( from http://www.sci-con.org/news/articles/20021202.html )

Atoms of thought: The microstate hypothesis of conscious states

By Bernard J Baars

William James famously pointed out that consciousness seems continuous, but is rather like the flight of a bird, composed of brief flights and “perchings�. “As we take… a general view of the wonderful stream of consciousness, what strikes us first is this different pace of its parts. Like a bird’s life, it seems to be made of an alternation of flights and perchings.� (James, 1890, p. 243)



For the past ten years a team of researchers led by Swiss scientist Dietrich Lehmann has been building a case that conscious experiences may be reflected in “microstates� of brain activity. These are like James’ metaphorical birds perching on a tree branch for a moment, before flying away again. Each microstate, they claim, lasts 100-200 milliseconds. Each one reflects the activation of a different network of neurons, and each corresponds to different conscious contents, from visual imagery to abstract thought, emotional feelings and face perception.



Lehmann and colleagues have now published a dozen experimental articles on microstates (see References). For example, Koenig, Kochi & Lehmann (1998) found evidence that electrical microstates differ between visual imagery and abstract meanings. Easily visualizable words like “chair,� “fire,� and “tree� differed systematically from abstract terms like “nation,� “assumption,� and “democracy.� Obviously the word types were matched in length and frequency. They wrote that “significantly different microstates were found in both subject groups at 286-354 ms where imagery-type words were more right-lateralized than abstract-type words, and at 550-606 ms and 606-666 ms where anterior-posterior differences occurred.� Thus imagery seems to be processed more in the right hemisphere, and abstractions involved more frontal regions, as might be expected. Abstractions also took longer to process, again as might be expected.



How do we know that the microstates reflect conscious experiences? Lehmann et al (1998) asked subjects to recall experiences in a spontaneous way – to report any memory that came to mind. A fraction of a second before the report, abstract experiences showed more left-side microstates compared to visual experiences. But two seconds before the report there was no difference in brain states. They argue that the results are due to the conscious experience immediately before the moment of reporting.



Are microstates different between different people? Koenig, Lehmann et al (1999) reported that acute schizophrenics who were not taking medication showed deviant patterns in EEG microstates. “One microstate class displayed significantly different field configurations and shorter durations in patients than controls; degree of shortening correlated with severity of paranoid symptomatology.�



Scientists are professional skeptics, and there are many questions to be asked about the microstate hypothesis. The Lehmann team has approached the “atoms of thought� hypothesis by systematically laying the groundwork for the methodology, and by tying microstates to clearly conscious events. However, each moment of visual imagery involves both conscious and unconscious brain processes, and it is not clear that the possibility of unconscious correlates has been ruled out in these studies. Other recent brain experiments have made an effort to do that, by using visual backward masking and other methods. Nevertheless, the possibility of brain microstates supporting conscious “flight and perches� is intriguing. We may see a consensus emerging if the evidence holds up.

References


(NOTE: More references are available by searching under “lehmann d� at PubMed.)



James, William (1890/1983) The principles of psychology. Cambridge, MA: Harvard University Press.

_____________________



A deviant EEG brain microstate in acute, neuroleptic-naive schizophrenics at rest. Eur Arch Psychiatry Clin Neurosci 1999;249(4):205-11


Koenig T, Lehmann D, Merlo MC, Kochi K, Hell D, Koukkou M.

The KEY Institute for Brain-Mind Research, University Hospital of Psychiatry, P.O.B. 68, CH-8029 Zurich, Switzerland. thomas.koenig@puk.unibe.ch

Momentary brain electric field configurations are manifestations of momentary global functional states of the brain. Field configurations tend to persist over some time in the sub-second range ("microstates") and concentrate within few classes of configurations. Accordingly, brain field data can be reduced efficiently into sequences of re-occurring classes of brain microstates, not overlapping in time. Different configurations must have been caused by different active neural ensembles, and thus different microstates assumably implement different functions. The question arises whether the aberrant schizophrenic mentation is associated with specific changes in the repertory of microstates. Continuous sequences of brain electric field maps (multichannel EEG resting data) from 9 neuroleptic-naive, first-episode, acute schizophrenics and from 18 matched controls were analyzed. The map series were assigned to four individual microstate classes; these were tested for differences between groups. One microstate class displayed significantly different field configurations and shorter durations in patients than controls; degree of shortening correlated with severity of paranoid symptomatology. The three other microstate classes showed no group differences related to psychopathology. Schizophrenic thinking apparently is not a continuous bias in brain functions, but consists of intermittent occurrences of inappropriate brain microstates that open access to inadequate processing strategies and context information

_____________________



Event-related electric microstates of the brain differ between words with visual and abstract meaning.

Electroencephalogr Clin Neurophysiol 1998 Jun;106(6):535-46


Koenig T, Kochi K, Lehmann D.

The KEY Institute for Brain-Mind Research, University Hospital of Psychiatry, Zurich, Switzerland. tkoenig@key.unizh.ch

The present study shows that different neural activity during mental imagery and abstract mentation can be assigned to well-defined steps of the brain's information-processing. During randomized visual presentation of single, imagery-type and abstract-type words, 27 channel event-related potential (ERP) field maps were obtained from 25 subjects (sequence-divided into a first and second group for statistics). The brain field map series showed a sequence of typical map configurations that were quasi-stable for brief time periods (microstates). The microstates were concatenated by rapid map changes. As different map configurations must result from different spatial patterns of neural activity, each microstate represents different active neural networks. Accordingly, microstates are assumed to correspond to discrete steps of information-processing. Comparing microstate topographies (using centroids) between imagery- and abstract-type words, significantly different microstates were found in both subject groups at 286-354 ms where imagery-type words were more right-lateralized than abstract-type words, and at 550-606 ms and 606-666 ms where anterior-posterior differences occurred. We conclude that language-processing consists of several, well-defined steps and that the brain-states incorporating those steps are altered by the stimuli's capacities to generate mental imagery or abstract mentation in a state-dependent manner.

_____________________



Brain electric microstates and momentary conscious mind states as building blocks of spontaneous thinking: I. Visual imagery and abstract thoughts.

Int J Psychophysiol 1998 Jun;29(1):1-11


Lehmann D, Strik WK, Henggeler B, Koenig T, Koukkou M.

The KEY Institute for Brain-Mind Research, University Hospital of Psychiatry, Zurich, Switzerland. dlehmann@key.unizh.ch

Prompted reports of recall of spontaneous, conscious experiences were collected in a no-input, no-task, no-response paradigm (30 random prompts to each of 13 healthy volunteers). The mentation reports were classified into visual imagery and abstract thought. Spontaneous 19-channel brain electric activity (EEG) was continuously recorded, viewed as series of momentary spatial distributions (maps) of the brain electric field and segmented into microstates, i.e. into time segments characterized by quasi-stable landscapes of potential distribution maps which showed varying durations in the sub-second range. Microstate segmentation used a data-driven strategy. Different microstates, i.e. different brain electric landscapes must have been generated by activity of different neural assemblies and therefore are hypothesized to constitute different functions. The two types of reported experiences were associated with significantly different microstates (mean duration 121 ms) immediately preceding the prompts; these microstates showed, across subjects, for abstract thought (compared to visual imagery) a shift of the electric gravity center to the left and a clockwise rotation of the field axis. Contrariwise, the microstates 2 s before the prompt did not differ between the two types of experiences. The results support the hypothesis that different microstates of the brain as recognized in its electric field implement different conscious, reportable mind states, i.e. different classes (types) of thoughts (mentations); thus, the microstates might be candidates for the 'atoms of thought'.