Abstract
It has been suggested that visual attention may be allocated at points in time by either a stochastic clock or coupled oscillator. Behavioral data and functional neuroimaging have hinted that both mechanisms may operate in parallel, but evidence has been ambiguous. To further distinguish between possible mechanisms of dynamic visual attention, 11 observers were given a perceptually difficult visual motion direction discrimination task in three cueing conditions: alerting, isochronous countdown, and symbolic cueing. In the alerting condition, a single cue alerted participants that the target was about to occur. In the isochronous countdown condition, multiple flashes counted down to the target onset, affording entrainment of a coupled oscillator mechanism, timing by a stochastic clock, or both. In the symbolic cueing condition, a single letter cued the time of target onset (one of two possible intervals), precluding entrainment of a coupled oscillator but affording an unambiguous signal for a stochastic timer. The stimuli were random dot cinematographs moving with net motion directions that differed by 45°, with variable amounts of noise. In addition to analyzing response latency and accuracy, a diffusion model of perceptual decisions was used to estimate the rate of information accumulation, boundary separation, and non-decision time. The isochronous countdown had little effect on latency or accuracy, but observers were slower and more accurate after a symbolic cue, indicating that the different cueing conditions induced a speed-accuracy tradeoff. Diffusion model analysis indicated that symbolic cues increased the boundary separation of the decision process and non-decision time, consistent with the idea that participants were slower because they strategically accumulated more information before committing to a response. The isochronous countdown, in contrast, had little effect on the perceptual decision process. The results contradict the idea that temporally structured cues and symbolic cues operate via the same mechanism.