Intermanual and intramanual tactual grating discrimination

For intermanual tactual discrimination to occur, it is thought that neural signals have to cross the corpus callosum in some way at least once. In this note we address the question of whether this interhemispheric transfer affects grating discrimination in active touch. Eight right-handed participants made intermanual and intramanual tactual discriminations of sinusoidal gratings that had slightly different spatial periods. Intramanual discrimination included comparisons in which the gratings were felt with the same finger, as well as comparisons made with two different fingers of the same hand. During intermanual discrimination the gratings were felt with corresponding fingers or with non-corresponding fingers of different hands. We found that thresholds for intramanual conditions were lower than for intermanual conditions in active dynamic touch. This suggests that there is a component of the task that is unilateral, as might be mediated by a somatosensory region that has predominantly or exclusively contralateral receptive fields.

The contributions of egocentric and allocentric reference frames in haptic spatial tasks

The influence of egocentric and allocentric reference frames on performance in haptic spatial tasks, was tested in three conditions. Blindfolded subjects had to make two bars haptically parallel, perpendicular or mirrored in the midsagittal plane. The hypothesis is that the contributions of egocentric and allocentric reference frames are combined, resulting in settings that lie in between the allo-representation and the ego-representation. This leads to different predictions for the outcome of different conditions. All findings were consistent with the hypothesis. In addition, for subjects with large deviations a reversal of the oblique effect was found once again, which provides extra support for the hypothesis.

The perception of doubly curved surfaces from anisotropic textures

Most existing computational models of the visual perception of three-dimensional shape from texture are based on assumed constraints about how texture is distributed on visible surfaces. The research described in the present article was designed to investigate how violations of these assumptions influence human perception. Observers were presented with images of smoothly curved surfaces depicted with different types of texture, whose distribution of surface markings could be both anisotropic and inhomogeneous. Observers judged the pattern of ordinal depth on each object by marking local maxima and minima along designated scan lines. They also judged the apparent magnitudes of relative depth between designated probe points on the surface. The results revealed a high degree of accuracy and reliability in all conditions, except for a systematic underestimation of the overall magnitude of surface relief. These findings suggest that human perception of three-dimensional shape from texture is much more robust than would be reasonable to expect based on current computational models of this phenomenon.

Perception of surface reflectance of 3-D geometrical shapes: influence of the lighting mode

We examined in three experiments whether the perception of surface reflectance in 3-D geometrical shapes is influenced by the mode of lighting, the level of surface reflectance, the number of polyhedral faces, and the regularity of polyhedral shape. As stimuli, we used simulations of regular polyhedra in a 2AFC constant-stimuli procedure to measure how large variations in the reflectance needed to be for observers to be able to tell whether the whole surfaces were uniform or non-uniform. In almost all the cases observers were more successful in discriminating non-uniform polyhedra in the hemispherical diffuse lighting conditions than in the collimated lighting conditions. The different levels of reflectance had no effect on the discrimination of non-uniformity. Second, as the number of faces of the shape increased, discrimination thresholds became lower, indicating that the interpretation of reflectance variations was better when a shape had many faces. Finally, the regularity of the shape, such as the central symmetry, was not an important factor. These results suggest that the interpretation of surface reflectance is more precise when surfaces have less variance in relative strengths between the face luminance values; these surfaces were the ones obtained under hemispherical diffuse lighting.

Detection of amplitude modulation and frequency modulation in tactual gratings: a critical bandwidth for active touch

Since most natural surfaces are complex and vary in amplitude and spatial frequency, it might be interesting to consider gratings not in the spatial domain, but in the spatial-frequency domain. Detection thresholds for amplitude modulation (AM) and frequency modulation (FM) in sinusoidal gratings were measured for seven participants. Participants moved their fingers actively across the gratings. Although the two types of modulation are quite different in the spatial domain, they have many features in common in the frequency domain. In previous research (Nefs et al 2001 Perception 30 1263-1274) we measured the discrimination thresholds for amplitude and frequency for sinusoidal gratings. We hypothesised then that these thresholds could be used to predict the discriminability of other types of gratings. In the present study, we did indeed find that the FM and AM detection thresholds can be understood quite well by these discrimination thresholds. The results indicate that the tactual system contains parallel psychophysical channels that filter and integrate the power of stimuli within critical bands. With these results, we are also able to calculate the critical bandwidth for active dynamic touch. We estimated the critical bandwidth surrounding the spatial frequency of 2 cycles cm(-1) to be about 125% of that spatial frequency. This value for the critical band for spatial frequency is incompatible with previous findings for temporal frequencies in vibrotactile research. This indicates that dynamic spatial-frequency discrimination is not likely to be done by temporal frequency.

