The discrimination of heading from optic flow is not retinally invariant.

Three experiments were conducted to determine whether the discrimination of heading from optic flow is retinally invariant and to determine the importance of acuity in accounting for heading eccentricity effects. In the first experiment, observers were presented with radial flow fields simulating forward translation through a three-dimensional volume of dots. The flow fields subtended 10 degrees of visual angle and were presented at 0 degree, 10 degrees, 20 degrees, and 40 degrees of retinal eccentricity. The observers were asked to indicate whether the simulated movement was to the right or the left of a target that appeared at the end of the display sequence. Eye movements were monitored with an electrooculogram apparatus. In a second experiment, static acuity thresholds were derived for each of the observers at the same retinal eccentricities. There was a significant increase in heading detection thresholds with retinal eccentricity (from 0.92 degree at 0 degree retinal eccentricity to 3.47 degrees at 40 degrees). An analysis of covariance indicated that the variation in sensitivity to radial flow, as a function of retinal eccentricity, is independent of acuity. Similar results were obtained when the Vernier acuity of observers was measured. These results suggest that the discrimination of heading from radial flow is not retinally invariant.     

Depth discrimination from optic flow

A simple scheme for deriving relative depth (time-to-collision, or TTC) from optic flow is developed in which the total flow is first sensed by unconnected motion (imperfect filter) sensors and then the rotational component is subtracted to yield the translational component. Only the latter component yields depth information. This scheme is contrasted with one where the TTC sensors respond only to the translational component at the initial registration of the flow (perfect filter sensors or looming detectors). The simple scheme predicts the results of three experiments on discrimination of TTC: discrimination thresholds are elevated if the objects withdraw from rather than approach the observer, thresholds are elevated if a rotational component is added to the flow, and the amount of threshold elevation resulting from the addition of a rotational component is reduced by prior adaptation to a pure rotational flow. These results confirm the simple model and disconfirm predictions based on the looming detector scheme.     

Perception of heading without retinal optic flow

How do we determine where we are heading during visually controlled locomotion? Psychophysical research has shown that humans are quite good at judging their travel direction, or heading, from retinal optic flow. Here we show that retinal optic flow is sufficient, but not necessary, for determining heading. By using a purely cyclopean stimulus (random dot cinematogram), we demonstrate heading perception without retinal optic flow. We also show that heading judgments are equally accurate for the cyclopean stimulus and a conventional optic flow stimulus, when the two are matched for motion visibility. The human visual system thus demonstrates flexible, robust use of available visual cues for perceiving heading direction.     

Improvement in line orientation discrimination is retinally local but dependent on cognitive set

The ability of human observers to discriminate the orientation of a pair of straight lines differing by 3° improved with practice. The improvement did not transfer across hemifield or across quadrants within the same hemifield. The practice effect occurred whether or not observers were given feedback. However, orientation discrimination did not improve when observers attended to brightness rather than orientation of the lines. This suggests that cognitive set affects tuning in retinally local orientation channels (perhaps by guiding some form of unsupervised learning mechanism) and that retinotopic feature extraction may not be wholly preattentive.     

Improvement in line orientation discrimination is retinally local but dependent on cognitive set.

The ability of human observers to discriminate the orientation of a pair of straight lines differing by 3 degrees improved with practice. The improvement did not transfer across hemifield or across quadrants within the same hemifield. The practice effect occurred whether or not observers were given feedback. However, orientation discrimination did not improve when observers attended to brightness rather than orientation of the lines. This suggests that cognitive set affects tuning in retinally local orientation channels (perhaps by guiding some form of unsupervised learning mechanism) and that retinotopic feature extraction may not be wholly preattentive.     

