Texture perception in sighted and blind observers

Prospective comparative study on enhancing geometrical mental representation and anatomical learning in medical students through modeling clay as an assessment tool

While traditional anatomy education often emphasizes passive learning and rote memorization, it seldom employs constructivist theories that focus on active, hands-on experiences for effective learning. This study tests the hypothesis that tactile experiences with modeling clay enhance geometric mental representation more effectively than verbal descriptions. We examine the potential of clay modeling to reflect and evaluate students' mental representation of anatomical structures. Utilizing a prospective randomized, open, blind endpoint (PROBE) design, we engaged 36 participants divided into three groups: blind, visually impaired, and a control group. The primary outcome measure was the difference between post-touch score and pre-touch score, which gauged the efficacy of tactile intervention in enhancing the participants' capacity to mentally reconstruct a well-designed anatomical fiction structure. Scoring was executed using a double-blind double-evaluation methodology, positioning clay model reconstruction as both an objective reliable assessment and a pedagogical tool. The analysis demonstrated a pronounced improvement in mental representation of anatomical structures, particularly in spatial comprehension and detailed textural discernment, with the mean score soaring from 47.81 pre-touch to 85.09 post-touch. Distinct cognitive processing adaptations were evident across groups, with the blind group excelling in tactile acuity and the visually impaired group showing the most significant gains in spatial understanding post-intervention (Cohen's d = 2.74). The application of modeling clay as a tactile intervention tool can significantly improve the learning of anatomy among medical students. The study underscores the value of active, multisensory engagement and customized teaching strategies for effective anatomy education among this demographic.

Neural underpinnings of the interplay between actual touch and action imagination in social contexts

While there is established evidence supporting the involvement of the sense of touch in various actions, the neural underpinnings of touch and action interplay in a social context remain poorly understood. To prospectively investigate this phenomenon and offer further insights, we employed a combination of motor and sensory components by asking participants to imagine exerting force with the index finger while experiencing their own touch, the touch of one another individual, the touch of a surface, and no touch. Based on the assumption that the patterns of activation in the motor system are similar when action is imagined or actually performed, we proceeded to apply a single-pulse transcranial magnetic stimulation over the primary motor cortex (M1) while participants engaged in the act of imagination. Touch experience was associated with higher M1 excitability in the presence and in the absence of force production imagination, but only during force production imagination M1 excitability differed among the types of touch: both biological sources, the self-touch and the touch of one other individual, elicited a significant increase in motor system activity when compared to touching a non-living surface or in the absence of touch. A strong correlation between individual touch avoidance questionnaire values and facilitation in the motor system was present while touching another person, indicating a social aspect for touch in action. The present study unveils the motor system correlates when the sensory/motor components of touch are considered in social contexts.

Multisensory Texture Perception in Individuals with Williams Syndrome

The sensory profile of people with Williams syndrome (WS) is characterised by atypical visual and auditory perceptions that affect their daily lives and learning. However, no research has been carried out on the haptic perception, in particular in multisensory (visual and haptic) situations. The aim of this study was to evaluate the communication of texture information from one modality to the other in people with WS. Children and adults with WS were included, as well as typically developing (TD) participants matched on chronological age (TD-CA), and TD children matched on mental age (TD-MA). All participants (N = 69) completed three matching tasks in which they had to compare two fabrics (same or different): visual, haptic and visuo-haptic. When the textures were different, the haptic and visual performances of people with WS were similar to those of TD-MA participants. Moreover, their visuo-haptic performances were lower than those of the two TD groups. These results suggest a delay in the acquisition of multisensory transfer abilities in individuals with WS. A positive link between MA and visual and visuo-haptic abilities only in people with WS suggests that they could benefit from an early intervention to develop their abilities to process and transfer multisensory information.

Late Blindness and Deafness are Associated with Decreased Tactile Sensitivity, But Early Blindness is Not

Perceptual experience is shaped by a complex interaction between our sensory systems in which each sense conveys information on specific properties of our surroundings. This multisensory processing of complementary information improves the accuracy of our perceptual judgments and leads to more precise and faster reactions. Sensory impairment or loss in one modality leads to information deficiency that can impact other senses in various ways. For early auditory or visual loss, impairment and/or compensatory increase of the sensitivity of other senses are equally well described. Investigating individuals with deafness (N = 73), early (N = 51), late blindness (N = 49) and corresponding controls, we compared tactile sensitivity using the standard monofilament test on two locations, the finger and handback. Results indicate lower tactile sensitivity in people with deafness and late blindness but not in people with early blindness compared to respective controls, irrespective of stimulation location, gender, and age. Results indicate that neither sensory compensation nor simple use-dependency or a hindered development of the tactile sensory system is sufficient to explain changes in somatosensation after the sensory loss but that a complex interaction of effects is present.

Establishing haptic texture attribute space and predicting haptic attributes from image features using 1D-CNN

The current study strives to provide a haptic attribute space where texture surfaces are located based on their haptic attributes. The main aim of the haptic attribute space is to come up with a standardized model for representing and identifying haptic textures analogous to the RGB model for colors. To this end, a four dimensional haptic attribute space is established by conducting a psychophysical experiment where human participants rate 100 real-life texture surfaces according to their haptic attributes. The four dimensions of the haptic attribute space are rough-smooth, flat-bumpy, sticky-slippery, and hard-soft. The generalization and scalability of the haptic attribute space is achieved by training a 1D-CNN model for predicting attributes of haptic textures. The 1D-CNN is trained using the attribute data from psychophysical experiments and image features collected from the images of real textures. The prediction power granted by the 1D-CNN renders scalability to the haptic attribute space. The prediction accuracy of the proposed 1D-CNN model is compared against other machine learning and deep learning algorithms. The results show that the proposed method outperforms the other models on MAE and RMSE metrics.