Large-scale visual frontoparallels under full-cue conditions

We determined the curvature of apparent frontoparallels in a natural scene (a large lawn in broad daylight). Data on frontoparallels in these conditions are very sparse and reveal idiosyncratic curvatures of frontoparallels and irregular variation with distance. We used a method of bisection of linear segments indicated through pairs of stakes at angular separations (from the vantage point) of up to 120 deg. Distances of 2 m and 10 m (in the forward direction) were used. The bisection was carried out by the observer through maneuvering a radio-controlled vehicle carrying a third stake. Four observers participated in the experiment; they had no problems with the task and yielded mutually consistent results. We found that the frontoparallels are significantly curved and are concave towards the observer. Surprisingly, the sign of the curvature is opposite to that found when the frontoparallels are defined through an exocentric pointing task. Available theory (Luneburg's) does not predict this, but the theory is hardly applicable to the case of vision in natural scenes. This interesting discrepancy has not been reported before.

Large systematic deviations in a bimanual parallelity task: further analysis of contributing factors

Previous studies showed that what subjects haptically perceive as parallel deviates largely from what is actually physically parallel [Perception 28 (1999) 1001; Acta Psychol. 109 (2002) 25; Perception 28 (1999) 781]. It also turned out that the deviations were strongly subject-dependent. It was hypothesized that what is haptically parallel is decided in a frame of reference intermediate to an allocentric and an egocentric one. The purposes of the present study were to collect more evidence for this hypothesis and to investigate the factor(s) that determines the specific weighting between the two reference frames. We found a highly significant reversal of a haptic oblique effect (in context: larger systematic deviations for oblique orientations) for subjects with large deviations. This reversal provides convincing evidence that an intermediate frame of reference is used for the decision of haptic parallelity. Contrary to common expectation, several factors that might have been of influence on the weighting of the two frames of reference, such as arm length, arm span, shoulder width, turned out to be irrelevant. Surprisingly, the only factors that seem to be of influence are gender and job experience or education.

Perceptual representation of visible surfaces

Two experiments are reported in which we examined the ability of observers to identify landmarks on surfaces from different vantage points. In Experiment 1, observers were asked to mark the local maxima and minima of surface depth, whereas in Experiment 2, they were asked to mark the ridges and valleys on a surface. In both experiments, the marked locations were consistent across different observers and remained reliably stable over different viewing directions. These findings indicate that randomly generated smooth surface patches contain perceptually salient landmarks that have a high degree of viewpoint invariance. Implications of these findings are considered for the recognition of smooth surface patches and for the depiction of such surfaces in line drawings.

Illumination direction from texture shading

We investigate the ability of human observers to judge the direction of illumination from image texture. Photographs of 61 real surfaces were used, taken from the Columbia-Utrecht Reflectance and Texture (Curet) database (http:/www.cs.columbia.edu/CAVE/curet). All samples were normally viewed but obliquely illuminated, the elevation of the source being 22.5 degrees, 45.0 degrees, or 67.5 degrees. The illumination was with a collimated, parallel beam. Stimuli were presented in random orientation, and observers had to judge both the elevation and the azimuth of the source. Observers judged the azimuth within approximately 15 degrees, except for the fact that they committed random (with approximately 50% probability) sign flips (180 degrees flips). Connected with this finding is the fact that observers judged the illumination to be from above rather than below in the overwhelming majority of cases, despite the fact that each case occurred with equal probability. The elevation of the illumination can be judged to some extent but is not far above chance level. The data are in good agreement with a simple model that bases the estimate of illumination direction on the second-order statistics of local luminance gradients. This locates the locus of the probable mechanism very early in the visual stream.

Delay improves performance on a haptic spatial matching task

Systematic deviations occur when blindfolded subjects set a test bar parallel to a reference bar in the horizontal plane using haptic information (Kappers and Koenderink 1999, Perception 28:781-795; Kappers 1999, Perception 28:1001-1012). These deviations are assumed to reflect the use of a combination of a biasing egocentric reference frame and an allocentric, more cognitive one (Kappers 2002, Acta Psychol 109:25-40). In two experiments, we have examined the effect of delay between the perception of a reference bar and the parallel setting of a test bar. In both experiments a 10-s delay improved performance. The improvement increased with a larger horizontal (left-right) distance between the bars. This improvement was interpreted as a shift from the egocentric towards the allocentric reference frame during the delay period.

