Performance of blind and sighted humans on a tactile grating detection task

We compared the abilities of blind and sighted humans to distinguish grooved from smooth surfaces pressed against the stationary index fingertip. Ranging in age from 20 to 72 years, 37 blind and 47 sighted subjects participated in an automated two-alternative forced-choice tactile grating detection task. The tactile acuity of blind and sighted subjects declined with age at equivalent rates (0.011-mm threshold increase per year), but the blind subjects were able to perceive significantly thinner grooves than were their sighted peers (the average difference between blind and sighted subjects of the same age and gender was 0.267 mm). The blind Braille readers performed no better than the blind nonreaders, and the congenitally blind subjects performed equivalently to those with adult-onset blindness. The superior tactile acuity of blind persons may result from the involvement of normally visually responsive cerebrocortical areas in tactile processing, as shown by functional-imaging studies.     

Tactile perception in blind Braille readers: a psychophysical study of acuity and hyperacuity using gratings and dot patterns

It is not clear whether the blind are generally superior to the sighted on measures of tactile sensitivity or whether they excel only on certain tests owing to the specifics of their tactile experience. We compared the discrimination performance of blind Braille readers and age-matched sighted subjects on three tactile tasks using precisely specified stimuli. Initially, the blind significantly outperformed the sighted at a hyperacuity task using Braille-like dot patterns, although, with practice, both groups performed equally well. On two other tasks, hyperacute discrimination of gratings that differed in ridge width and spatial-acuity-dependent discrimination of grating orientation, the performance of the blind did not differ significantly from that of sighted subjects. These results probably reflect the specificity of perceptual learning due to Braille-reading experience.     

Journeying beyond classical somatosensory cortex.

Visual cortical areas are involved in a variety of somatosensory tasks in the sighted, including tactile perception of two-dimensional patterns and motion, and haptic perception of three-dimensional objects. It is still unresolved whether visual imagery or modality-independent representations can better explain such cross-modal recruitment. However, these explanations are not necessarily in conflict with each other and might both be true, if imagery processes can access modality-independent representations. Greater visual cortical engagement in blind compared to sighted people is commonplace during language tasks, and also seems to occur during processing of tactile spatial information. Such engagement is even greater in the congenitally blind compared to the late blind, indicative of enhanced cross-modal plasticity during early development. At the other extreme, short-term visual deprivation of the normally sighted also leads to cross-modal plasticity. Altogether, the boundaries between sensory modalities appear to be flexible rather than immutable.     

Tactile pattern recognition by graphic display: Importance of 3-D information for haptic perception of familiar objects

Haptic recognition of familiar objects by the early blind, the late blind, and the sighted was investigated with two-dimensional (2-D) and three-dimensional (3-D) stimuli produced by small tactor-pins. The 2-D stimulus was an outline of an object that was depicted by raising tactor-pins to 1.5 mm. The 3-D stimulus was a relief that was produced by raising the tactors up to 10 mm, corresponding to the height of the object. Mean recognition times for correct answers to the 3-D stimuli were faster than those for the 2-D stimuli, in all three subject groups. No statistically significant differences in percentage of correct responses between the 2-D and the 3-D stimuli were found for the late-blind and sighted groups, but the early-blind group demonstrated a significant difference. In addition, the haptic legibility for the quality of depiction of the object, without regard to whether or not the stimulus was understood, was measured. The haptic legibility of the 3-D stimuli was significantly higher than that of the 2-D stimuli for all the groups. These results suggest that 3-D presentation seems to promise a way to overcome the limitations of 2-D graphic display.     

