G. Mulder

Assessment of baroreceptor reflex sensitivity by means of spectral analysis.

A method of determining baroreceptor reflex sensitivity is proposed that is based on spectral analysis of systolic pressure values and RR interval times, namely, the modulus (or gain) in the mid frequency band (0.07-0.14 Hz) between these two signals. Results using this method were highly correlated (0.94; n = 8) with results of the phenylephrine method. In addition, compared with the values for the preceding rest period, the modulus decreased during mental challenge, as might be expected from the literature.

USE OF PARTIAL STIMULUS INFORMATION IN RESPONSE PROCESSING

We examined the reaction time benefit that is obtained when salient features of the stimulus set and response set correspond. Components of the event-related brain potentials were used to measure the timing of stimulus-related and response-related processes in order to determine the locus of this effect. Of particular importance was the development of a new index of selective response preparation, the corrected motor asymmetry (CMA). We found no evidence for the use of preliminary, partial stimulus information in response preparation. These results suggest that the benefit is located primarily in response selection processes and probably reflects a more efficient algorithm for stimulus-response translation. Also, we found trial-to-trial variability in the duration of response selection to be the major determinant of variability in reaction time, whereas the durations of subsequent response-related processes were relatively invariant. Implications of these results for discrete and continuous models of choice reaction performance are discussed.

Memory-search and visual spatial attention - an event-related brain potential analysis

Event-related brain potentials (ERPs) were recorded from subjects as they attended to one diagonal of a visual display. The task was to respond only to memory set items (targets) at the attended diagonal and to ignore stimuli at the other diagonal. The probability that the display contained either an attended or an unattended target was 0.30. The spatial separation between attended and unattended stimuli was 1.6°. The ERP elicited by stimuli at the unattended diagonal contained a sequence of phasic components. The early N170 and P250 components were elicited by the onset of the display and the later components N480 and P550 by the offset of the display. The presence of masks delayed N170 and P250. The ERPs elicited by attended non-targets, in addition, contained an increased N350 (Cz, Fz) and P410 (P3a, Pz, Cz). The ERPs elicited by attended and unattended non-targets started to differ after 200 msec. This finding suggested that selection is relatively late if selection must be based on a conjunction of features (location and orientation) and if the spatial separation between attended and unattended stimuli is small. Memory-set size affected the ERPs after 250 msec. The ERPs elicited by attended stimuli contained a broadly distributed (Fz, Cz, Pz) negative endogeneous component. The amplitude of this component was related to memory-set size. Finally, the ERPs elicited by attended targets contained a large P3b (Pz, Oz) with a peak latency around 600 msec. The ERP results suggested the existence of three processing stages: (1) orienting to the attended stimuli; (2) controlled search, and (3) target decision.

LATE POSITIVE COMPONENTS AND STIMULUS EVALUATION TIME

The amplitude and latency of late positive components were, together with reaction time (RT), studied in a task which combines visual and memory search. The visual display contained either one, two or four letters, as did the memory set. Six load combinations, resulting in one, four, eight and 16 comparisons, were examined. The reaction time data indicated a self-terminating search process. Three late positive components were present in the evoked potential: one at 375 msec after the onset of the display, one at 375 msec after the offset of the display and one around 600 msec. Only the latter component appeared to be sensitive to the number of comparisons. Adaptive averaging was applied to this latter component. The latency of this P300 suggested, in contrast to the RT data, an exhaustive search process. In addition there was a negligible correlation between the response latency and P300 latency at single trial level. Several hypotheses are suggested for what P300 could have to tell us.

THE P3-COMPLEX AS AN INDEX OF INFORMATION-PROCESSING - THE EFFECTS OF RESPONSE PROBABILITY

The amplitude and latency of the late positive (P3b) component of the cortical evoked potential were studied in a visual search task. The main independent variables were memory load, response type and relative response frequency. In the experiment we used the varied mapping procedure, a condition in which the target and distractor stimuli were thoroughly mixed across trials. The main goal of the study was to establish the locus of relative response frequency. In many models of choice reaction time the effect of this task variable is believed to occur in the response selection stage. If the latency of the P3b only indexes stimulus evaluation, independent of response selection and organization processes, relative response frequency should only affect reaction time and not the latency of P3b. The results indicated that the latency of P3b was very sensitive to the number of comparisons, response type and relative response frequency. In addition, only a subset of subjects selectively prepared for the most probable response. Only these subjects showed the effects of relative response frequency in their reaction times too. Response preparation, however, did not affect the latency of P3b. It was concluded that the latency of P3b is a very sensitive index of the search and comparison process, the binary decision process, and the probability of the type of the decision, but insensitive to motor preparation.

