Haptic shape discrimination in humans: insight into haptic frames of reference

Radial trunk-centred reference frame in haptic perception

The shape of objects is typically identified through active touch. The accrual of spatial information by the hand over time requires the continuous integration of tactile and movement information. Sensory inputs arising from one single sensory source gives rise to an infinite number of possible touched locations in space. This observation raises the question of the determination of a common reference frame that might be employed by humans to resolve spatial ambiguity. Here, we employ a paradigm where observers reconstruct the spatial attributes of a triangle from tactile inputs applied to a stationary hand correlated with the voluntary movements of the other hand. We varied the orientation of the hands with respect to one another and to the trunk, and tested three distinct hypotheses regarding a reference frame used for integration: a hand-centred, a trunk-centred or an allocentric reference frame. The results indicated strongly that the integration of movement information and tactile inputs was performed in a radial trunk-centred reference frame.

Haptic two-dimensional angle categorization and discrimination

This study examined the extent to which haptic perception of two-dimensional (2-D) shape is modified by the design of the perceptual task (single-interval categorization vs. two-interval discrimination), the orientation of the angles in space (oblique vs. horizontal), and the exploration strategy (one or two passes over the angle). Subjects (n = 12) explored 2-D angles using the index finger of the outstretched arm. In the categorization task, subjects scanned individual angles, categorizing each as "large" or "small" (2 angles presented in each block of trials; range 80° vs. 100° to 89° vs. 91°; implicit standard 90°). In the discrimination task, a pair of angles was scanned (standard 90°; comparison 91-103°) and subjects identified the larger angle. The threshold for 2-D angle categorization was significantly lower than for 2-D angle discrimination, 4° versus 7.2°. Performance in the categorization task did not vary with either the orientation of the angles (horizontal vs. oblique, 3.9° vs. 4°) or the number of passes over the angle (1 vs. 2 passes, 3.9° vs. 4°). We suggest that the lower threshold with angle categorization likely reflects the reduced cognitive demands of this task. We found no evidence for a haptic oblique effect (higher threshold with oblique angles), likely reflecting the presence of an explicit external frame of reference formed by the intersection of the two bars forming the 2-D angles. Although one-interval haptic categorization is a more sensitive method for assessing 2-D haptic angle perception, perceptual invariances for exploratory strategy and angle orientation were, nevertheless, task-independent.

On the edge: haptic discrimination of edge sharpness

The increasing ubiquity of haptic displays (e.g., smart phones and tablets) necessitates a better understanding of the perceptual capabilities of the human haptic system. Haptic displays will soon be capable of locally deforming to create simple 3D shapes. This study investigated the sensitivity of our haptic system to a fundamental component of shapes: edges. A novel set of eight high quality shape stimuli with test edges that varied in sharpness were fabricated in a 3D printer. In a two alternative, forced choice task, blindfolded participants were presented with two of these shapes side by side (one the reference, the other selected randomly from the remaining set of seven) and after actively exploring the test edge of each shape with the tip of their index finger, reported which shape had the sharper edge. We used a model selection approach to fit optimal psychometric functions to performance data, and from these obtained just noticeable differences and Weber fractions. In Experiment 1, participants performed the task with four different references. With sharpness defined as the angle at which one surface meets the horizontal plane, the four JNDs closely followed Weber's Law, giving a Weber fraction of 0.11. Comparisons to previously reported Weber fractions from other haptic manipulations (e.g. amplitude of vibration) suggests we are sufficiently sensitive to changes in edge sharpness for this to be of potential utility in the design of future haptic displays. In Experiment 2, two groups of participants performed the task with a single reference but different exploration strategies; one was limited to a single touch, the other unconstrained and free to explore as they wished. As predicted, the JND in the free exploration condition was lower than that in the single touch condition, indicating exploration strategy affects sensitivity to edge sharpness.

Exploratory Strategies in Haptic Softness Discrimination Are Tuned to Achieve High Levels of Task Performance

Haptic perception essentially depends on the executed exploratory movements. It has been speculated that spontaneously executed movements are optimized for the computation of associated haptic properties. We investigated to what extent people strategically execute movements that are tuned for softness discrimination of objects with deformable surfaces. In Experiment 1, we investigated how movement parameters depend on expected stimulus compliance. In a discrimination task, we measured exploratory forces for less compliant (hard) stimuli and for more compliant (soft) stimuli. In Experiment 2, we investigated whether exploratory force also depends on the expected compliance difference between the two stimuli. The results indicate that participants apply higher forces when expecting harder objects as compared to softer objects, and they apply higher forces for smaller compliance differences than for larger ones. Experiment 3 examined how applied force influences differential sensitivity for softness as assessed by just noticeable differences (JNDs). For soft stimuli, JNDs did not depend on force. For hard stimuli, JNDs were "worse" (higher) if participants applied less force than they use naturally. We conclude that applying high force is a robust strategy to obtain high differential sensitivity, and that participants used this strategy if it was required for successful discrimination performance.

Spatial patterns in tactile perception: is there a tactile field?

