Location selection in the visual domain.

According to A.H.C. Van der Heijden (1992), attentional selection of visual stimuli can be considered as location selection. Depending on the type of task, location selection can be considered to be automatic (e.g., in case of abrupt onsets), directly controlled (e.g., in case of symbolic precues), or indirectly controlled (e.g., in case of visual search). In tasks with symbolic precues a contralateral enhancement of the P1 event-related potential (ERP) component has been found, whereas in visual search tasks a contralateral enhancement of the N2 component has been found. We hypothesized that both lateralized effects reflect the influence of location selection, which differs only in its moment of activation. Linear arrays (a target among five distractors, presented at two eccentricities) requiring left or right responses, were preceded by cues indicating either the precise target location or the side of the target, or by bilateral cues that indicated all array positions. Array-evoked ERPs, corrected for cue-evoked and interactive effects, showed not only lateralized but also target location-specific effects for both components, which supports the view that, depending on the type of task, either the P1 or the N2 component reflect the influence of location selection.

The influence of peripheral precues on the tendency to react towards a lateral relevant stimulus with multiple-item arrays.

Responses are faster when the side of the stimulus and response correspond than when they do not correspond. This has been ascribed to the influence of a spatial code on response selection. Regarding the origin of this code, three hypotheses were proposed that all assume that spatial codes decay as a function of time: the attention shift hypothesis, the static reference hypothesis, and the dynamic reference hypothesis. Arrays (a target among five distractors) requiring left or right responses, were preceded by peripheral cues indicating either the target position, the side of the target, or all array positions. Reaction times (RT), proportions correct, the response-locked lateralized readiness potential (LRP), and RT distributions were examined. A correspondence effect was present on RTs when all positions or the relevant side were cued, independently of response speed. No correspondence effect was found when the cue indicated the target position. The LRP results indicate that the correspondence effect is at least partially due to an effect at the level of motor preparation. The results favor the attention shift hypothesis which assumes that spatial codes are generated whenever there is a shift of attention.

Modulation of early ERP components with peripheral precues: a trend analysis.

In the visual domain, involuntary allocation of attentional resources can be induced by using peripheral cues. An enlargement of the P1 ERP-component has been reported in connection with voluntary allocation of resources induced by symbolic cues, but until recently, it has not been reported in connection with involuntary allocation of resources. However, involuntary allocation of resources was only investigated with long cue-target intervals (SOAs) of about 600 ms. Therefore, an experiment was conducted with SOAs between 100 and 300 ms. After a 100% valid peripheral cue a bilateral multi-item array was presented. Trend analyses, which were employed to estimate the ERP elicited by the array corrected for linear and nonlinear contribution of the cue, showed a contralateral enhancement for the posterior P150 and the N230 component. Hence, involuntary allocation of resources with short SOAs might invoke the same level as voluntary allocation of resources. The P150 enhancement may be interpreted as a reflection of allocated resources at a specific location, whereas the N230 enhancement might reflect elaborated processing at the contralateral hemisphere.

Information processing in children with minor neurological dysfunction: behavioural and neurophysiological indices.

Minor neurological dysfunction (MND) refers to deviant function of the central nervous system in the absence of localizable neurological disorders. Children with no signs (n = 28) and with varying grades of MND (n = 48), classified according to failure on circumscript neurological subsystems, were administered selective and sustained attention tasks at the age of twelve. During the execution of one of the tasks, electrocortical activity of the brain was recorded at the Fz, Cz, Pz and Oz scalp locations. Of main interest were behavioural and electrophysiological indices of deficits in attentional control. With respect to the latter category, the investigation was focused on differences in event-related potential amplitudes reflecting subprocesses of cognitive processing (processing negativity, P300). Following a linear stage model of information processing, it was found that children who failed on three or more neurological subsystems (in particular on fine manipulation and coordination), exhibited deficits in the encoding, search and decision stages of processing. Furthermore, the children with MND showed a reduced positive parietal shift on target presentation. Under complex task conditions, children without MND showed a decrease in P300 amplitude which reflects the impact of processing negativity as a result of increased task demands; this effect was absent in children with MND. These electrocortical differences suggest imbalances in the external and internal neural regulation of the flow of information in the brain.

