Directional biases in whole hand motion perception revealed by mid-air tactile stimulation

Abstract

Many emerging technologies are attempting to leverage the tactile domain to convey complex spatiotemporal information translated directly from the visual domain, such as shape and motion. Despite the intuitive appeal of touch for communication, we do not know to what extent the hand can substitute for the retina in this way. Here we ask whether the tactile system can be used to perceive complex whole hand motion stimuli, and whether it exhibits the same kind of established perceptual biases as reported in the visual domain. Using ultrasound stimulation, we were able to project complex moving dot percepts onto the palm in mid-air, over 30 cm above an emitter device. We generated dot kinetogram stimuli involving motion in three different directional axes ('Horizontal', 'Vertical', and 'Oblique') on the ventral surface of the hand. Using Bayesian statistics, we found clear evidence that participants were able to discriminate tactile motion direction. Furthermore, there was a marked directional bias in motion perception: participants were both better and more confident at discriminating motion in the vertical and horizontal axes of the hand, compared to those stimuli moving obliquely. This pattern directly mirrors the perceptional biases that have been robustly reported in the visual field, termed the 'Oblique Effect'. These data demonstrate the existence of biases in motion perception that transcend sensory modality. Furthermore, we extend the Oblique Effect to a whole hand scale, using motion stimuli presented on the broad and relatively low acuity surface of the palm, away from the densely innervated and much studied fingertips. These findings highlight targeted ultrasound stimulation as a versatile method to convey potentially complex spatial and temporal information without the need for a user to wear or touch a device.

Keywords: Confidence; Haptics; Human–computer interaction; Somatosensory; Touch.

Fig. 1

Overview of mid-air tactile experimental setup. A. Participants were seated with their hands above an array of ultrasound actuators and a infra-red camera. B. The combination of real time hand tracking and focused ultrasound can project discrete points onto the user's unadorned hand (Carter et al., 2013). C. Users experienced a series of moving dot stimuli (Fig. 2) in differing directions. D. Stimuli were delivered using an Ultrahaptics device (UltraLeap, Bristol).

Fig. 2

Overview of a single experimental trial presenting mid-air tactile stimuli in a two-interval-forced-choice task.

Fig. 3

Evidence of an oblique effect in whole hand tactile motion perception. Performance on the experimental two-interval-forced-choice task for the three conditions combined (grey) as well as separate for vertical (red), horizontal (blue), and oblique (green) tactile motion stimuli presented on the palm. A. Group distributions of % correct (left), with each dot in a distribution showing the performance of one participant, with the accompanying BF₁₀ from the one-sample t-tests presented above. The BF₁₀ show extreme evidence that % correct exceeds chance level. The right panel shows the change in BF₁₀ as a function of participant recruitment, reflecting the accumulation of evidence as the sample size increased. B. Bayes Factor (BF_inclusion) of effect of condition a function of participant recruitment, showing the cut-offs for the minimally to be collected sample size (n = 12 or BF_inclusion for condition > 6) C. Plot of the group mean of each condition with the within-subject error bars - reflecting the within-subject differences across the three conditions - with the BF₁₀ of the post-hoc tests above. Post-hoc tests indicate a clear existence of an “Oblique” effect in the data, such that participants performed statistically better in perceptual discrimination in the horizontal and vertical axes compared with the oblique axis.

Fig. 4

Overview of results of the meta-cognitive measures. A. Mean confidence rating (top panel) and mean meta-cognitive ability (AROC; bottom panel) for the vertical (red), horizontal (blue), and oblique (green) condition over the ten different stimuli durations (logarithmically spaced between 200 msec and 8 sec). Error bars indicate the within-subject error across conditions. There was a main effect of condition and duration on confidence rating, but not on meta-cognitive ability. On the right, the BF₁₀ from the post-hoc tests on condition are shown. Again, there is a clear oblique effect, with confidence in the oblique condition being worse than in the horizontal and vertical condition. B. The BF₁₀ from the post-hoc comparisons of duration on confidence ratings. Dark blue colours indicate more evidence for the null-hypothesis (i.e., no difference in confidence rating between these two durations), while lighter blue to red colours indicate gradual higher evidence for the alternative hypothesis (i.e., evidence for a statistical difference in confidence ratings between the two durations). For interpretation purposes, the colours in the heatmap are log-scaled. The cut-offs for indeterminate evidence ($\frac{1}{3}$ and 3) are shown on the legend by respectively the lower and upper arrow. Overall, confidence is lowest in the shortest and in the longest durations.

Fig. 5

Distributions of the explained variance (R²; top panel) and accompanying Bayes Factors (bottom panel) of the correlation analyses and ANOVA of individual differences and task effects. Analyses were conducted for each condition separately as well as combined, with task performance (star), confidence ratings (triangle), and AROC (square) - resulting in 12 R²-values in each violin. Analyses are separated between those conducted on the between-subject (left) and on the within-subject level (right). In the bottom panel, values above the upper red line indicate more evidence for the null-hypothesis (BF₀₁ > 3). Values between the two red lines are typically interpreted as indeterminate, while evidence below the red line (BF₀₁ <$\frac{1}{3}$) indicate evidence for the presence of a correlation/effect. Overall, we find evidence against systematic individual differences and task effects for age, palm size, order, and hand temperature. The evidence for effects of time remain largely indeterminate.