Improvements of perceptual performance following the presentation of peripheral cues have been ascribed to accelerated accrual of information, enhanced contrast perception, and decision bias. We investigated effects of peripheral cues on the perception of Gabor and letter stimuli. Non-predictive, peripheral cues improved perceptual accuracy when the stimuli were masked. In contrast, peripheral cues degraded perception of low-contrast letters and did not affect the perception of low-contrast Gabors. The results suggest that involuntary attention accelerates accrual of information but are not entirely consistent with the idea that involuntary attention enhances subjective contrast. Rather, peripheral cues may cause crowding with single letter targets of low contrast. Further, we investigated the effect of the amount of uncertainty on involuntary attention. Cueing effects were (initially) larger when there were more possible target locations. In addition, cueing effects were larger when error feedback was absent and observers had no knowledge of results. Despite these strategic factors, location uncertainty was not sufficient to produce cueing effects, showing that location uncertainty paired with non-predictive cues reveals perceptual and not (only) decisional processes.

External transients, such as a flash or a startling sound, are believed to capture attention. Bauer, Cheadle, Parton, Müller, and Usher reported that attention can also be captured by a stimulus that flickers subliminally at 50 Hz, presumably by entrainment of neurons to the flicker frequency. In their reaction time (RT) task, participants had to locate a subtle change in the spatial frequency content of one of three Gabors (the target). Prior to target onset, presumably subliminal 50-Hz flicker in one of the Gabors served as a spatial cue. Bauer et al. found faster RTs when the cued location was congruent with the target location than when the cue was incongruent with the target location. In their experiments, the cue stopped to flicker at 50 Hz at target onset and was replaced by a stimulus flickering at 100 Hz (i.e., the screen refresh rate). In the present study, we show that the transition from 50 to 100 Hz results in a flash-like impression that can be localized above chance. We suggest that the illusory transition flash interfered with the localization of the subtle target, which contributed to the congruency effect. In support of this view, participants selected the flickering object more often than the non-flickering object when they failed to respond to the target. Further, no cueing effects were observed when the cue continued to flicker until the end of the trial or when the target was a salient change in polarity. In our view, the cueing effect occurs because observers confuse the illusory transition flash with the target when the two are similar. When truly subliminal flicker is used (70-Hz flicker), very small cueing effects persist in the absence of an illusory transition flash but may be accounted for by small effects on reaction time unrelated to attention.

In a previous study (M. Carrasco, S. Ling, & S. Read, 2004), observers selected one of two Gabors that appeared to have higher contrast (comparative judgment). A peripheral cue preceded one of the Gabors by 120 ms. Results showed that the cue increased the perceived contrast of the adjacent Gabor. We replicated the experiment and found correlations between the precision of judgments and perceptual cueing effects. Larger cueing effects occurred in conditions with less precise judgments and in observers who saw less difference between the stimuli. Further, we asked observers to judge whether the two Gabors were equal or different (equality judgment). This method avoids decision biases but turned out to be less efficient. Cueing effects were absent for equality judgments and trained observers but present for untrained observers with poor ability to discriminate the stimuli. In another experiment, we showed that when observers' responses became more rapid over the course of the experiment, a cueing effect in brightness perception emerged that is unlikely to reflect perceptual changes. Overall, the results show that large effects of peripheral cues on appearance correlate with poor ability to discriminate the stimuli. We suggest that the cue biased observers' decision but did not affect their perception.

Among other theories, visible persistence has been suggested to explain the flash-lag effect (FLE). According to this account, the flash is not compared to the moving object at its perceived onset, but at a later time while it is still subjectively visible. Therefore, it is reported to lag the moving object. We show that observers' perceptual set determines whether the persisting image of the flash or its onset is used to judge relative position. Spontaneously, observers use the persisting image, such that a flash-lag results. When forced to focus on the onset of the flash because flashes and stationary onset-only stimuli are mixed, observers base their judgments on the onset and the FLE is abolished. We found that perceptual set affected the FLE in the flash-initiated and in the continuous-motion paradigm. Finally, we showed that the position of the moving object was perceived without any blur and that the flash persisted subjectively for at least 60-80ms. Changes of perceptual set and visible persistence may underlie many of the previously reported modulations of the FLE.

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