Haptic integration of object properties: texture, hardness, and planar contour

Planar objects varying in shape, texture, and hardness were classified under haptic exploration. Classes were defined by values on one dimension, or redundantly, by two or three dimensions. Response times and exploratory procedures (Lederman & Klatzky, 1987) were recorded. Experiment 1 showed that a second dimension speeded responses for all combinations (redundancy gain), but a third dimension produced no further effect. In Experiments 2 and 3, classification trials began with two redundant dimensions, and subsequently one was withdrawn (held constant). When texture and hardness varied redundantly, withdrawal of either increased response time--even when subjects were initially instructed to focus on one dimension. Joint exploration for texture and hardness dominated whenever the two varied redundantly and persisted despite withdrawal. Redundancy gains (Experiment 1), but not substantial withdrawal effects (Experiments 2 and 3), were observed for combinations of texture or hardness with planar contour, indicating less integration than between substance dimensions. Compatibility of exploratory procedures appears to constrain dimensional integration.     

Haptic perception of texture gradients

To pick up 3-D aspects of pictures is arguably the most difficult problem concerning tactile pictorial perception by the blind. The aim of the experiments reported was to examine the potential utility of texture gradients in this context. Since there is no theoretical basis for predicting absolute values of 3-D properties from 2-D patterns read by the finger pads, the abilities of participants to perceive gradients lying between known maxima and minima were assessed. Experiment 1 involved blindfolded sighted participants making verbal magnitude estimations of texture-gradient magnitudes corresponding to plane surfaces at different slants. In experiment 2 the participants' task was to orient a surface at a slant corresponding to the texture gradients depicted tactually, and experiment 3 required early-blind participants to attempt the same task. The results revealed that participants can scale the magnitudes of texture gradients with high precision and that they can also accurately produce surface slants from depictions, providing the extreme conditions are clearly defined and there are opportunities for learning. Texture gradients appear as informative to the blind as they do to the sighted. To what extent these data can be generalised to other gradients and textures or to other projections of 3-D scenes remains to be investigated.     

Haptic estimates of discordant visual-haptic size vary developmentally.

In two size-conflict experiments, children viewed various squares through a reducing (1/2) lens while manually grasping them through a hand-concealing cloth. Then, using either vision or touch, they selected a match from a set of comparison squares. Forty 6-, 9-, and 12-year-olds participated in Experiment 1. Vision dominated the visual estimates of all three age groups; however, for the haptic estimates, the dominant modality varied developmentally: Vision dominated the 6-year-olds' haptic estimates, whereas neither modality dominated the 9-year-olds' haptic estimates, and touch dominated the 12-year-olds' haptic estimates. In Experiment 2, 24 six-year-olds were tested, as in Experiment 1; however, half of them were shown the size-distorting effects of the lens just prior to testing. Although this reduced the visual dominance of their haptic estimates, the effect was weak and short-lived. The haptic dominance seen in the data of the 12-year-olds was conspicuously absent.     

Processing time of contour integration: the role of colour, contrast, and curvature

We investigated the temporal properties of the red - green, blue-yellow, and luminance mechanisms in a contour-integration task which required the linking of orientation across space to detect a 'path'. Reaction times were obtained for simple detection of the stimulus regardless of the presence of a path, and for path detection measured by a yes/no procedure with path and no-path stimuli randomly presented. Additional processing times for contour integration were calculated as the difference between reaction times for simple stimulus detection and path detection, and were measured as a function of stimulus contrast for straight and curved paths. We found that processing time shows effects not apparent in choice reaction-time measurements. (i) Processing time for curved paths is longer than for straight paths. (ii) For straight paths, the achromatic mechanism is faster than the two chromatic ones, with no difference between the red-green and blue-yellow mechanisms. For curved paths there is no difference in processing time between mechanisms. (iii) The extra processing time required to detect curved compared to straight paths is longest for the achromatic mechanism, and similar for the red - green and blue-yellow mechanisms. (iv) Detection of the absence of a path requires at least 50 ms of additional time independently of chromaticity, contrast, and path curvature. The significance of these differences and similarities between postreceptoral mechanisms is discussed.     

Reading acquisition enhances an early visual process of contour integration

The acquisition of reading has an extensive impact on the developing brain and leads to enhanced abilities in phonological processing and visual letter perception. Could this expertise also extend to early visual abilities outside the reading domain? Here we studied the performance of illiterate, ex‐illiterate and literate adults closely matched in age, socioeconomic and cultural characteristics, on a contour integration task known to depend on early visual processing. Stimuli consisted of a closed egg‐shaped contour made of disconnected Gabor patches, within a background of randomly oriented Gabor stimuli. Subjects had to decide whether the egg was pointing left or right. Difficulty was varied by jittering the orientation of the Gabor patches forming the contour. Contour integration performance was lower in illiterates than in both ex‐illiterate and literate controls. We argue that this difference in contour perception must reflect a genuine difference in visual function. According to this view, the intensive perceptual training that accompanies reading acquisition also improves early visual abilities, suggesting that the impact of literacy on the visual system is more widespread than originally proposed.     

