CONSCIOUSNESS AND COGNITION MAY BE MEDIATED BY MULTIPLE INDEPENDENT COHERENT ENSEMBLES

Short-term or working memory (WM) provides temporary storage of inform ation in the brain after an experience and is associated with consciou s awareness. Neurons sensitive to the multiple stimulus attributes com prising an experience are distributed within many brain regions. Such distributed cell assemblies, activated by an event, are the most plaus ible system to represent the WM of that event. Studies with a variety of imaging technologies have implicated widespread brain regions in th e mediation of WM for different categories of information. Each kind o f WM may thus be expected to involve many brain regions rather than a local, uniquely dedicated set of cells. Neurons in a distributed ''cel l assembly'' may be self-selected by their temporally coherent activat ions. The process by which this fragmented representation of the recen t past is reassembled to accomplish essentially automatic and reliable recognition of a recurrent event constitutes an important problem. On e plausible mechanism to achieve the identification of past with previ ous events would require that the representational system mediating WM must coexist in spatial extent and somehow overlap in temporal activa tion with cell ensembles registering input from subsequent events. The detection of such a postulated mechanism required an experimental app roach which would focus upon spatial patterns of coherent activation w hile information about different events was stored in WM and retrieved , rather than focusing upon the temporal sequences of activation in lo calized regions of interest. For this purpose, the familiar delayed ma tching from sample (DMS) task was modified. A series of information fr ee flashes, or ''noncontingent probes,'' was presented before an initi al series of visual information items, the Priming Sample, which were to be held in WM during a Delay Period. A second series of visual info rmation items were then presented, the Matching Sample. The task requi red detection of any item in the second series which had been absent f rom the initial series. Thirty such trials with a particular category of Visual information constituted a single task. Several DMS tasks wit h this standardized design, but with different categories of visual in formation, were presented within each test session. The information ca tegories included letters of the alphabet, single digit numbers, or fa ces from a school yearbook. Event-related potentials (ERPs), were comp uted from 21 standardized electrode placements, separately for informa tion-free probes and for information items in each interval of the tri als within a task. Because each electrode is particularly sensitive to coherent activation of neurons in the immediately underlying brain re gions, topographic maps were constructed and interpolated across the s urface of the scalp. The momentary fluctuations of the resulting volta ge ''landscapes'' throughout the task were then subjected to quantitat ive analysis. Distinctive landscapes sometimes persisted for prolonged periods, implying sustained engagement of very large populations of n eurons. ''Difference landscapes'' were constructed by subtraction of t opographic maps evoked by noncontingent probes during the Delay Period from maps of probe ERPs before the presentation of the initial inform ation in the Priming Sample. Such probe difference landscapes displaye d recurrent high similarity to momentary landscapes elicited during su bsequent presentation of the information items in the Matching Sample. It seemed as if the distributed cell assembly continuously engaged by mediation of WM of the diverse attributes of the initial stimuli was being dynamically compared to the ensembles engaged by registration of the subsequent stimuli. Spatial Principal Component Analysis was appl ied to the sequences of momentary voltage landscapes observed througho ut trials of each task. This method sought a small number of spatial p atterns with which these large sets of inhomogeneous spatial distribut ions of voltage could be reconstructed. This is the spatial analog of the reconstruction of local ERPs by temporal principal components, as often described previously. Five Spatial Principal Components (SPCs) w ere found which accounted for about 90% of the total variance of volta ge across the surface of the scalp throughout every task. The loadings , or distinguishing topographic features, of these SPCs, were highly s imilar during every cognitive task for every subject. However, factor scores, or relative average contribution to the overall voltage distri butions, of the different SPCs varied substantially among subjects bet ween the tasks and momentarily within successive intervals of each tas k. These five SPCs may reflect coherent activation of huge distributed ensembles of neurons which comprise independent but interacting funct ional brain subsystems. These subsystems may correspond to basic resou rces available to individuals for allocation to mediate conscious eval uation of information during cognitive activity, providing a filter to bind together fractionated representations of the past to evaluate th e present. (C) 1997 Academic Press.