Abstract
Attending to a feature (e.g., orientation, motion direction) enhances visual processing of that particular feature, indexed by both psychophysical and neurophysiological measures. However, it is not known how many features one can attend simultaneously. Given that one can attend to multiple spatial locations (e.g., Awh & Pashler, 2000; McMains & Somers, 2004), we might expect that one can also attend to multiple features. We tested this conjecture by measuring motion detection threshold with a cueing protocol. Each trial contained two intervals, with a random dot motion stimulus shown in each interval. One stimulus (noise) had 0% coherence (no net motion), while the other stimulus (signal) moved with varying levels of coherence in a particular direction. Subjects reported which interval contained the signal (two-interval forced choice) in one of three cueing conditions. In the 1-cue condition, a small line segment preceded the stimuli indicating the direction of the signal with 100% validity. In the 2-cue condition, two lines preceded the stimuli, indicating the signal would move in one of the two cued directions. In the no-cue condition, no line segment appeared before the dot stimuli. The three conditions were blocked and block order was counter-balanced. In several experiments, we consistently observed a lower detection threshold in the 1-cue condition than the no-cue condition, showing subjects can orient their attention to a single feature. However, detection threshold was consistently higher for 2-cue condition than 1-cue condition, indicating subjects could not prepare for two motion directions as effectively as one direction. This finding revealed a severe capacity limit in our ability to orient attention to multiple features.
Meeting abstract presented at VSS 2012