Tactual perception: a review of experimental variables and procedures <s> Touch and vision <s> The issues of visual-haptic perceptual equivalence and the impact of nonequivalence upon cross-modal performance were explored. A measure of cross-modal nonequivalence was developed from multidimensional scaling models of the perceptual structures of 24 nonrepresentative three-dimensional stimuli. In Experiment 1 the visual and haptic perceptual structures and measures of cross-modal nonequivalence were shown to be replicable. Experiments 2 and 3 employed sets of stimuli selected as cross modally similar or dissimilar (based upon the results of Experiment 1) and tested the impact of perceptual nonequivalence upon cross-modal performance with shape information. The experiments used somewhat different tasks and produced converging results. There was poorer cross-modal performance when cross modally dissimilar stimuli were involved than when only cross modally similar stimuli were involved, but there was no such pattern for intramodal performance. The findings are related to the theoretical notions of perceptual equivalence (Gibson, 1966; Marks, 1978) and the theoretical and practical importance of understanding the perceptual properties of stimuli used in cognitive tasks (Garner, 1970; Monahan & Lockhead, 1970). Language: en <s> BIB001 </s> Tactual perception: a review of experimental variables and procedures <s> Touch and vision <s> Explicit memory tests such as recognition typically access semantic, modality-independent representations, while perceptual implicit memory tests typically access presemantic, modality-specific representations. By demonstrating comparable cross- and within-modal priming using vision and haptics with verbal materials (Easton, Srinivas, & Greene, 1997), we recently questioned whether the representations underlying perceptual implicit tests were modality specific. Unlike vision and audition, with vision and haptics verbal information can be presented in geometric terms to both modalities. The present experiments extend this line of research by assessing implicit and explicit memory within and between vision and haptics in the nonverbal domain, using both 2-D patterns and 3-D objects. Implicit test results revealed robust cross-modal priming for both 2-D patterns and 3-D objects, indicating that vision and haptics shared abstract representations of object shape and structure. Explicit test results for 3-D objects revealed modality specificity, indicating that the recognition system keeps track of the modality through which an object is experienced. <s> BIB002 </s> Tactual perception: a review of experimental variables and procedures <s> Touch and vision <s> Older (age 60-75) and younger adults (age 20-35) participated in a within-subject experiment. All participants were given four lists of 17 items, a recall test followed each list. Each participant was given a separate list under each of the following conditions, at the rate of 1 item every 10 s. (1) control: list of nouns; (2) visual-only: visually shown objects; (3) visual-and-haptic: given objects to hold and visually examine; (4) haptic-only: blindfolded, given objects to hold. A final surprise recall test was given of all items presented in the study. Data indicated that younger adults recalled more than older adults under all conditions. However, both age groups had the highest recall under the haptic conditions. <s> BIB003 </s> Tactual perception: a review of experimental variables and procedures <s> Touch and vision <s> Cross-modal illusory conjunctions (ICs) happen when, under conditions of divided attention, felt textures are reported as being seen or vice versa. Experiments provided evidence for these errors, demonstrated that ICs are more frequent if tactile and visual stimuli are in the same hemispace, and showed that ICs still occur under forced-choice conditions but do not occur when attention to the felt texture is increased. Cross-modal ICs were also found in a patient with parietal damage even with relatively long presentations of visual stimuli. The data are consistent with there being cross-modal integration of sensory information, with the modality of origin sometimes being misattributed when attention is constrained. The empirical conclusions from the experiments are supported by formal models. <s> BIB004 </s> Tactual perception: a review of experimental variables and procedures <s> Touch and vision <s> When a person looks at an object while exploring it with their hand, vision and touch both provide information for estimating the properties of the object. Vision frequently dominates the integrated visual-haptic percept, for example when judging size, shape or position, but in some circumstances the percept is clearly affected by haptics. Here we propose that a general principle, which minimizes variance in the final estimate, determines the degree to which vision or haptics dominates. This principle is realized by using maximum-likelihood estimation to combine the inputs. To investigate cue combination quantitatively, we first measured the variances associated with visual and haptic estimation of height. We then used these measurements to construct a maximum-likelihood integrator. This model behaved very similarly to humans in a visual-haptic task. Thus, the nervous system seems to combine visual and haptic information in a fashion that is similar to a maximum-likelihood integrator. Visual dominance occurs when the variance associated with visual estimation is lower than that associated with haptic estimation. <s> BIB005 </s> Tactual perception: a review of experimental variables