Brain potential differences related to spatial attention in migraineurs with and without aura symptoms support supposed differences in activation.

It is to be expected that differences in electrical activity of the brain between migraine patients with aura and those without aura can only be revealed by stimuli that provoke visual spatial processing, i.e. stimuli that trigger so called endogenous Visual Evoked Potential (VEP) activity. This is not the case for the flashes and checkerboard reversals. Those stimuli elicited exogenous activity only. During and between attacks the blood flow of migraineurs with aura changes in the posterior cerebral part of the brain, which is assumed to be specialized in the processing of spatial aspects of visual stimuli. Reaction times (RTs), early and late Event Related Potential (ERP) differences were compared at 12 scalp positions for two groups of migraineurs (with and without aura) and a control group. They had to perform a passive attention task, checkerboard reversals, and an active attention task, where attention was either divided into or focussed at spatial locations. In agreement with many studies on migraine, checkerboard stimuli did not differ on any early components. However, RTs were faster for migraineurs with aura and their early components were different when stimuli were highly attended. This is probably because these stimuli can relatively easily trigger cortical activity due to an over activated central mechanism and an enhanced level of attention.

Removal of ocular artifacts from the REM sleep EEG.

The present report concerns the first study in which electrooculographic (EOG) contamination of electroencephalographic (EEG) recordings in rapid eye movement (REM) sleep is systematically investigated. Contamination of REM sleep EEG recordings in six subjects was evaluated in the frequency domain. REM-active and REM-quiet series were obtained for each subject. Transfer coefficients and power spectra of EOG and EEG indicated that (a) increases in transfer coefficients beyond 4.5 Hz are brought about by residual EEG in the EOG, and (b) EOG-EEG contamination in the delta band is most pronounced in frontal, intermediate in central and negligible in occipital leads. It was found that correction of the REM-active series resulted in significant (c) reductions in power, (d) increases in interhemispheric coherences and (e) reductions in degree of lateral asymmetry. These effects were largest for frontal leads, but still marked for central ones. The results are discussed in the light of previous findings concerning models of hemispheric functioning during REM sleep.

Learning where to look: electrophysiological and behavioral indices of visual search in young and old subjects.

The present investigation explores the way young and elderly subjects use regularities in target location in a visual display to guide search for targets. Although both young and old subjects show efficient use of search strategies, slight but reliable differences in reaction times suggest decreased ability in the elderly to use complex cues. Event-related potentials were very different for the young and the old. In the young, P3 amplitudes were larger on trials where the rule that governed the location of the target became evident; this was interpreted as an effect of memory updating. Enhanced positive Slow Wave amplitude indicated uncertainty in random search conditions. Elderly subjects' P3 and SW, however, seemed unrelated to behavioral performance, and they showed a large negative Slow Wave at central and parietal sites to randomly located targets. The latter finding was tentatively interpreted as a sign of increased effort in the elderly to allocate attention in visual space. This pattern of behavioral and ERP results suggests that age-related differences in search tasks can be understood in terms of changes in the strategy of allocating visual attention.

Eye-movements in a two-dimensional plane: a method for calibration and analysis using the vertical and horizontal EOG.

A method for calibration, orthogonalization and standardization of eye movements is described. The method is based on linear transformation of the horizontal and vertical EOG. With this method it is possible to measure the locus of eye fixation on a TV screen and its associated fixation time. Because of the used time constant (TC) of 36 sec, subjects must look at the centre of the TV screen or at random places on the screen during the interstimulus intervals (ISIs). For the EOG registration pre-amplifiers with a long TC were used to cut off slow changes in polarization caused by the electrode-electrolyte combination. In the calibration procedure only four orthogonal stimulus points were used. In an experiment to evaluate this method, subjects had to fixate letters on a TV screen in 12 different pre-determined positions. The distance between the measured locus of fixation and the coordinates of the stimulus was measured. The mean horizontal errors ranged from 1.4-2.2 degrees of arc and the mean vertical errors from 2.4-3.8 degrees of arc across subjects. It was concluded that accuracy was within acceptable limits, despite the fact that eye movement behaviour during the ISI was free. It was shown that the method is insensitive to non-orthogonality of the vertical and horizontal EOG. Calibration and transformation can be done by any real time computer system. The method is suitable for measuring, e.g. the visual orienting reaction (VOR), and it can also be applied in event related potential (ERP) studies where ocular fixation is used.

