Misperceptions of Trajectories of Dots Moving through the Blind Spot

The trajectories of objects moving along simple paths can be grossly misperceived by the human visual system (Dodge 1904, 1910; Carr 1907; Ford 1910; Brown and Voth 1937; Mack 1986). In a recent study of dots moving along trajectories that were either linear or bilinear (ie having two linear segments), we reported that a bilinear trajectory in the presence of other linear trajectories is grossly misperceived (Tripathy and Barrett 2003). Pursuit eye movements can account for some misperceptions, as was cleverly demonstrated by Dodge in his `̀ pendular-whiplash illusion''öpursuing a luminous point at one end of a counterbalanced pendulum, oscillating about its centre, causes a luminous point at the other end to be perceived to reverse at a later time than the pursued light (Dodge 1904; Mack 1986). Involuntary movement of the retinal image can also distort straight lines in static images such as in MacKay's well-known `ray' pattern (MacKay 1958). In our earlier study (Tripathy and Barrett 2003) we could not rule out the contribution of eye movements in the perceived distortions of straightline trajectories. In the present study, we use a dot that had a size comparable to the physiological blind spot within each observer's projection of the blind spot in the right eye (eg Horton 1992; Ramachandran 1992) to ensure that he/she does not make large eye movements or pursue the trajectories (Tripathy and Levi 1994; Tripathy et al 1995). We find that observers can grossly misperceive simple bilinear trajectories even in the absence of pursuit eye movements and when the actual shape of the trajectory is known in advance. The experiment was conducted in a room that was normally lit, with two fluorescent ceiling lights. The ambient lighting ensured that the trajectory traces resulting from screen persistence were not visible. Chin and forehead rests were used to stabilise the observer's head and eye movements. The two authors and a graduate student acted as observers. The stimuli were presented on a Gateway Vivitron monitor, at a frame rate of 60 Hz (16.67 ms per frame). At the viewing distance of 64.5 cm, each pixel subtended a visual angle of 2 min of arc in the horizontal and vertical directions. The stimuli (shown schematically in figure 1) consisted of a small number of dots moving from left to right along straight-line trajectories. Each dot had dimensions of 20 min of arc620 min of arc, with a luminance of 69.9 cd mÿ2 on a background luminance of 5.3 cd mÿ2. Two vertical markers indicated the horizontal midline of the screen. All dots reached this midline on the same frame. On this frame, if there was more than one dot, the centre-to-centre separation between any adjacent dots was 70 ^ 90 min of arc, with the mean separation (averaged across trials) being 80 min of arc. The target dot, on reaching the midline of the screen, deviated by 328, either clockwise or counterclockwise; any other dots present, the distractors, continued without deviation, along straight lines. All angles representing orientations and deviations of trajectories were measured with reference to the horizontal, in the plane of the monitor; counterclockwise deviations of trajectories are considered positive. The dots were presented for exactly 37 frames and moved at velocities of 24 deg sÿ1 or 32 deg sÿ1, yielding trajectory lengths of 14.4 deg or 19.2 deg, respectively. The high velocities used ensured continuity of motion across the blind spot. On any trial, the distractor trajectories had orientations randomly varying between 778 and 138, with a mean orientation (averaged across trials) of 458 with respect to the horizontal. The target trajectory had a fixed orientation of 558 Last but not least Perception, 2006, volume 35, pages 137 ^ 142