Previous studies of tactile spatial perception focussed either on a single point of stimulation, on local patterns within a single skin region such as the fingertip, on tactile motion, or on active touch. It remains unclear whether we should speak of a tactile field, analogous to the visual field, and supporting spatial relations between stimulus locations. Here we investigate this question by studying perception of large-scale tactile spatial patterns on the hand, arm and back. Experiment 1 investigated the relation between perception of tactile patterns and the identification of subsets of those patterns. The results suggest that perception of tactile spatial patterns is based on representing the spatial relations between locations of individual stimuli. Experiment 2 investigated the spatial and temporal organising principles underlying these relations. Experiment 3 showed that tactile pattern perception makes reference to structural representations of the body, such as body parts separated by joints. Experiment 4 found that precision of pattern perception is poorer for tactile patterns that extend across the midline, compared to unilateral patterns. Overall, the results suggest that the human sense of touch involves a tactile field, analogous to the visual field. The tactile field supports computation of spatial relations between individual stimulus locations, and thus underlies tactile pattern perception.

Raised-angle discrimination under passive finger movement

The characteristics of raised-line drawing discrimination can be defined as the sum of the discriminability of the length, curvature, and angles of the edges. The size of the angle between two edges constitutes an important feature of these tactile stimuli. In the first experiment, five standard angles (30 degrees, 60 degrees, 90 degrees, 120 degrees, and 150 degrees) and twenty comparison angles for each standard angle were used to investigate the human capacity for tactile discrimination of raised angles by passive finger movement. The subjects in this study were asked to identify the larger angle of each pair by passive finger movement. We found that the threshold doubled when the standard angle was increased from 30 degrees to 90 degrees; however, the threshold remained unchanged when the standard angle was greater than 90 degrees. In the second experiment, to investigate the influence of the endpoints on angle discriminability, we used one standard angle (60 degrees) and seven comparison angles that changed in four bisector orientations. The results indicate that cutaneous feedback from the local apex and endpoints of the angle contributed to the discrimination of acute angles. Taken together, these results suggest that, when an acute angle is presented, both local apex and endpoint informations are used, while cutaneous mechanoreceptors rely more on apex information to discriminate the angle size when an obtuse angle is presented.

Curvature discrimination in various finger conditions

The ability of humans to discriminate curvature was investigated for different finger conditions. The experiments were conducted in which subjects explored cylindrically curved stimuli by touch. Using a 2-alternative forced-choice procedure, discrimination thresholds and biases were measured for several conditions. In 1-finger conditions, reference and test stimulus were explored with the same finger, whereas in 2-finger conditions these stimuli were felt with different fingers. Similar thresholds were obtained for the 1-finger conditions, in which either the preferred or the non-preferred index finger or the thumb was employed. However, significantly higher thresholds were found for the conditions in which subjects used two fingers, either of the same hand or of different hands. Interestingly, even higher thresholds were obtained for a 2-finger condition in which subjects explored the stimuli simultaneously instead of sequentially. In addition, subject-dependent biases were found in the 2-finger conditions. We conclude that the number of fingers and the mode of exploration have a considerable effect on performance in a haptic task such as curvature discrimination.

Haptic discrimination of two-dimensional angles: influence of exploratory strategy

The aim of this study was to define the relative contribution of self-generated cutaneous and proprioceptive feedback to haptic shape discrimination by systematically constraining the exploratory strategy. Subjects (n = 23) explored pairs of two-dimensional (2-D) angles (standard angle, 90 degrees; comparison angles, 91 degrees -103 degrees) placed at arm's length from the subject, and identified the larger angle of each pair. The exploratory strategies included a reference condition, dynamic scan of the index finger over the entire object [combined cutaneous and proprioceptive (shoulder) feedback], and modified conditions, static touch of the intersection of the two bars that formed the angle using the index finger (cutaneous feedback) and dynamic scans of the object using a hand-held tool (proprioceptive feedback, shoulder). Discrimination thresholds (75% correct) were very similar for dynamic and static touch with the index finger. Thresholds varied as a function of the static contact duration (<1 s, 7.2 degrees +/- 0.6 degrees; approximately 3 s, 4.2 degrees +/- 0.5 degrees), but were not different from the reference condition (6.0 degrees +/- 0.9 degrees). The higher threshold with short static touch likely reflects movement-related gating of self-generated tactile inputs. Together, the results suggested that cutaneous feedback alone may be sufficient to explain 2-D angle discrimination, because the added proprioceptive feedback did not improve performance. Also, threshold did not vary with the number of dynamic scans (one or two), suggesting that the critical information was gathered on the first pass over the angle. In contrast, when the angles were explored with the tool, the threshold increased relative to the corresponding reference condition from the same session (tool, 9.6 degrees +/- 0.9 degrees; dynamic scan with the finger, 6.2 degrees +/- 1.0 degrees). Thus, performance was poorer with proprioceptive feedback alone, suggesting that cutaneous feedback was relatively more important for 2-D haptic angle discrimination in the present experiment.