Our perceptual world comprises a wide variety of objects, arts, and images that are rich in texture. Textures, an essential element of sensory inputs, provide us not only cues to perceptual discrimination [9, 51] but also significant information about the aesthetic qualities of objects, industrial products, or artifacts. Therefore, textures are widely used in product design, art, and architecture to convey specific aesthetic information [110], to evoke aesthetic emotions and set moods in humans [14, 35]. In everyday life, we choose to purchase objects or products, such as clothes or curtains designed with certain textures, while discarding many alternatives designed with different textures. Thus, it is important to understand how people perceive and appreciate the beauty of textured products or objects in the environment.

Surface textures are usually described as smooth or rough, coarse or fine, soft or hard, matt or glossy, and so forth. Depending on the sensory modality we use to sense and perceive them, textures fall into two distinct categories: visual textures and tactile textures. Visual textures give us an implied sense of surface composition related to local spatial variations of simple stimuli like color, orientation, and intensity. Tactile textures, on the contrary, are rendered to provide real information about the physical surface qualities of objects or products, such as sculpture and architecture materials that can not only be seen but felt by touch as well. In the last few decades or so, research has exclusively focused on the perception and aesthetics of visual textures (e.g., texture perception: [16, 22, 41, 48, 53, 54, 71, 76, 90, 95, 109, 111–113, 116, 121]; texture aesthetics: [14, 28, 50, 67, 68, 77, 78, 87, 94, 107, 110]) with scant attention to tactile textures. The intent of this study was to fill this gap by studying the perception and aesthetics of tactilely textured objects in individuals who underwent typical sensory development in comparison with those who experienced deprivation of the visual sense. Perception is the capacity of an individual to detect slight differences in environmental stimulation using a sensory system, whereas aesthetics is defined as the emotional feelings or affective reactions that s/he uses to appraise/appreciate the quality or richness of an object or event in the environment.

Like visual objects or products, tactile objects or products have also discriminative attributes and hedonic attributes that produce pleasant or unpleasant sensations [8, 96]. The hedonic attributes of tactile stimulation are important for emotional wellbeing [21, 56, 96], and interpersonal relationships between individuals, such as parents and infants, close friends, and romantic partners [66, 88, 93, 118]. Thus, hedonic touch forms a cornerstone of social, affiliative behavior in humans [85] and is critical in physical and mental development [24, 42]. Prosaically, we encounter the touch of clothing against our bodies every day, and this contact determines the comfort of the garments we wear [18]. Thus hedonic touch likely influences the estimated product quality [39] and guides consumer behavior and attitudes [17, 97–99]. Hedonic tactile stimulations (pleasant or unpleasant) are powerful motivators that facilitate product evaluation, product preferences, and purchase decisions (see [3, 23, 27, 80, 83, 98]). Thus, retailers can directly benefit from allowing customers to touch their products as it positively affects customers’ choices (see [2, 3, 23, 39, 100]).

Touch is particularly important for the evaluation of a product’s substance/material properties, such as roughness, hardness, temperature, and weight [52, 60, 70, 98], as it provides unique information that cannot be obtained through visual inspection [75]. Among these substance properties, roughness and softness are two prominent dimensions of tactile textures [49, 92] that have received growing amount of interest in recent years. One line of research has investigated the psychophysical relationships between subjective experience of tactile roughness or softness and the physical magnitude of surface roughness or object compliance (a physical correlate of softness) in sighted humans [30, 96, 115]. These studies showed that the perceived roughness of a stimulus surface was approximately a power function of the physical magnitude of roughness [30, 115] and that the perceived magnitude of softness increased monotonically as a function of increasing object compliance, leveling off around the end of the stimulus range [96]. A second line of research investigated how these discriminative texture parameters are related to the implicit hedonic aspects of touch in sighted humans [58, 63, 96, 115]. These studies revealed that the perceived magnitude of pleasantness of tactile sensation was inversely related to the physical or estimated magnitude of surface roughness [58, 63, 115], or increased monotonically with softness estimates or object compliance [96]. Because smooth or soft stimuli likely engender less friction [31, 64, 65], other studies in sighted humans demonstrated that people rate smooth and soft stimuli (e.g., silk material, cosmetic brushes) as more pleasing than rough and hard stimuli (e.g., burlap material, plastic mesh, polyester, sandpaper, sponge, cotton) under both active [32, 79, 102] and passive [29, 31, 32] touch conditions. Thus, it has been shown that stimulus preference increases (proportionally) with increasing magnitude of smoothness or softness [30, 47].