and procedures <s> Touch and vision <s> In a previous experiment, we showed that bistable visual object motion was partially disambiguated by tactile input. Here, we investigated this effect further by employing a more potent visuotactile stimulus. Monocular viewing of a tangible wire-frame sphere (TS) rotating about its vertical axis produced bistable alternations of direction. Touching the TS biased simultaneous and subsequent visual perception of motion. Both of these biases were in the direction of the tactile stimulation and, therefore, constituted facilitation or priming, as opposed to interference or adaptation. Although touching the TS biased visual perception, tactile stimulation was not able to override the ambiguous visual percept. This led to periods of sensory conflict, during which visual and tactile motion percepts were incongruent. Visual and tactile inputs can sometimes be fused to form a coherent percept of object motion but, when they are in extreme conflict, can also remain independent. <s> BIB006 </s> Tactual perception: a review of experimental variables and procedures <s> Touch and vision <s> The present study examined the extent to which vision and touch are perceptually equivalent for texture information in adults. Using Garbin's method, we selected two sets of textures having high versus low cross-modal dissimilarity values between vision and touch (Experiment 1). The two sets of textures were then used as material in a cross-modal matching task (Experiment 2). Results showed that asymmetries occurred in the performances when the stimuli had high cross-modal dissimilarity values, but not when the stimuli had low cross-modal dissimilarity values. These results extend Garbin's findings on shape information to the texture domain and support the idea that partial perceptual equivalence exists between vision and touch. Language: en <s> BIB007 </s> Tactual perception: a review of experimental variables and procedures <s> Touch and vision <s> Change blindness is the name given to people's inability to detect changes introduced between two consecutively-presented scenes when they are separated by a distractor that masks the transients that are typically associated with change. Change blindness has been reported within vision, audition, and touch, but has never before been investigated when successive patterns are presented to different sensory modalities. In the study reported here, we investigated change detection performance when the two to-be-compared stimulus patterns were presented in the same sensory modality (i.e., both visual or both tactile) and when one stimulus pattern was tactile while the other was presented visually or vice versa. The two to-be-compared patterns were presented consecutively, separated by an empty interval, or else separated by a masked interval. In the latter case, the masked interval could either be tactile or visual. The first experiment investigated visual-tactile and tactile-visual change detection performance. The results showed that in the absence of masking, participants detected changes in position accurately, despite the fact that the two to-be-compared displays were presented in different sensory modalities. Furthermore, when a mask was presented between the two to-be-compared displays, crossmodal change blindness was elicited no matter whether the mask was visual or tactile. The results of two further experiments showed that performance was better overall in the unimodal (visual or tactile) conditions than in the crossmodal conditions. These results suggest that certain of the processes underlying change blindness are multisensory in nature. We discuss these findings in relation to recent claims regarding the crossmodal nature of spatial attention. <s> BIB008 </s> Tactual perception: a review of experimental variables and procedures <s> Touch and vision <s> The relationship between visually and haptically derived representations of objects is an important question in multisensory processing and, increasingly, in mental representation. We review evidence for the format and properties of these representations, and address possible theoretical models. We explore the relevance of visual imagery processes and highlight areas for further research, including the neglected question of asymmetric performance in the visuo –haptic cross-modal memory paradigm. We conclude that the weight of evidence suggests the existence of a multisensory representation, spatial in format, and flexibly accessible by both bottom – up and top – down inputs, although efficient comparison between modality-specific representations cannot entirely be ruled out. <s> BIB009 </s> Tactual perception: a review of experimental variables and procedures <s> Touch and vision <s> Sequences of auditory beeps and tactile taps were simultaneously presented and participants were instructed to focus on one of these modalities and to ignore the other. We tested whether (i) the two sensory channels bias one another and (ii) the interaction depends on the relative reliability of the channels. Audition biased tactile perception and touch biased auditory perception. Lowering the reliability of the auditory channel (i.e. the intensity of the beeps) decreased the effect of audition on touch and increased the effect of touch on audition. These results show that simultaneous auditory and tactile stimuli tend to be automatically integrated in a reliability-dependent manner. <s> BIB010 </s> Tactual perception: a review of experimental variables and procedures <s> Touch and vision <s> In the following review, we discuss the evidence for the role of vision and touch in