RESUMEN

Because of the importance of oscillations as a general phenomenon of neuronal activity the use of EEG spectral analysis is among the most important approaches for studying human information processing. Usually, oscillations at different frequencies occur simultaneously during information processing. Thus, the question for synchronisation of different frequencies by phase coupling and its possible functional significance is of primary importance. An answer may be given by bispectral analysis. Estimation of the (cross-) bispectrum allows to identify synchronised frequencies and possibly, the existence of non-linear phase coupling of different oscillators. Previous studies have demonstrated the simultaneous occurrence of slow (4-7 Hz) and fast (20-30 Hz) oscillations at frontal and prefrontal electrode positions during memory processing. However, interrelations between these rhythms have not been investigated up to now. In order to test short-term memory, the Sternberg task with random figures and number words was carried out on 10 female subjects. During the task EEG was recorded. Power and bispectral analyses from frontal, prefrontal and frontopolar regions were performed off-line. Increased power was found in both the theta and the gamma bands. Strong phase-coupling between theta at Fz and gamma at F3 and at Fp1, respectively, was shown for memorising number words by means of cross-bicoherence. A possible reason for this is an amplitude modulation of gamma frequencies by slow oscillations. The correspondent coherence analysis between the envelope of gamma frequencies at Fp1 and the raw EEG at Fz supports this presumption. This finding is interpreted as an EEG aspect of the functional linking between the prefrontal areas and the G.cinguli (as part of the limbic system), which are both extremely important for memory functions.

Asunto(s)

RESUMEN

An investigation of the cortical response (EEG) to periodically presented stimuli producing an ambiguity between long-range apparent motion and flicker is reported. ERPs to stimulus onsets differed slightly between the two percepts, in accordance with the results of Manning et al. (1988), Selmes et al. (1997). Induced rhythms exhibited a strong increase in induced beta and gamma powers at electrode positions T7 and T8 during the perception of apparent motion in two out of 10 participants. In addition, a small overall increase in alpha power at 12-13 Hz and a decrease in delta power below 3.5 Hz during perceived motion were found. The results indicate that a variety of different neural rhythms are involved in the perception of long-range apparent motion.

Asunto(s)

RESUMEN

Elaborating on a paradigm of Basar in experiments with periodically presented auditory or visual stimuli, 10 subjects performed tasks in which omitted stimuli had to be anticipated and their (virtual) onset time had either to be marked (a) mentally ('mental marking' MM) or (b) by pressing a button ('button pressing' BP). EEG was obtained from 11 sites in frontal, central, temporal, parietal and occipital areas. The experiments were carried out in triplets of MM, BP, MM measurements. Mean temporal errors of motor responses served as the basis for a functional interpretation of EEG patterns. Correlation analysis of sweeps for each individual revealed brief intervals of phase ordering of EEG patterns in the alpha range lasting approximately three periods. For frontal and vertex derivations a close congruence of the location of the phase-ordered patterns to mean errors of motor responding was shown. These results corroborate the claim that 'emitted' alpha oscillations represent intentional processing. The close agreement between temporal locations of phase-ordered EEG segments and those of the motor responses strongly suggests that EEG phases represent memory retrieval of period duration as a common functional component of MM and BP. A lack of topographical correspondence with results from a passive 'missing stimulus' paradigm underlines the specific intentional origin of the observed phase ordering. Theoretical implications of the results are discussed.

Asunto(s)

RESUMEN

Conditions for the disappearance of long-range apparent movement were investigated. In an experiment on beta motion, critical interstimulus intervals (ISIs) of downward simultaneity thresholds for stimuli presented in continuous alternation were determined for exposure durations (EDs) varying from 3 to 160 msec. Each subject performed each test twice. Data were collected in three sessions, each for one of three angular separations (3 degrees, 6 degrees, and 12 degrees) and the full set of EDs. The distribution of critical ISIs collapsed across subjects, EDs, and angular separations shows sharp maxima at regular distances within a range of 0-110 msec ISI. Significant or near-significant peaks were found at ISIs of 5, 9, 22, 27, 43, 55, and 107 msec. Although mean critical ISIs shifted with spatial separation, no essential shift of the main maxima occurred. Evidence of a periodic modulation with a period duration of 4.5 msec was obtained from the distributions of differences between critical ISIs of the first tests and their replications, which exhibit extremely low standard deviations (< 10 msec). These results agree well with previous analyses (Geissler, 1987, 1992) that led to a taxonomic model of quantal timing, briefly summarized in this paper. Further consequences are discussed and related to earlier developments (Geissler, 1991, 1992, 1997).

Asunto(s)

Asunto(s)

RESUMEN

Temporal code invariants as possible links between psychological and physiological characteristics of cognition: a tentative time quantum approach. Relationships between alpha activity and short-term memory performance are discussed referring to an approach suggested by Lebedev and Lutzky (1973). These authors suggest a secondary relationship in considering processing as well as storing of information as a result of the superposition of oscillatory processes which differ from one another an elementary period (relative refractoriness) specific to individuals. Some specifications of scan rate and STM span suggested by the authors seem to be untenable for empirical and logical reasons. Alternative explanations are proposed referring to the time quantum model (TQM) by Geissler (1985, 1987, 1990, 1991). Straight forward expansions of this model lead to the hypothesis of an "alpha band" consisting of 9 discrete frequencies. Predictions on scanning rhythms can be derived from this via the assumption that these represent segmentations caused by pairwise resonance. A modification of the Lebedev-Lutzky assumption on spans is proposed assuming an optimum code for stored information.

Asunto(s)

Asunto(s)

Asunto(s)

Asunto(s)