The upper spatial limit for perception of displacement is affected by preceding motion.

The upper spatial limit D(max) for perception of apparent motion of a random dot pattern may be strongly affected by another, collinear, motion that precedes it [Mateeff, S., Stefanova, M., &. Hohnsbein, J. (2007). Perceived global direction of a compound of real and apparent motion. Vision Research, 47, 1455-1463]. In the present study this phenomenon was studied with two-dimensional motion stimuli. A random dot pattern moved alternately in the vertical and oblique direction (zig-zag motion). The vertical motion was of 1.04 degrees length; it was produced by three discrete spatial steps of the dots. Thereafter the dots were displaced by a single spatial step in oblique direction. Each motion lasted for 57ms. The upper spatial limit for perception of the oblique motion was measured under two conditions: the vertical component of the oblique motion and the vertical motion were either in the same or in opposite directions. It was found that the perception of the oblique motion was strongly influenced by the relative direction of the vertical motion that preceded it; in the "same" condition the upper spatial limit was much shorter than in the "opposite" condition. Decreasing the speed of the vertical motion reversed this effect. Interpretations based on networks of motion detectors and on Gestalt theory are discussed.

Flanker interference in young and older participants as reflected in event-related potentials.

The present study investigates behavioural and event-related potential (ERP) differences between young and older participants in two variants of a flanker task. Flankers preceded the target by 100 ms (Experiment 1) or were presented simultaneously with the target (Experiment 2). In both experiments the response times showed an age-related slowing and a compatibility effect, which did not differ significantly across age. The older participants committed only half as many errors as the young ones. The visual ERPs revealed that the speed of visual perception was similar between groups. In addition the processing of the targets, but not of the flankers, appeared to be enhanced in the older participants. Moreover the lateralized readiness potential (LRP) started later and was larger for old vs. young participants, which was most pronounced for the incorrect LRP activation due to the flankers. The LRP amplitude effect is due to an enhanced activation over contralateral motor areas, which appears to be a general finding unrelated to the error rate. In summary, in the present study we could not find evidence for enhanced flanker interference in the performance of older compared to young participants. The reduced error rates for older participants are likely due to enhanced processing of the targets and delayed transmission of flanker information from visual to motor areas.

Perceived global direction of a compound of real and apparent motion.

We presented a random dot pattern that moved horizontally for 1.6s within a stationary invisible aperture. The dots were periodically visible for 50 ms (visible phase) with lengths of the real motion 0-1.34 deg; for the next 50 ms their luminance was zero (invisible phase). The pattern was seen to translate in the direction of the vector sum of the displacements in the two phases, when the displacement in the invisible phase was shorter than an upper limit. When the motions in both phases were in the same direction, the upper limit decreased with increasing length of real motion. When the motions in both phases were in opposite directions, the upper limit increased with increasing length of real motion. We suggest that during the visible phase 'motion streaks' occur at an early level of the motion processing [Geisler, W. S. (1999). Motion streaks provide a spatial code for motion direction. Nature, 400, 65-69]. The pattern is seen in short-range apparent motion when the displacement of the streaks, rather than of the dots, is below an upper limit. The data show that this limit remains nearly the same, about 1.5 deg, irrespective of the length and direction of the real motion in the visible phase.

Effects of ageing on cognitive task preparation as reflected by event-related potentials.

OBJECTIVE: The anticipation of complex cognitive tasks involves effortful preparation being reflected in the contingent negative variation (CNV) of the event-related potential. In the literature there are contradictory results concerning the effect of age on this potential. We wanted to investigate effects of age, time-on-task, and task difficulty on the CNV. METHOD: Young and middle-aged participants performed a visual search and a non-search task during an early and a late phase of a 6-h session. RESULTS: Performance data revealed increased response times and error rates for middle-aged vs. young participants. Most importantly, an increased frontal CNV amplitude was found for the older participants, especially pronounced in the search task. A late positivity which was elicited to the offset of the preceding stimulus was increased for the middle-aged vs. young group in the visual search task only. There was no effect of time-on-task on performance, but the CNV became larger with time-on-task in the search task while it became smaller in the non-search task. CONCLUSIONS: The results suggest an enhancement of effortful task preparation for middle-aged participants especially when the task is difficult. SIGNIFICANCE: This underlines the role of the CNV as a neurophysiological indicator for effortful cognitive preparation.

Visual analysis of changes of motion in reaction-time tasks.

Subjects observed a random-dot pattern moving uniformly in the vertical direction (vector V1). The motion vector abruptly changed to V2, both in speed and direction simultaneously. It was found that the time of simple reaction to such changes V1 --> V2 can be described by a function of a single variable, [w(V1 - V2c) + (1 - w) V2N], 0 < w < 0.5, where V2c and V2N are the components of V2 collinear with and normal to V1. The choice-reaction time for changes in direction that are accompanied by changes in speed can be described by a function solely of the absolute value of V2N. Unlike the simple-reaction time, the choice-reaction time was independent of the initial speed of motion. The processes that may be engaged in simple and choice reactions to motion are discussed.