Visual experience shapes the Bouba-Kiki effect and the size-weight illusion upon sight restoration from congenital blindness

The Bouba-Kiki effect is the systematic mapping between round/spiky shapes and speech sounds ("Bouba"/"Kiki"). In the size-weight illusion, participants judge the smaller of two equally-weighted objects as being heavier. Here we investigated the contribution of visual experience to the development of these phenomena. We compared three groups: early blind individuals (no visual experience), individuals treated for congenital cataracts years after birth (late visual experience), and typically sighted controls (visual experience from birth). We found that, in cataract-treated participants (tested visually/visuo-haptically), both phenomena are absent shortly after sight onset, just like in blind individuals (tested haptically). However, they emerge within months following surgery, becoming statistically indistinguishable from the sighted controls. This suggests a pivotal role of visual experience and refutes the existence of an early sensitive period: A short period of experience, even when gained only years after birth, is sufficient for participants to visually pick-up regularities in the environment, contributing to the development of these phenomena.

Crossmodal Texture Perception Is Illumination-Dependent

Visually perceived roughness of 3D textures varies with illumination direction. Surfaces appear rougher when the illumination angle is lowered resulting in a lack of roughness constancy. Here we aimed to investigate whether the visual system also relies on illumination-dependent features when judging roughness in a crossmodal matching task or whether it can access illumination-invariant surface features that can also be evaluated by the tactile system. Participants ( N = 32) explored an abrasive paper of medium physical roughness either tactually, or visually under two different illumination conditions (top vs oblique angle). Subsequently, they had to judge if a comparison stimulus (varying in physical roughness) matched the previously explored standard. Matching was either performed using the same modality as during exploration (intramodal) or using a different modality (crossmodal). In the intramodal conditions, participants performed equally well independent of the modality or illumination employed. In the crossmodal conditions, participants selected rougher tactile matches after exploring the standard visually under oblique illumination than under top illumination. Conversely, after tactile exploration, they selected smoother visual matches under oblique than under top illumination. These findings confirm that visual roughness perception depends on illumination direction and show, for the first time, that this failure of roughness constancy also transfers to judgements made crossmodally.

Individuals with and without Visual Impairments Use a Force Feedback Device to Identify the Friction and Hardness of Haptic Surfaces

The general purpose of this study is to promote access to haptic virtual environments. Using a haptic device, people with and without visual impairments (VI) are able to feel different textures and compare these textures based on different surface properties, i.e., friction and hardness. The objectives of this study were to examine the following: (a) whether the variables of friction and hardness were identifiable through the Touch device (Phantom Omni) and could therefore function as 3D haptic variables; (b) if there were differences between people with VI and sighted individuals in terms of their performance; (c) the differences that should exist between the values of each variable so that the virtual surfaces could be identified as different to each other; and (d) if the individual characteristics of participants have an impact on their performance. The results showed that it is necessary to use surfaces which are differentiated based on the degree of friction and hardness because the haptic properties of a virtual object are then better perceived. Individuals with VI need more time and more effort to understand friction and hardness, respectively. With the motivation of increasing access to object perception for people with VI in a virtual environment, accessibility advisors and experts can extract useful information for the development of functional and efficient 3D objects for haptic perception.

Roughness perception: A multisensory/crossmodal perspective

Roughness is a perceptual attribute typically associated with certain stimuli that are presented in one of the spatial senses. In auditory research, the term is typically used to describe the harsh effects that are induced by particular sound qualities (i.e., dissonance) and human/animal vocalizations (e.g., screams, distress cries). In the tactile domain, roughness is a crucial factor determining the perceptual features of a surface. The same feature can also be ascertained visually, by means of the extraction of pattern features that determine the haptic quality of surfaces, such as grain size and density. By contrast, the term roughness has rarely been applied to the description of those stimuli perceived via the chemical senses. In this review, we take a critical look at the putative meaning(s) of the term roughness, when used in both unisensory and multisensory contexts, in an attempt to answer two key questions: (1) Is the use of the term 'roughness' the same in each modality when considered individually? and (2) Do crossmodal correspondences involving roughness match distinct perceptual features or (at least on certain occasions) do they merely pick-up on an amodal property? We start by examining the use of the term in the auditory domain. Next, we summarize the ways in which the term roughness has been used in the literature on tactile and visual perception, and in the domain of olfaction and gustation. Then, we move on to the crossmodal context, reviewing the literature on the perception of roughness in the audiovisual, audiotactile, and auditory-gustatory/olfactory domains. Finally, we highlight some limitations of the reviewed literature and we outline a number of key directions for future empirical research in roughness perception.

Predictive attenuation of touch and tactile gating are distinct perceptual phenomena

In recent decades, research on somatosensory perception has led to two important observations. First, self-generated touches that are predicted by voluntary movements become attenuated compared with externally generated touches of the same intensity (attenuation). Second, externally generated touches feel weaker and are more difficult to detect during movement than at rest (gating). At present, researchers often consider gating and attenuation the same suppression process; however, this assumption is unwarranted because, despite more than 40 years of research, no study has combined them in a single paradigm. We quantified how people perceive self-generated and externally generated touches during movement and rest. We show that whereas voluntary movement gates the precision of both self-generated and externally generated touch, the amplitude of self-generated touch is robustly attenuated compared with externally generated touch. Furthermore, attenuation and gating do not interact and are not correlated, and we conclude that they represent distinct perceptual phenomena.

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We tested the perception of self-generated and external touch during movement and rest

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The intensity of self-generated touch is reduced during movement and rest (attenuation)

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The precision of self-generated and external touch is reduced during movement (gating)

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Attenuation and gating neither interact nor correlate, and are distinct phenomena

The Role of Spatial and Modality Cues on Visual and Haptic Memory

This study investigates the ability to remember a sequence of stimuli in two basic conditions: haptic and visual. Participants rely on a combination of modal and/or spatial information to perform a memory task. For this purpose, an experimental setup was developed based on the "Simon Says" memory game. Individuals receive a series of sensory stimuli and need to remember the sequence and repeat it. The stimuli in visual conditions are colored or white lights, and the stimuli in haptic conditions are vibration, hot, cold, and skin stretch. Results demonstrate that participants retained longer sequences in spatial conditions compared to the modal conditions. It is also demonstrated that participants performed better in visual conditions compared to haptic conditions. Participants were able to retain more complex spatial patterns and remember them faster in visual conditions compared to haptic conditions. A spatial difficulty ranking system was developed, indicating how easily each spatial pattern can be retained visually and haptically.