VISUAL-SEARCH AND SPATIAL ATTENTION - ERPS IN FOCUSED AND DIVIDED ATTENTION CONDITIONS

ERPs and performance were measured in divided and focussed attention visual search tasks. In focussed attention tasks, to-be-attended and to-be-ignored letters were presented simultaneously. We varied display load, mapping conditions and display size. RT, P3b-latency and negativity in the ERP associated with controlled search all increased with display load. Each of these measures showed selectivity of controlled search, in that they decreased with focussing of attention. An occipital N230, on the other hand, was not sensitive to focussing of attention, but was primarily affected by display load. ERPs to both attended and unattended targets in focussed attention conditions showed and N2 compared to nontargets, suggesting that both automatic and controlled letter classifications are possible. These effects were not affected by display size. Consistent mapping resulted in shorter RT and P3b-latency in divided attention conditions, compared to varied mapping conditions, but had no effect in focussed attention conditions.

Differential psychophysiological reactivity of city bus drivers

Two groups of six city bus drivers whose absence rate differed during the previous year were studied. Catecholamine (adrenaline, noradrenaline and dopamine) excretion rates were measured from urine samples obtained at 3-5-hour intervals over 2 working days and, for control purposes, over the first and second consecutive free days. After 3-5-hour periods of work heart rate variability was studied in laboratory conditions. In addition self-assessed workload was measured. As a major result differential neuro-endocrine and cardiovascular reactivity was found.

Stage analysis of the reaction process using brain-evoked potentials and reaction time

SummaryMotor processes partly determine reaction time (RT) in both choice reaction time and in binary classification tasks. These latter tasks are popular in cognitive psychology because the experimenter believes that he has kept the motor component simple and constant and therefore can attribute changes in RT to perceptual or cognitive processes. In this paper we used the P3 component of the event-related potential (ERP) as a time marker indicating the duration of perceptual and cognitive processes. The latency of this component is believed to reflect stimulus evaluation time independent of response selection and organization time.Two types of tasks were used: a choice-reaction time task and a binary classification task. Signal similarity and S-R compatibility additively affected RT, but only signal similarity affected P3 latency. The number of items in the positive set and response type affected both P3 latency and RT. Relative response frequency changed the bias in the cognitive evaluation of the stimulus, reflected in the latency of the P3 component, and affected RT only if the subjects preset their motor system (indexed by the late CNV). A model was presented which proposes that motor processes may partially overlap with the perceptual and cognitive evaluation of the stimulus. Both ERPs and RT are necessary tools in the study of the relative timing of these processes.

Heart rate variability in a binary choice reaction task: an evaluation of some scoring methods

Abstract Several scoring methods of heart rate variability have been investigated in a binary choice task. It was concluded that in this type of task the number of waves in the cardiotachogram and the sum of absolute differences between successive R-R intervals divided by the number of waves are the best indicators of the task levels (20, 30, 40, 50 and 60 binary choices per min). A detailed analysis of these scoring methods in different types of information processing tasks is recommended.

THE ROLE OF NEUROIMAGING IN THE DISCOVERY OF PROCESSING STAGES - A REVIEW

In this contribution we show how neuroimaging methods can augment behavioural methods to discover processing stages. Event Related Brain Potentials (ERPs), Brain Electrical Source Analysis (BESA) and regional changes in cerebral blood flow (rCBF) do not necessarily require behavioural responses. With the aid of rCBF we are able to discover several cortical and subcortical brain systems (processors) active in selective attention and memory search tasks. BESA describes cortical activity with high temporal resolution in terms of a limited number of neural generators within these brain systems. The combination of behavioural methods and neuroimaging provides a picture of the functional architecture of the brain. The review is organized around three processors: the Visual, Cognitive and Manual Motor Processors.