Central and parietal event-related lateralizations in a flanker task.

Recently Wascher et al. (1999) reported that in a flanker task with arrow stimuli not only the known lateralized readiness potential (LRP) that reflects lateralized response activation was induced, but also a parietal lateralized activation (direction encoding lateralization; DEL) that was interpreted as reflecting an earlier coding of a response side. However, the Wascher study did not exclude that the DEL could have also been due to lateralized stimulus- or attention-related factors. In the present study we used vertically directed arrow stimuli, and had our subjects perform responses in the vertical dimension. To separate flanker-induced from target-induced lateralizations the delay between the presentations of irrelevant and relevant stimuli (stimulus onset asynchrony; SOA) was manipulated. Apart from the usual LRPs we obtained clear DELs that varied in a similar way with the experimental variables, but peaked earlier and had a more posterior topography than the LRP. These results indicate that the DEL reflects premotor response representation.

Effects of spatial attention on the brain stem frequency-following potential.

Can spatial attention or orienting affect human auditory information processing as peripheral as on the brain stem level? More specifically, is the reduction of the latency of the frequency-following potential (FFP; an evoked lower brain stem response) that we described in an earlier Neuroreport article really specifically attention-related? Here we demonstrate that, indeed, exogenous intramodal (auditory) spatial orienting, but not a transient modulation of general arousal, reduced the latency of the FFP by 27 micros; there were no effects on the FFP-amplitude. Although it might seem small, this reduction may be relevant in spatial hearing. We conclude that under certain conditions spatial attention can affect auditory information processing already on the brain stem level.

Parallel systems of error processing in the brain.

Major neurophysiological principles of performance monitoring are not precisely known. It is a current debate in cognitive neuroscience if an error-detection neural system is involved in behavioral control and adaptation. Such a system should generate error-specific signals, but their existence is questioned by observations that correct and incorrect reactions may elicit similar neuroelectric potentials. A new approach based on a time-frequency decomposition of event-related brain potentials was applied to extract covert sub-components from the classical error-related negativity (Ne) and correct-response-related negativity (Nc) in humans. A unique error-specific sub-component from the delta (1.5-3.5 Hz) frequency band was revealed only for Ne, which was associated with error detection at the level of overall performance monitoring. A sub-component from the theta frequency band (4-8 Hz) was associated with motor response execution, but this sub-component also differentiated error from correct reactions indicating error detection at the level of movement monitoring. It is demonstrated that error-specific signals do exist in the brain. More importantly, error detection may occur in multiple functional systems operating in parallel at different levels of behavioral control.

Sensorimotor slowing with ageing is mediated by a functional dysregulation of motor-generation processes: evidence from high-resolution event-related potentials.

The objective of the present study was to identify the origin(s) of ageing-related behavioural slowing in sensorimotor tasks. For this aim, event-related potentials (ERPs) were analysed at 64 electrodes to evaluate the strength and timing of different stages of information processing in the brain. Electrophysiological indices of stimulus processing, sensorimotor integration/response selection and motor-related processing were used to compare the processing speed of young (n = 13, mean age = 22.5 years) and older adults (n = 14, mean age = 58.3 years) in simple- and choice-reaction tasks presented in two modalities, auditory and visual. The behavioural results showed significant ageing-related slowing, but only in the choice-reaction task. The quantification of separate central processing stages, in combination with advanced ERP methodology, helped to reveal that this slowing did not originate from the early processes of stimulus processing and response selection. Instead, it was produced by slower activation patterns over the contralateral motor cortex underlying response generation. It is concluded that ageing is accompanied by a functional dysregulation of motor cortex excitability during sensorimotor processing, with this deficit becoming progressively evident with greater task complexity.

Short-term mobilization of processing resources is revealed in the event-related potential.

This study investigates whether an occasional effortful improvement of performance, as asked for by a precue, is reflected in event-related potential (ERP) changes. To estimate the limits of possible effort-induced behavioral and ERP changes, we manipulated the time between precue and imperative stimulus (IS; precue interval, PCI). The subjects could, in fact, improve their performance in the effort trials, with all but the shortest PCI. The postcue ERP revealed a fronto-central contingent negative variation (CNV), which was preceded by a frontal positive/occipital negative wave (P2/N2). Both the P2/N2 and the CNV were larger for effort than for standard trials for all PCIs. For the shortest PCI (300 ms), the CNV increase was seen after the IS. The CNV increase for PCIs 600 and 300 began at about 400 ms postcue. The results suggest that effortful performance improvement is associated with prior increase of a frontocentral CNV and a preceding P2/N2. The CNV increase is thought to reflect the activity of a frontal executive process by which additional processing resources can be mobilized on a trial-to-trial basis within less than 500 ms.