Preattentive texture segmentation: The role of line terminations,size, and filter wavelength

The texel pair represents a dilemma in texture discrimination because, despite having the same component orientations, discrimination is still possible (Julesz, 1981), showing a performance asymmetry. Other possible element properties that could influence this task are line terminations, closure, and the size of these elements. We found that line terminators are critical for the task; however, results from double-task experiments indicated that terminator-based discrimination requires the use of attention. When attention is not available for the task, “size” of the elements (with the considered slightly larger) seems to be critical for this discrimination and for the asymmetric performance. To generalize the concept of “size” to textures in general, further experiments were performed with textures of different-sized elements. Results showed, as past literature has indicated, that there is a performance asymmetry, with the larger of the elements being more visible when in the foreground. This asymmetry was additionally shown to reverse itself (i.e., the smaller element became the more visible) as the scale of the elements increased (while interelement distance remained fixed). A filter analysis was developed in order to measure the apparent size of these elements within textures (texsize), defined as the response weighted average of the filter wavelength, $\bar \lambda $ , for a group of elements. The calculation of $\bar \lambda $ was attained by introducing a nonlinearity after the second stage of filtering (or spatial averaging of filter responses). This analysis showed high correlation between the texture with the larger $\bar \lambda $ and the more visible texture. On the basis of this correlation, a wavelength-dependent noise is proposed, having more internal noise for low-spatial-frequency filters and less for high-spatial-frequency filters.     

Spatiotemporal integration and contour interpolation revealed by a dot localization task with serial presentation paradigm

The visual system seems to integrate information that is presented over time in a spatially fragmented fashion, with the result that observers are able to report the whole shape of objects. This research considers relations in space and time that allow the integrated percepts of complete objects. Specifically, temporal characteristics for spatiotemporal integration of illusory contour and spatial characteristics of interpolated contour are examined. A serial presentation paradigm and a dot localization task were used in two experiments; observers localized a probe dot relative to a perceived contour of an illusory object. Each of four inducing figures was briefly presented in a serial order to observers and the total time of the series was manipulated. In Experiment 1 short time ranges varied up to 180 ms, whereas longer times were examined in Experiment 2. Overall, the results demonstrate that a short time allows spatiotemporal integration, and that the perceived location of contour consistently shifts with time range. These experiments suggest that the mechanism of spatiotemporal integration operates on spatial integration as a limiting case.     

Pictorial perspective: perception of size, linear, and texture perspective in children and adults

Perception of size, linear, and texture perspective was investigated in third-grade and sixth-grade children and in college adults in three separate studies. A matching task required the observer to choose from a set of four alternative real scenes the correct match for the test stimulus, which was either a picture or a real scene. Correct performance required that the subject utilize perspective information for both size and distance distance relations. Erroneous choices available to the subject indicated errors in size judgement, in distance judgment, or in both simultaneously. View was either restricted at the correct station point or was free, with head motion. There were no significant effects of grade level. For all three groups, mean percent correct was nearly 100% with the real-scene test stimuli, and significantly below the chance level with the picture test stimuli. Errors in size judgment occurred most frequently, indicating that the geometrically correct rate of perspective convergence was too rapid to be seen by the subjects as perceptually acceptable. With size-perspective information alone, the number of size plus distance errors also increased significantly. There was no significant effect of viewing condition.     

Perception of texture by vision and touch: multidimensionality and intersensory integration

A series of six experiments offers converging evidence that there is no fixed dominance hierarchy for the perception of textured patterns, and in doing so, highlights the importance of recognizing the multidimensionality of texture perception. The relative bias between vision and touch was reversed or considerably altered using both discrepancy and nondiscrepancy paradigms. This shift was achieved merely by directing observers to judge different dimensions of the same textured surface. Experiments 1, 4, and 5 showed relatively strong emphasis on visual as opposed to tactual cues regarding the spatial density of raised dot patterns. In contrast, Experiments 2, 3, and 6 demonstrated considerably greater emphasis on the tactual as opposed to visual cues when observers were instructed to judge the roughness of the same surfaces. The results of the experiments were discussed in terms of a modality appropriateness interpretation of intersensory bias. A weighted averaging model appeared to describe the nature of the intersensory integration process for both spatial density and roughness perception.     

