What Makes an Oddball Odd? Evidence from a Spatially Predictable Temporal Oddball Paradigm

AbstractThe temporal oddball effect (Birngruber, Schröter, and Ulrich, in press; Pariyadath and Eagleman, 2007; Schindel, Rowlands, and Arnold, 2011) describes the finding that rare, deviant stimuli (oddballs) are temporally overestimated as compared to standards of equal physical duration. In a typical oddball paradigm, oddballs are presented at a random position within a stream of repeated standard stimuli. While the standards’ duration is constant, oddball duration varies from trial to trial. After each trial, participants are asked to judge whether the oddball was shorter or longer in duration than the standards. In order to explain the oddball effect, it has been assumed that oddballs as compared to standards either attract more attention (Tse, Intriligator, Rivest, & Cavanagh, 2004) or increase the level of arousal (Ulrich, Nitschke, & Rammsayer, 2006). Nevertheless, it is not entirely clear which aspects make an oddball “odd”.In the present study, we modified the classical paradigm in order to disentangle different factors that could drive the oddball effect. In each trial, a stream of five stimuli was presented, whereby the first four stimuli were always standards (e.g., blue disks) of a constant standard duration (800ms) and the last stimulus was a comparison stimulus of varying duration (480-1120ms; in nine steps). The comparison stimulus could either be another standard (e.g., another blue disk) or an oddball (e.g., a red disk). Participants’ task was to judge whether the last stimulus in the stream was shorter or longer than the four preceding stimuli. In order to make counting of the stimuli unnecessary, we presented the stimuli in a spatially predictable format on the screen, that is, on an imagined clock face. Stimuli appeared in clockwise order at the 12, 3, 6, and 9 o’clock positions (standards), and again at the 12 o’clock position (comparison). In half of all trials, a standard stimulus was presented as comparison whereas in the other half of trials an oddball stimulus was presented as comparison. This allowed for a direct comparison of the subjective duration of oddballs and standards. In two separate blocks of trials, we manipulated how many different oddball colors could appear. In one block of trials, only a single oddball color was used (e.g., red) whereas in the other block of trials, four different oddball colors were used (cyan, green, magenta, and yellow) and presented equally often. Points of subjective equality (PSE) were derived from the individual psychometric functions as measures of perceived duration. Preliminary data show that even though all stimuli evoked PSEs smaller than the objective standard duration, no oddball effect was present in either block. Possible explanations for this result pattern will be discussed