The perceived duration of gratings

Perceived time is spatial frequency dependent

We investigated whether changes in low-level image characteristics, in this case spatial frequency, were capable of generating a well-known expansion in the perceived duration of an infrequent “oddball” stimulus relative to a repeatedly-presented “standard” stimulus. Our standard and oddball stimuli were Gabor patches that differed from each other in spatial frequency by two octaves. All stimuli were equated for visibility. Rather than the expected “subjective time expansion” found in previous studies, we obtained an equal and opposite expansion or contraction of perceived time dependent upon the spatial frequency relationship of the standard and oddball stimulus. Subsequent experiments using equi-visible stimuli reveal that mid-range spatial frequencies (ca. 2 c/deg) are consistently perceived as having longer durations than low (0.5 c/deg) or high (8 c/deg) spatial frequencies, despite having the same physical duration. Rather than forming a fixed proportion of baseline duration, this bias is constant in additive terms and implicates systematic variations in visual persistence across spatial frequency. Our results have implications for the widely cited finding that auditory stimuli are judged to be longer in duration than visual stimuli.

Attention delays perceived stimulus offset

Yeshurun and Levy (2003) [Transient spatial attention degrades temporal resolution. Psychological Science, 14, 225 -231.] have suggested that visual attention enhances the activation of the parvocellular system and thus delays the perceived offset of a stimulus. We tested this assumption in a spatial cueing paradigm in which participants responded to stimulus offset. Consistent with this assumption, offset reaction time (RT) was prolonged for attended compared to unattended stimuli. For onset RT, however, we confirmed the well-known spatial cueing effect that attention speeds up the detection of stimulus onset. The results provide direct evidence for the proposal made by Yeshurun and Levy.

Response latencies to the onset and offset of visual stimuli

Simple response times (RTs) are known to be slower to the offset of a visual stimulus than to its onset. This is called the onset advantage. In the first of four experiments, we discovered that a spurious onset advantage can be produced by the long persistence of P31 phosphor. In the remaining three experiments, we found that offset RTs were slower only when they were made in a context in which responses to the abrupt onset of some stimuli had to be suppressed. We tested this hypothesis of response suppression in two ways: (1) by mixing regular onset trials with other trials on which a response to an onset had to be suppressed, and (2) by ramping the emergence of "offset" stimuli over time, so that offsets were the only abrupt events in the display. In both cases, we found that the onset advantage depended critically on whether the responses were made in a context of response suppression. We conclude that the onset advantage is mediated not by sensory factors such as visible persistence, but by response programming factors that are strongly affected by contextual events.