the perception of texture with the particular aim of evaluating whether these systems contribute information to texture perception in either an independent or integrated manner. Although texture is perceived by both vision and touch, several behavioural and neuroimaging studies have suggested that texture information is processed in qualitatively different ways, such that the type of information encoded and the manner in which it is encoded differs across these sensory systems. Furthermore, vision and touch perceive different aspects of texture information with vision the more appropriate modality for discriminating texture boundaries and touch the more appropriate modality for discriminating stimulus roughness and compliance. Differences also exist in the neural structures and functions which underlie these systems, further suggesting independence of these modalities in the perception of texture. Accordingly, recent evidence suggests that behavioural tasks involving texture perception do not benefit from integration across the senses, suggesting that qualitatively different information is represented across vision and touch which cannot be combined to enhance perceptual performance. Research using more familiar objects, however, does suggest a role for sensory integration on texture perception, although these effects may be due to more cognitive influences rather than basic sensory encoding. In sum, we suggest that unlike shape perception, vision and touch contribute information to the perception of texture in an independent but complementary manner. <s> BIB011 </s> Tactual perception: a review of experimental variables and procedures <s> Touch and vision <s> A variety of similarities between visual and haptic object recognition suggests that the two modalities may share common representations. However, it is unclear whether such common representations preserve low-level perceptual features or whether transfer between vision and haptics is mediated by high-level, abstract representations. Two experiments used a sequential shape-matching task to examine the effects of size changes on unimodal and crossmodal visual and haptic object recognition. Participants felt or saw 3D plastic models of familiar objects. The two objects presented on a trial were either the same size or different sizes and were the same shape or different but similar shapes. Participants were told to ignore size changes and to match on shape alone. In Experiment 1, size changes on same-shape trials impaired performance similarly for both visual-to-visual and haptic-to-haptic shape matching. In Experiment 2, size changes impaired performance on both visual-to-haptic and haptic-to-visual shape matching and there was no interaction between the cost of size changes and direction of transfer. Together the unimodal and crossmodal matching results suggest that the same, size-specific perceptual representations underlie both visual and haptic object recognition, and indicate that crossmodal memory for objects must be at least partly based on common perceptual representations. <s> BIB012 </s> Tactual perception: a review of experimental variables and procedures <s> Touch and vision <s> Deviations in both haptic and visual spatial experiments are thought to be caused by a biasing influence of an egocentric reference frame. The strength of this influence is strongly participant-dependent. By using a parallelity test, it is studied whether this strength is modality-independent. In both haptic and visual conditions, large, systematic and participant-dependent deviations were found. However, although the correlation between the haptic and visual deviations was significant, the explained variance due to a common factor was only 20%. Therefore, the degree to which a participant is “egocentric” depends on modality and possibly even more generally, on experimental condition. <s> BIB013 </s> Tactual perception: a review of experimental variables and procedures <s> Touch and vision <s> In six experiments, subjects judged the sizes of squares that were presented visually and/or haptically, in unimodal or bimodal conditions. We were interested in which mode most affected size judgments in the bimodal condition when the squares presented to each mode actually differed in size. Three factors varied: whether haptic exploration was passive or active, whether the choice set from which the subjects selected their responses was visual or haptic, and whether cutaneous information was provided in addition to kinesthetic information. To match the task for each mode, visual presentations consisted of a cursor that moved along a square pathway to correspond to the haptic experience of successive segments revealed during exploration. We found that the visual influence on size judgments was greater than the influence of haptics when the haptic experience involved only kinesthesis, passive movement, and a visual choice set. However, when cutaneous input was added to kinesthetic information, size judgments were most influenced by the haptic mode. The results support hypotheses of sensory integration, rather than capture of one sense by the other. <s> BIB014 </s> Tactual perception: a review of experimental variables and procedures <s> Touch and vision <s> This research examined whether visual and haptic map learning yield functionally equivalent spatial images in working memory, as evidenced by similar encoding bias and updating performance. In 3 experiments, participants learned 4-point routes either by seeing or feeling the maps. At test, blindfolded participants made spatial judgments about the maps from imagined perspectives that were either aligned or misaligned with the maps as represented in working memory. Results from Experiments 1 and 2 revealed a highly similar pattern of latencies and errors between visual and haptic conditions. These findings extend the well-known alignment biases for visual map learning to haptic map learning, provide further evidence of haptic updating, and most important, show that learning from the 2 modalities yields very similar performance across all conditions. Experiment 3 found the same encoding biases and updating performance with blind individuals, demonstrating that functional equivalence cannot be due to visual recoding and is consistent with an amodal hypothesis of spatial images. <s> BIB015
