Charles C.-F. Or

The role of luminance contrast in the detection of global structure in static and dynamic, same- and opposite-polarity, Glass patterns

Perception of global structure conveyed in static Glass patterns is difficult, though not impossible, when the constituent dipoles are formed by partnering opposite polarity dots. We investigate whether the addition of motion signals to opposite-polarity Glass patterns can act to restore the perception of global structure. The stimuli were concentric Glass patterns consisting of 200 dipoles concentrically orientated, or oriented at random orientations, placed on a grey background. For each dipole, one luminance-increment dot (Weber contrast of 1) was paired with another dot set to a contrast ranging between luminance increment and luminance decrement (i.e., a Weber contrast range of approximately -1 to 1). Dipoles were either stationary (Experiment 1), or randomly re-positioned at 17Hz (Experiment 2), on each frame transition. A two-interval forced-choice paradigm, in conjunction with an adaptive staircase, was used to obtain Glass-pattern detection thresholds. The task required observers to identify the interval that contained concentric Glass structure; the other interval contained randomly orientated dipoles. Generally, lower global form thresholds were observed for dynamic and same-polarity Glass patterns than for static and opposite-polarity Glass patterns. In particular, for dynamic presentations improvement in sensitivity was more evident for opposite-polarity than for same-polarity Glass patterns. These findings suggest that motion plays an important role in the detection of global structure in dynamic Glass patterns.

Moving Glass Patterns: Asymmetric Interaction between Motion and form

The perceived motion direction of a moving Glass pattern is influenced by the orientation of the dot pairs (dipoles) that generate the pattern (Krekelberg et al, 2003 Nature 424 674–677; Ross, 2004 Vision Research 44 441–448). Here, we investigate how the motion vector and the dipole orientation of moving Glass patterns influence the perceived orientation of each. We employed 1 s movie presentations of sequences of linear Glass patterns, each consisting of 200 dot pairs. Signal pairs, aligned in a common orientation, moved in a common direction. The observers task was to indicate either the perceived direction of motion, or the perceived dipole orientation of Glass patterns that consisted of either same-polarity dipoles, or opposite-polarity dipoles. Perceived orientation or motion direction was measured as a function of the angular difference between the orientation and the motion direction of the dipoles. We found that the apparent global direction of motion was attracted by approximately 4° towards the dipole orientation for small (15°, 23°) angular differences between dipole motion-direction and dipole orientation, regardless of dipole polarity. However, under the same stimulus conditions, the apparent global orientation was much less affected by the direction of motion, suggesting that motion and form interact asymmetrically. Global form influences global motion-direction perception more powerfully than global motion influences global form perception.

Face recognition: Are viewpoint and identity processed after face detection?

Previous research has suggested that an objects category is retrieved as soon as it is detected (Grill-Spector & Kanwisher, 2005). Here we examined whether face views and identities are likewise treated as categories. We measured behavioural performance on three tasks: face detection, recognition of face view within identity, and within-view face identification, by using the method of constant stimuli combined with a two-alternative forced-choice (2AFC) paradigm. Stimulus duration was varied between 13 ms and 133 ms in order to estimate the time required for 75%-correct discrimination in each task. The results showed, respectively, 24- and 31-ms shorter threshold durations for face detection than for viewpoint recognition and face identification, while similar threshold durations for viewpoint recognition and face identification. We demonstrated that face view and identity are retrieved after face detection, and importantly, the view-based categorical analysis takes almost as long as the face identification process. Thus, additional processing is essential for viewpoint and identity extraction as opposed to face detection.

Discrimination and identification of periodic motion trajectories.

