Postfusional latency in stereoscopic slant perception and the primitives of stereopsis.

Random dot stereograms of slanted surfaces were constructed, each representing one or two slanted surfaces in different relative arrangements and with different axes. Latency to fusion and from fusion to stereoscopic resolution was measured for each stimulus. It was found that latency to fusion was always very brief but that latency to stereoscopic resolution varied markedly, depending upon the orientation and arrangement of the stereoscopic surfaces. A gradient of discontinuities at a surface boundary produced an instant slant response for that surface, whereas a gradient of absolute disparities across the surface did not, except under conditions where vertical declination (a form of orientation disparity) was present. We conclude that stereopsis is not based on the primitives used in matching the images for fusion and that it is, at least initially, a response to disparity discontinuities which play no role in the fusion process. We also conclude that vertical declination is responded to globally as a slant around a horizontal axis but that other forms of orientation disparity are ineffective. The evidence from our experiments does not support the existence of a stereoscopic ability to respond globally to differences in magnification (or spatial frequency). It is suggested that stereoscopic perception of slant around a vertical axis is slow because it results from the integration of local processes. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Plaid slant and inclination thresholds can be predicted from components

We investigated whether stereoscopic slant and inclination thresholds for surfaces defined by two component plaids could be predicted from the interocular differences in their individual component gratings. Thresholds were measured for binocular images defined by single sinusoidal gratings and two component plaids. In both cases thresholds showed a marked dependence on component orientation. For absolute component orientations greater than 45 deg we found that inclination thresholds were smaller than slant thresholds. However, for absolute component orientations less than 45 deg, we found a reversal: slant thresholds were smaller than inclination thresholds. We considered three models that might account for these data. One assumed that thresholds stemmed from interocular position differences of corresponding image points. The other two assumed a combination of position, orientation and/or spatial-frequency differences. The best fits were obtained from those models that explicitly represented orientation differences. From the model combining orientation and spatial-frequency differences, we estimated the relative cue sensitivity to be 1.7:1, respectively. For plaids, we found that thresholds obtained from the individual components could be used to predict thresholds for plaids, even though an additional disparity cue from the contrast beat was available.     

Discrimination of the direction and speed of motion in depth of a monocularly visible target from binocular information alone

Thresholds for discriminating a monocularly visible object's direction of motion in depth and speed of motion in depth were measured using only binocular cues. Observers could discriminate the direction of motion in depth while totally ignoring speed and discriminate the speed of motion in depth while totally ignoring direction. Direction discrimination thresholds were the same for motion in depth within the vertical and horizontal meridians, even though a cue to trajectory was available for motion within the horizontal meridian that is not available for motion within the vertical meridian. Speed discrimination thresholds also were the same for motion in depth within the vertical and horizontal meridians. For the 3 observers the lowest direction discrimination thresholds were 0.14 degree, 0.18 degree, and 0.22 degree (means of horizontal and vertical thresholds).     

Speed gradients and the perception of surface slant: analysis is two-dimensional not one-dimensional

Motion parallax provides cues to the three-dimensional layout of a viewed scene and, in particular, to surface tilt and slant. For example, as a textured surface, inclined around a horizontal axis, translates horizontally relative to an observer's view point, then, in the absence of head and eye movements, the observer's retinal flow will contain a one-dimensional (1D) vertical speed gradient. The direction of this gradient indicates the direction of surface tilt, and its magnitude and sign can be used in calculating the magnitude and sign of the surface slant. Alternatively, the same retinal flow contains a 1D translating component, plus a two-dimensional (2D) component of rotation (curl), and a 2D component of deformation (def). On this view, the direction of surface tilt is related to the orientation of def and the magnitude and sign of the surface slant is related to the magnitude and sign of def. We used computer generated random dot patterns as stimuli to determine whether the human visual system employs a 1D analysis (i.e. 1D speed gradients) or a 2D analysis (i.e. deformation) of surface slant from motion parallax. Using a matching technique we found compelling impressions of slant when we vector summed a translation field with (i) vertical shear, horizontal shear or deformation (made from vertical and horizontal shear), but not rotation; and (ii) vertical compression, horizontal compression or deformation (made from vertical and horizontal compression), but much less so for expansion. In both cases, the first three conditions contain def, but the fourth does not, and the last three conditions contain 1D speed gradients orthogonal to the perceived axis of inclination, but the first one does not. Therefore, the results from the first and fourth conditions distinguish between the two processing strategies. They support the idea that surface slant is coded by combining both horizontal and vertical speed gradients in a way similar to the 2D differential invariant def and oppose the view that surface slant is encoded by a 1D analysis of motion in a direction orthogonal to the perceived axis of inclination. In a further experiment, we found essentially no effect of reducing the field size from 18 to 9 deg.     