Active haptic detection and discrimination of shape

In previous research, we have shown that detection thresholds for Gaussian shapes increase with a power of 1.3 of spatial width. In the present three experiments, we generalized this finding to more complex shapes and to discrimination tasks. In Experiment 1, we found that the slope of the psychometric function for detection (i.e., distinguishing curved from flat surfaces) was independent of surface shape. In Experiment 3, we found the same result for discrimination of two different curved shapes. In Experiment 2, we found that detection and discrimination functions had the same dependence on spatial width, except that discrimination thresholds were two to four times larger. Possible neural mechanisms underlying these results are discussed.

Frequency discrimination between and within line gratings by dynamic touch

Line gratings were used to investigate the tactual discrimination thresholds for line frequency. In Experiment 1, participants were asked to discriminate between two gratings, each with a different line frequency. We used four standard frequencies in the eightfold range from 0.5 to 4 lines/cm. Thresholds were found to be constant at about 10.6%. In this experiment, we also measured hand speed and contact force. Hand speed was roughly in the range between 0.12 and 0.44 m/sec; contact force ranged from 0.62 to 2.76 N. In Experiment 2, we determined discrimination thresholds for line frequency transitions within a single grating. We used two frequencies and three transition lengths. The transition length had no effect on the threshold. In a third experiment, line frequency was modulated periodically. Varying the standard frequency and the size of the modulation period was found to have no effect on the discrimination thresholds. We conclude three things. First, Weber fractions for line frequency discrimination decrease as a function of line frequency within the experimental range. Second, discrimination thresholds are not altered by the length of the transition between two adjacent gratings with different line frequencies. And finally, the size of a modulation period in periodically modulated gratings is of no influence on the modulation detection threshold.

Haptic discrimination of stimuli varying in amplitude and width

We studied active haptic discrimination of the geometrical features of an object. The geometrical parameters under investigation were the amplitude and width of a gaussian-shaped surface. Haptic discrimination thresholds were measured with regard to three values of these geometrical parameters. We found that humans discriminate up to about 300 shapes when both amplitude and width are extrapolated to the range between 1 micro m and 1 m. Over this range, which covers the span of the arms, the number of discriminations is small compared to the number across the full range of chromaticities in vision. Roughly speaking, humans are far better at discriminating sharp (extensive amplitude and little width) gaussian surfaces from smooth (small amplitude and extensive width) ones than they are at discriminating small (small amplitude and width) surfaces from large (extensive amplitude and width) ones. Our main conclusion is that discrimination in the geometrical domain is poorest when the proportion between amplitude and width is roughly the same for both shapes. Our results are in close agreement with results of earlier experiments on detection thresholds. This indicates that similar, or even the same, neural mechanisms were used for detection and discrimination of the geometrical parameters under investigation.

Scale invariance in near space: pointing under influence of context

This paper describes the results of a pointing task close to the observer. Unlike most research reported in literature, the stimuli were composed of real solid objects in an unambiguous context, free of any cue conflict. Moreover, the stimuli were either within arm-reach or just beyond arm-reach, thus having strong binocular depth cues. Surprisingly, systematic errors up to four times the standard deviation were found. These errors depended mainly on the variations in context and hardly on egocentric distance. In good first approximation, the results were scale invariant. These results are in direct conflict with classic theories about visual space. The stimuli were unambiguous real 3-D equivalents of the computer-generated stimuli we used in a previous experiment. A comparison of the results did not reveal a clear effect of a depth cue conflict between accommodation and disparity.

Visual perception of collinearity

In a metrical space, there exists an intimate relation between collinearity and parallelity. In particular, in a Riemannian space collinearity is just a special case of parallelity. Is this true for visual space as well? We investigated the visual perception of collinearity by having subjects align two bars in the horizontal plane at eye height. The distances of the bars from the subject and the angles at which they were placed were varied. We found deviations of up to 22 degrees. The deviations of the left and right bars could be split into two independent components: namely, the sum and the difference of the deviations of the left and right bars. We found that the former depended only on the ratio between the distances of each bar from the subject, whereas the latter was largely independent of the positions of the bars. The difference in deviations corresponded to the deviation from parallelity. Compared with the results in the parallelity task (Cuijpers, Kappers, & Koenderink, 2000b), the deviations from parallel were much smaller. As a consequence, the results of the two experiments cannot be described by the same Riemannian geometry. This indicates that the intrinsic geometry of visual space differs across tasks. This is conceivable if the intrinsic geometry of visual space is operationally defined.

