Tactual perception: a review of experimental variables and procedures <s> Relevance of movement in touch <s> Abstract Two experiments establish links between desired knowledge about objects and hand movements during haptic object exploration. Experiment 1 used a match-to-sample task, in which blindfolded subjects were directed to match objects on a particular dimension (e.g., texture). Hand movements during object exploration were reliably classified as “exploratory procedures,” each procedure defined by its invariant and typical properties. The movement profile, i.e., the distribution of exploratory procedures, was directly related to the desired object knowledge that was required for the match. Experiment 2 addressed the reasons for the specific links between exploratory procedures and knowledge goals. Hand movements were constrained, and performance on various matching tasks was assessed. The procedures were considered in terms of their necessity, sufficiency, and optimality of performance for each task. The results establish that in free exploration, a procedure is generally used to acquire information about an object property, not because it is merely sufficient, but because it is optimal or even necessary. Hand movements can serve as “windows,” through which it is possible to learn about the underlying representation of objects in memory and the processes by which such representations are derived and utilized. <s> BIB001 </s> Tactual perception: a review of experimental variables and procedures <s> Relevance of movement in touch <s> In a yes/no identification task using touch alone, subjects indicated whether an object belonged to a named category. Previously, we found that subjects explored in two stages—first grasping and lifting the object, then executing further exploratory procedures (Lederman & Klatzky, 1990b). We proposed that Stage 1 (grasp/lift) was sufficient to extract coarse information about multiple object properties, whereas Stage 2 was directed toward precise information about particularly diagnostic properties. the current study, subjects were initially constrained to grasping and lifting, after which they could explore further. Accuracy was above chance after Stage 1, confirming our assumption that the grasp/lift combination was broadly useful. Stage 2 increased accuracy and confidence. It primarily elicited exploratory procedures associated with object geometry, but exploration was also influenced by diagnostic object properties. <s> BIB002
The relevance of movement in touch was valued since the first efforts to systematically study this modality.  ,   and Weber (1834 Weber ( /1996 Weber ( ) and 1905 Weber ( /1996 understood the relevance of dynamic contact with the stimuli from their early experiments and considered the sense of movement crucial in tactile perception. Later, the works of Lederman and Klatzky (e.g.,  brought a first effort to give some structure to our knowledge about movement in touch. Nonetheless, despite the importance of movement to touch, most studies with tactile stimuli are still conducted with passive stimulation (e.g.,  . In fact, the variability of hand movements that people perform to explore an object presented by touch only makes it harder to control for exploration times and inter-stimulus intervals and introduces some individual variability in the way stimuli are explored, even though general movement patterns are always detectable . Object manipulation is determinant for the acquisition of object properties like texture, shape, weight or volume. The movements performed while touching an object are pertinent to the perception of the object in space (Kaas et al. 2008) , and grasping has been shown to be a crucial stage in object identification as well BIB002 , showing the relevance of active manipulation for touch. BIB001 provided a systematic classification of manual object exploration. In this study, participants were asked to touch everyday objects, without sight, and to evaluate specific features like weight. Participants were allowed to explore the objects freely with both hands. Through a complex movement analysis that involved video examinations by naive evaluators, the authors were able to discriminate between six essential movement patterns, each one specified in collecting particular information from the objects: lateral motion (texture), pressure (hardness), static contact (temperature), unsupported holding (weight), enclosure (volume, global shape) and contour following (exact shape). The authors called these movement patterns as exploratory procedures and defined them as a stereotyped movement pattern having certain characteristics that are variant and others that are highly typical (Lederman and Klatzky 1987, p. 344) . By this definition, the authors explained that it is possible to identify general patterns of movement, equivalent in all participants (for instance, rubbing an object to evaluate its texture), although there are some variations in the individual exploration (for instance, some participants rub the object with only one finger while other might use two fingers or even the whole hand). More recently, using a movement-tracking device, Kappers and colleagues (Bergmann-Tiest et al. 2010;  initiated research on the relevance of movements while exploring two-dimensional stimuli.  were able to extract specific patterns of exploration from the movement coordinates recorded through the experiment and were able to establish relations between these patterns and specific properties' evaluations, namely hardness, coldness, texture and texture orientation.  work showed that participants performed the same type of movements when trying to answer to the same questions exploring a stimuli matrix (e.g., Which sample is the warmest?). These studies showed that haptic movements are specific and directed to feature evaluation and that the movement patterns noted by BIB001 regarding three-dimensional everyday objects also emerge with two-dimensional stimuli, although the movements that characterise each pattern might suffer some alteration due to stimuli specificity (e.g., global shape and shape contours correspond to the same procedure for two-dimensional objects, because participants cannot hold these type of stimuli).