Co-operative interactions between first- and second-order mechanisms in the processing of structure from motion

Structure from motion (SFM) is the ability to perceive three-dimensional structure from stimuli containing only two-dimensional motion signals and this ability seems to be a result of high-level cortical processes. It has long been thought that local motion signals defined by second-order cues only weakly contribute to perception of SFM since performance on purely second-order SFM tasks is poor, relative to first-order stimuli. We hypothesized that the mechanisms responsible for deriving SFM were insensitive to low-level stimulus attributes such as the first- or second-order nature of the dots composing the stimulus, in other words: that they were "cue-invariant", but that large differences in sensitivity to local first- and second-order motions were responsible for previous findings. By manipulating the relative strength of first-order dots in an SFM stimulus that combines first- and second-order dots, we show that the two types of motion can separately support SFM and co-operatively interact to produce vivid three-dimensional percepts. This provides strong support that the mechanisms underlying SFM are cue-invariant.     

Interocular transfer of the motion aftereffect in strabismus

The interocular transfer of the motion aftereffect (MAE) was measured in three groups of strabismic subjects (six with monofixation, nine with alternation, four with anomalous retinal correspondence) and compared with a group of four normal subjects. The duration of the MAE was measured using sinusoidal gratings of 0.5 c/deg subtending a visual angle of 8. Contrary to previous findings, a substantial amount of MAE transfer was found in some stereoblind individuals with strabismus. The mean amount of transfer for each group was found to correlate with binocularity; the order of decreasing transfer being: normal, monofixation, alternation, and anomalous retinal correspondence (ARC). This reduction in transfer which accompanied the loss of binocularity was asymmetric, that is. right-left transfer did not equal left-right transfer. The amount of asymmetry was found to correlate inversely with the amount of transfer. The direction in which the greatest transfer occurred did not correlate with visual acuity. These data substantiate the overall poor binocularity in subjects with ARC. and the relatively good binocularity of subjects with monofixation. Furthermore, the substantial amount of transfer found in subjects with alternating strabismus may be a measure of potentially achievable binocularity.     

Three-dimensional motion aftereffects reveal distinct direction-selective mechanisms for binocular processing of motion through depth

Motion aftereffects are historically considered evidence for neuronal populations tuned to specific directions of motion. Despite a wealth of motion aftereffect studies investigating 2D (frontoparallel) motion mechanisms, there is a remarkable dearth of psychophysical evidence for neuronal populations selective for the direction of motion through depth (i.e., tuned to 3D motion). We compared the effects of prolonged viewing of unidirectional motion under dichoptic and monocular conditions and found large 3D motion aftereffects that could not be explained by simple inheritance of 2D monocular aftereffects. These results (1) demonstrate the existence of neurons tuned to 3D motion as distinct from monocular 2D mechanisms, (2) show that distinct 3D direction selectivity arises from both interocular velocity differences and changing disparities over time, and (3) provide a straightforward psychophysical tool for further probing 3D motion mechanisms.     

Contrast-reversing global-motion stimuli reveal local interactions between first- and second-order motion signals

Motion perception appears to be mediated by, at least, two systems: a first-order and a second-order system. To investigate the degree of interaction between these systems, we used a contrast-reversing global-motion stimulus in which the signal dots reverse their contrast polarity as they move. In response to such a stimulus, fullwave-rectifying second-order units would signal motion in the displacement direction and first-order units would signal motion in the opposite direction (reverse-phi motion). If these signals were of equal strength, then any inhibitory interaction between them would lead to motion nulling. Such a situation would account for the failure to perceive coherent motion with such a stimulus in a previous study [Vis. Res. 34 (1994) 2849]. In order to test for this possibility we manipulated the stimulus in order to reduce the strength of the second-order response relative to the first-order response. This was achieved by: decreasing dot contrast; increasing stimulus eccentricity; and increasing dot speed. These manipulations resulted in an increase in the perception of (first-order mediated) reverse-phi motion. We conclude that interaction between first- and second-order motion signals occur at the local-motion-pooling level.     

Contributions of the opponent mechanisms to brightness and nonlinear models

Additivity of heterochromatic brightness matching was investigated between unique-green and red (660 nm), and between unique-blue and unique-yellow with 2-deg bipartite field composed of a 100 td white reference field and a bichromatic mixture field. Ten subjects participated in the measurements and the results showed wide varieties among subjects. Previous models for brightness perception based on the contributions of red-green and yellow-blue opponent channels can explain some properties of the results, but they fail to explain other aspects. To overcome the difficulties, a new model is proposed for the brightness perception at photopic levels.     

