Purpose. A number of recent studies have demonstrated that the processing of visual sensory information along the cortical pathways is modulated by attention. The same visual stimulus can elicit differential activation of even early cortical areas, depending upon the behavioral significance of the stimuli. Here we demonstrate a psychophysical correlate of this differential activation with moving stimuli.

Methods. In tasks involving rapid serial visual presentation of moving random dot patterns subjects were asked to perform two-alternative forced choice direction- (or color-) discriminations of either one or two target items in a temporal string of distractor items.

Results. When viewing simple uni-directional stimuli the correct direction discrimination of the second target was impaired when it appeared within a couple hundred ms of a similar first target, regardless of whether the subject also had to perform a discrimination of a first target feature. Since this impairment occurs even when no, or only a color, judgement is required for the first target and only when the first and second target contain similar motion axes, it appears to reflect simple sensory masking. In contrast, if the first target consists of signal motion embedded in motion noise, discrimination of the second target was much more severe and prolonged when the subjects also had to judge the direction of the first target. In this condition the difference between the physiological response to the attended and unattended first target is maximized.

Conclusions. These results demonstrate that the ability of a moving stimulus to influence the processing of subsequent stimuli can be affected by attention. We suggest that this impairment reflects altered neural processing of the attended stimulus rather than a simple increase in attentional load or task difficuly.