The look and feel of soft are similar across different softness dimensions

The softness of objects can be perceived through several senses. For instance, to judge the softness of a cat's fur, we do not only look at it, we often also run our fingers through its coat. Recently, we have shown that haptically perceived softness covaries with the compliance, viscosity, granularity, and furriness of materials (Dovencioglu, Üstün, Doerschner, & Drewing, 2020). However, it is unknown whether vision can provide similar information about the various aspects of perceived softness. Here, we investigated this question in an experiment with three conditions: in the haptic condition, blindfolded participants explored materials with their hands, in the static visual condition participants were presented with close-up photographs of the same materials, and in the dynamic visual condition participants watched videos of the hand-material interactions that were recorded in the haptic condition. After haptically or visually exploring the materials, participants rated them on various attributes. Our results show a high overall perceptual correspondence among the three experimental conditions. With a few exceptions, this correspondence tended to be strongest between haptic and dynamic visual conditions. These results are discussed with respect to information potentially available through the senses, or through prior experience, when judging the softness of materials.

The roles of lower- and higher-order surface statistics in tactile texture perception

Humans can haptically discriminate surface textures when there is a significant difference in the statistics of the surface profile. Previous studies on tactile texture discrimination have emphasized the perceptual effects of lower-order statistical features such as carving depth, inter-ridge distance, and anisotropy, which can be characterized by local amplitude spectra or spatial-frequency/orientation subband histograms. However, the real-world surfaces we encounter in everyday life also differ in the higher-order statistics, such as statistics about correlations of nearby spatial-frequencies/orientations. For another modality, vision, the human brain has the ability to use the textural differences in both higher- and lower-order image statistics. In this work, we examined whether the haptic texture perception can use higher-order surface statistics as visual texture perception does, by three-dimensional (3-D)-printing textured surfaces transcribed from different "photos" of natural scenes such as stones and leaves. Even though the maximum carving depth was well above the haptic detection threshold, some texture pairs were hard to discriminate. Specifically, those texture pairs with similar amplitude spectra were difficult to discriminate, which suggests that the lower-order statistics have the dominant effect on tactile texture discrimination. To directly test the poor sensitivity of the tactile texture perception to higher-order surface statistics, we matched the lower-order statistics across different textures using a texture synthesis algorithm and found that haptic discrimination of the matched textures was nearly impossible unless the stimuli contained salient local features. We found no evidence for the ability of the human tactile system to use higher-order surface statistics for texture discrimination.NEW & NOTEWORTHY Humans can discriminate subtle spatial patterns differences in the surrounding world through their hands, but the underlying computation remains poorly understood. Here, we 3-D-printed textured surfaces and analyzed the tactile discrimination performance regarding the sensitivity to surface statistics. The results suggest that observers have sensitivity to lower-order statistics whereas not to higher-order statistics. That is, touch differs from vision not only in spatiotemporal resolution but also in (in)sensitivity to high-level surface statistics.

Developmental plasticity of texture discrimination following early vision loss in the marsupial Monodelphis domestica

Behavioral strategies that depend on sensory information are not immutable; rather they can be shaped by the specific sensory context in which animals develop. This behavioral plasticity depends on the remarkable capacity of the brain to reorganize in response to alterations in the sensory environment, particularly when changes in sensory input occur at an early age. To study this phenomenon, we utilize the short-tailed opossum, a marsupial that has been a valuable animal model to study developmental plasticity due to the extremely immature state of its nervous system at birth. Previous studies in opossums have demonstrated that removal of retinal inputs early in development results in profound alterations to cortical connectivity and functional organization of visual and somatosensory cortex; however, behavioral consequences of this plasticity are not well understood. We trained early blind and sighted control opossums to perform a two-alternative forced choice texture discrimination task. Whisker trimming caused an acute deficit in discrimination accuracy for both groups, indicating the use of a primarily whisker-based strategy to guide choices based on tactile cues. Mystacial whiskers were important for performance in both groups; however, genal whiskers only contributed to behavioral performance in early blind animals. Early blind opossums significantly outperformed their sighted counterparts in discrimination accuracy, with discrimination thresholds that were lower by ∼75 μm. Our results support behavioral compensation following early blindness using tactile inputs, especially the whisker system.

Tribology of an assembly of hairs: Influence of multiscale surface chemistry and structure on sensorial tactile properties

BACKGROUND

Hair fibers may be either oriented in a common direction or randomly arranged. Fiber arrangement as well as cosmetic treatment control the sensorial perception. The present study explores the respective influence of these two aspects by predicting the product performance in terms of tactile perception.

MATERIALS AND METHODS

Friction forces between hair swatches of different curl patterns using a finger-like probe have been measured to better mimic real-life hair/finger contact. Measurements of fiber alignment, hair diameter (thickness), and compression tests were performed on natural and treated swatches to assess the respective weight of these parameters.

RESULTS

Conditioned hair exhibit an adhesive behavior measured at the start of the frictional movement. Conversely, natural hair is influenced by fiber reorientation. After a few seconds, friction-related signals stabilize. Thus, the averaged friction forces do not only depend on hair thickness, but increase with a decreased alignment of the fibers.

CONCLUSIONS

Intrinsic (diameter/curliness) and external (orientation/ friction/compression) characteristics allow to define a model of "macroscopic" roughness linked to the sensorial characterization. As friction of hair swatches depends upon fiber alignment and coating, this combined approach is potentially a very useful in vitro test, as an alternative or complementary method to sensory tests.