Pappus in optical space

Optical space differs from physical space. The structure of optical space has generally been assumed to be metrical. In contradistinction, we do not assume any metric, but only incidence relations (i.e., we assume that optical points and lines exist and that two points define a unique line, and two lines a unique point). (The incidence relations have generally been assumed implicitly by earlier authors.) The condition that makes such an incidence structure into a projective space is the Pappus condition. The Pappus condition describes a projective relation between three collinear triples of points, whose validity can--in principle--be verified empirically. The Pappus condition is a necessary condition for optical space to be a homogeneous space (Lobatchevski hyperbolic or Riemann elliptic space) as assumed by, for example, the well-known Luneburg theory. We test the Pappus condition in a full-cue situation (open field, broad daylight, distances of up to 20 m, visual fields of up to 160 degrees diameter). We found that although optical space is definitely not veridical, even under full-cue conditions, violations of the Pappus condition are the exception. Apparently optical space is not totally different from a homogeneous space, although it is in no way close to Euclidean.

Haptic perception of parallelity in the midsagittal plane

Previous studies [Perception 28 (1999) 1001; Perception 28 (1999) 781] on the haptic perception of parallelity on a horizontal plane showed that what subjects haptically perceive as being parallel deviates considerably from what is physically parallel. The deviations could be described with a subject-dependent orientation gradient in the left-right direction. The gradients found in the bimanual conditions were significantly larger (about 70%) than those in the unimanual conditions. The questions to be answered in the present study are the following: (1) Does the haptic perception of parallelity in the midsagittal plane also show systematic deviations from veridicality? (2) Are the unimanual and bimanual performances again quantitatively but not qualitatively different? The set-up consisted of a plate positioned in the midsagittal plane of the subject. The subject touched the right side of the plate with his/her right hand and the left side with the left hand. The results show again large systematic deviations. The major part of the deviations can be described by means of a subject-dependent orientation gradient in the vertical direction. The quantitative (but not qualitative) difference between the unimanual and the bimanual conditions is much larger in the midsagittal plane than in the horizontal plane.

Detection of Relative Motion

We measured human sensitivity to the relative motion of blobs moving in the peripheral visual field. The stimuli consisted of one or two blobs (Gaussian luminance profiles) oscillating relative to an (invisible) reference frame which rotated with constant angular velocity about a fixation point. We determined thresholds for detecting the oscillation of different configurations of one or two blobs as a function of velocity, eccentricity (viewing distance), and temporal frequency. By determining thresholds as a function of frequency the temporal characteristics of the detection system could be revealed.

Thresholds are higher for oscillations in the motion direction of the reference frame than perpendicular to it. No influence has been found of the position of the blobs in the frame of reference. The thresholds are scale-invariant. For low frequencies (<2 Hz) the threshold amplitude of the velocity modulation is constant whereas for high frequencies (>2 Hz) the threshold amplitude of the position modulation is constant. This behaviour can be well described by a model which detects the oscillations whenever, within a critical time (of about 200 ms for two blobs), the relative displacement is larger than a critical distance. The critical distance shows the same dependence on velocity as the span in the bilocal detector model of Koenderink et al (1985 Journal of the Optical Society of America A2 252 – 259).

Indicating the Singular Point in First-Order Optical Flow Fields

We investigated the accuracy with which subjects can indicate the singular point in a first-order optical flow field. This singular point might be important in navigation and orientation. The stimuli were expanding or rotating sparse random-dot patterns consisting of 80 dark dots on a light background. The stimulus window was circular with a diameter of 20 deg arc. The singular point could be at one of 48 different locations. Subjects had to indicate the location of this singular point with a cursor, while fixating in the centre of the stimulus. Presentation time was unlimited, though each dot had a limited lifetime (114 ms) to avoid density cues. Both veridicality and reproducibility for our subjects increased with increasing values of expansion or rotation in a nonlinear way. We did not find any systematic differences between expansion and rotation. When we blocked either the outer rim or the central part of the stimulus, performance remained the same for singular points that were within the visible part of the stimulus. For singular points outside this visible part, the reproducibility also remained the same, but subjects tended to locate the singular points closer to the rim of the visible part of the stimulus.