularly on free-recall tasks. The focus of our study was to examine neurophysiological correlates of working memory processes and to identify the specific components responsible for the working memory deficits reported in multiple sclerosis. Event-related brain potentials (ERPs) were recorded from the scalp of mildly afflicted multiple sclerosis patients and their matched normal controls during the performance of phonological and visuo-spatial working memory tasks. Neuropsychological test, behavioural performance and ERP data all indicated that verbal working memory is especially susceptible to impairment by multiple sclerosis, while visuospatial working memory is less susceptible. The pattern of results further indicated that the verbal working memory dysfunction in multiple sclerosis is at least partially due to impairment in the phonological loop, a rehearsal mechanism for retaining verbal information in working memory.

Multiple sources of P3b associated with different types of information.

This experiment investigated how the P3a, P3b, and Slow Wave components of the event-related brain potential (ERP) respond to manipulations of the nature, timing, and extent of information delivery. There were two experiments in which the total amount of task information was distributed between pairs of successive stimuli (S1 and S2) within each trail. The task was to predict the relation between S1 and S2. In Experiment 1, the S1 could resolve no, partial, or all uncertainty with respect to the prediction outcome (correct or incorrect). Each S1 delivered three types of information: 1) outcome information--which resolved the subjects' uncertainty about the correctness of their prediction; 2) procedural information--which resolved uncertainty about how much outcome information would be delivered by S1; and 3) memory information--the identity of S1, which had to be stored for subsequent comparison with S2. In Experiment 2, the activity of these components was contrasted in two conditions in which the S1 delivered either memory information alone or both memory and procedural information. P3a and Slow Wave were sensitive only to outcome information. P3b was sensitive to all three types of information, and its scalp topography varied as a function of the type of information. The topographic variations indicate that P3b is not a unitary phenomenon but rather is a composite of activity arising from multiple intracranial sources of bioelectric activity.