Perceiving heading with different retinal regions and types of optic flow

We examined the ability to use optic flow to judge heading when different parts of the retina are stimulated and when the specified heading is in different directions relative to the display. To do so, we manipulated retinal eccentricity (the angle between the fovea and the center of the stimulus) and heading eccentricity (the angle between the specified heading and the center of the stimulus) independently. Observers viewed two sequences of moving dots that simulated translation through a random cloud of dots. They reported whether the direction of translation—the heading—in the second sequence was to the left or right of the direction in the first sequence. The results revealed a large and consistent effect of heading eccentricity: Judgments were much more accurate with radial flow fields (small heading eccentricities) than with lamellar fields (large heading eccentricities), regardless of the part of the retina being stimulated. The results also revealeda smaller and less consistent effect of retinal eccentricity: With radial flow (small heading eccentricities), judgments were more accurate when the stimulus was presented near the fovea. The variation of heading thresholds from radial to lamellar flow fields is predicted by a simple model of two-dimensional motion discrimination. The fact that the predictions are accurate implies that the human visual system is equally efficient at processing radial and lamellar flow fields. In addition, efficiency is reasonably constant no matter what part of the retina is being stimulated.     

Stability in young infants' discrimination of optic flow

Although considerable progress has been made in understanding how adults perceive their direction of self-motion, or heading, from optic flow, little is known about how these perceptual processes develop in infants. In 3 experiments, the authors explored how well 3- to 6-month-old infants could discriminate between optic flow patterns that simulated changes in heading direction. The results suggest that (a) prior to the onset of locomotion, the majority of infants discriminate between optic flow displays that simulate only large (> 22 deg.) changes in heading, (b) there is minimal development in sensitivity between 3 and 6 months, and (c) optic flow alone is sufficient for infants to discriminate heading. These data suggest that spatial abilities associated with the dorsal visual stream undergo prolonged postnatal development and may depend on locomotor experience.     

A neural model of how the brain computes heading from optic flow in realistic scenes

Visually-based navigation is a key competence during spatial cognition. Animals avoid obstacles and approach goals in novel cluttered environments using optic flow to compute heading with respect to the environment. Most navigation models try either explain data, or to demonstrate navigational competence in real-world environments without regard to behavioral and neural substrates. The current article develops a model that does both. The ViSTARS neural model describes interactions among neurons in the primate magnocellular pathway, including V1, MT⁺, and MSTd. Model outputs are quantitatively similar to human heading data in response to complex natural scenes. The model estimates heading to within 1.5° in random dot or photo-realistically rendered scenes, and within 3° in video streams from driving in real-world environments. Simulated rotations of less than 1°/s do not affect heading estimates, but faster simulated rotation rates do, as in humans. The model is part of a larger navigational system that identifies and tracks objects while navigating in cluttered environments.     

Perception of translational heading from optical flow

Radial patterns of optical flow produced by observer translation could be used to perceive the direction of self-movement during locomotion, and a number of formal analyses of such patterns have recently appeared. However, there is comparatively little empirical research on the perception of heading from optical flow, and what data there are indicate surprisingly poor performance, with heading errors on the order of 5 degrees-10 degrees. We examined heading judgments during translation parallel, perpendicular, and at oblique angles to a random-dot plane, varying observer speed and dot density. Using a discrimination task, we found that heading accuracy improved by an order of magnitude, with 75%-correct thresholds of 0.66 degrees in the highest speed and density condition and 1.2 degrees generally. Performance remained high with displays of 63-10 dots, but it dropped significantly with only 2 dots; there was no consistent speed effect and no effect of angle of approach to the surface. The results are inconsistent with theories based on the local focus of outflow, local motion parallax, multiple fixations, differential motion parallax, and the local maximum of divergence. But they are consistent with Gibson's (1950) original global radial outflow hypothesis for perception of heading during translation.     

Perception of circular heading from optical flow

Observers viewed random-dot optical flow displays that simulated self-motion on a circular path and judged whether they would pass to the right or left of a target at 16 m. Two dots in two frames are theoretically sufficient to specify circular heading if the orientation of the rotation axis is known. Heading accuracies were better than 1.5 degrees with a ground surface, wall surface, and 3D cloud of dots, and were constant over densities down to 2 dots, consistent with the theory. However, there was an inverse relation between the radius of the observer's path and constant heading error, such that at small radii observers reported heading 3 degrees to the outside of the actual path with the ground and to the inside with the wall and cloud. This may be an artifact of a small display screen.     