Haptic research has been mostly conducted under multimodal visuo-tactile paradigms, either comparing performance on touch and vision (Bergmann-Tiest and Kappers 2007; BIB006 BIB013 or exploring shared representations between the two modalities (Easton et al. 1997a, b; BIB009 BIB011 . BIB009 conducted a review on vision and touch interactions attending to crossmodal memory studies from a theoretical perspective. The authors argued that, in memory tasks, participants have to retrieve a representation of the object, and as such these tasks would be ideal to test the nature of crossmodal representations. The authors concluded that there is enough evidence to assume that both modalities share cognitive resources and processes, as shown by processes such as viewpoint dependence on vision and touch and the equivalence of spatial representations in the two modalities. These representations were described as dynamic and could be affected by both topdown and bottom-up processes. Even though the authors highlighted the relevance of research that has reported differences between the two modalities and has pointed to the existence of unimodal coding and retrieval processes, for instance, better performance in memory tasks when the presentation modality is the same as the retrieval modality. As such, they concluded that further research was needed to clarify these modality-specific processes BIB009 ). These descriptions of shared generic components, marked by modality specificity, are the most frequent report in crossmodal visuo-tactile studies, as will be illustrated in this section. A central aspect of the connection between vision and touch is the shared spatial component, present in both modalities, but mostly explored in vision. It seems that there is an equivalence between touch and vision for spatial processing, and many researchers have explored this topic. Easton et al. (1997a, b ) present a study consisting in the presentation of words either visually in a computer screen or haptically through printed raised letters. Participants previously studied 10 words visually and 10 words haptically and at test had to perform either a word-stem task or a cued-recall task with 30 words (20 studied and 10 nonstudied) in one of the modalities. The authors reported no differences between modalities and argued in favour of shared representations in vision and touch. In another study, BIB002 , aware of the relevance of verbal processing in the presentation of words, explored visual and haptic representations in a similar paradigm, but testing participants with two-dimensional patterns and three-dimensional objects. The participants had to study part of the stimuli visually or haptically, and then their task was to either perceptually identify item or explicitly recognise it. Results showed that for two-dimensional patterns, cross-modality priming was strong (meaning that visually studied objects were haptically identified or recognised better than non-studied objects and vice-versa). With three-dimensional objects, modality-specificity was observed indicating that both shared and unique processes determined crossmodal integration. In a study with blind and sighted participants, BIB015 asked participants to learn four-point routes in a map either visually or haptically and then to reproduce the path in the test phase. Results showed that spatial images were shared between vision and touch. Results for the blind participants' group suggested that this equivalence was not a consequence of visual recoding of haptic information, since the pattern was equivalent for both participants' groups. Likewise, asked participants to memorise a number of locations in a 5 9 5 matrix either visually or haptically and to report them in an empty matrix. Results showed that visual performance was overall better than haptic performance, that representations were modality-dependent and that some information was shared between modalities, indicating that both supra-modal and modality-specificity processes were present. Besides spatial equivalence, other similarities have been reported for visual and haptic processing. Imamiya (2003, 2004) reported that colours affected perceived haptic roughness in surfaces. In texture perception, BIB007 reported partial equivalence between vision and touch in a crossmodal matching task, in which participants had to match stimuli that were previously rated as presenting a high-or low-crossmodal dissimilarity. This study broadened the findings of BIB001 on visuo-haptic equivalence for shape perception. A review on texture perception by vision and touch BIB011 concluded that, unlike shape perception, texture perception by vision and touch was not equivalent, but complementary. The authors pointed to research providing no evidence for visuo-haptic integration in behavioural tasks with familiar objects (e.g., Bergmann-Tiest and and suggested that this lack of crossmodal correspondence