Interocular transfer of the motion after-effect is not reduced by binocular rivalry

Motion after-effects (MAEs) were measured intraocularly (adaptation, test stimuli to same eye) and interocularly (adaptation, test stimuli to opposite eyes) when (a) a rival stimulus caused perceptual suppression of the adaptation stimulus; (b) no rival stimulus was presented for the entire adaptation duration; and (c) non-rival adaptation was limited to the duration and adaptation stimulus was dominant in (a). Intraocular MAEs were greater than interocular MAEs; furthermore, both intraocular and interocular MAEs were similar following conditions (a) and (b) and reduced following (c). This pattern occurred with gratings of 1, 2 and 4c/deg, but not 8c/deg. Data are discussed in terms of mechanisms of rivalry and MAEs.     

Large shifts in perceived motion direction reveal multiple global motion solutions

Moving objects are thought to be decomposed into one-dimensional motion components by early cortical visual processing. Two rules describing how these components might be re-combined to produce coherent object motion are the intersection of constraints and the vector average rules. Using stimuli for which these combination rules predict different directional solutions, we found that adapting one of the solutions through motion adaptation switched perceived direction to the other solution. The effects were symmetrical: shifts from IOC to VA, and from VA to IOC, were observed following adaptation. These large shifts indicate that multiple solutions to global motion processing coexist and compete to determine perceived motion direction.     

Color opponent slow potential interactions in the frontal organ of the frog:Rana pipiens

Slow potentials recorded from the frog (Rana pipiens) Stirnorgan in response to a step stimulus of dim violet light, can increase linearly for at least a minute. Such a long integration time is remarkable. Either a violet or a green conditioning stimulus will cause a reduction in the response to a subsequent violet test stimulus, even with separation times of up to 2 min. The decay time constant of this effect is about 35 sec, twice as long as the decay of the response to the violet conditioning flash itself. Such interaction can be accounted for by a proposed model in which transmitters block post synaptic sites as well as directly inactivate opponent transmitter. In the absence of conditioning stimuli, the stimulus-response curve for violet stimuli is linear with log intensity over the two log units recordable. Possibly, the violet-green interaction in the frog Stirnorgan permits time of day (or season) estimation by determining the angle of the sun.     

Intact "biological motion" and "structure from motion" perception in a patient with impaired motion mechanisms: a case study

A series of psychophysical tests examining early and later aspects of image-motion processing were conducted in a patient with bilateral lesions involving the posterior visual pathways, affecting the lateral parietal-temporal-occipital cortex and the underlying white matter (as shown by magnetic resonance imaging studies and confirmed by neuro-ophthalmological and neuropsychological examinations). Visual acuity, form discrimination, color, and contrast-sensitivity discrimination were normal whereas spatial localization, line bisection, depth, and binocular stereopsis were severely impaired. Performance on early motion tasks was very poor. These include seeing coherent motion in random noise (Newsome & Paré, 1988), speed discrimination, and seeing two-dimensional form from relative speed of motion. However, on higher-order motion tasks the patient was able to identify actions from the evolving pattern of dots placed at the joints of a human actor (Johansson, 1973) as well as discriminating three-dimensional structure of a cylinder from motion in a dynamic random-dot field. The pattern of these results is at odds with the hypothesis that precise metrical comparison of early motion measurements is necessary for higher-order "structure from motion" tasks.     

Interocular interactions during acuity measurement in children and adults, and in adults with amblyopia

The binocular interactions that occur during dichoptic and binocular viewing were investigated using a letter acuity task in normally sighted children (age range 6-14 years) and adults, and in adults with anisometropic amblyopia. Our aims were to investigate the nature of binocular interactions that occur in each group, and the extent to which the characteristics of binocular interactions differ across the groups. The non-tested eye was occluded during monocular (baseline) viewing, and was allowed to view a uniform stimulus with fusion lock in dichoptic viewing. In adults and children with normal vision, acuity under dichoptic viewing was unchanged relative to monocular baseline in the dominant eyes, while acuity of the non-dominant eye improved under dichoptic viewing relative to baseline. The magnitude of dichoptic change in the non-dominant eyes was similar in the two normally sighted groups, but the dichoptic advantage was found to decrease with increasing age within the children tested. Binocular acuity was better than monocular acuity in normal subjects, and a decrease in binocular summation with age was noted within the age range of the children tested. In contrast, the amblyopic observers showed no change in acuity with viewing conditions. The results demonstrate development of interocular interactions during childhood, and wide inter-individual variation in pattern of interocular interactions among anisometropic amblyopic adults.     

