Perception of mechanical material qualities through haptic and visual explorations
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Material perception is a crucial part of our everyday life. This is more evident, when deciding where to put our step forward while walking under the rain or when applying enough force to a soap to grasp it without it slipping through our fingers. Over the past decade, material perception attracted increasingly more attention. Yet many questions remain unanswered. Material perception is a complex problem with numerous entry points. For instance, in haptic research, softness is generally equated to the compliance of the objects. However, a recent study has shown that this is not the only case. Perceived material dimensions underlying softness (Dovencioglu et al., 2021) include granularity, viscosity, surface softness, and deformability of the materials. Moreover, people adapt their hand movements according to the material. Another open question would be in addition to extrinsic material properties whether our purpose (i.e., information to be gained) affects hand movements when haptically perceiving different softness dimensions. To this extend, in Study 1 we investigated whether the task and the explored material modulate the exploratory movements. Firstly, our findings replicated the previously reported multiple perceptual dimensions of softness (Dovencioglu et al., 2021). More importantly, our results extend the literature by showing that people adapt their movements based on the material, task, and the interaction between the two. Another entry point to the material perception would be to ask how different modalities provide information on the same material. In daily life, we usually see what we touch and touch what we see. Whether vision provide similar information to haptic about the various aspects of softness is an intriguing question. For instance, in order to judge the softness of a rabbit’s fur, we can inspect its softness by looking at a picture or by touching the rabbit’s fur. Another source of information could be, watching someone else petting the rabbit. In contrast to the merely looking at the rabbit’s fur, watching someone else’s action not only provides the visual feature of the fur but also reveals how the material reacts to the hand movements. It is elusive whether these three examples would yield similar interpretations of softness as a multidimensional construct or not. In Study 2, we investigated to what extent the perceived softness dimensions are similar in vision compared to haptics. Our results showed high overall consistency across haptics, static visual information (i.e., images), and dynamic visual information (i.e., hand movement of other exploring materials). These similarities were the strongest between availability of haptic and dynamic visual information. In our daily experience, we do not only touch objects with bare hands but also sometimes through intermediate surfaces (e.g., wearing gloves). Perceiving materials over another layer of material could reduce some of the haptic information such as thermal properties of the material in question. Despite our regular interaction with materials under restrained conditions (i.e., wearing gloves in winter), it is mostly unknown to what extend these restrictions affect our perception of different aspects of softness. Therefore, another entry point to understanding material perception would be to understand how material perception is affected by physical constraints. It is almost ironic that the augmented or mixed reality technologies for haptics generally construct the haptic experience through gloves or other restrictive proxies. Hence, understanding how physical constraints affect material perception is an important question concerning both theoretical and practical research. In Study 3, we seek to understand how haptic constraints affect perceived softness. Participants explored haptic stimuli under four conditions: bare hand, open-fingered glove, open-fingered glove with rigid sensors, and full glove. General results suggest that softness perception was overall highly similar across conditions. However, in a closer inspection, we found that glove condition differed from the others especially in terms of surface softness. So far, the discussed entry points to material perception scrutinized the material perception in sensory domains. However, as in other topics in perception, the material perception - in addition to the sensation - depends on the agent’s cognitive state such as their motivation, emotion, etc. In a similar vein, the previous studies have shown that sensory and affective properties are related. For instance, fine grained materials like sand feel pleasant while rough materials such as sandpaper feel unpleasant (Drewing et al., 2018). The origin of these relationships is another piece of the puzzle. To remedy this gap, in Study IV, we investigated whether the relationship between sensory materials properties (i.e., granular) and affective responses (i.e., feeling pleasant) can be modified by learning. We further investigated previously observed relationships: positive relationship between granular and pleasantness, negative relationship between roughness and valence. With a classical conditioning paradigm, instead of participants’ existing material-emotion associations the opposite affective relationship was reinforced. The results have shown a significantly decreased relationship between valence and granularity in the experimental group compared to the control group. However, valence and roughness relationships did not differ between the experimental and the control groups. The results suggest that not all affective associations of the perceived material dimensions could be modified. We explain these results with the difference in learned and hard-wired connections.