Single trial ERP estimation in the frequency domain using orthogonal polynomial trend analysis (OPTA): estimation of individual habituation.

A new Event Related Potential (ERP) model is described in which ERPs are estimated as single cortical responses. This model is based on an Orthogonal Polynomial Trend Analysis of Variance (OPTA) in the frequency domain. The orthogonal polynomials are limited to the fifth order in this study. The means and trend-coefficients which are used to estimate the ERPs are tested for significance with the F-statistics. Coefficients not reaching a preset significance criterion are set to zero. Subsequently significant means and trend-coefficients multiplied with the polynomials are inversely transformed to the time domain for each trial. The method is demonstrated with artificial signal-in-noise EEG (electroencephalogram) series in which the signal component (the ERP) decreases in a relatively short time span. The estimated signals approximate closely the input signals and a signal-to-noise improvement on single trial level is reached of about a factor 10. Comparison with the statistical 'Wiener' filtering method shows that the OPTA detects weaker signals. Single trial ERPs estimated with the OPTA from EEGs collected in a habituation paradigm suggest that the averaged evoked potential model may be inadequate in detecting ERP change over a few trials. It is shown that the frontal P3b wave habituated in 7-9 trials, a habituation speed which is comparable to the habituation rate of the visual orienting reaction and the electrodermal orienting reaction.

Stimulus information and habituation of the visual event related potential and the skin conductance reaction under task-relevance conditions.

The influence of information value of visual stimuli on habituation of event related potentials (ERPs) at Fz, Cz, Pz and Oz and of the skin conductance reaction (SRC) was investigated under task-relevance condition. An improved Wiener filter was used to reduce the number of stimulus presentations to estimate an ERP. Twenty-six subjects received two times a block of 36 stimulus presentations. Half of the subjects received a stimulus with a high information value in terms of complexity and then a stimulus with a low information value. The other half of the subjects received the reversed order. Wiener filtered ERPs and SCRs were determined over ensembles of six stimulus presentations. The habituation of the P300 component was restricted to the fronto-central leads, and was delayed when compared to the results of a former study (Woestenburg, Verbaten and Slangen, 1981b) where non-signal stimuli were used. Also information effects were noticed on these fronto-central leads, but not on the SCR. This reaction habituated as in the former non-signal study. The P300 at the parieto-occipital leads showed larger amplitudes than the fronto-central P300 and these components did not habituate. At the Oz lead early waves habituated and late waves increased during ensemble 1 to 6.

The removal of the eye-movement artifact from the EEG by regression analysis in the frequency domain.

A new method to remove the eye-movement artifact from the electro-encephalogram (EEG) is described. It is based on complex regression analysis. Transfer of eye-movement activity to EEG can have frequency dependent amplitude and phase characteristics. The proposed method is suitable for handling such transfer because the regression formula is used in the frequency domain. The method is demonstrated with artificial signal-in-noise EOG (electro-oculogram) and EEG series. In the EEG noise an event related potential (ERP) is buried as a constant signal and in the EOG noise a changing EOG response (saccadic eye-movement) is simulated before adding the whole series to the EEG series. Also decreasing levels of transfer from EOG on EEG leads are simulated as the EOG artifact diminishes from the frontal to the occipital area. Because of the possible frequency dependent phase characteristics also a 24 msec time-shift of the EOG is simulated. The complex regression coefficient (P(jw)) of the EOG and EEG series is first calculated and then 'common' regression is removed from the EEG. To do this the estimated P(jw) is tested for significance with the F-statistic and significant EOG activity is subtracted in the frequency domain from the EEG on a single trial base. Then each EEG record is inversely transformed to the time domain. It was found that the complex P(jw)s as used in the subtraction formula, corrects very accurately the influence of the EOG artifact on EEG activity. Finally an application of the method in a real life situation is reported.