To measure the aforementioned variables, we used a ranking scale approach that allowed respondents to judge a set of 2D tactile surfaces or 3D tactile objects in relation to one another. This would not be possible in other types of scales, such as a Likert type rating scale which asks respondents simply to rate aesthetics of individual objects, and cannot give comparable information about the aesthetic qualities of objects. For example, when someone assigns a score of 4 to an object of certain qualities s/he may assign the same weight to a second object of worse or better qualities as s/he does not have information of the first object while making judgments of the second one. The problem intensifies when respondents are required to rate several objects on a large number of items/questions. Thus, a rating scale technique likely produces inaccurate and unreliable quantitative data about aesthetic preferences. The ranking scale is free from this kind of potential problem. Thus, though a ranking method likely gives qualitative data, they are more accurate and more reliable at least for relative aesthetics of objects. Also, in a real life setting, we prefer products of a particular brand as compared to others, and even within the same preferred brand, we choose a particular product to purchase by comparing its quality with a number of alternatives. Thus, the use of a ranking scale approach was an appropriate and precise choice in this study as it primarily intended to examine relative aesthetics of tactile objects.

Two experiments were conducted, each one using a mixed experimental design, a type of semi-repeated measures design, in which groups of differential visual experiences were repeatedly tested under varying levels of smoothness of 2D tactile stimuli or softness of 3D tactile stimuli.

Using a novel behavioral (rank ordering) approach, we conducted two exploratory experiments to examine for the first time the effects of visual experience and texture parameters on tactile stimulus preference and tactile aesthetic appreciation, in addition to replicating the effects of these factors on basic tactile discrimination.

Analysis of the data in appropriate statistics demonstrated significant effects of textures but no effect of visual experience on all these behavioral judgments. Specifically, it was shown that the basic sensory discrimination of tactile smoothness or softness, smoothness or softness preference, and appreciation of smoothness or softness on three affective dimensions, Relaxation, Hedonics, and Arousal, did not differ between the congenitally blind, late blind, and blindfolded sighted populations. This implies that visual experience is not required for (or does not affect) the emergence of basic, preferred, and aesthetic mental representations of tactile textures. However, specific texture parameters, such as 2D tactile smoothness and 3D tactile softness, were found to have significant impacts on basic tactile discrimination, tactile stimulus preference, and all three affective dimensions of tactile aesthetics. More specifically, we found that tactile stimuli received proportionally higher smoothness or softness ranks with the increasing physical magnitude of 2D smoothness or 3D softness, indicating finer tactile discriminability of humans irrespective of visual experience. Second, physically smoother or softer stimuli were associated with higher ranks in stimulus preference (Figs. 3(b), 4(b), 7(b), 8(b)) and stimulus appraisal on Relaxation and Hedonics dimensions of aesthetics (Figs. 5, 6, 9, 10). However, in contrast to the 2D smoother stimuli that were associated with lower ranks on Arousal (Figs. 5, 6) the 3D softer stimuli were associated with higher ranks on this dimension (Figs. 9, 10).