Improvements and Degradation to Spatial Tactile Acuity Among Blind and Deaf Individuals

Cross-modal reorganization takes place for sensory cortices when there is no more primary input. For instance, the visual cortex in blind individuals which receives no visual input starts responding to auditory and tactile stimuli. Reorganization may improve or degrade processing of other modality inputs, via bottom-up compensational processes and top-down updating. In two experiments, we measured the spatial tactile response in a large sample of early- (N = 49) and late-blind (N = 51) individuals with varying levels of Braille proficiencies, and early-deaf (N = 69) with varying levels of hearing devices against separate hearing and sighted controls. Spatial tactile responses were measured using a standard gradient orientation task on two locations, the finger and tongue. Experiments show limited to no advantage in passive tactile response for blind individuals and degradation for deaf individuals at the finger. However, the use of hearing devices decreased the tactile impairment in early-deaf individuals. Also, no differences in age-related decline in both sensory-impaired groups were shown. Results show less tactile acuity differences between blind and sighted than previously reported, but supports recent reports of tactile impairment among the early-deaf.

Detection of Simulated Tactile Gratings by Electro-Static Friction Show a Dependency on Bar Width for Blind and Sighted Observers, and Preliminary Neural Correlates in Sighted Observers

The three-dimensional micro-structure of physical surfaces produces frictional forces that provide sensory cues about properties of felt surfaces such as roughness. This tactile information activates somatosensory cortices, and frontal and temporal brain regions. Recent advances in haptic-feedback technologies allow the simulation of surface micro-structures via electro-static friction to produce touch sensations on otherwise flat screens. These sensations may benefit those with visual impairment or blindness. The primary aim of the current study was to test blind and sighted participants' perceptual sensitivity to simulated tactile gratings. A secondary aim was to explore which brain regions were involved in simulated touch to further understand the somatosensory brain network for touch. We used a haptic-feedback touchscreen which simulated tactile gratings using digitally manipulated electro-static friction. In Experiment 1, we compared blind and sighted participants' ability to detect the gratings by touch alone as a function of their spatial frequency (bar width) and intensity. Both blind and sighted participants showed high sensitivity to detect simulated tactile gratings, and their tactile sensitivity functions showed both linear and quadratic dependency on spatial frequency. In Experiment 2, using functional magnetic resonance imaging, we conducted a preliminary investigation to explore whether brain activation to physical vibrations correlated with blindfolded (but sighted) participants' performance with simulated tactile gratings outside the scanner. At the neural level, blindfolded (but sighted) participants' detection performance correlated with brain activation in bi-lateral supplementary motor cortex, left frontal cortex and right occipital cortex. Taken together with previous studies, these results suggest that there are similar perceptual and neural mechanisms for real and simulated touch sensations.

Organized Toe Maps in Extreme Foot Users

Although the fine-grained features of topographic maps in the somatosensory cortex can be shaped by everyday experience, it is unknown whether behavior can support the expression of somatotopic maps where they do not typically occur. Unlike the fingers, represented in all primates, individuated toe maps have only been found in non-human primates. Using 1-mm resolution fMRI, we identify organized toe maps in two individuals born without either upper limb who use their feet to substitute missing hand function and even support their profession as foot artists. We demonstrate that the ordering and structure of the artists’ toe representation mimics typical hand representation. We further reveal “hand-like” features of activity patterns, not only in the foot area but also similarly in the missing hand area. We suggest humans may have an innate capacity for forming additional topographic maps that can be expressed with appropriate experience.

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We ask if extreme behavior can cause the (re)emergence of somatotopic maps

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We investigated two foot artists, born without arms

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7T fMRI shows individuated maps of up to 5 toes in the artists but not controls

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Activity in artists’ foot and hand areas was more “hand-like” than in controls

Visual motion information modulates tactile roughness perception

We perceive the roughness of an object through our eyes and hands. Many crossmodal studies have reported that there is no clear visuo-tactile interaction in roughness perception using static visual cues. One exception is that the visual observation of task-irrelevant hand movements, not the texture of task-relevant objects, can enhance the performance of tactile roughness discrimination. Our study investigated whether task-irrelevant visual motion without either object roughness or bodily cues can influence tactile roughness perception. Participants were asked to touch abrasive papers while moving their hand laterally and viewing moving or static sine wave gratings without being able to see their hand, and to estimate the roughness magnitude of the tactile stimuli. Moving gratings with a low spatial frequency induced smoother roughness perceptions than static visual stimuli when the visual grating moved in the direction opposite the hand movements. The effects of visual motion did not appear when the visual stimuli had a high spatial frequency or when the participants touched the tactile stimuli passively. These results indicate that simple task-irrelevant visual movement without object roughness or bodily cues can modulate tactile roughness perception with active body movements in a spatial-frequency-selective manner.

Perception of surface stickiness in different sensory modalities: an functional MRI study

Surface texture can be perceived not only from tactile, but also from auditory and visual sensory cues. In our previous psychophysical study, we demonstrated that humans can recognize surface stickiness using only one kind of sensory modality without any difficulty. However, the brain regions that would be activated by non-corresponding sensory cues, for example, auditory and visual cues, remain unknown. In this human functional MRI study, we explored brain regions associated with surface stickiness perception in each of three different sensory modalities, and sought for common neural activities across modalities. In the tactile condition, participants actually touched a sticky surface with their right index finger. In the auditory and visual conditions, audio and video clips of tactile explorations of a sticky surface were presented and participants were asked to recall the perceived stickiness as vividly as possible. Our results, based on a general linear model analysis, showed that somatosensory cortices including postcentral gyrus, anterior insula, and anterior intraparietal sulcus were significantly activated across all modalities. Moreover, we observed significant activation of primary sensory regions of each modality. A follow-up conjunction analysis identified that postcentral gyrus, anterior intraparietal sulcus, precentral gyrus, and supplementary motor area were activated in common. These findings could deepen our understanding of the surface stickiness perception in the human brain.