Picture and pattern perception in the sighted and the blind: the advantage of the late blind

Two experiments are reported on the contribution of visual experience to tactile perception. In the first experiment, sighted, congenitally blind, and late blind individuals made tactual matches to tangible embossed shapes. In the second experiment, the same subjects attempted tactile identification of raised-line drawings. The three groups did not differ in the accuracy of shape matching, but both groups of blind subjects were much faster than the sighted. Late blind observers were far better than the sighted or congenitally blind at tactile picture identification. Four of the twelve pictures were correctly identified by most of the late blind subjects. The sighted and congenitally blind performed at comparable levels in picture naming. There was no evidence that visual experience alone aided the sighted in the tactile task under investigation, since they performed no better than did the early blind. The superiority of the late blind suggests that visual exposure to drawings and the rules of pictorial representation may help tactile picture identification when combined with a history of tactual experience.     

Perception of weight and volume of functional objects as judged by the sighted and blind

Summary The manner in which sighted, sighted-but-blindfolded, late-blind (subsequent to 7 years of age), and congenitally blind (blind since birth) persons employ physical parameters in determining weight and volume of functional objects was studied. Eight stimuli each having a unique combination of physical specifications were judged, using both a match and estimate procedure. The results indicated that information derived from mass, displacement, and density provides the basis for volumetric and weight judgements of sighted persons. Non-sighted individuals employ this information differently than do sighted persons. Likewise, individuals after 7 years of age retain information provided by early visual experience and thus may employ a combination of kinesthetic and visual cues. The results also show that sighted and late-blind groups may employ visual perception or memory in determining volume, whereas sighted-but-blindfolded persons and the congenitally blind may employ mass in volumetric tasks. All groups may employ mass (and perhaps density) in determining weight, and although the weight judgements of non-sighted groups correlate highly with mass, sighted subjects are more accurate in judgement (i.e. have the smaller constant error). Sighted subjects apparently employ a complex system in determining weight. Age at which blindness occurred and the number of years of blindness had no effect for the late-blind group.     

Retention of high tactile acuity throughout the life span in blindness

Previous studies of tactile acuity on the fingertip, using passive touch, have demonstrated an age-related decline in spatial resolution for both sighted and blind subjects. We have reexamined this age dependence with two newly designed tactile-acuity charts that require active exploration of the test symbols. One chart used dot patterns similar to braille, and the other used embossed Landolt rings. Groups of blind braille readers and sighted subjects ranging from 12 to 85 years old were tested in two experiments. We replicated previous findings for sighted subjects by showing an age-related decrease in tactile acuity by nearly 1% per year. Surprisingly, the blind subjects retained high acuity into old age, showing no age-related decline. For the blind subjects, tactile acuity did not correlate with braille reading speed, the amount of daily reading, or the age at which braille was learned. We conclude that when measured with active touch, blind subjects retain high tactile acuity into old age, unlike their aging sighted peers. We propose that blind people's use of active touch in daily activities, not specifically braille reading, results in preservation of tactile acuity across the life span.     

Haptic orientation perception benefits from visual experience: evidence from early-blind, late-blind, and sighted people

Early-blind, late-blind, and blindfolded sighted participants were presented with two haptic allocentric spatial tasks: a parallel-setting task, in an immediate and a 10-sec delay condition, and a task in which the orientation of a single bar was judged verbally. With respect to deviation size, the data suggest that mental visual processing filled a beneficial role in both tasks. In the parallel-setting task, the early blind performed more variably and showed no improvement with delay, whereas the late blind did improve, but less than the sighted did. In the verbal judgment task, both early- and late-blind participants displayed larger deviations than the sighted controls. Differences between the groups were absent or much weaker with respect to the haptic oblique effect, a finding that reinforces the view that this effect is not of visual origin. The role of visual processing mechanisms and visual experience in haptic spatial tasks is discussed.     

Numerical estimation in blind subjects: evidence of the impact of blindness and its following experience

Vision was for a long time considered to be essential in the elaboration of the semantic numerical representation. However, early visual deprivation does not seem to preclude the development of a spatial continuum oriented from left to right to represent numbers (J. Castronovo & X. Seron, 2007; D. Szücs & V. Csépe, 2005). The authors investigated the impact of blindness and its following experience on a 3rd property of the mental number line: its obedience to Weber's law. A group of blind subjects and a group of sighted subjects were submitted to 2 numerical estimation tasks: (a) a keypress estimation task and (b) an auditory events estimation task. Blind and sighted subjects' performance obeyed Weber's law. However, blind subjects demonstrated better numerical estimation abilities than did sighted subjects, especially in contexts involving proprioception, indicating the existence of better mapping abilities between the symbolic representations of numbers and their corresponding magnitude representations, obeying Weber's law (e.g., J. S. Lipton & E. Spelke, 2005). These findings suggest that blindness and its following experience with numbers might result in better accuracy in numerical processing.     