might be due to more elaborate cognitive processes than basic perception. Crossmodal illusory conjunctions (considering that an item presented in one modality was actually presented in another) were also observed within touch and vision BIB004 . Participants saw visual stimuli while touching tactile stimuli and after a mask presentation had to report the shape and texture of the seen object or the orientation of the touched stimuli. Results showed that participants reported felt textures as seen and vice-versa, and these misguided reports were more frequent when visual and haptic stimuli were presented in the same hemispace. BIB008 also described equivalence between touch and vision in a crossmodal change blindness paradigm. Change blindness is a phenomenon characterised by the inability to detect differences in two consecutive stimuli, due to the presentation of a mask between them. This phenomenon has been studied in vision, audition and touch BIB008 . In this study, change blindness was reported even when stimuli were presented in different modalities (haptic and visual), and regardless of the mask presentation modality, although the phenomenon was stronger in within-modality presentations. According to the authors, these results indicate that multisensory processes could explain change blindness. BIB012 argued that size perception was shared between vision and haptics after a crossmodal study in which participants were asked to recognise three-dimensional objects based on shape alone, ignoring size changes. Results revealed that size variation impaired crossmodal recognition, suggesting shared representations of size between modalities. The study did not allow a clarification on the nature of these shared representations, and the authors hypothesised that they might be either perceptual (low-level) or mediated by high-level processes. Another group of studies has focused on the differences between touch and vision in perception and recognition tasks.  evaluated depth rotation in visual and haptic conditions with morphs of everyday objects in a sequential-matching task in within modal and crossmodal conditions. The difficulty of the task was manipulated Cogn Process (2012) 13:285-301 295 through the discriminability of the presented stimuli (from presenting unrelated stimuli-bed, lizard-to presenting related stimuli-bed, chair-and finally presenting very similar stimuli-bed, bed-chair morph). Results showed that for within modal and visuo-haptic trials, object discriminability and orientation changes impaired recognition, while in haptic-visual trials orientation changes had no effect. Also, orientation changes and discriminability had additive effects for visuo-haptic trials, but haptic-visual trials did not reveal orientation change effects. This latter result suggests that different processes underlie haptic and visual orientation processing. In a study analysing the perception of shape and size by vision and touch through a computer screen and a Phantom system, BIB014 asked participants to evaluate the size of touched shapes (squares and lines), selecting the touched shape from a haptic or visual set. Haptic exploration was either active (participants actively moved the Phantom) or passive (guided by Phantom) and could present cutaneous information (direct contact with a raised line) along with kinaesthetic information (movement through the stimulus). Performance in the visual task was better than in the haptic task when participants were allowed to passively explore the objects, but when cutaneous information was allowed in active exploration, haptic performance was better than visual. In another study, examination of visual and haptic perception of shape for three-dimensional objects resulted in a partial functional equivalence between modalities, with noticeable unimodal representations  . In this experiment, participants were able to recognise objects in crossmodal conditions with high levels of accuracy; nonetheless, performance was better in unimodal conditions. BIB003 explored visual and haptic recognition in function of age and concluded that regardless of sensory modality, young adults (20-35 years old) were better than older adults (60-75 years old) in object recognition. Also, haptic recognition supported the higher recall in both groups (younger and older participants). BIB005 developed a multisensory integration mathematical model applicable to vision and touch interactions, as well as other multimodal associations (e.g., BIB010 . This model is based on Bayesian statistics and uses maximum likelihood estimation, assuming that visual and tactile inputs are integrated in a statistically optimal fashion BIB005 . According to this model, when two perceptual modalities are capable of processing the same stimuli, the most influential modality will be the one that presents the lesser variance in perceptual estimation. This model allows a better understanding of apparently contradictory experimental findings in which vision dominates for some tasks and haptics for others. According to the maximum likelihood model, the dominant modality will be the one with more reliable information for each particular task. Previous studies focused essentially on the analysis of visuo-haptic interactions for stimuli features that can be encoded by both modalities: location, patterns, texture, shape, size and orientation. For these common features, partially shared processing is described in the literature, with many features presenting equivalent manipulation and representation in both modalities (e.g., location), but others maintaining modality specificity (e.g., texture).