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What structures and processes mediate the operation of selective visual attention in the human brain? One hypoth-esis suggests that object representations at different locations compete for processing capacity in the visual system, and that attention works by biasing this competition in favour of the attended location. Recordings of single cells in awake monkeys have been consistent with this idea1xAttentional modulation of visual motion processing in cortical areas MT and MST. Treue, S. and Maunsell, J.H.R. Nature. 1996; : 382–385PubMedSee all References1. The response of a neuron to an optimal stimulus is reduced substantially when an irrelevant stimulus is presented simultaneously at another location in the receptive field. However, if the animal directs its attention to one of the competing stimuli in the receptive field, the responses are as large as when the stimulus is presented alone. Now, Kastner and colleagues2xMechanisms of directed attention in the human extrastriate cortex as revealed by functional MRI. Kastner, S., De Weerd, P., Desimone, R., and Ungerleider, L.G. Science. 1998; 282: 108–111Crossref | PubMed | Scopus (605)See all References2 provide evidence that a similar mechanism might operate in humans. Using functional MRI, they examined cortical responses to four adjacent objects presented either simultaneously, or one at a time in rapid succession. Although the total amount of retinal stimulation (integrated over time) was the same in the two experimental conditions, cortical responses differed. Simultaneous presentation evoked less activity than successive presentation, and this difference was more pronounced in ‘higher’ cortical areas (those with larger receptive fields). Moreover, the reduction with simultaneous presentation was much less severe when attention was directed to one of the four peripheral stimuli. The authors argue that these findings are consistent with the notion that attention is ‘protecting’ a representation of the target item from competition. However, it is also possible that the difference between simultaneous and successive stimulation could have arisen as a result of other factors, such as the difference in presentation rate between the two conditions. In a control experiment to test this possibility, the authors presented stimuli at a constant rate and showed that the response to a single peripheral item presented alone was lowered when it was presented simultaneously with three other items. This suggests that the difference between successive and simultaneous stimulation is not simply related to different presentation rates and so could be a mechanism of attention that filters out unwanted information in cluttered visual scenes. Assessing the significance of these findings will come from directly addressing the mechanisms of directed attention – the intriguing parallels between single-cell and functional-imaging results warrant further investigation.

In two experiments, we investigated the effects of exogenous cueing on visual motion processing. The first experiment shows that the typical pattern of reaction time (RT) effects, namely early facilitation and later inhibition of return (IOR), can be obtained using a color change as exogenous cue and a direction change as target. In the second experiment, we manipulated the validity of the cue independently with respect to location and feature using transparent motion stimuli. Facilitation of RTs with short cue-target interstimulus-intervals (ISIs) was only evident for targets with both the valid location and the valid feature. Furthermore, at longer cue-target intervals, RTs were prolonged for targets at the cued location, irrespective of the cued feature. These results demonstrate spatial and feature-based components of early facilitation and purely spatial IOR.

The ability of infants to discriminate between opposite directions of motion was examined in infant control habituation experiments. A group of 3–5-week-olds showed no evidence of discrimination between a random-dot pattern which was segregated into regions that moved in opposite directions, and a uniform pattern in which the dots all moved in the same direction. However, they did discriminate between segregated and uniform patterns in two additional conditions, neither of which required sensitivity to direction: in the first of these, segregation was based on the contrast between coherently moving and stationary dots, while in the second the contrast was between coherently and incoherently moving dots. Unlike the younger infants, a slightly older group of 6–8-weeks-olds proved capable of discriminating between the segregated and uniform patterns when the contrast was between opposite directions of motion. These results confirm and extend the results from preferential looking [Wattam-Bell, J. (1996). Motion processing in one-month-old infants: Preferential looking experiments. Vision Research, 36 , 1671–1677]; they suggest that direction discrimination may not emerge until around 6–8 weeks of age. The apparent lack of direction discrimination before 6 weeks may reflect an inability to use directional cues to visually segment the segregated pattern, rather than an insensitivity to direction as such. To examine this, a further set of infants was tested for absolute direction discrimination—i.e., between leftwards- and rightwards-moving uniform patterns. However, in this case neither 3–5- nor 6–8-week-olds showed any evidence of discrimination, which suggests that direction discrimination may first emerge for relative motion. Possible reasons for this are discussed.