Late‐ but not early‐onset blindness impairs the development of audio‐haptic multisensory integration

Integrating different senses to reduce sensory uncertainty and increase perceptual precision can have an important compensatory function for individuals with visual impairment and blindness. However, how visual impairment and blindness impact the development of optimal multisensory integration in the remaining senses is currently unknown. Here we first examined how audio‐haptic integration develops and changes across the life span in 92 sighted (blindfolded) individuals between 7 and 70 years of age. We used a child‐friendly task in which participants had to discriminate different object sizes by touching them and/or listening to them. We assessed whether audio‐haptic performance resulted in a reduction of perceptual uncertainty compared to auditory‐only and haptic‐only performance as predicted by maximum‐likelihood estimation model. We then compared how this ability develops in 28 children and adults with different levels of visual experience, focussing on low‐vision individuals and blind individuals that lost their sight at different ages during development. Our results show that in sighted individuals, adult‐like audio‐haptic integration develops around 13–15 years of age, and remains stable until late adulthood. While early‐blind individuals, even at the youngest ages, integrate audio‐haptic information in an optimal fashion, late‐blind individuals do not. Optimal integration in low‐vision individuals follows a similar developmental trajectory as that of sighted individuals. These findings demonstrate that visual experience is not necessary for optimal audio‐haptic integration to emerge, but that consistency of sensory information across development is key for the functional outcome of optimal multisensory integration.     

Mechanisms in the haptic horizontal-vertical illusion: Evidence from sighted and blind subjects

The haptic horizontal-vertical illusion was studied in two experiments. In Experiment 1, the illusion was relatively weak in sighted subjects and depended on stimulus size and the nature of the figure, that is, whether the pattern was an inverted-T or L shape. Experiment 2 compared early blind and late blind subjects. The illusion was present for an inverted-T figure but absent for an L figure in late blind subjects. However, the early blind subjects treated both the L and T figures as similar and showed the illusion to both. These results support the idea that visual experience may alter haptic judgments in sighted and late blind subjects.     

Smell and taste function in the visually impaired

Surprisingly few quantitative studies have addressed the question of whether visually impaired individuals evidence, perhaps in compensation for their loss of vision, increased acuteness in their other senses. In this experiment we sought to determine whether blind subjects outperform sighted subjects on a number of basic tests of chemosensory function. Over 50 blind and 75 sighted subjects were administered the following olfactory and gustatory tests: the University of Pennsylvania Smell Identification Test (UPSIT); a 16-item odor discrimination test; and a suprathreshold taste test in which measures of taste-quality identification and ratings of the perceived intensity and pleasantness of sucrose, citric acid, sodium chloride, and caffeine were obtained. In addition, 39 blind subjects and 77 sighted subjects were administered a single staircase phenyl ethyl alcohol (PEA) odor detection threshold test. Twenty-three of the sighted subjects were employed by the Philadelphia Water Department and trained to serve on its water quality evaluation panel. The primary findings of the study were that (a) the blind subjects did not outperform sighted subjects on any test of chemosensory function and (b) the trained subjects significantly outperformed the other two groups on the odor detection, odor discrimination, and taste identification tests, and nearly outperformed the blind subjects on the UPSIT. The citric acid concentrations received larger pleasantness ratings from the trained panel members than from the blind subjects, whose ratings did not differ significantly from those of the untrained sighted subjects. Overall, the data imply that blindness, per se, has little influence on chemosensory function and add further support to the notion that specialized training enhances performance on a number of chemosensory tasks.     