Blocking tactile input to one finger using anaesthetic enhances touch perception and learning in other fingers

Brain plasticity is a key mechanism for learning and recovery. A striking example of plasticity in the adult brain occurs following input loss, for example, following amputation, whereby the deprived zone is “invaded” by new representations. Although it has long been assumed that such reorganization leads to functional benefits for the invading representation, the behavioral evidence is controversial. Here, we investigate whether a temporary period of somatosensory input loss to one finger, induced by anesthetic block, is sufficient to cause improvements in touch perception (“direct” effects of deafferentation). Further, we determine whether this deprivation can improve touch perception by enhancing sensory learning processes, for example, by training (“interactive” effects). Importantly, we explore whether direct and interactive effects of deprivation are dissociable by directly comparing their effects on touch perception. Using psychophysical thresholds, we found brief deprivation alone caused improvements in tactile perception of a finger adjacent to the blocked finger but not to non-neighboring fingers. Two additional groups underwent minimal tactile training to one finger either during anesthetic block of the neighboring finger or a sham block with saline. Deprivation significantly enhanced the effects of tactile perceptual training, causing greater learning transfer compared with sham block. That is, following deafferentation and training, learning gains were seen in fingers normally outside the boundaries of topographic transfer of tactile perceptual learning. Our results demonstrate that sensory deprivation can improve perceptual abilities, both directly and interactively, when combined with sensory learning. This dissociation provides novel opportunities for future clinical interventions to improve sensation.

An Evaluation of Substrates for Tactile Maps and Diagrams: Scanning Speed and Users’ Preferences

This study evaluated the relative suitability of a range of base materials for producing tactile maps and diagrams via a new ink-jet process. The visually impaired and sighted participants tactilely scanned arrays of symbols that were printed on seven substrate materials, including paper, plastic, and aluminum. In general, the rougher substrates were scanned faster than the smoother substrates, and the majority of participants preferred the rougher substrates over the smoother ones.

Tactile Functioning in Children who are Blind: A Clinical Perspective

This study of 48 children with congenital blindness who attended mainstream schools focused on the tactile and haptic skills they needed in typical academic and everyday tasks. The results showed that, in general, the children mastered such tactile tasks, but some items posed special problems.

Edge orientation perception during active touch

Previous studies investigating the perceptual attributes of tactile edge orientation processing have applied their stimuli to an immobilized fingertip. Here we tested the perceptual attributes of edge orientation processing when participants actively touched the stimulus. Our participants moved their finger over two pairs of edges, one pair parallel and the other nonparallel to varying degrees, and were asked to identify which of the two pairs was nonparallel. In addition to the psychophysical estimates of edge orientation acuity, we measured the speed at which participants moved their finger and the forces they exerted when moving their finger over the stimulus. We report four main findings. First, edge orientation acuity during active touch averaged 12.4°, similar to that previously reported during passive touch. Second, on average, participants moved their finger over the stimuli at ~20 mm/s and exerted contact forces of ~0.3 N. Third, there was no clear relationship between how people moved their finger or how they pressed on the stimulus and their edge orientation acuity. Fourth, consistent with previous work testing tactile spatial acuity, we found a significant correlation between fingertip size and orientation acuity such that people with smaller fingertips tended to have better orientation acuity. NEW & NOTEWORTHY Edge orientation acuity expressed by the motor system during manipulation is many times better than edge orientation acuity assessed in psychophysical studies where stimuli are applied to a passive fingertip. Here we show that this advantage is not because of movement per se because edge orientation acuity assessed in a psychophysical task, where participants actively move their finger over the stimuli, yields results similar to previous passive psychophysical studies.

Mental Imagery Follows Similar Cortical Reorganization as Perception: Intra-Modal and Cross-Modal Plasticity in Congenitally Blind

Cortical plasticity in congenitally blind individuals leads to cross-modal activation of the visual cortex and may lead to superior perceptual processing in the intact sensory domains. Although mental imagery is often defined as a quasi-perceptual experience, it is unknown whether it follows similar cortical reorganization as perception in blind individuals. In this study, we show that auditory versus tactile perception evokes similar intra-modal discriminative patterns in congenitally blind compared with sighted participants. These results indicate that cortical plasticity following visual deprivation does not influence broad intra-modal organization of auditory and tactile perception as measured by our task. Furthermore, not only the blind, but also the sighted participants showed cross-modal discriminative patterns for perception modality in the visual cortex. During mental imagery, both groups showed similar decoding accuracies for imagery modality in the intra-modal primary sensory cortices. However, no cross-modal discriminative information for imagery modality was found in early visual cortex of blind participants, in contrast to the sighted participants. We did find evidence of cross-modal activation of higher visual areas in blind participants, including the representation of specific-imagined auditory features in visual area V4.

Haptic aesthetics in the blind: A behavioral and fMRI investigation

Understanding perception and aesthetic appeal of arts and environmental objects, what is appreciated, liked, or preferred, and why, is of prime importance for improving the functional capacity of the blind and visually impaired and the ergonomic design for their environment, which however so far, has been examined only in sighted individuals. This paper provides a general overview of the first experimental study of tactile aesthetics as a function of visual experience and level of visual deprivation, using both behavioral and brain imaging techniques. We investigated how blind people perceive 3D tactile objects, how they characterize them, and whether the tactile perception, and tactile shape preference (liking or disliking) and tactile aesthetic appreciation (judging tactile qualities of an object, such as pleasantness, comfortableness etc.) of 3D tactile objects can be affected by the level of visual experience. The study employed innovative behavioral measures, such as new forms of aesthetic preference-appreciation and perceptual discrimination questionnaires, in combination with advanced functional Magnetic Resonance Imaging (fMRI) techniques, and compared congenitally blind, late-onset blind and blindfolded (sighted) participants. Behavioral results demonstrated that both blind and blindfolded-sighted participants assessed curved or rounded 3D tactile objects as significantly more pleasing than sharp 3D tactile objects, and symmetric 3D tactile objects as significantly more pleasing than asymmetric 3D tactile objects. However, as compared to the sighted, blind people showed better skills in tactile discrimination as demonstrated by accuracy and speed of discrimination. Functional MRI results demonstrated that there was a large overlap and characteristic differences in the aesthetic appreciation brain networks in the blind and the sighted. As demonstrated both populations commonly recruited the somatosensory and motor areas of the brain, but with stronger activations in the blind as compared to the sighted. Secondly, sighted people recruited more frontal regions whereas blind people, in particular, the congenitally blind, paradoxically recruited more 'visual' areas of the brain. These differences were more pronounced between the sighted and the congenitally blind rather than between the sighted and the late-onset blind, indicating the key influence of the onset time of visual deprivation. Understanding of the underlying brain mechanisms should have a wide range of important implications for a generalized cross-sensory theory and practice in the rapidly evolving field of neuroaesthetics, as well as for 'cutting-edge' rehabilitation technologies for the blind and the visually impaired.