Modeling spatial integration and contrast invariance in visual pattern discrimination

PURPOSE: Human pattern discrimination performance has been reported to be largely independent of stimulus contrast but to depend on stimulus area. The authors propose a model that combines the effects of spatial integration and contrast. The model is based on the computation of similarity between pattern templates in memory and signals to be discriminated using normalized correlation. There are also two sources of additive noise, one before and one after the computation of correlation. The model was compared with human observers in an orientation discrimination task. METHODS: Orientation discrimination thresholds of human observers were measured for sinusoidal gratings of various areas, contrasts, and spatial frequencies. A two-interval, forced-choice methods was used. The performance of the model was determined by using computer simulations. RESULTS: It was found that the effects of contrast and grating area were interrelated. The decrease of orientation thresholds as a function of grating area was considerably larger at low than at high contrast. On the other hand, orientation thresholds decreased clearly as a function of contrast at the smallest grating areas but hardly at all at the largest grating areas. The model accounted well for the experimental findings. CONCLUSIONS: Because the invariance of orientation discrimination with respect to stimulus contrast depended on area, the cause of the invariance appeared to occur after spatial integration. The model explains this so that, with increasing contrast or area, the normalized correlation gradually approached a constant value. The proportion of pretemplate noise became negligible compared to the constant posttemplate noise. Thus, total noise also approached a constant value. Hence, the signal-to-noise ratio and discrimination performance became constant.     

Competition and sharing of processing resources in visual discrimination.

Discrimination thresholds for spatial frequency and contrast tested individually were compared with dual discrimination of contrast and spatial frequency, and dual discrimination of 2 contrast or spatial frequency components. The components were presented overlapping, forming a compound grating or as side-by-side simple gratings. When observers had to judge contrast and spatial frequency simultaneously, discrimination thresholds increased by an amount predicted by a model of stimulus uncertainty for orthogonal dimensions (1.7); when they had to judge 2 frequency or contrast components, discrimination thresholds increased by a factor of 3–6 compared with the single-judgment task. The relative spatial location of the components did not interact with task complexity. The results are consistent with a model assuming a set of parallel special-purpose attentional mechanisms. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Parrel processing in visual short-term memory.

Visual short-term memory for the contrast and spatial frequency of sinusoidal gratings was measured in a delayed discrimination task in which the 2 stimuli to be compared were separated in time by 1-10 s interstimulus intervals (ISIs). Delayed discrimination thresholds for spatial frequency and contrast were compared, both when the 2 types of thresholds were measured in separate blocks of trials and when the 2 types of measures were randomly intermixed in an uncertainty paradigm, which required participants to process information about both dimensions on each trial. In both cases, accuracy of memory for spatial frequency was independent of ISI, but memory for contrast decreased as ISI increased. Performance was lower in the uncertainty case, but only by an amount predicted by statistical decision theory for independent sources. The results are consistent with a model assuming a set of parallel special-purpose visual discrimination and short-term memory mechanisms. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Feature asymmetries in visual search: effects of display duration, target eccentricity, orientation and spatial frequency

In Experiments 1-3, we monitored search performance as a function of target eccentricity under display durations that either allowed or precluded eye movements. The display was present either until observers responded, for 104 msec, or for 62 msec. In all three experiments an orientation asymmetry emerged: observers detected a tilted target among vertical distracters more efficiently than a vertical target among vertical distracters. As target eccentricity increased, reaction times and errors augmented, and the set size effect became more pronounced, more so for vertical than tilted targets. In Experiments 4-7, the stimulus spatial properties were manipulated: spatial frequency; size; and orientation. The eccentricity effect was more pronounced for vertical than tilted targets and for high- than low-spatial frequency targets. This effect was eliminated when either the size, the size and orientation, or the size and spatial frequency were magnified (M-cortical factor). By increasing the signal-to-noise ratio, magnification reduced the extent of both asymmetries; it aided more the detection of tilted than vertical and of high- than low-spatial frequency targets. Experiments 4-7 indicate that performance improvement in the magnified conditions was due to the specific pairing of stimulus size with retinal eccentricity and not to the larger stimulus size of the magnified conditions. We conclude that stimulus size, orientation and spatial frequency influence the extent of the eccentricity effect and the efficiency of search performance.     

Position and orientation features and the development of spatial concepts.