Early- and late-onset blindness both curb audiotactile integration on the parchment-skin illusion

It has been shown that congenital blindness can lead to anomalies in the integration of auditory and tactile information, at least under certain conditions. In the present study, we used the parchment-skin illusion, a robust illustration of sound-biased perception of touch based on changes in frequency, to investigate the specificities of audiotactile interactions in early- and late-onset blind individuals. Blind individuals in both groups did not experience any illusory change in tactile perception when the frequency of the auditory signal was modified, whereas sighted individuals consistently experienced the illusion. This demonstration that blind individuals had reduced susceptibility to an auditory-tactile illusion suggests either that vision is necessary for the establishment of audiotactile interactions or that auditory and tactile information can be processed more independently in blind individuals than in sighted individuals. In addition, the results obtained in late-onset blind participants suggest that visual input may play a role in the maintenance of audiotactile integration.     

Imagery in the congenitally blind: how visual are visual images?

Three experiments compared congenitally blind and sighted adults and children on tasks presumed to involve visual imagery in memory. In all three, the blind subjects' performances were remarkably similar to the sighted. The first two experiments examined Paivio's (1971) modality-specific imagery hypothesis. Experiment 1 used a paired-associate task with words whose referents were high in either visual or auditory imagery. The blind, like the sighted, recalled more high-visual-imagery pairs than any others. Experiment 2 used a free-recall task for words grouped according to modality-specific attributes, such as color and sound. The blind performed as well as the sighted on words grouped by color. In fact, the only consistent deficit in both experiments occurred for the sighted in recall of words whose referents are primarily auditory. These results challenge Paivio's theory and suggest either (a) that the visual imagery used by the sighted is no more facilitating than the abstract semantic representations used by the blind or (b) that the sighted are not using visual imagery. Experiment 3 used Neisser and Kerr's (1973) imaging task. Subjects formed images of scenes in which target objects were described as either visible in the picture plane or concealed by another object and thus not visible. On an incidental recall test for the target objects, the blind, like the sighted, recalled more pictorial than concealed targets. This finding suggests that the haptic images of the blind maintain occlusion just as the visual images of the sighted do.     

Hand preferences and hand ability in congenitally blind children

Congenitally blind and sighted blindfolded children between the ages of 6 and 14 years were tested for hand preference with performance tasks. There were no differences between the groups in direction or degree of hand preference. The degree of handedness increased with age and was essentially linear though the blind seemed to be somewhat less lateralized at the younger ages.

When the same groups were required to match three-dimensional bricks for height, depth, breadth, and volume, no hand advantages were found for either group. Both groups of children improved in their accuracy of spatial discriminations with age.

Further, the degree of lateralization on the handedness task did not relate to ability on the tactile task or to differences between the right and left hands on the tactile task. Thus, there is no effect of blindness on tactile matching ability nor is there an effect of the hand used in the task.     

Visuo-spatial processing in congenitally blind people: is there a gender-related preference?

Several studies have shown that blind and sighted individuals are equally able to generate and process visuo-spatial images. However, differences emerge when active manipulation of information is required. A similar distinction has been reported in studies investigating gender-related preferences in visuo-spatial tasks indicating a male advantage in active tasks only. The present study investigates gender-related visuo-spatial preference in congenitally blind people by using active and passive working memory tasks. The results show significant effects due to gender and blindness; nevertheless, gender effects are more pronounced in the blind group and maximised in the active task. These data are interpreted within a theoretical framework that accounts for the different factors modulating the development of visuo-spatial abilities.     