The demonstration that visual experience plays no significant role in shaping tactile texture perception is consistent with both theoretical views [59, 62, 69] and prior empirical observations [7, 43]. For example, Lederman and Klatzky [69] theorized that the tactile perception of surface structure does not always require mediation by visual imagery. They posit that touch may provide optimal performance when people attempt to identify multidimensional stimuli like common 3D objects that can vary along a number of substance-related dimensions [61]. The sense of touch may perform well in the apprehension of texture (e.g., roughness, hardness), thermal properties and weight, and thus direct tactile encoding likely operates for these qualities, without requiring any visual imagery. In line with this theoretical argument, prior empirical studies showed that visual imagery or experience is not necessary for texture perception as proven by the lack of differences between blind and blindfolded sighted participants in tactile roughness or hardness perception [7, 43]. Though visual experience is not mandatory or advantageous for the development of tactile discriminability, some studies suggested that the lack of visual experience in the blind might lead to superior performance in tactile acuity (e.g., Braille recognition, grating acuity; [12, 37, 72, 91]) and 3D tactile shape discrimination [37, 55, 91] tasks as it is thought to be compensated by heightened tactile experience in their everyday life. Here, we did not find any evidence for this kind of compensatory tactile benefit in the blind. Research has suggested that blind people are able to perform tactile tasks much faster than sighted people [44, 45]. In the present study, we did not test participant’s performance speed; instead, we allocated equal amount of time to both the blindfolded sighted and blind groups for stimulus exploration. Thus, we contend that the equal amount of time might have enabled the sighted group to perform to a level equal to the performance level of the blind groups.

As outlined above, visual experience cannot mediate Relaxation, Hedonics, and Arousal dimensions that likely underlie texture preference and aesthetic appraisal of textures. Here, it is important to stress on the limitation of the current literature that neither theoretical views nor empirical findings tell us anything about texture preference and texture aesthetics. The findings of the present study fill this gap by explicitly demonstrating that visual experience is not necessary for shaping preference for and appreciation of tactile objects with such microspatial property as smoothness and softness. In line with this, one prior study demonstrated that the preference for and appreciation of tactile objects with macrospatial properties (shape, pattern) were not dependent on the level of visual experience [55]. Thus, combining the findings of the present study together with those of that prior study [55], we conclude that visual coding of tactile stimuli or objects is neither necessary nor advantageous for tactile preference and tactile appreciation, and that this notion may equally apply to the stimuli with macrospatial properties (e.g., shape, size, orientation), and those with microspatial properties (e.g., roughness, hardness) as well. This novel notion derived from the findings of our present and prior controlled experimentations challenge the claim of an early uncontrolled experiment that tactile aesthetic preference and appreciation can be associated with visual experience [103]. The present evidence against the role of visual experience in tactile aesthetic preference and appreciation further rules out the prior notion that the visually impaired individuals tend to be more “subjective”, and the sighted children tend to be fairly “objective” in tactile aesthetic preference and appreciation [103], and lends direct support to Diderot’s philosophical supposition that the blind can be as practical and objective as the sighted in the assessment of tactile objects [25, 86]. However, preclusion of the role of visual experience does not necessarily deny experience-driven tactile aesthetic preference and appreciation (e.g., prior experience associated with emotional valence or aesthetically pleasant/unpleasant tactile objects) as can be speculated from a pool of prior research in visual arts and aesthetics [57, 84, 101, 117, 119, 120].

Analysis of the effects of texture parameters reflects some important aspects of our study. One such aspect is that the basic tactile discrimination varied highly proportionally as a function of each of the two physical stimulus properties: 2D tactile smoothness/roughness and 3D tactile softness/hardness. Irrespective of visual experience, participants were able to discriminate not only between the smoothest and roughest 2D tactile surfaces but between the 2D surfaces with intermediate levels of smoothness as well (Figs. 3(a), 4(a), Table I). Similarly, they were able to discriminate not only between the softest and hardest stimuli, but between the stimuli of intermediate levels of softness as well (Figs. 7(a), 8(a), Table IV). These findings are supported by the findings of a number of prior studies. For example, as discussed earlier, some studies demonstrated that the perceived roughness of a stimulus surface varies as an approximate power function of the physical magnitude of roughness [30, 115], while other studies revealed that the perceived magnitude of softness increased monotonically as a function of increasing object compliance, leveling off around the end of the stimulus range [96].

