Intermanual and intramanual tactual grating discrimination

Enabling visually impaired people to learn three-dimensional tactile graphics with a 3DOF haptic mouse

Background

In this work, we present a novel sensory substitution system that enables to learn three dimensional digital information via touch when vision is unavailable. The system is based on a mouse-shaped device, designed to jointly perceive, with one finger only, local tactile height and inclination cues of arbitrary scalar fields. The device hosts a tactile actuator with three degrees of freedom: elevation, roll and pitch. The actuator approximates the tactile interaction with a plane tangential to the contact point between the finger and the field. Spatial information can therefore be mentally constructed by integrating local and global tactile cues: the actuator provides local cues, whereas proprioception associated with the mouse motion provides the global cues.

Methods

The efficacy of the system is measured by a virtual/real object-matching task. Twenty-four gender and age-matched participants (one blind and one blindfolded sighted group) matched a tactile dictionary of virtual objects with their 3D-printed solid version. The exploration of the virtual objects happened in three conditions, i.e., with isolated or combined height and inclination cues. We investigated the performance and the mental cost of approximating virtual objects in these tactile conditions.

Results

In both groups, elevation and inclination cues were sufficient to recognize the tactile dictionary, but their combination worked at best. The presence of elevation decreased a subjective estimate of mental effort. Interestingly, only visually impaired participants were aware of their performance and were able to predict it.

Conclusions

The proposed technology could facilitate the learning of science, engineering and mathematics in absence of vision, being also an industrial low-cost solution to make graphical user interfaces accessible for people with vision loss.

Haptic shape discrimination and interhemispheric communication

In three experiments participants haptically discriminated object shape using unimanual (single hand explored two objects) and bimanual exploration (both hands were used, but each hand, left or right, explored a separate object). Such haptic exploration (one versus two hands) requires somatosensory processing in either only one or both cerebral hemispheres; previous studies related to the perception of shape/curvature found superior performance for unimanual exploration, indicating that shape comparison is more effective when only one hemisphere is utilized. The current results, obtained for naturally shaped solid objects (bell peppers, Capsicum annuum) and simple cylindrical surfaces demonstrate otherwise: bimanual haptic exploration can be as effective as unimanual exploration, showing that there is no necessary reduction in ability when haptic shape comparison requires interhemispheric communication. We found that while successive bimanual exploration produced high shape discriminability, the participants’ bimanual performance deteriorated for simultaneous shape comparisons. This outcome suggests that either interhemispheric interference or the need to attend to multiple objects simultaneously reduces shape discrimination ability. The current results also reveal a significant effect of age: older adults’ shape discrimination abilities are moderately reduced relative to younger adults, regardless of how objects are manipulated (left hand only, right hand only, or bimanual exploration).

When kinesthesia becomes visual: a theoretical justification for executing motor tasks in visual space

Several experimental studies in the literature have shown that even when performing purely kinesthetic tasks, such as reaching for a kinesthetically felt target with a hidden hand, the brain reconstructs a visual representation of the movement. In our previous studies, however, we did not observe any role of a visual representation of the movement in a purely kinesthetic task. This apparent contradiction could be related to a fundamental difference between the studied tasks. In our study subjects used the same hand to both feel the target and to perform the movement, whereas in most other studies, pointing to a kinesthetic target consisted of pointing with one hand to the finger of the other, or to some other body part. We hypothesize, therefore, that it is the necessity of performing inter-limb transformations that induces a visual representation of purely kinesthetic tasks. To test this hypothesis we asked subjects to perform the same purely kinesthetic task in two conditions: INTRA and INTER. In the former they used the right hand to both perceive the target and to reproduce its orientation. In the latter, subjects perceived the target with the left hand and responded with the right. To quantify the use of a visual representation of the movement we measured deviations induced by an imperceptible conflict that was generated between visual and kinesthetic reference frames. Our hypothesis was confirmed by the observed deviations of responses due to the conflict in the INTER, but not in the INTRA, condition. To reconcile these observations with recent theories of sensori-motor integration based on maximum likelihood estimation, we propose here a new model formulation that explicitly considers the effects of covariance between sensory signals that are directly available and internal representations that are ‘reconstructed’ from those inputs through sensori-motor transformations.

Do Left and Right Matter for Haptic Recognition of Familiar Objects?

Two experiments were carried out to examine the effects of dominant right versus non-dominant left exploration hand and left versus right object orientation on haptic recognition of familiar objects. In experiment 1, participants named 48 familiar objects in two blocks. There was no dominant-hand advantage to naming objects haptically and there was no interaction between exploration hand and object orientation. Furthermore, priming of naming was not reduced by changes of either object orientation or exploration hand. To test whether these results were attributable to a failure to encode object orientation and exploration hand, experiment 2 replicated experiment 1 except that the unexpected task in the second block was to decide whether either exploration hand or object orientation had changed relative to the initial naming block. Performance on both tasks was above chance, demonstrating that this information had been encoded into long-term haptic representations following the initial block of naming. Thus when identifying familiar objects, the haptic processing system can achieve object constancy efficiently across hand changes and object-orientation changes, although this information is often stored even when it is task-irrelevant.

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.