The perceived onset of visual events
26.309, Saturday, May 11, 2:45 - 6:45 pm, Royal Ballroom 6-8
Chris Paffen1, Ryota Kanai2; 1Experimental Psychology & Helmholtz Institute, Utrecht University, 2School of Psychology, Sackler Centre for Consciousness Science, University of Sussex
What is the moment at which a visual stimulus reaches visual awareness? Previously, this question has been addressed by varying stimulus attributes such as contrast and intensity. These manipulations have shown differential effects on reaction times (RTs) and temporal order judgments (TOJs), which usually go in the same direction: stimulus manipulations that make RTs shorter also shorten perceptual latency for the stimulus measured in TOJ tasks. In our experiments, we manipulated the size of Gaussian blobs or Gabors of different spatial frequency presented left or right of fixation. In RT experiments, participants were instructed to indicate as soon as possible whether a target stimulus appeared left or right of fixation. In TOJ experiments, participants indicated which of two stimuli presented left and right of fixation appeared earlier. The results show that RTs are generally shorter for large stimuli, for both Gaussian blobs and Gabors of various spatial frequencies. On the other hand, the perceived timing of larger stimuli was delayed in TOJ experiments. Thus, when a large stimulus (e.g. 8 degrees in diameter) was presented simultaneously with a smaller stimulus (e.g. 1 degree in diameter), the larger stimulus was perceived to appear later. We suggest that the differential effects of stimulus size on RT and TOJ reflect separate neural mechanisms being used in both tasks. The effect of size on RT can be attributed to the increased signal of a larger stimulus. For the TOJ task, however, we suggest that the judgment of perceived timing is made based on synchronized activity over a larger cortical surface. The perceived timing of larger stimuli will be delayed, since increasing stimulus size will increase the cortical surface over which activity needs to be synchronized.