Z. Bohdanecký

EEG alpha and non-alpha intervals alternation.

In human subjects with closed eyes and limited external influences the spontaneous alternation of alpha and non-alpha intervals was recorded. It was found that duration of alpha periods fits the shifted exponential distribution. A mathematical model based upon the theory of queuing has been designed explaining the empirical data.

Scanning open and closed polygons

The process of selection of target points during voluntary eye movements when contoured polygonal shapes with open and closed angles are observed was analysed in humans by means of an eye recording technique. It has been found for both types of stimuli that the majority of fixation points where located at angles and that the mean value of distribution was found to be more distant from the vertex for acute angles than for obtuse ones. In this respect a difference was found between open and closed contours. In the former case the mean values of fixation point distribution were more distant from the vertex and the dispersion of eye fixation was greater.

An integral measure of the coherence function between pairs of EEG recordings

Abstract An integral measure of the coherence function between pairs of EEG derivation (I) is suggested; it gives an easy and well-arranged presentation of results and facilitates further statistical evaluation. Its computation is relatively easy, if the coherence estimate is available (negligible time necessary for integral measure computations when the coherence is known) and the frequency-related integral coherence measure (sub I) can also be computed. Higher I values were found between frontal leads compared with occipital ones.

Target point selection during scanning eye movements

The process of selection of target points during voluntary eye movements when polygonal random shapes are observed was analysed in humans by means of an eye movement recording technique. A computer model was constructed with the aim to explain the empirical results. It has been found that the majority of fixation points were located at the angles. The marginal distribution over the x-axis of the individual angles depends on the angles size: the maximal value of distribution was found for acute angles more distant from the vertex than in obtuse ones. The distribution of output activity of the “ganglion cells” in a computer model, reproducing some basic features of the retina, is in good agreement with the empirical results.

Acoustic stimulus threshold related to EEG alpha and non-alpha epochs☆

In humans sitting in the dark with their eyes closed the presence of EEG alpha and non-alpha epochs was registered automatically and the threshold detection of weak acoustic stimuli and reaction time of responses were measured according to the principles of the von Békésy audiometer under both conditions. The threshold was higher during alpha epochs in comparison with non-alpha ones. The reaction time of responses under both conditions was equal. Presentation of the stimulus did not influence the duration of corresponding alpha or non-alpha epochs significantly.

EEG SPECTRAL CHANGES DURING ONE-DIMENSIONAL HAND-TRACKING '

For our study of change in spectral power in the EEG (alpha and beta bands) when subjects were on and off target during one-dimensional hand-tracking, 11 healthy paid subjects, 8 men and 3 women of mean age 23.4 yr., volunteered to participate in the experiments. During 3-min. tracking task (1) the subject had to keep a joystick on a light-spot inside the given target (two vertical bars) which moved horizontally across the CRT screen. The instant a subject failed to keep the light-spot within target limits was scored as a tracking error (OUTevent). The return into the target was evaluated as error correction (IN event). EEG was recorded during the whole series of 8 tracking tasks, differing in target size (10 and 20 rnm), type of target movement (continuous and interrupted), and target speed (constant and feedback modified) (2) from O,, 0, and C,, C, (according to the 10120 system of electrode locations) by means of nonpolarizing AgtAgC1 electrodes. Linked earlobe electrodes served as the common reference. The power spectra in alpha and beta bands were chosen as main spectral characteristics of EEG as several subjects showed suppression of EEG alpha activity recorded on 0, and 0, positions after tracking error in pilot experiments. To describe the relationship between the tracking (IN/OUT) events and occurrence of EEG alpha and beta activity, the following method was adopted. Using a Fast Fourier Transformation (FFT) procedure, pairs of power spectra were calculated in averaged l-sec. EEG intervals with IN and OUT events located in their center. Then the ratios of power spectra for the alpha and beta frequency bands to the total power spectra were calculated. The frequency bands were set to 8-13 Hz for alpha and 13-24 Hz for beta activity. It followed from the comparisons of all values obtained (pooled together as the results were similar on all the tasks) that in 65% of cases the power of EEG alpha activity on 0, and 0, was lower immediately after OUT events than before them (by 5.8% p< .05), whereas in 62% the power of beta activity became lower on C, and C, immediately after IN events than before them (by 1.2%). It might be assumed that stereotyped manual cracking is often combined with a prevalence of alpha activity in EEG whereas on an out-ol-larger s~tuation voluntary effort toward error correction is accompanied by an increase of power in be~a band.

OCULOMOTOR AND COGNITIVE ASPECTS OF VISUAL PERCEPTION

Publisher Summary This chapter discusses the oculomotor and cognitive aspects of visual perception. It analyzes objectively the internal processes during human visual perception and cognition, which are of subjective nature. The experimental design in perception and cognition research is usually based upon the assumption that the perceiving system can be analyzed by comparing the responses recorded with the stimuli varied by the experimenter according to certain rules. Analyzing statistically serial dependences concerning the length of time intervals corresponding to both types of subjective interpretation of the Necker cube patterns, it has been found empirically that neighboring intervals tend to be of similar duration. This is valid for both types of interpretation separately, and for the neighboring intervals of different type. As both types of processes take place within the same system, their mutual influence and interrelations do not require the existence of any mediating interface. As conscious phenomena are attributes of the complex neuronal matter, they cannot be dissociated from it.

EEG ALPHA ACTIVITY PROCESSING AND MODELLING

Abstract On-line as well as off-line EEG alpha activity evaluation is described. The methods of detection of EEG alpha activity and its scoring are mentioned. A filtered EEG activity is fed into a computer where by sequential approach the value of median of probability distribution of their local extremes has been estimated. This value serves as a threshold for the threshold detection itself. Integral detection method is based on the computation of the area under this filtered signal. Model of alpha and non-alpha alternation based upon the analogy with the queueing process is suggested.