Motion extrapolation into the blind spot : Research report

The flash-lag effect, inwhich amoving object is perceived ahead of a colocalized flash, has led to keen empirical and theoretical debates. To test the proposal that a predictive mechanism overcomes neural delays in vision by shifting objects spatially, we asked observers to judge the final position of a barmoving into the retinal blind spot. The bar was perceived to disappear in positions well inside the unstimulated area. Given that photoreceptors are absent in the blind spot, the perceived shift must be based on the history of themoving object. Such predictive overshoots are suppressed when a moving object disappears abruptly from the retina, triggering retinal transient signals. No such transient-driven suppression occurs when the object disappears by virtue of moving into the blind spot. The extrapolated position of the moving bar revealed in this manner provides converging support for visual prediction. Objects moving across the visual field constantly change their position over time. Neural responses to moving stimuli are delayed, and persist for a significant duration after stimulation ceases. The delay in the neural response should cause a moving object to be seen in a position lagging its physical position, and response persistence should cause the moving object to appear smeared. However, moving objects appear less smeared than expected (Burr, 1980), and the instantaneous perceived position of a moving object is shifted forward in the direction of motion, as illustrated in a class of visual phenomena, most prominently in the flash-lag effect (Nijhawan, 1994). If a brief flash is presented in alignment with a moving object, then the object is seen to be ahead of the flash. Several theories have been brought forward to explain this forward shift (reviewed in Krekelberg & Lappe, 2001; Nijhawan, 2002). Temporal integration theories state that the visual system samples positions of a moving object over an extended period of time and produces an average position, possibly weighted towards more recently sampled positions (Brenner, van Beers, Rotman, & Smeets, 2006; Krekelberg & Lappe, 2000; Roulston, Self, & Zeki, 2006). The postdiction account additionally assumes that a flash resets this integration process and uses mainly positions from after the flash to produce the averaged output (Eagleman & Sejnowski, 2000). More recently, the same authors argued for a slightly different account, proposing that motion signals from after the flash bias the localization of objects towards the direction of motion (Eagleman & Sejnowski, 2007). Alternatively, motion extrapolation posits that the visual system uses motion information from the previous trajectory to predict the moving object’s position, thus compensating for neural processing delays in the visual pathway (Nijhawan, 1994, 2008). One particular observation concerning the flash-lag effect has been used to argue against the visual prediction model: When the moving object disappears (Eagleman & Sejnowski, 2000) or reverses direction (Whitney & Murakami, 1998) at the time of the flash, it does not perceptually overshoot the point of disappearance or reversal. Generally, the final position of abruptly disappearing moving objects is perceived accurately when observers keep steady fixation (Kerzel, 2000). This absence of a predictive overshoot has been a major difficulty for the motion extrapolation account. However, abrupt offsets and direction changes of moving objects elicit retinal transient signals (Schwartz, Taylor, Fisher, Harris, & Berry, 2007), which carry precise positional information. We argue that these signals can suppress the visibility of an extrapolated object representation and thus facilitate accurate localization of the object’s final position despite predictive mechanisms (Maus & Nijhawan, 2006, in press; Nijhawan, 2002, 2008). When a moving object disappears from view without eliciting a retinal transient signal, it should be seen to disappear in an extrapolated position. For example, when a moving object gradually decreases in luminance contrast, it disappears from view in positions beyond its detection threshold (Maus & Nijhawan, 2006). The object is seen in positions where luminance contrast alone is insufficient to produce a percept. Here we present converging evidence for extrapolation in the absence of transient signals by showing Address correspondence to Gerrit Maus, Psychology Department, University of Sussex, Falmer, Brighton BN1 9QH, United Kingdom, e-mail: G.W. Maus@sussex.ac. uk. PSYCHOLOGICAL SCIENCE Volume 19—Number 11 1087 Copyright r 2008 Association for Psychological Science motion extrapolation into the retinal blind spot. This finding cannot be readily explained by a temporal integration mechanism or by a retrospective position bias based on motion signals. Observers judged the last visible position of a bar moving into the blind spot in relation to the last visible position of a second bar that was abruptly switched off. In monocular viewing, one bar, which was ipsilateral to the viewing eye, moved into the blind spot. A comparison bar on the contralateral side of the viewing eye was presented in mirror-image positions of the first bar and was switched off near the mirror image of the blind spot border (see Fig. 1). Observers performed a temporal-order-judgment task, indicating which bar they perceived as disappearing first. Because the two bars occupied exactly mirrored positions, the point of subjective simultaneity for the disappearances also gave the last-seen position of the bar on the blind-spot side. In an additional condition, one bar startedmoving inside the blind spot, while the other bar abruptly startedmoving over intact retina. The task for observers was to indicate which bar they saw first. In contrast to an object disappearing in full view of photoreceptors, an object moving into the blind spot does not elicit a transient retinal off-signal carrying precise position information. Therefore, we predict that if the perceived position of the object is extrapolated during continuous motion, then it should be seen as disappearing in a position shifted forward past the blind-spot boundary into the blind area. Temporal integration and postdiction would predict the object to perceptually disappear at (or slightly before) the blind-spot boundary, because positions cannot be sampled from unstimulated retinal areas.