Processing of haptic texture information over sequential exploration movements

Where textures are defined by repetitive small spatial structures, exploration covering a greater extent will lead to signal repetition. We investigated how sensory estimates derived from these signals are integrated. In Experiment 1, participants stroked with the index finger one to eight times across two virtual gratings. Half of the participants discriminated according to ridge amplitude, the other half according to ridge spatial period. In both tasks, just noticeable differences (JNDs) decreased with an increasing number of strokes. Those gains from additional exploration were more than three times smaller than predicted for optimal observers who have access to equally reliable, and therefore equally weighted, estimates for the entire exploration. We assume that the sequential nature of the exploration leads to memory decay of sensory estimates. Thus, participants compare an overall estimate of the first stimulus, which is affected by memory decay, to stroke-specific estimates during the exploration of the second stimulus. This was tested in Experiments 2 and 3. The spatial period of one stroke across either the first or second of two sequentially presented gratings was slightly discrepant from periods in all other strokes. This allowed calculating weights of stroke-specific estimates in the overall percept. As predicted, weights were approximately equal for all strokes in the first stimulus, while weights decreased during the exploration of the second stimulus. A quantitative Kalman filter model of our assumptions was consistent with the data. Hence, our results support an optimal integration model for sequential information given that memory decay affects comparison processes.

Neurophysiological evidence for enhanced tactile acuity in early blindness in some but not all haptic tasks

Previous research assessing the presence of enhanced tactile skills in early-blind (EB) population obtained conflicting results. Most of the studies relied on behavioral measures with which different mechanisms leading to the same outcome go unnoticed. Moreover, the scarce electrophysiological research that has been conducted focused exclusively on the processing of microgeometric properties. To clarify the extent of superior tactile abilities in EBs using high-density multichannel electrophysiological recordings, the present study compared the electrophysiological correlates of EBs and sighted controls (CON) in two tactile discrimination tasks that targeted microgeometric (texture) and macrogeometric (shape) properties. After a restricted exploration (haptic glance), participants judged whether a touched stimulus corresponded to an expected stimulus whose name had been previously presented aurally. In the texture discrimination task, differences between groups emerged at ∼75 ms (early perceptual processing stages) whereas we found no between-group differences during shape discrimination. Furthermore, for the first time, we were able to determine the latency at which EBs started to discriminate micro- (EB: 170 ms; CON: 230 ms) and macrogeometric (EB: 250 ms; CON: 270 ms) properties. Altogether, the results suggest different electrophysiological signatures during texture (but not shape) discrimination in EBs, possibly due to cortical reorganization in occipital areas and their increased connectivity with S1.

Linkage between Free Exploratory Movements and Subjective Tactile Ratings

We actively move our hands and eyes when exploring the external world and gaining information about object's attributes. Previous studies showing that how we touch might be related to how we felt led us to consider whether we could decode observers' subjective tactile experiences only by analyzing their exploratory movements without explicitly asking how they perceived. However, in those studies, explicit judgment tasks were performed about specific tactile attributes that were prearranged by experimenters. Here, we systematically investigated whether exploratory movements can explain tactile ratings even when participants do not need to judge any tactile attributes. While measuring both hand and eye movements, we asked participants to touch materials freely without judging any specific tactile attributes (free-touch task) or to evaluate one of four tactile attributes (roughness, hardness, slipperiness, and temperature). We found that tactile ratings in the judgment tasks correlated with exploratory movements even in the free-touch task and that eye movements as well as hand movements correlated with tactile ratings. These results might open up the possibility of decoding tactile experiences by exploratory movements.

Not only age but also tactile perception influences the preference for cosmetic creams applied to the forearm

OBJECTIVE

We aimed to examine whether and how age as well as tactile sensitivity and perception had an impact on how women liked richer and lighter creams. Furthermore, the question arose if age and tactile perception had an influence on the ability to distinguish between the creams and how the ability to distinguish between creams influenced the liking of these creams.

METHODS

A total of 299 female participants were invited to rate how much they liked four different cosmetic creams applied to their forearms. The creams were based on the same base formula but differed with respect to the texture. In order to arouse the impression of more lightness (quasi-light) or more richness (quasi-rich), polyethylene particles of different sizes were added to the base formula. First of all, the participants were tested for their tactile sensitivity and perception. Tactile sensitivity was tested by Von Frey filaments, tactile spatial perception by the tactile Landolt ring test and the ability to discriminate surface structures by a sandpaper test. Furthermore, the participants rated the creams with respect to the acceptance, the subjective skin feeling after application and performed paired-comparison tests. Analyses of variance and regression analyses were applied to the data.

RESULTS

In general, participants liked quasi-rich creams less than quasi-light creams. However, older women compared to younger women and women with lower tactile performance in comparison with women with higher tactile performance revealed a weaker influence of cream type-specific acceptance ratings. Further results revealed that young participants perceived the quasi-light creams (with particles of ~50 μm diameter) as soft and quasi-rich creams (with particles of ~100 μm and ~165 μm diameter), as coarse. In contrast, this subjective skin feeling after application in participants at age 50 and older did not differ much.

CONCLUSION

Age and tactile perceptual abilities have additive effects on the acceptance of creams with different textures when applied to the forearm.