Enhanced optic flow speed discrimination while walking: contextual tuning of visual coding

We tested the hypothesis that long-term adaptation to the normal contingencies between walking and its multisensory consequences (including optic flow) leads to enhanced discrimination of appropriate visual speeds during self-motion. In experiments 1 (task 1) and 2 a two-interval forced-choice procedure was used to compare the perceived speed of a simulated visual flow field viewed while walking with the perceived speed of a flow field viewed while standing. Both experiments demonstrated subtractive reductions in apparent speed. In experiments 1 and 3 discrimination thresholds were measured for optic flow speed while walking and while standing. Consistent with the optimal-coding hypothesis, speed discrimination for visual speeds near walking speed was enhanced during walking. Reduced sensitivity was found for slower visual speeds. The multisensory context of walking alters the coding of optic flow in a way that enhances speed discrimination in the expected range of flow speeds.     

Perceiving surface slant from deformation of optic flow

Perceived surface orientation and angular velocity were investigated for orthographic projections of 3-D rotating random-dot planes. It was found that (a) tilt was accurately perceived and (b) slant and angular velocity were systematically misperceived. It was hypothesized that these misperceptions are the product of a heuristic analysis based on the deformation, one of the differential invariants of the first-order optic flow. According to this heuristic, surface attitude and angular velocity are recovered by determining the magnitudes of these parameters that most likely produce the deformation of the velocity field, under the assumption that all slant and angular velocity magnitudes have the same a priori probability. The results of the present investigation support this hypothesis. Residual orientation anisotropies not accounted for by the proposed heuristic were also found.     

Detection of three-dimensional surfaces from optic flow: The effects of noise

Previous research (Andersen, 1989) has suggested that the recovery of 3-D shape from nonsmooth optic flow (motion transparency) can be performed by segregating surfaces according to the distributions of velocities present in the flow field. Five experiments were conducted to examine this hypothesis in a surface detection paradigm and to determine the limitations of human observers to detect 3-D surfaces in the presence of noise. Two display types were examined: a flow field that simulateda surface corrugated in depth and a flow field-that simulated a random volume. In addition, two types of noise were examined: a distribution of noise velocities that overlapped or did not overlap the velocity distribution that defined the -surface. Corrugation frequency and surface density were also examined. Detection performance increased with decreasing corrugation frequency, decreasing noise density, and decreasing surface density. Overall, the subjects demonstrated remarkable tolerance to the presence of noise and, for some conditions, could discriminate surface from random conditions when noise density was twice the-surface density. Discrimination accuracy was greater for the nonoverlapping than for the overlapping noise, providing support for an analysis based on the distribution of velocities.     

The optic trajectory is not a lot of use if you want to catch the ball

According to linear optic trajectory (LOT) theory, fielders use the direction of curvature of theoptic trajectory to control the way they run to intercept the ball. Data presented by D. M. Shaffer and M. K. McBeath (2002) as support for LOT theory show that the optic trajectory of balls that will fall behind the fielder provide the cue that LOT theory predicts would send the fielder running forward, not backward. In this article, the authors show that watching these balls would provide the fielder with the cue that the optic acceleration cancellation (OAC) theory of interception predicts would send the fielder running backward. It appears that the fielders studied by Shaffer and McBeath were following the cue predicted by OAC theory, not that predicted by LOT theory.     

The development of distance estimation in optic flow

In three experiments we tested the ability of children aged 8 to 12 years and adults to locate a target in an optic texture flow projected onto the ground. During the exposure phase, a static target (diode) was lit up at 6 m or 8 m on the ground in front of the subject. During the pointing phase, the subject was asked to indicate the perceived location of the target with a laser pointer as soon as the target was switched off. In the first experiment, during both phases the optic texture (environment) was either motionless or approaching the subject. Results showed that target locations were significantly more underestimated within the moving texture than within the still texture. In the second experiment, a detailed error analysis showed that the differences of performance between children and adults were not due to differences in eye height. Errors can be described by a linear fit with the retinal speed of the optic flow surrounding the targets. Distance judgments improved from the age of 8 years onwards. In the last experiment we found the same kind of results with a receding texture and without stimulation in central vision. Results are discussed in terms of subject's capacity to compensate for the effect of linear vection produced by the optic flow.     