The haptic oblique effect in the perception of rod orientation by blind adults

The haptic perception of vertical, horizontal, +45° oblique, and +135° oblique orientations was studied in completely blind adults. The purpose was to determine whether the variations of the gravitational cues provided by the arm-hand system during scanning would affect the manifestation of the oblique effect (lower performance in oblique orientations than in vertical-horizontal ones) as they did in blindfolded sighted people (Gentaz & Hatwell, 1996). In blindfolded sighted adults, the oblique effect was reduced or absent when the magnitude of gravitational cues was decreased. If visual experience participated in the haptic oblique effect, we should observe no oblique effect in early blind subjects in the conditions of manual exploration where late blind and blindfolded sighted manifest this effect. The magnitude of gravitational cues was therefore varied by changing gravity constraints, whereas the variability of these cues was varied by changing the plane in which the task was performed: horizontal (low variability) and frontal (high variability). Early and late blind adults were asked to explore haptically a rod and then to reproduce its orientation ipsilateraUy in one of two exploratory conditions in each plane. In the horizontal plane, the oblique effect was absent, whatever the gravity constraints, in both groups (early and late blind subjects). In the frontal plane, the oblique effect was present, whatever the gravity constraints, in both groups. Taken together, these results showed that, in blind people, the variability of gravitational cues played a role in the haptic oblique effect; no effect of previous visual experience was observed.     

Haptic concepts in the blind

We investigated and compared the acquisition of haptic concepts by the blind with the acquisition of haptic concepts by sighted controls. Each subject—blind, sighted but blindfolded, sighted and touching, and sighted only-initially classified eight objects into two categories using a study/test format, followed by a recognition/classification test involving old, new, and prototype forms. Each object varied along the dimensions of shape, size, and texture, with each dimension having five values. The categories were linearly separable in three dimensions, but no single dimension permitted 100% accurate classification. The results revealed that blind subjects learned the categories quickly and comparably with sighted controls. On the classification test, all groups performed equivalently, with the category prototype classified more accurately than the old or new stimuli. The blind subjects differed from the other subjects on the recognition test in two ways: They were least likely to false alarm to novel patterns that belonged to the category but most likely to false alarm to the category prototype, which they falsely called “old” 100% of the time. We discuss these results in terms of current views of categorization.     

Mental rotation in blind and sighted adolescents: The effects of haptic strategies

IntroductionWhen observing blind persons in a situation of mental rotation, it is more usual to emphasize their lack of visual experience than their expertise concerning tactile exploration. Nevertheless, it seems important to verify whether the procedures of tactile exploration developed by young blind persons enable them to recognize a shape whatever its orientation in space.ObjectiveThe objective of this study was to compare blind adolescents and sighted adolescents in a situation of mental rotation, with raised patterns.MethodIn this perspective, a group of blind adolescents and a group of sighted adolescents – matched for age and sex – have been observed during two task of mental rotation.ResultsThe results show that the blind adolescents are always faster, and they perform better when the rotation task concerns simple shapes. A fine analysis of the strategies of tactile exploration deployed by the subjects allows us to identify several procedures that are specific to the blind adolescents. These exploratory procedures allow them to compensate for the lack of visual experience, up to a certain level of complexity in the shapes.ConclusionThese results suggest that the recognition of 2D shapes after rotation in space is not necessarily problematic for the blind. These results are discussed in relation with recent work on the haptic competence of blind persons.     

Reference frame preferences in haptics differ for the blind and sighted in the horizontal but not in the vertical plane

We investigated which reference frames are preferred when matching spatial language to the haptic domain. Sighted, low-vision, and blind participants were tested on a haptic-sentence-verification task where participants had to haptically explore different configurations of a ball and a shoe and judge the relation between them. Results from the spatial relation "above", in the vertical plane, showed that various reference frames are available after haptic inspection of a configuration. Moreover, the pattern of results was similar for all three groups and resembled patterns found for the sighted on visual sentence-verification tasks. In contrast, when judging the spatial relation "in front", in the horizontal plane, the blind showed a markedly different response pattern. The sighted and low-vision participants did not show a clear preference for either the absolute/relative or the intrinsic reference frame when these frames were dissociated. The blind, on the other hand, showed a clear preference for the intrinsic reference frame. In the absence of a dominant cue, such as gravity in the vertical plane, the blind might emphasise the functional relationship between the objects owing to enhanced experience with haptic exploration of objects.