Crossmodal Processing of Haptic Inputs in Sighted and Blind Individuals

In a previous behavioral study, it was shown that early blind individuals were superior to sighted ones in discriminating two-dimensional (2D) tactile angle stimuli. The present study was designed to assess the neural substrate associated with a haptic 2D angle discrimination task in both sighted and blind individuals. Subjects performed tactile angle size discriminations in order to investigate whether the pattern of crossmodal occipital recruitment was lateralized as a function of the stimulated hand. Task-elicited activations were also compared across different difficulty levels to ascertain the potential modulatory role of task difficulty on crossmodal processing within occipital areas. We show that blind subjects had more widespread activation within the right lateral and superior occipital gyri when performing the haptic discrimination task. In contrast, the sighted activated the left cuneus and lingual gyrus more so than the blind when performing the task. Furthermore, activity within visual areas was shown to be predictive of tactile discrimination thresholds in the blind, but not in the sighted. Activity within parietal and occipital areas was modulated by task difficulty, where the easier angle comparison elicited more focal occipital activity along with bilateral posterior parietal activity, whereas the more difficult comparison produced more widespread occipital activity combined with reduced parietal activation. Finally, we show that crossmodal reorganization within the occipital cortex of blind individuals was primarily right lateralized, regardless of the stimulated hand, supporting previous evidence for a right-sided hemispheric specialization of the occipital cortex of blind individuals for the processing of tactile and haptic inputs.

Analysis of haptic information in the cerebral cortex

Haptic sensing of objects acquires information about a number of properties. This review summarizes current understanding about how these properties are processed in the cerebral cortex of macaques and humans. Nonnoxious somatosensory inputs, after initial processing in primary somatosensory cortex, are partially segregated into different pathways. A ventrally directed pathway carries information about surface texture into parietal opercular cortex and thence to medial occipital cortex. A dorsally directed pathway transmits information regarding the location of features on objects to the intraparietal sulcus and frontal eye fields. Shape processing occurs mainly in the intraparietal sulcus and lateral occipital complex, while orientation processing is distributed across primary somatosensory cortex, the parietal operculum, the anterior intraparietal sulcus, and a parieto-occipital region. For each of these properties, the respective areas outside primary somatosensory cortex also process corresponding visual information and are thus multisensory. Consistent with the distributed neural processing of haptic object properties, tactile spatial acuity depends on interaction between bottom-up tactile inputs and top-down attentional signals in a distributed neural network. Future work should clarify the roles of the various brain regions and how they interact at the network level.

Parahippocampal cortex is involved in material processing via echoes in blind echolocation experts

Some blind humans use sound to navigate by emitting mouth-clicks and listening to the echoes that reflect from silent objects and surfaces in their surroundings. These echoes contain information about the size, shape, location, and material properties of objects. Here we present results from an fMRI experiment that investigated the neural activity underlying the processing of materials through echolocation. Three blind echolocation experts (as well as three blind and three sighted non-echolocating control participants) took part in the experiment. First, we made binaural sound recordings in the ears of each echolocator while he produced clicks in the presence of one of three different materials (fleece, synthetic foliage, or whiteboard), or while he made clicks in an empty room. During fMRI scanning these recordings were played back to participants. Remarkably, all participants were able to identify each of the three materials reliably, as well as the empty room. Furthermore, a whole brain analysis, in which we isolated the processing of just the reflected echoes, revealed a material-related increase in BOLD activation in a region of left parahippocampal cortex in the echolocating participants, but not in the blind or sighted control participants. Our results, in combination with previous findings about brain areas involved in material processing, are consistent with the idea that material processing by means of echolocation relies on a multi-modal material processing area in parahippocampal cortex.

Compensatory plasticity and cross-modal reorganization following early visual deprivation

For human and non-human primates, vision is one of the most privileged sensory channels used to interact with the environment. The importance of vision is strongly embedded in the organization of the primate brain as about one third of its cortical surface is involved in visual functions. It is therefore not surprising that the absence of vision from birth, or the loss of vision later in life, has huge consequences, both anatomically and functionally. Studies in animals and humans, conducted over the past few decades, have demonstrated that the absence of vision causes massive structural changes that take place not only in the visually deprived cortex but also in other brain areas. These studies have further shown that the visually deprived cortex becomes responsive to a wide variety of non-visual sensory inputs. Recent studies even showed a role of the visually deprived cortex in cognitive processes. At the behavioral level, increases in acuity for auditory and tactile processes have been reported. The study of the congenitally blind brain also offers a unique model to gain better insights into the functioning of the normal sighted brain and to understand to what extent visual experience is necessary for the brain to develop its functional architecture. Finally, the study of the blind brain allows us to investigate how consciousness develops in the absence of vision. How does the brain of someone who has never had any visual perception form an image of the external world? In this paper, we discuss recent findings from animal studies as well as from behavioural and functional brain imaging studies in sighted and blind individuals that address these questions.

Roughness perception of unfamiliar dot pattern textures

Both vision and touch yield comparable results in terms of roughness estimation of familiar textures as was shown in earlier studies. To our knowledge, no research has been conducted on the effect of sensory familiarity with the stimulus material on roughness estimation of unfamiliar textures. The influence of sensory modality and familiarity on roughness perception of dot pattern textures was investigated in a series of five experiments. Participants estimated the roughness of textures varying in mean center-to-center dot spacing in experimental conditions providing visual, haptic and visual-haptic combined information. The findings indicate that roughness perception of unfamiliar dot pattern textures is well described by a bi-exponential function of inter-dot spacing, regardless of the sensory modality used. However, sensory modality appears to affect the maximum of the psychophysical roughness function, with visually perceived roughness peaking for a smaller inter-dot spacing than haptic roughness. We propose that this might be due to the better spatial acuity of the visual modality. Individuals appeared to use different visual roughness estimation strategies depending on their first sensory experience (visual vs. haptic) with the stimulus material, primarily in an experimental context which required the combination of visual and haptic information in a single bimodal roughness estimate. Furthermore, the similarity of findings in experimental settings using real and virtual visual textures indicates the suitability of the experimental setup for neuroimaging studies, creating a more direct link between behavioral and neuroimaging results.

