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Haptic research has frequently equated softness with the compliance of elastic objects. However, in a recent study we have suggested that compliance is not the only perceived material dimension underlying what is commonly called softness [1]. Here, we investigate, whether the different perceptual dimensions of softness affect how materials are haptically explored. Specifically, we tested whether also the task, i.e., the attribute that a material is being judged on, might affect how a material is explored. To this end we selected 15 adjectives and 19 materials that each associate with different softness dimensions for the study. In the experiment, while participants freely explored and rated the materials, we recorded their hand movements. These movements were subsequently categorized into distinct exploratory procedures (EPs) and analyzed in a multivariate analysis of variance (MANOVA). The results of this analysis suggest that the pattern of EPs depended not only on the material's softness dimension and the task (i.e. what attributes were rated), but also on an interaction between the two factors. Taken together, our findings support the notion of multiple perceptual dimensions of softness and suggest that participants actively adapt their EPs in a nuanced way when judging a particular softness dimensions for a given material.

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Softness is an important material property that can be judged directly, by interacting with an object, but also indirectly, by simply looking at an image of a material. The latter is likely possible by filling in relevant multisensory information from prior experiences with soft materials. Such experiences are thought to lead to associations that make up our representations about perceptual softness. Here, we investigate the structure of this representational space when activated by words, and compare it to haptic and visual perceptual spaces that we obtained in earlier work. To this end, we performed an online study where people rated different sensory aspects of soft materials, presented as written names. We compared the results with the previous studies where identical ratings were made on the basis of visual and haptic information. Correlation and Procrustes analyses show that, overall, the representational spaces of verbally presented materials were similar to those obtained from haptic and visual experiments. However, a classifier analysis showed that verbal representations could better be predicted from those obtained from visual than from haptic experiments. In a second study we rule out that these larger discrepancies in representations between verbal and haptic conditions could be due to difficulties in material identification in haptic experiments. We discuss the results with respect to the recent idea that at perceived softness is a multidimensional construct.

We interact with different types of soft materials on a daily basis such as salt, hand cream, etc. Recently we have shown that soft materials can be described using four perceptual dimensions which are deformability, granularity, viscosity, and surface softness [1]. Here, we investigated whether humans can actually perceive systematic differences in materials that selectively vary along one of these four dimensions as well as how judgments on the different dimensions are correlated to softness judgments. We selected at least two material classes per dimension (e.g., hair gel and hand cream for viscosity) and varied the corresponding feature (e.g., the viscosity of hair gel). Participants ordered four to ten materials from each material class according to their corresponding main feature, and in addition, according to their softness. Rank orders of materials according to the main feature were consistent across participants and repetitions. Rank orders according to softness were correlated either positively or negatively with the judgments along the associated four perceptual dimensions. These findings support our notion of multiple softness dimensions and demonstrate that people can reliably discriminate materials which are artificially varied along each of these softness dimensions.