Ebbinghaus illusion: context, contour, and age influence the judged size of a circle amidst circles

Subjects (groups of age 6, 8, 10, 12, and 21 years, with 60 or more persons per group) made comparative size judgments between a single circle and a standard circle that was flanked by four context circles. The context circles varied in diameter, proximity to the standard circle, and amount of circumference displayed. (When circumferences are complete, the display is the usual Ebbinghaus configuration. A variant consisted of the Ebbinghaus display with the outer three quarters of each context-circle circumference removed.) A contour-plus-context theory accounts for the data: Contours attract at every age level, and context (size of flanking circles relative to the standard circle) leads to constrast in judged size beyond age 6. Contour and context effects decreased with decreasing proximity between context circles and the standard circle.     

Variations in size judgements as a function of surface texture

Adult subjects made monocular size judgements in two experiments in which the independent variables of surface texture and restrictions on viewing conditions were manipulated. Texture density gradients of stimulation had a significant influence of size judgements only under the less reduced conditions of observation when subjects could see other textured surfaces beyond the surfaces over which judgements were made. Identical manipulations of surface texture had earlier been found to have a highly significant influence on relative distance judgements (Newman, 1971). The principally negative results were thus taken to imply that subjects extract different information from the texture density gradient when judging size from that extracted when judging relative distance.     

Haptic and visual perception of roughness

In this study, we are interested in the following two questions: (1) how does perceived roughness correlate with physical roughness, and (2) how do visually and haptically perceived roughness compare? We used 96 samples of everyday materials, such as wood, paper, glass, sandpaper, ceramics, foams, textiles, etc. The samples were characterized by various different physical roughness measures, all determined from accurately measured roughness profiles. These measures consisted of spectral densities measured at different spatial scales and industrial roughness standards (R(a), R(q) and R(z)). In separate haptic and visual conditions, 12 na?ve subjects were instructed to order the 96 samples according to perceived roughness. The rank orders of both conditions were correlated with the various physical roughness measures. With most physical roughness measures, haptic and visual correspondence with the physical ordering was about equal. With others, haptic correspondence was slightly better. It turned out that different subjects ordered the samples using different criteria; for some subjects the correlation was better with roughness measures that were based on higher spatial frequencies, while others seemed to be paying more attention to the lower spatial frequencies. Also, physical roughness was not found to be the same as perceived roughness.     

Judging the time to collision with a simulated textured object: Effect of mismatching rate of expansion of object size and of texture element size

We measured the accuracy with which subjects estimated the time to collision with a simulated textured object approaching at constant speed along the line of sight. The independent variable was the ratioR, whereR = (rate of dilation of the texture elements that covered the simulated object) / (rate of dilation of object size). When matching was perfect (i.e.,R = 1.0), the mean of 12 settings was close to the nominal value of 2,000 msec for both subjects. In addition, the standard error of 12 settings was only 25 and 52 msec in 2,000 msec for the 2 subjects. Discrimination threshold for time to collision was not significantly affected byR over the range investigated betweenR = 0 andR = 2.0. However, the accuracy of estimating time to collision was significantly affected byR. Estimated time to collision was a monotonic function ofR. For example, when the mismatch was only 10% (i.e.,R = 0.9) subjects judged time to collision would occur 178 msec later than the true time to collision of 2,000 msec.     

Number, texture and crowding

A recent study shows that selectivity for numerosity emerges as a natural property in deep networks of hierarchical generative models of visual perception. These results, together with recent conceptualizations of crowding and texture, suggest that the 'sense of number' is a fundamental visual property, independent of texture and seemingly related attributes.     

The effects of illumination level and retinal size on the depth stratification of subjective contour figures

The apparent stratification in depth of subjective contour figures over their backgrounds was investigated as a function of illumination level, figure size, and viewing distance. Magnitude estimation, with a real contour figure serving as the modulus, was used to measure the stratification in depth of a subjective contour figure over its background. Illumination level and retinal size both had significant effects on the depth stratification of the subjective contour figures. The greatest apparent depth differences were obtained for figures of small retinal size under low levels of illumination. These results paralleled previous findings for judgments of subjective contour strength. Consequently, both contour clarity and depth stratification of subjective contour figures are affected in similar ways by illumination level, figure size, and viewing distance. The implications of this response coupling are discussed in terms of current theories of subjective contours.     

Coloured overlays, text, and texture

In four studies children were asked to read aloud a passage of randomly ordered common words with and without a coloured sheet of plastic (overlay) placed upon the page. The children's rate of reading increased with the overlay, for some children more than for others. The children were also asked to undertake a test of texture segmentation in which targets consisting of a structured texture had to be distinguished from within a random background texture. The texture segmentation was improved when the overlay was used, again for some children more than for others. The improvement in texture segmentation was, in general, correlated with the improvement in rate of reading. Slower readers were generally poorer at texture segmentation. The implications for reading, for texture segmentation, and for clinical tests of vision are discussed.