Because touch is the first sense that humans develop and the first sense they use to explore and apprehend the environment [33], the aforementioned findings together with prior research findings [7, 30, 43, 96, 115] suggest that perhaps humans are inherently capable of fine texture detection using the tactile modality alone. This vision-independent high level of texture detection capability can be explained by spatial variations in the firing rate of mechanoreceptive neurons. Although there is no study thus far to interpret neural encoding of tactile softness sensation, studies addressing the neural codes underlying sensation of tactile roughness have shown that the perceived tactile roughness is related to spatial variations in the firing rate of slowly adapting Type I mechanoreceptive neurons [11, 19, 20]. One recent study demonstrated that a surface is felt rough to the extent that the activity varies across nerve fibers and across time within nerve fibers, and this activity variation-based neural code can account not only for magnitude estimates of roughness but for roughness discrimination performance as well [74]. A more recent study in rhesus macaques has shown that a subpopulation of somatosensory neurons preferentially encodes coarse textural features, whereas a second subpopulation of neurons preferentially encodes fine textural features in tactile stimuli [73]. We propose that such type of neural coding might also operate in humans, enabling fine textural discrimination.

A second important aspect of our study is that the tactile stimulus preference varied monotonically (not exactly proportionally) as a function of 2D smoothness/roughness or 3D softness/hardness. Irrespective of visual experience, participants preferred 2D smoother surfaces over 2D rougher surfaces (Figs. 3(b), 4(b), Table I), and softer cubes over harder cubes (Figs. 7(b), 8(b), Table IV), with strongest preference going for the softest cube, and weakest for the hardest cube as compared to preferences for all other cubes and 2D stimuli (Figs. 3(b), 4(b), 7(b), 8(b)). In support of the former part of this, prior studies in sighted individuals showed that stimulus preference (proportionally) increased with increasing magnitude of stimulus smoothness or softness [30, 47]. The current literature cannot tell anything about the second part. However, it was typical to exhibit strongest preference for the softest cube because all the roughest surfaces were in fact hard to a certain extent, and here, it has been evident that the preference was weakest for the hardest cube. These findings together suggest two related propositions about sensing tactile aesthetics. First, aesthetic preference for a textured stimulus is inversely related to texture sensitivity, which typically increases with the physical level of roughness or hardness (see [26, 31]). Second, because pleasure and interest are considered as two distinct routes to liking an object/event, the smoother or softer tactile stimuli appear to be more hedonic, that is, more pleasant and/or more interesting [38]. This latter notion has been further corroborated by the findings of our present study as discussed below.

A third important and novel aspect of our study is that it was the first to provide insight into the amodal nature of sensory aesthetics, suggesting that all three affective dimensions, such as Relaxation, Hedonics, and Arousal identified for visual arts and aesthetics ([82]; see also [107]) also likely underlie appreciation of tactile arts and aesthetics. Moreover, as indicated in the aesthetic profile for participants irrespective of visual experience, the Arousal dimension appears to be independent (does not mean uncorrelated) of the Relaxation and Hedonics dimensions in smoothness appreciation (Fig. 5(d)), though it is not clear in softness appreciation (Fig. 9(d)). These findings are partly consistent with the circumplex model, the most widely known dimensional model of emotion [104, 105], which distinguishes hedonic valence (pleasure–displeasure) and arousal (activating–relaxing) as two orthogonal dimensions of an emotional experience, and are inconsistent with Berlyne’s psychobiological model of aesthetics which posits an inverted-U shaped relationship between arousal potential of a stimulus and hedonic experience, with an intermediate level of arousal corresponding to maximum pleasure [10]. Thus, we conclude that unlike Berlyne’s view [10], the Hedonics and Arousal dimensions are not always positively associated; rather they might be orthogonal depending on stimulus properties. However, further quantitative studies are warranted to statistically validate these dimensions and their orthogonality in the tactile domain. Because of the ordinal nature of our data such a quantitative analysis (e.g., factor analysis) deemed to the extraction of response dimensions was not feasible in the current study.