Assessing Interocular Symmetry of the Foveal Cone Mosaic

PurposeTo test the hypothesis that foveal cone topography is symmetrical between contralateral eyes.MethodsWe used adaptive optics scanning light ophthalmoscopy to acquire images of the foveal cone mosaic in each eye of 58 subjects with normal vision (35 female, 23 male). Cones were semiautomatically identified over a 300 × 300-µm foveal area. From these cone coordinates, maps of cone density were derived, and we extracted estimates of peak cone density from each map. Mosaic regularity was assessed using Voronoi cell area regularity (VCAR). Average roundness and average area of the 70%, 75%, 80%, 85%, and 90% of peak density isodensity contours were evaluated.ResultsThe average peak cone density for right eyes was 180,286 cones/mm² (n = 49) and for left eyes was 182,397 cones/mm² (n = 45), with a mean absolute difference of 6363 cones/mm² (n = 43). Peak density, cone spacing, VCAR, and average area within the isodensity contours of fellow eyes were not significantly different (P = 0.60, P = 0.83, P = 0.30, and P = 0.39, respectively). However, the average roundness of the isodensity contours was 2% more circular in the right eyes than in the left eyes (P = 0.02).ConclusionsThere is interocular symmetry of peak foveal cone density, mosaic regularity, and area encompassing the most densely packed cells in subjects with normal vision. The origin and significance of the observed interocular difference in average roundness of the isodensity contours are unclear.     

Linear mechanisms can produce motion sharpening

Human observers are not normally conscious of blur from moving objects [Nature 284 (1980) 164]. Several recent reports have even shown that blurred images appear sharper when drifting than when stationary and have suggested different non-linear mechanisms to explain this phenomenon [Vision Res. 36 (1996) 2729; Vision Res. 38 (1998) 2099]. We demonstrate here that even though distortions of drifting narrow-band sine-wave gratings cannot be explained by linear mechanisms, these mechanisms may have an important role in sharpening of moving edges. We show first that the effective spatial filter for a moving object that is formed by a simple difference-of-Gaussians spatial filter and the typical biphasic temporal impulse response function can be approximated by a combination of Gaussian filters only. When this filter is applied to moving, Gaussian-blurred edges, regions of blurring and sharpening are found over the same ranges of blur widths and velocities where recent experimental findings have shown them to exist. In general, that means that the output of the filter shows blurring in response to small blur widths and sharpening in response to larger blur widths.     

Form overshadows ‘opponent motion’ information in processing of biological motion from point light walker stimuli

The point light walker (PLW) has been taken to demonstrate the existence of mechanisms specialised in the processing of biological motion, but the roles of form and motion information in such processing remain unclear. While processing is robust to distortion and exclusion of the local motion signals of the individual elements of the PLW, the motion relationships between the elements - referred to as opponent motion - have been suggested to be crucial. By using Gabor patches oriented in relation to the opponent motion paths as the elements of the PLW, the influence of form and opponent motion information on biological motion processing can be compared. In both a detection in noise, and a novel form distortion task, performance was improved by orienting the elements orthogonally to the opponent motion paths - strengthening the opponent motion signal - compared to orienting them collinearly. However, similar benefits were found with static tasks presentations. Orienting the Gabor patches orthogonally to their opponent motion also benefits contour integration mechanisms by aligning neighbouring elements along the limbs of the PLW. During static presentations this enhanced form cue could account for all the changes in performance, and the lack of additional improvement in moving presentations suggests that the strengthened opponent motion signal may not be affecting performance. We suggest the results demonstrate the primacy of form information over that of opponent motion in the processing of biological motion from PLW stimuli.     