Judgments of path, not heading, guide locomotion

To steer a course through the world, people are almost entirely dependent on visual information, of which a key component is optic flow. In many models of locomotion, heading is described as the fundamental control variable; however, it has also been shown that fixating points along or near one's future path could be the basis of an efficient control solution. Here, the authors aim to establish how well observers can pinpoint instantaneous heading and path, by measuring their accuracy when looking at these features while traveling along straight and curved paths. The results showed that observers could identify both heading and path accurately (approximately 3 degrees ) when traveling along straight paths, but on curved paths they were more accurate at identifying a point on their future path (approximately 5 degrees ) than indicating their instantaneous heading (approximately 13 degrees ). Furthermore, whereas participants could track changes in the tightness of their path, they were unable to accurately track the rate of change of heading. In light of these results, the authors suggest it is unlikely that heading is primarily used by the visual system to support active steering.     

Spatial orientation from optic flow in the central visual field

Previous research has shown that stimulation of the central visual field with radial flow patterns (produced by forward motion) can induce perceived self-motion, but has failed to demonstrate effects on postural stability of either radial flow patterns or lamellar flow patterns (produced by horizontal translation) in the central visual field. The present study examined the effects of lamellar and radial flow on postural stability when stimulation was restricted to the central visual field. Displays simulating observer motion through a volume of randomly positioned points were observed binocularly through a window that limited the field of view to 15 degrees. The velocity of each display varied according to the sum of four sine functions of prime frequencies. Changes in posture were used to measure changes in perceived spatial orientation. A frequency analysis of postural sway indicated that increased sway occurred at the frequencies of motion simulated in the display for both lamellar and radial flow. These results suggest that both radial and lamellar optic flow are effective for determining spatial orientation when stimulation is limited to the central visual field.     

Smoothness of the velocity field and three-dimensional surface detection from optic flow

Five experiments were conducted to determine the importance of smoothness of the velocity field in detecting 3-D surfaces from optic flow. Subjects were presented with optic flow displays simulating either points positioned on a corrugated 3-D surface or points randomly positioned within a 3-D volume. The subject’s task was to indicate whether or not the display appeared to be a 3-D surface. Smoothness of the velocity field was examined by systematically varying the speed of individual velocities in the flow field according to a Gaussian distribution withM = 0 andSD = σ. Variations in frequency, amplitude, density, and surface complexity were also examined. Detection of the corrugated surfaces systematically declined with an increase in σ. An increase in frequency of the corrugation for simple (single-frequency corrugation) surfaces resulted in a decrease in surface detection accuracy. Accuracy increased with an increase in density and amplitude for both simple and complex (multiplefrequency corrugation) surfaces. An analysis of the deformation of the displays predicted performance on the basis of human observers, providing further support for the importance of deformation for 3-D surface detection.     

Detection and discrimination of optical flow components

Observers viewed animation sequences consisting of random dots, some of which moved coherently in a given direction (signal dots) and the rest of which moved randomly (noise dots). Using forced-choice procedures, detectability of weak signals within noise was measured for translation, rotation, and expansion/contraction. Sensitivity to all motion types was approximately equal, with practiced observers reliably detecting coherent motion at signal levels as low as 4%. Observers were able to identify the motion structure presented on a given trial at signal levels corresponding to the detection threshold, implying that the neural signals supporting detection are labeled for motion type. Results are discussed in the context of hierarchical analysis of optic flow in which all motion types are registered as patterns of activity among neurons comprising a single mechanism.     

Path integration from optic flow and body senses in a homing task

We examined the roles of information from optic flow and body senses (eg vestibular and proprioceptive information) for path integration, using a triangle completion task in a virtual environment. In two experiments, the contribution of optic flow was isolated by using a joystick control. Five circular arenas were used for testing: (B) both floor and wall texture; (F) floor texture only, reducing information for rotation; (W) wall texture only, reducing information for translation; (N) a no texture control condition, and (P) an array of posts. The results indicate that humans can use optic flow for path integration and are differentially influenced by rotational and translational flow. In a third experiment, participants actively walked in arenas B, F, and N, so body senses were also available. Performance shifted from a pattern of underturning to overturning and exhibited decreased variability, similar responses with and without optic flow, and no attrition. The results indicate that path integration can be performed by integrating optic flow, but when information from body senses is available it appears to dominate.