The influence of information on habituation of the "Wiener" filtered visual even related potential and the skin conductance reaction.

In this study the influence of the information value of visual stimuli on habituation in event related potentials (ERPs) at Fz, Cz and Oz locations and the skin conductance reaction (SCR) was investigated. An improved 'Wiener' filter was used to reduce the number of stimulus presentations. This enabled us to study P300 and SCR decrement in the same period of time. SCRs and Wiener filtered ERPs were determined over six stimulus presentations. Half of the 26 subjects received for 36 times a stimulus with a high information value in terms of complexity and the other half of the subjects 36 stimuli with a low information value. The results of this study show that decrement of the P300 amplitude occurs in about the same period of time as the SCR magnitude decrement. P300 habituation was manifested only in the fronto-central leads, the occipital P300 did not show habituation. A second difference in the topography of the P300 was that the effect of stimulus information was only found in the fronto-central leads and not in the occipital lead. As earlier studies, we did not find an effect of information on SCR habituation. The results show that the fronto-central P300 showed two important characteristics of the orienting response (OR). They are that habituation occurs in about the same period of time as in which a classical OR index as the SCR habituates and P300 amplitudes are larger when the information value of the stimuli is higher.

A statistical Wiener filter using complex analysis of variance.

The Wiener filter is a digital filter in the frequency domain which enables the detection of signals in noise on the basis of a relatively small number (N) of signals-in-noise series. However, the variance associated with the Wiener filter increases sharply with low N or low signal-to-noise ratio, sometimes resulting in large negative transfer coefficients (H(w)'s). This causes an unacceptable distortion of the extracted signal. The classical solution for this problem is rather arbitrary; clipping all H(w)'s less than zero. It can be shown that this approach has limited practical value and does not lead to optimal filtering of the signal. The solution presented in this paper is based on the fact that the large negative H(w)'s in the case of a small N are caused by using an estimate for the noise-power which deviates from the actual noise-power. This makes the use of a statistical procedure feasible. By using complex analysis of variance it is possible to test which part of the spectrum is due to the signal and which part of the spectrum is due to the noise. H(w)'s not reaching a preset significance criterion are set to zero. The statistical Wiener filter acts as an adaptive multi-band-pass filter with band-passes determined by (a posteriori) detected signal components. It is demonstrated that this procedure leads to better suppression of the noise in signal-in-noise series.

The influence of task relevance and stimulus information on habituation of the visual and the skin conductance orienting reaction.

This study investigated the hypothesis that task-relevant stimuli induce orienting reactions (ORs) that are stronger and more resistant to habituation when their information content is high than when it is low. Task-relevance was given to the stimuli by rewarding the subjects for correct recognition at the end of the experiment. The dependent variables in this study were the visual orienting reaction (VOR), the skin conductance reaction (SCR), their habituation scores and the number of spontaneous fluctuations in skin conductance (SFs). 28 subjects received two blocks of 14 trials. Half the subjects received the higher information condition first and then the lower information condition, while the other half received the reversed order. The VOR habituated quickly and was not significantly influenced by information value. SCR amplitudes were larger and SCR-habituation slower to stimuli containing more information. The results were interpreted as supporting the hypothesis that the SCR is associated with a secondary phase of the orienting process.

The influence of visual information on habituation of the electrodermal and the visual orienting reaction.

In this study the influence of the information value of visual stimuli on habituation of the visual orienting reaction (VOR) and the skin conductance reaction (SCR) was investigated. 28 subjects received two blocks of 14 trials. Half the subjects received the higher information condition first and then the lower information, the other half received the conditions in the reversed order. Subjects fixated the stimuli with the higher information value longer than the stimuli with the lower information value during the 14 trials and habituated slower. This effect of information was absent in the second block and VOR habituation was also faster in this block. There was no difference in SCR amplitudes nor in SCR habituation scores between the two information conditions, neither in block 1 nor in block 2. The results of this study are discussed in relation with a two-stage model of the OR.