Metaphorically feeling: comprehending textural metaphors activates somatosensory cortex

Conceptual metaphor theory suggests that knowledge is structured around metaphorical mappings derived from physical experience. Segregated processing of object properties in sensory cortex allows testing of the hypothesis that metaphor processing recruits activity in domain-specific sensory cortex. Using functional magnetic resonance imaging (fMRI) we show that texture-selective somatosensory cortex in the parietal operculum is activated when processing sentences containing textural metaphors, compared to literal sentences matched for meaning. This finding supports the idea that comprehension of metaphors is perceptually grounded.

The neural substrate for working memory of tactile surface texture

Fine surface texture is best discriminated by touch, in contrast to macro geometric features like shape. We used functional magnetic resonance imaging and a delayed match-to-sample task to investigate the neural substrate for working memory of tactile surface texture. Blindfolded right-handed males encoded the texture or location of up to four sandpaper stimuli using the dominant or non-dominant hand. They maintained the information for 10-12 s and then answered whether a probe stimulus matched the memory array. Analyses of variance with the factors Hand, Task, and Load were performed on the estimated percent signal change for the encoding and delay phase. During encoding, contralateral effects of Hand were found in sensorimotor regions, whereas Load effects were observed in bilateral postcentral sulcus (BA2), secondary somatosensory cortex (S2), pre-SMA, dorsolateral prefrontal cortex (dlPFC), and superior parietal lobule (SPL). During encoding and delay, Task effects (texture > location) were found in central sulcus, S2, pre-SMA, dlPFC, and SPL. The Task and Load effects found in hand- and modality-specific regions BA2 and S2 indicate involvement of these regions in the tactile encoding and maintenance of fine surface textures. Similar effects in hand- and modality-unspecific areas dlPFC, pre-SMA and SPL suggest that these regions contribute to the cognitive monitoring required to encode and maintain multiple items. Our findings stress both the particular importance of S2 for the encoding and maintenance of tactile surface texture, as well as the supramodal nature of parieto-frontal networks involved in cognitive control.

Are surface properties integrated into visuohaptic object representations?

Object recognition studies have almost exclusively involved vision, focusing on shape rather than surface properties such as color. Visual object representations are thought to integrate shape and color information because changing the color of studied objects impairs their subsequent recognition. However, little is known about integration of surface properties into visuohaptic multisensory representations. Here, participants studied objects with distinct patterns of surface properties (color in Experiment 1, texture in Experiments 2 and 3) and had to discriminate between object shapes when color or texture schemes were altered in within-modal (visual and haptic) and cross-modal (visual study followed by haptic test and vice versa) conditions. In Experiment 1, color changes impaired within-modal visual recognition but had no effect on cross-modal recognition, suggesting that the multisensory representation was not influenced by modality-specific surface properties. In Experiment 2, texture changes impaired recognition in all conditions, suggesting that both unisensory and multisensory representations integrated modality-independent surface properties. However, the cross-modal impairment might have reflected either the texture change or a failure to form the multisensory representation. Experiment 3 attempted to distinguish between these possibilities by combining changes in texture with changes in orientation, taking advantage of the known view-independence of the multisensory representation, but the results were not conclusive owing to the overwhelming effect of texture change. The simplest account is that the multisensory representation integrates shape and modality-independent surface properties. However, more work is required to investigate this and the conditions under which multisensory integration of structural and surface properties occurs.

Active and passive touch differentially activate somatosensory cortex in texture perception

The neural mechanisms behind active and passive touch are not yet fully understood. Using fMRI we investigated the brain correlates of these exploratory procedures using a roughness categorization task. Participants either actively explored a surface (active touch) or the surface was moved under the participant's stationary finger (passive touch). The stimuli consisted of three different grades of sandpaper which participants were required to categorize as either coarse, medium, or fine. Exploratory procedure did not affect performance although the coarse and fine surfaces were more easily categorized than the medium surface. An initial whole brain analysis revealed activation of sensory and cognitive areas, including post-central gyrus and prefrontal cortical areas, in line with areas reported in previous studies. Our main analysis revealed greater activation during active than passive touch in the contralateral primary somatosensory region but no effect of stimulus roughness. In contrast, activation in the parietal operculum (OP) was significantly affected by stimulus roughness but not by exploration procedure. Active touch also elicited greater and more distributed brain activity compared with passive touch in areas outside the somatosensory region, possibly due to the motor component of the task. Our results reveal that different cortical areas may be involved in the processing of surface exploration and surface texture, with exploration procedures affecting activations in the primary somatosensory cortex and stimulus properties affecting relatively higher cortical areas within the somatosensory system.

Cross-modal plasticity of tactile perception in blindness

This review focusses on cross-modal plasticity resulting from visual deprivation. This is viewed against the background of task-specific visual cortical recruitment that is routine during tactile tasks in the sighted and that may depend in part on visual imagery. Superior tactile perceptual performance in the blind may be practice-related, although there are unresolved questions regarding the effects of Braille-reading experience and the age of onset of blindness. While visual cortical areas are clearly more involved in tactile microspatial processing in the blind than in the sighted, it still remains unclear how to reconcile these tactile processes with the growing literature implicating visual cortical activity in a wide range of cognitive tasks in the blind, including those involving language, or with studies of short-term, reversible visual deprivation in the normally sighted that reveal plastic changes even over periods of hours or days.

A colorful albino: the first documented case of synaesthesia, by Georg Tobias Ludwig Sachs in 1812

In 1812, Georg Sachs published a medical dissertation concerning his own albinism and that of his sister. However, he also goes on to describe another phenomenon--namely synaesthesia involving colors for music and simple sequences (including numbers, days, and letters). Most contemporary researchers of synaesthesia fail to cite the case when offering a history of the subject and fewer still will have read it (the original was published in Latin). In this article, we argue that Sachs's case is the first convincing account of synaesthesia; we provide the first English translation of his description of it; we discuss the influence of the case in early theories about synaesthesia and its resonance with contemporary research findings.