A fourth interesting and novel aspect of our study is the demonstration that ranking aesthetic quality on each of the proposed dimensions (the degree to which Relaxational, Hedonic and Arousal attributes are present or absent in a tangible object) varied monotonically, with a negative or positive relationship, as a function of stimulus roughness or hardness. The monotonic functions of the Relaxation and Hedonics dimensions comprised a negative relationship, indicating that the rank assigned on each of these two dimensions decreased with increasing roughness (Fig. 5) or hardness (Fig. 9). These findings are consistent with prior studies that were limited to examining tactile Hedonics only. As discussed earlier, studies of sighted individuals in the tactile domain revealed that the perceived magnitude of pleasantness of tactile sensation was inversely related to the physical or estimated magnitude of surface roughness [58, 63, 115], and positively with softness estimates or object compliance [96]. Because physically smooth or soft stimuli engender less friction [31, 64, 65], other studies in sighted humans demonstrated that people rate physically smooth and soft stimuli as more pleasing than physically rough and hard stimuli under both active [32, 79, 102] and passive [29, 31, 32] touch conditions.

A fifth, more interesting and novel aspect of our study is that the monotonic function of Arousal was positive with stimulus roughness in contrast to negative with hardness, which indicates that the felt Arousal increased with physical roughness but decreased with physical hardness. This difference between the two Arousal trends might be due to the fact that roughness is a source of irritation and pain, but hardness is not much like that. However, there are also some similarities between the smoothness and softness dimensions. As demonstrated for the blindfolded sighted (Figs. 5(c) and 9(c)) and for all participants irrespective of visual experience (Figs. 5(d) and 9(d); Table III), the physically softest and softer 3D stimuli, like the physically smoothest and smoother 2D stimuli, received higher ranks on the Relaxation and Hedonics dimensions as compared to the Arousal dimension, and on the contrary, the physically hardest and harder stimuli, like the physically roughest and rougher stimuli, received higher ranks on the Arousal dimension as compared to both the Relaxation and Hedonics dimensions. However, the differences between the Arousal dimension on the one hand and Relaxation and Hedonics dimensions on the other at the softest, softer, hardest, and harder stimulus levels was substantially apparent in the blindfolded sighted group (Fig. 9(c)), with a little tendency in the late (Fig. 9(b)) but not in the congenitally blind group (Fig. 9(a)). This implies that in 3D softness appreciation task, the blindfolded sighted group differed from both the blind groups on the affective dimensions across the physical levels of 3D softness/hardness; however, this kind of difference was not observed in 2D smoothness appreciation task (Figs. 5(a),(b),(c)). Thus the difference between the sighted and blind individuals on the affective dimensions here cannot be attributed to visual experience but to such stimulus property as texture type.

Inspection of Fig. 9 further indicates that the line charts for Relaxation and Hedonics dimensions perfectly overlapped with one another across all the physical softness/hardness levels of 3D stimuli; however, such an overlap was not present across the physical smoothness/roughness levels of 2D stimuli (Fig. 5). Relaxation is sometimes thought to be a component of hedonic orientation (see [4–6, 13, 15, 36]). Thus, it is likely that what is hedonically pleasant can simultaneously be equally relaxing or calming in a particular context but not in another context as happened in the present study.