Humans are extremely sensitive to radial deformations of static circular contours (F. Wilkinson, H. R. Wilson, & C. Habak, 1998). Here, we investigate detection and identification of periodic motion trajectories defined by these radial frequency (RF) patterns over a range of radial frequencies of 2-5 cycles. We showed that the average detection thresholds for RF trajectories range from 1 to 4 min of arc and performance improves as a power-law function of radial frequency. RF trajectories are also detected for a range of speeds. We also showed that spatiotemporal global processing is involved in trajectory detection, as improvement in detection performance with increasing radial deformation displayed cannot be accounted for by local probability summation. Finally, identification of RF trajectories is possible over this RF range. Overall thresholds are about 6 times higher than previously reported for static stimuli. These novel stimuli should be a useful tool to investigate motion trajectory learning and discrimination in humans and other primates.

Oriented texture detection: Ideal observer modelling and classification image analysis

Perception of visual texture flows contributes to object segmentation, shape perception, and object recognition. To better understand the visual mechanisms underlying texture flow perception, we studied the factors limiting detection of simple forms of texture flows composed of local dot dipoles (Glass patterns) and related stimuli. To provide a benchmark for human performance, we derived an ideal observer for this task. We found that human detection thresholds were 8.0 times higher than ideal. We considered three factors that might account for this performance gap: (1) false matches between dipole dots (correspondence errors), (2) loss of sensitivity with increasing eccentricity, and (3) local orientation bandwidth. To estimate the effect of correspondence errors, we compared detection of Glass patterns with detection of matched line-segment stimuli, where no correspondence uncertainty exists. We found that eliminating correspondence errors reduced human thresholds by a factor of 1.8. We used a novel form of classification image analysis to directly estimate loss of sensitivity with eccentricity and local orientation bandwidth. Incorporating the eccentricity effects into the ideal observer model increased ideal thresholds by a factor of 2.9. Interestingly, estimated orientation bandwidth increased ideal thresholds by only 8%. Taking all three factors into account, human thresholds were only 58% higher than model thresholds. Our findings suggest that correspondence errors and eccentricity losses account for the great majority of the perceptual loss in the visual processing of Glass patterns.

Increment thresholds for radial frequency trajectories produce a dipper function

Radial frequency (RF) trajectories are a new class of stimuli that have been developed to study the visual perception of periodic motion (Or et al., 2011). These stimuli are described by a moving dot that traces a distorted path through space with periodic radial deformations whose frequency, amplitude, and phase can be independently specified. Here, we extend Or et al.s findings by investigating how the discrimination of RF amplitude changes as a function of different reference amplitudes in a two-interval forced choice task. Using an RF3 trajectory (a pattern with three cycles of deformation along its trajectory), increment thresholds were measured at six different reference amplitudes: Detection (discriminating a circle from RF3), 1X (discriminating a pair of RF3 patterns, with the amplitude of one member of this pair set to (1X) threshold obtained from the detection condition), 2.5X, 5X, 10X, and 15X. Data show that sensitivity to changes in amplitude improves at 2.5X by a factor of about 2, recovers to detection threshold levels at 5X, and continues to rise at 10X and 15X. These results generalize across both radial frequency and the angular speed of the trajectory, and persist with low contrast trajectories. Our findings point to the existence of a neural mechanism that is sensitive to deviations from circular motion trajectories.

Detection of periodic motion trajectories: Effects of frequency and radius

Periodic trajectories are an important component of biological motion. Or, Thabet, Wilkinson, and Wilson (2011) studied radial frequency (RF) motion trajectory detection and concluded that, for RF2-5 trajectories, the threshold function paralleled that of static RF patterns. We have extended Or et al.s (2011) findings to a broader range of RFs (three to 24 cycles) and across a 4-fold range of radii (1°-4°). We report that (a) thresholds for RF trajectories decrease as a power function of RF for low RF trajectories (three to six cycles) before approaching an asymptote at high RFs (12-24 cycles); (b) detection thresholds for RF trajectories scale proportionally with radius; and (c) there is no lower versus upper field advantage in the parafoveal field for stimuli displaced from fixation on the vertical midline. The results are compared to earlier findings for static RF thresholds, and we argue that our findings support the existence of parallel spatial and temporal processing channels that may contribute to both action perception and production.