Tested the speed with which spatial information is classified into categories by asking 34 6-yr-olds, 35 10-yr-olds, 30 14-yr-olds, and 30 undergraduates to sort 6 decks of cards, each requiring a spatial judgment. Results indicate that (1) oblique discriminations required more processing time than did vertical–horizontal discriminations; (2) stimuli along the vertical axis were not processed significantly more quickly than stimuli along the horizontal axis; and (3) position variations slowed orientation discriminations, especially if the lines were of oblique orientation. The effect of position variations was particularly marked for the 6-yr-olds. The role of positional cues in spatial organization, as well as aspects of developmental changes in spatial concepts and strategies, are discussed. (French abstract) (11 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Types of size disparity and the perception of surface slant

We examined (i) perceived slant of a textured surface about a vertical axis as a function of disparity magnitude for horizontal-size disparity, vertical-size disparity, and overall-size disparity; and (ii) interactions between patterns with various types and magnitudes of size disparity and superimposed or adjacent zero-disparity stimuli. Horizontal-size disparity produced slant which increased with increasing disparity, was enhanced by superimposed zero-disparity stimuli, and induced contrasting slant in superimposed or adjacent zero-disparity stimuli. Vertical-size disparity produced opposite slant (induced effect) which was reduced to near zero by a superimposed zero-disparity pattern and both patterns appeared as one surface. Adjacent vertical-size-disparity and zero-disparity patterns appeared as separate surfaces with a wide curved boundary. Overall-size disparity produced slant which was enhanced by a superimposed zero-disparity pattern and less so by a zero-disparity line, and induced more slant in a zero-disparity line than in a zero-disparity pattern. The results are discussed in terms of depth underestimation of isolated surfaces, depth enhancement, depth contrast, and the processing of deformation disparity.     

Orientation dependency for foveal line stimuli: detection and intensity discrimination, resolution, orientation discrimination and vernier acuity

Thresholds were measured for five tasks: line detection, intensity discrimination, two-line resolution, vernier acuity and line-orientation discrimination. For each task, 30 arcmin lines were presented foveally in eight retinal meridians to assess similarities in orientation anisotropies across tasks in the same observer. Three observers were tested. The pattern of the orientation anisotropy differs across tasks. Meridional anisotropies exist in detection, increment discrimination thresholds, and vernier acuity but the classical oblique effect is consistently found only in orientation discrimination.     

Spatial associations for musical stimuli: A piano in the head?

This study was aimed at examining whether pitch height and pitch change are mentally represented along spatial axes. A series of experiments explored, for isolated tones and 2-note intervals, the occurrence of effects analogous to the spatial numerical association of response codes (SNARC) effect. Response device orientation (horizontal vs. vertical), task, and musical expertise of the participants were manipulated. The pitch of isolated tones triggered the automatic activation of a vertical axis independently of musical expertise, but the contour of melodic intervals did not. By contrast, automatic associations with the horizontal axis seemed linked to music training for pitch and, to a lower extent, for intervals. These results, discussed in the light of studies on number representation, provide a new example of the effects of musical expertise on music cognition. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Perception of rigid motion in depth from the optical deformations of shadows and occlusion boundaries.

Three experiments were designed to examine the abilities of observers to determine an object's 3-dimensional structure and motion from various types of optical deformations. Observers were required to discriminate whether pairs of moving ellipsoids were rotating rigidly about a single axis or nonrigidly about different axes that varied in slant. Discrimination thresholds were significantly influenced by whether the ellipsoids were intersecting or nonintersecting and whether they contained identifiable texture elements. Performance was unaffected by precession movements of the axis of rotation, by increasing the number of intersecting ellipsoids beyond 2, or by replacing the deforming silhouettes with the projected motions of cast shadows presented in isolation against a planar background. Findings indicate that observers can perceive structure from motion based on several different types of optical deformation, including the deformations of shadows and silhouettes that do not contain identifiable features on which most existing theoretical analyses are designed to operate. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Spatial uncertainty and sampling efficiency in amblyopic position acuity

Spatial uncertainty and undersampling are two of the major hypotheses for the losses of amblyopic spatial vision. To test these two hypotheses, equivalent spatial uncertainty and spatial integration efficiency in spatial position judgments were quantified with a spatial perturbation paradigm. Specifically, three-line bisection thresholds were measured for the amblyopic eyes of two strabismic and two anisometropic amblyopes, and for normal controls. The horizontal stimulus lines comprised discrete dark dots distributed randomly around the mean line position according to a gaussian function. Line separation, the number of dots on each line (N), stimulus contrast (C), and the vertical standard deviation (sigma e) of the dot distribution were varied. An ideal observer analysis quantified the magnitude of equivalent spatial uncertainty (sigma s), the effective number of dots used (k), and spatial integration efficiency (k/N). At the optimal separation, equivalent spatial uncertainty (sigma s) is approximately ten-fold higher in both types of amblyopic visual systems than in control observers, even when stimulus visibility is accounted for. This apparent increase in sigma s is largely due to a shift in spatial scale of analysis in the amblyopic eye. Integration efficiency (k/N) increases in proportion to stimulus contrast or visibility (in units of detection threshold). Unlike sigma s, k/N is different between the two types of amblyopia. For the anisometropic observers, k/N is quantitatively similar to that of control observers. For the strabismic observers, on the other hand, k/N is reduced even after taking stimulus visibility into account. The decreased spatial integration efficiency in the strabismic visual system suggests that spatial undersampling may occur at a secondary stage of visual processing, beyond the detection stage.     