Position shifts following crowded second-order motion adaptation reveal processing of local and global motion without awareness

Adaptation to first-order (luminance defined) motion produces not only a motion aftereffect but also a position aftereffect, in which a target pattern's perceived location is shifted opposite the direction of adaptation. These aftereffects can occur passively (when the direction of motion adaptation cannot be detected) and remotely (when the target is not at the site of adaptation). Although second-order (contrast defined) motion produces these aftereffects, it is unclear whether they can occur passively or remotely. To address these questions, we conducted two experiments. In the first, we used crowding to remove a local adapter's second-order motion from awareness and still found a significant position aftereffect. In the second experiment, we found that the direction of motion in one region of a crowded array could produce a position aftereffect in an unadapted, spatially separated region of the crowded array. The results suggest that second-order motion influences perceived position over a large spatial range even without awareness.     

Additivity of opponent chromatic valence

The additivity of chromatic valences that were produced by two different wavelengths λ₁ and λ₂ was investigated for four chromatic sensations, red, green, yellow, and blue, by the summation index method. Linear additivity was found for the green and the blue chromatic valence giving the summation index of 0.30. The linear additivity was also found for the red sensation if both stimuli were in the same region of long wavelengths or of short wavelengths. A slight additivity failure of the enhancement type was, however, found for λ₁ and λ₂ pairs when λ₁ was chosen in the short wavelength region while λ₂ in the long wavelength region, respectively. The additivity failure of the same type was found for the yellow chromatic valence for two subjects when λ₁ was 607 nm and λ₂ was 570, 550, and 553 nm, the last combination giving a summation index of as much as 0.64. The results may imply the existence of two different cones whose responses do not add linearly to yield the yellow sensation.     

Spatial interactions reveal inhibitory cortical networks in human amblyopia

Humans with amblyopia have a well-documented loss of sensitivity for first-order, or luminance defined, visual information. Recent studies show that they also display a specific loss of sensitivity for second-order, or contrast defined, visual information; a type of image structure encoded by neurons found predominantly in visual area A18/V2. In the present study, we investigate whether amblyopia disrupts the normal architecture of spatial interactions in V2 by determining the contrast detection threshold of a second-order target in the presence of second-order flanking stimuli. Adjacent flanks facilitated second-order detectability in normal observers. However, in marked contrast, they suppressed detection in each eye of the majority of amblyopic observers. Furthermore, strabismic observers with no loss of visual acuity show a similar pattern of detection suppression. We speculate that amblyopia results in predominantly inhibitory cortical interactions between second-order neurons.     

Colour‐opponent mechanisms are not affected by age‐related chromatic sensitivity changes

The purpose of this study was to assess whether age‐related chromatic sensitivity changes are associated with corresponding changes in hue perception in a large sample of colour‐normal observers over a wide age range (n = 185; age range: 18–75 years). In these observers we determined both the sensitivity along the protan, deutan and tritan line; and settings for the four unique hues, from which the characteristics of the higher‐order colour mechanisms can be derived. We found a significant decrease in chromatic sensitivity due to ageing, in particular along the tritan line. From the unique hue settings we derived the cone weightings associated with the colour mechanisms that are at equilibrium for the four unique hues. We found that the relative cone weightings (w_L/w_M and w_L/w_S) associated with the unique hues were independent of age. Our results are consistent with previous findings that the unique hues are rather constant with age while chromatic sensitivity declines. They also provide evidence in favour of the hypothesis that higher‐order colour mechanisms are equipped with flexible cone weightings, as opposed to fixed weights. The mechanism underlying this compensation is still poorly understood.     

Visual motion mechanisms under low retinal illuminance revealed by motion reversal

The aim of this study is to determine what kinds of motion mechanisms operate at low luminance levels. We used a motion reversal phenomenon in which the perceived direction of motion is reversed when a blank inter-stimulus interval (ISI) frame is inserted between two image frames of similar mean luminance. At low luminance levels, we found that motion reversal was perceived when the moving pattern was presented in the retinal periphery, but no motion reversal was observed when the stimulus was presented in the central retina. When a large stimulus that covers both central and peripheral visual fields was presented, motion reversal did not occur. We conclude that as retinal illuminance decreases, the relative contribution of a feature-tracking mechanism in the central retina becomes larger, while motion perception in the peripheral retina continues to depend on a biphasic, first-order motion mechanism. When both central and peripheral visual fields are stimulated simultaneously, the motion mechanism that dominates in the central retina determines the perceived direction of motion at low luminance levels.