Tactual perception: a review of experimental variables and procedures <s> Two-dimensional and three-dimensional stimuli <s> In 8 experiments college students felt 32 geometric objects and were tested in a signal-detection framework to same or distractor items. Retention intervals and intervening experiences were also manipulated following initial touching. In all instances performance was high, and there was no evidence of a decline in haptic sensitivity over the retention intervals employed. These surprising results were interpreted as consistent with the 1985 contention of Klatzky, Lederman, and Metzger that the haptic modality constitutes an expert system. <s> BIB001 </s> Tactual perception: a review of experimental variables and procedures <s> Two-dimensional and three-dimensional stimuli <s> College students actively felt complex objects for 3 sec, and, after delay intervals of 5, 15, 30, or 45 sec, they received either the same object or a different object for comparison. In a signal detection framework, it was observed that d′ scores were significantly higher for the 5- and 15-sec intervals than for the 30- and 45-sec intervals. Although a clearly negatively accelerated function was not obtained, this is one of the rare instances in which any decay function has been observed for haptic memory. Although it seems counterintuitive that the entire short-term-memory decay function occupies the narrow band extending from 15 to 30 sec, it is clear from previous studies that short-term haptic memory is unique. Results were discussed in terms of findings in both active- and passive-touch experiments. <s> BIB002 </s> Tactual perception: a review of experimental variables and procedures <s> Two-dimensional and three-dimensional stimuli <s> We studied active haptic discrimination of the geometrical features of an object. The geometrical parameters under investigation were the amplitude and width of a gaussian-shaped surface. Haptic discrimination thresholds were measured with regard to three values of these geometrical parameters. We found that humans discriminate up to about 300 shapes when both amplitude and width are extrapolated to the range between 1 μm and 1 m. Over this range, which covers the span of the arms, the number of discriminations is small compared to the number across the full range of chromaticities in vision. Roughly speaking, humans are far better at discriminating sharp (extensive amplitude and little width) gaussian surfaces from smooth (small amplitude and extensive width) ones than they are at discriminating small (small amplitude and width) surfaces from large (extensive amplitude and width) ones. Our main conclusion is that discrimination in the geometrical domain is poorest when the proportion between amplitude and width is roughly the same for both shapes. Our results are in close agreement with results of earlier experiments on detection thresholds. This indicates that similar, or even the same, neural mechanisms were used for detection and discrimination of the geometrical parameters under investigation. <s> BIB003 </s> Tactual perception: a review of experimental variables and procedures <s> Two-dimensional and three-dimensional stimuli <s> Visually, a red item is easily detected among green items, whereas a mirrored S among normal Ss is not. In visual search, the former is known as the pop-out effect. In daily life, people often also conduct haptic (tactual) searches, for instance, when trying to find keys in their pocket. The aim of the present research was to determine whether there is a haptic version of the pop-out effect. Blindfolded subjects had to search for a target item which differed in roughness from the surrounding distractor items. We report reaction time slopes as low as 20 ms/item. When target and distractor identities were interchanged the slopes increased indicating a search asymmetry. Furthermore, we show that differences in search slope were accompanied by search strategy differences. In some conditions a single-hand sweep over the display was sufficient, while in others a more detailed search strategy was used. By relating haptic search slopes to parallel and serial search strategies we show, for the first time, that pop-out effects occur under free manual exploration. <s> BIB004
Aiming to evaluate tactual perception and cognition in isolation, many researchers have chosen to collect data using artificial stimuli created in the laboratory. These stimuli, either two-or three-dimensional, allow absolute control of stimuli properties and are very useful to understand how people discriminate single features of the objects, or how different properties are weighted and integrated. Regarding two-dimensional stimuli, two main types of stimuli can be found: raised line drawings or haptic pictures, which consist of the presentation of the contours of a drawing in a higher level to the background; and tactile scenes, presentation of matrices or other complex stimuli that can only be explored in two dimensions. BIB004 have explored the pop-out effect on haptics. The pop-out effect has been studied in vision and refers to the easier detection of a stimulus among others in a visual search: for instance, detecting a red stimulus among green ones. In this first study on haptic pop-out BIB004 , it was shown that a rough item among fine items in a matrix was detected faster than the inverse, demonstrating the existence of a pop-out effect in haptics. Analysing bilateral symmetry, Ballesteros et al. (1997) reported that for raised line shapes, perception of asymmetry was better than perception of symmetry when participants could explore the item with only one finger and that bimanual exploration facilitated symmetry judgments, but not asymmetry ones. For unfamiliar three-dimensional stimuli, single hand and bimanual explorations did not differ, and symmetry judgments were very accurate. The variability of three-dimensional abstract stimuli is higher than in two-dimensional stimuli, and these types of stimuli can be geometrical or abstract forms built in various materials (e.g., plastic, wood, metal) or stimuli created from the combination of toy construction blocks. BIB001 used three-dimensional uncommon (hardly nameable) geometrical plastic shapes (stimuli from the ''Superfection'' game) and reported a hit rate of 93 % in haptic recognition, with false alarms being virtually nonexistent even for delays as large as 40 s between item presentation and recognition for a total of 30 objects. In a later study BIB002 , the authors reported high discriminability measures, even when the stimuli were attached to a base, and the participants only had 3 s to explore them. Regarding the discrimination of Gaussian-like shapes, attending to amplitude and width parameters BIB003 showed that participants were able to haptically discriminate up to 300 shapes, being better at discriminating sharp from smooth Gaussian curves than at discriminating between small and large ones. The authors concluded that the estimation of width was central for the discrimination of shape. Using three-dimensional objects,  explored the effect of curvature in haptic perception. The participants were asked to differentiate cylinders with a circular versus an elliptical base and to discriminate square from rectangular cuboids. Results showed that the base ratio to perceive objects with curvature was smaller than the needed ratio to perceive cuboid differences, suggesting that curvature is a relevant cue in haptic experience, facilitating object perception. According to  , this group of studies on touch using abstract stimulation, might be contributing to an underestimation of haptic abilities. To these authors, a main distinction between abstract and everyday objects' perception and recognition rises from the concept of pattern recognition. Everyday objects are perceived as patterns and analysed as a whole, while abstract stimuli are perceived as a sum of different features  . They argue that haptic recognition of everyday objects entails an apprehension of tactile patterns that is achieved through both bottom-up and top-down processes, and these aspects differentiate haptic perception from the simple categorisation that is typical in the perception of abstract stimuli  . Experiments with two-and three-dimensional abstract stimuli are very useful to analyse haptic evaluation of specific properties, allowing an estimation of the impact of each tactual feature in the overall perception of the stimuli, and as such contributing to a deeper understanding of how people perceive and process everyday tactual stimuli. This section presented an overall scenario of research with abstract stimuli, showing that these type of stimuli contribute to a detailed analysis of tactual processes, allowing for a greater control of experimental conditions, such as manipulation times or stimuli variability. However, these techniques remove crucial cues from haptics such as material properties (e.g., texture or weight information), and as such might be resulting in an estimation of tactual ability that does not necessarily correspond to performance in optimal conditions with free manipulation and complex objects. The following section will explore performance with concrete stimuli.