Kinetic depth effect and optic flow -- I. 3D shape from Fourier motion

Fifty-three different 3D shapes were defined by sequences of 2D views (frames) of dots on a rotating 3D surface. (1) Subjects ’ accuracy of shape identifications dropped from over 90 % to less than 10 % when either the polarity of the stimulus dots was alternated from light-on-gray to dark-on-gray on successive frames or when neutral gray interframe intervals were interposed. Roth manipulations interfere with motion extraction by spatio-temporal (Fourier) and gradient first-order detectors. Second-order (non-Fourier) detectors that use full-wave rectification are unaffected by alternating-polarity but disrupted by interposed gray frames. (2) To equate the accuracy of two-alternative forced-choice (ZAFC) planar dir~tion-of-motion ~~ri~nation in standard and zloty-alternated stimuli, standard contrast was reduced. 3D shape discrimination survived contrast reduction in standard stimuli whereas it failed completely with polarity-alternation even at full contrast. (3) When individual dots were permitted to remain in the image sequence for only two frames, performance showed little loss compared to standard displays where individual dots had an expected lifetime of 20 frames, showing that 3D shape identification does not require continuity of stimulus tokens. (4) Performance in all discrimination tasks is predicted (up to a monotone transformation) by considering the quality of first-order information (as given by a simple computation on Fourier power) and the number of locations at which motion information is required. Perceptual first-order analysis of optic flow is the primary substrate for st~cture-from-motion computations in random dot displays because only it offers suBicient quality of perceptual motion at a sufficient number of locations. Kinetic depth effect Structure from motion Shape identification Fourier motio

How to study the kinetic depth effect experimentally

with 2D artifactual cues removed and with full feedback (FB) to the subjects to measure KDE and to circumvent algorithmically equivalent KDE-alternative computations and artifactual non-KDE processing. (1) The 2D velocity flow-field was necessary and sufficient for true KDE. (2) Only the first-order (Fourier-based) perceptual motion system could solve our task because the second-order (rectifying) system could not simultaneously process more than two locations. (3) To ensure first-order motion processing, KDE tasks must require simultaneous processing at more than two locations. (4) Practice with FB is essential to measure ultimate capacity (aptitude) and, thereby, to enable comparisons with ideal observers. Experiments without FB measure ecological achievementmthe ability of subjects to extrapolate their past experience to the current stimuli. Our article (Sperling, Landy, Dosher, & Perkins, 1989, henceforth, the source article) proposed the following: (1) An objective task that involves 53 different shapes to measure shape identification performance in kinetic depth effect (KDE) experiments; (2) an algorithm for the structure-from

Spatial attention excludes external noise at the target location.

To investigate the nature of external noise exclusion, we compared central spatial precuing effects in 16 conditions that varied the amount of external noise, the number of signal stimuli, the number of locations masked by external noise, and the number and style of frames surrounding potential target locations. In the absence of external noise, precuing produced only marginal performance improvements in a small number of display conditions. In the presence of high external noise, precuing improved task performance in all the display conditions. The magnitude of these spatial attention effects, as gauged by contrast threshold reduction, is nearly constant across all the display conditions. This suggests that spatial attention mostly excludes external noise at the target location; the presence of external noise and/or signal stimuli in non-target regions has little effect on spatial performance when location uncertainty is eliminated by report cues. However, the presence of other potential locations for the target is critical, because if target location is known in advance, attention can be focused on that location with or without a cue

Ratings of kinetic depth in multidot displays

Subjects saw kinetic depth displays whose shape (sphere or cylinder) was defined by luminous dots distributed randomly on the surface or in the volume of the object. Subjects rated perceived 3-D depth, rigidity, and coherence. Despite individual differences, all 3 ratings increased with the number of dots. Dots in the volume yielded ratings equal to or greater than surface dots. Each rating varied with 3 of 4 factors (shape, distribution, numerosity, and perspective), but the ratings either between trials or between conditions were often uncorrelated. Object shape affected rigidity but not depth ratings. Veridically perceived polar displays had slightly lower rigidity but higher depth ratings than parallel projection displays. (Reversed polar displays were always grossly nonrigid.) The interaction of ratings and stimulus parameters requires theories and experiments in which different KDE ratings are not treated interchangeably