It follows from the above findings that irrespective of visual experience, the 2D smoother stimuli were more strongly characterized by Relaxation-oriented slightly intense affective attributes, such as calming, relaxing, soothing, comfortable, or by Hedonics-oriented moderately intense affective attributes, such as pleasant, desirable, enjoyable, sensual, and appealing (Figs. 5(d), 6(a),(b)). On the contrary, the rougher tactile stimuli were more strongly characterized by Arousal-oriented highly intense affective attributes, such as enlivening, exciting, thrilling, and irritating (Figs. 5(d), 6(c)). However, a different picture was observed for softer–harder comparison in the aesthetic profile. Overall, the physically softer stimuli were characterized more strongly by the Hedonics- and Relaxation-oriented affective attributes than by the Arousal-oriented affective attributes (Figs. 9(d) and 10; Table VI). On the contrary, the physically harder stimuli showed a tendency to be characterized more strongly by the Arousal-oriented highly intense affective attributes than by the other two sorts of affective attributes (Figs. 9(d) and 10; Table VI). Thus tactile aesthetic appreciation is determined by physical properties of objects or stimuli.

To summarize, the aforementioned striking differences between the forms of appreciation of 2D smoothness and 3D softness across the affective dimensions of the aesthetic profile suggest unanticipated substructures in the nascent field of tactile aesthetics. We conclude that by examining a variety of implicit attributes encompassing all three affective dimensions and relating each dimension with the physical magnitude of 2D smoothness and 3D softness, the two prominent forms of tactile textures, the present study gives us a full-fledged understanding of the affective structure of tactile aesthetics that apply not only for the sighted but for the blind as well.

To the best of our knowledge, this exploratory study was the first to experimentally investigate aesthetic preference for and aesthetic appreciation of tactile objects as a function of visual experience and tangible texture. This gave us an opportunity to examine for the first time the applicability of three-dimensional affective structure of visual aesthetics (Relaxation, Hedonics, and Arousal; [82]) to the understanding of tactile aesthetics. Studying groups of congenitally blind, late blind, and temporarily blinded (blindfolded) sighted individuals by employing a novel behavioral approach (rank ordering), we revealed that contrary to what might be expected, visual experience is not required for shaping tactile aesthetic preference and tactile aesthetic appreciation, in addition to showing the same for basic tactile discrimination.

We further showed the evidence that tactile texture parameters, such as 2D smoothness and 3D softness, can significantly impact aesthetic preference and the aforementioned three affective dimensions of aesthetic sensation, in addition to impacting basic tactile discrimination. Based on these novel and interesting findings, we conclude that the three-dimensional affective structure of visual arts and aesthetics ([82]; see also [107]) is amodal and applicable to tactile arts and aesthetics not only in the sighted but in the blind as well. However, analysis of the aesthetic profile across the affective dimensions reveals striking differences between the forms of appreciation of 2D smoothness and 3D softness, suggesting unanticipated substructures in the nascent field of tactile aesthetics. Depending on the nature of object textures we experience, the tactile modality differentially senses and perceives environmental objects, making differential affective responses to them. As found in the present study, our felt relaxation and hedonics may highly overlap in sensing and appreciating objects of varying softness levels, but markedly differ in sensing and appreciating objects of varying smoothness levels, with the arousal having an inverse relationship with smoothness level but a positive relationship with softness level.

The novel and interesting findings presented here can have a wide range of theoretical and practical implications. One theoretical implication is that there might be an inherent nature to the coding of tactile perception, tactile preferences, and tactile aesthetics, independent of visual experience. Practically, the findings can inform decisions in future design of tactile arts, tactile objects/products, tools, and technology sensitive to the blind community, yet applicable to the sighted as well. Designing the tactile environments using such a nondifferential approach will strengthen mood and confidence of the blind, enhancing their capability and productivity. The findings also provide novel and highly informative insights to guide the design of future research on the affective structure of tactile aesthetics in relation to a variety of tactile properties. Finally, this study opens the door to expanded experimental investigation of tactile preferences and tactile aesthetics in the blind and the sighted.