Abstract
The claim that temporal aspects of perception are sensitive to top-down influences is supported by several pieces of evidence regarding perception of temporal order. This evidence includes modulations of temporal displacements by attention (the prior entry effect; Titchener, 1908) and by schemata obtained from experience (Pechenkova, 2008; Caldwell-Harris & Morris, 2008). Would there be similar evidence in perception of simultaneity and succession? To answer this question, we tested potential effects of two methods of top-down stimuli grouping.
In the first experiment, left and right halves of 6-letter strings (Russian nouns and nonword anagrams of the same words) were presented for 25 ms each with SOAs from −59 to +59 ms or simultaneously. Participants performed a 2AFC task, indicating whether they saw two parts of a string as simultaneous or successive. 75% threshold for perceived simultaneity and difference thresholds were measured by method of constant stimuli. The results showed that when letters form a word, subjects tend to report ‘simultaneity’ on larger SOAs than for nonwords. Comparison of difference thresholds shows that this effect is not just a response bias.
The second experiment tested whether biasing the perceptual organization of a bistable figure (Rubin vase) would affect the perception of simultaneity or succession of its component parts. Bias to see either a central vase or two faces was created within two separate blocks of trials by both verbal instruction and introducing real vases or faces as filler images. The presentation timing and task were identical to the first experiment. There was a slight tendency for more ‘simultaneity’ reports in face than vase condition.
Results from both experiments indicate that top-down modulations can change perception of simultaneity vs. succession. However, different materials and ways of grouping produce controversial effects. Possible explanations of these effects and reasons of the discrepancy will be discussed.