Integration of binocular stereopsis and structure from motion in the discrimination of noisy surfaces

Integration of stereo and motion information was measured on the basis of observers' ability to discriminate between triangle- and sine-wave corrugated surfaces or sinusoidal surfaces of different spatial frequency. Discrimination performance for the triangle-sine task was consistent with independent processing of motion and stereo, but the spatial frequency discrimination task led to performance superior to that predicted by an independent combination of motion and stereo signals, indicating that the integration of stereo and motion depends on the type of 3-dimensional structure observers are required to discriminate. This pattern of results is consistent with the existence of multiple stereoscopic mechanisms suggested by psychophysical and neurophysiological data.     

Significance of presence of teratomatous elements in the primary tumour of testicular cancer

Foveal discrimination thresholds were measured for orientation, vernier alignment, bisection, displacement detection and stereoscopic acuity using simple line stimuli and also sinusoidal grating patches with gaussian intensity modulation (Gabor stimuli). Stimulus parameters such as luminance, exposure duration, component separation and, as far as possible, length were identical for both. Orientation discrimination as a function of length is almost identical for the two classes of stimuli, some observers performing slightly better with Gabor patches. Thresholds for displacement detection are also the same. Vernier, stereo and bisection acuities, however, are considerable better with line than with Gabor stimuli.     

Refraction, aliasing, and the absence of motion reversals in peripheral vision

Reversals in perceived direction of motion of a grating when its spatial frequency exceeds half that of the sampling mosaic provide a potential tool for estimating sampling frequency in peripheral retina. We used two-alternative forced-choice tasks to measure performance of three observers detecting or discriminating direction of motion of high contrast horizontal or vertical sinusoidal luminance gratings presented either 20 or 40 deg from the fovea along the horizontal meridian. A foveal target at a comfortable viewing distance aided fixation and accommodation. A Maxwellian view optometer with 3 mm artificial pupil was used to correct the refraction of the peripheral grating, which was presented in a circular patch, 1.8 deg in diameter, in a surround of similar colour and mean luminance (47.5 cd.m-2). The refractive correction at each eccentricity was measured by recording the aerial image of a point after a double pass through the eye. The highest frequency which can reliably be detected (7-14 c/deg at 20 deg, 5.5-7.5 c/deg at 40 deg) depends critically on refraction. Refraction differs by up to 5 D from the fovea to periphery, and by up to 6 D from horizontal to vertical. Direction discrimination performance shows no consistent reversals, and depends less on refraction. It falls to chance at frequencies as low as one-third of the highest that can be detected. Gratings which can be detected but whose direction of motion cannot be discriminated appear as irregular speckle patterns whose direction of motion varies from trial to trial.(ABSTRACT TRUNCATED AT 250 WORDS)     

Contrast masking effects change with practice

Contrast detection thresholds are known to increase with background contrast, a phenomenon called contrast masking. We found that, under some conditions, observers improved their masked detection performance by repetitive practice of a masking experiment. This learning effect resulted in a cancellation of suprathreshold contrast masking within the contrast range measured. A two-alternative forced-choice discrimination paradigm was used, with stimuli consisting of Gabor signals as maskers and target, presented at the same location and time. Untrained observers showed increased detection thresholds with increasing mask contrast for suprathreshold mask contrasts, but perceptual learning caused an elimination of this classical effect, with masked thresholds reaching the no-mask level and below. Learning did not decrease, but rather somewhat increased, discrimination thresholds when target and mask shared the same Gabor signal parameters. Performance improvement was found to be specific for orientation and mask configurations, though it did transfer between mirror symmetric mask configurations and between eyes. These results argue against a static transducer function-based account for contrast masking and are consistent with a theory assuming multiple feature-based interactive network capable of long-term gain modifications.     

Influence of shape on haptic curvature perception

Curvature discrimination of hand-sized doubly curved surfaces by means of static touch was investigated. Stimuli consisted of hyperbolical, cylindrical, elliptical and spherical surfaces of various curvatures. In the first experiment subjects had to discriminate the curvature along a specified orientation (the discrimination orientation) of a doubly curved surface from a flat surface. The curvature to be discriminated was oriented either along the middle finger or across the middle finger of the right hand. Independent of the shape of the surface, thresholds were found to be about 1.6 times smaller along the middle finger than across the middle finger. Discrimination biases were found to be strongly influenced by the shape of the surface; subjects judged a curvature to be more convex when the perpendicular curvature was convex than when this curvature was concave. With the results of the second experiment it could be ruled out that the influence of shape on curvature perception was simply due to a systematic error made by the subject regarding the discrimination orientation.