13-24 – How does attention keep up with moving stimuli? Exploring the effect of object speed on attentional facilitation in early visual processing

Our capacity to track moving targets is inherently limited and influenced by both the number of targets and their speeds. Previous research has shown that increasing the number of tracked targets weakens attentional selection in early visual areas, but it remains unclear whether similar attentional constraints apply to tracking faster-moving objects. To investigate this, we systematically varied motion speed in a multiple object tracking (MOT) task while measuring the strength of attentional selection using steady-state visual evoked potentials (SSVEPs).
Participants were presented with eight identical discs at random screen locations, flickering at two different frequencies. Following a cueing period where a subset of discs was highlighted for tracking, all discs moved along randomly chosen linear trajectories, bouncing off the borders of the viewing area and each other. Depending on the condition, the discs moved at one of three constant speeds (slow/medium/fast). At the end of each trial, participants were asked to select the discs they were tracking by clicking on them. The strength of attentional selection was assessed in the early visual cortex as the difference in SSVEP amplitudes elicited by targets versus distractors.
Behavioural results indicated that the MOT task became increasingly difficult as object speed increased, paralleling the challenge of tracking a larger number of objects. However, the attentional enhancement of targets, as reflected by SSVEP amplitudes in predefined electrode locations, did not vary across speed conditions. To further dissect the contributions of different visual areas to the overall SSVEP signal, we performed a functionally defined EEG template analysis. This analysis confirmed that V4 is the primary contributor to attentional enhancement of SSVEPs, and its contribution was speed invariant. This suggests that the tracking errors associated with increased speed originate at later stages of visual processing.