Orientation tuning of receptive fields is well documented in the spatial domain, but considerable variability exists amongst published estimates of orientation sensitivity of motion receptive fields. We used a two-frame motion sequence, in which one frame was binary noise and the other was a horizontally displaced and filtered version of the same noise field, to examine the orientation sensitivity of human motion mechanisms. Initially, orientations orthogonal to the direction of motion were removed from each filtered frame. Observers indicated perceived direction of motion in a single interval, binary choice task. D max was determined for different amounts of removed orientations, and found to remain constant across the removal of energy up to approximately ±60 deg from vertical. In a second experiment, the orientations removed were now parallel to the direction of motion of the stimulus. D max fell as a cosine function with increasing removal of orientation information, in agreement with off-orientation looking or matched filtering predictions. The two experiments show the presence of mechanisms both broadly tuned and more narrowly tuned for orientation. A control experiment introduced an interstimulus interval between the two frames of our motion sequence. Performance on the direction discrimination task was severely degraded, indicating that the original results are not explicable in terms of a feature-tracking or long-range motion process. The presence of both broadly and narrowly tuned mechanisms implies multiple possible solutions to the processing of coherent plaid motion.

When our eyes are tracking a target that is moving in front of a structured background, global motion of equal speed is induced in the opposite direction. This effect has been termed reafference, which, astonishingly, does not significantly affect the execution of such pursuit eye movements. Employing brief and unexpected injections of full-field motion during ongoing human smooth pursuit, we demonstrate that the sensitivity for full-field motion is reduced strongly in the direction opposite to the eye movement, i.e. the direction of reafferent background motion. Our experiments further characterize this asymmetry in visual motion processing and provide a preliminary explanation for the accuracy of the pursuit system despite self-induced motion.

The goal of the research described in this paper was to study the spatio-temporal properties of visual motion mechanisms and to apply the knowledge gained to the evaluation/design of visual displays. Objective measurements of an observer’s sensitivity to the order of onset of stimuli in two-point motion sequences were obtained. The basic research findings were consistent with the idea that the spatial “grain” of the retina increases with retinal eccentricity, but that the temporal, resolving power of motion mechanisms at different retinal locations remains relatively constant. There are at least two important, applied implications of the research results. First, a visual display which is updated every 16 msec (60-Hz refresh rate), as are many raster-scan systems, will “filter out” motion information capable of being perceived by a human. Second, the critical flicker frequency (cff) is not a completely adequate criterion upon which to base the design of a display intended to simulate the motion of real-world objects.

Performance deficits and diminished brain activity during cognitive control and error processing are frequently reported in Attention Deficit/Hyperactivity Disorder (ADHD), indicating a “top-down” deficit in executive attention. So far, these findings are almost exclusively based on the processing of static visual forms, neglecting the importance of visual motion processing in everyday life as well as important attentional and neuroanatomical differences between processing static forms and visual motion. For the current study, we contrasted performance and electrophysiological parameters associated with cognitive control from two Flanker-Tasks using static stimuli and moving random dot patterns. Behavioral data and event-related potentials were recorded from 16 boys with ADHD (combined type) and 26 controls (aged 8 to 15 years). The ADHD group showed less accuracy especially for moving stimuli, and prolonged response times for both stimulus types. Analyses of electrophysiological parameters of cognitive control revealed trends for diminished N2-enhancements and smaller error-negativities (indicating medium effect sizes), and we detected significantly lower error positivities (large effect sizes) compared to controls, similarly for both static and moving stimuli. Taken together, the study supports evidence that motion processing is not fully developed in childhood and that the cognitive control deficit in ADHD is of higher order and independent of stimulus type.