Vibrotactile spatial acuity on the back.

Vibrotactile feedback can be built into clothing such as vests. This means that often vibrotactile information is presented to the back. It is known that the back has a relatively low spatial acuity. Spatial acuity varies across different limbs and sometimes with different locations on a limb. These known anisotropies suggest that there might be systematic variations in vibrotactile spatial acuity for different areas of the back and also for different orientations (i.e. horizontal vs. vertical). Here we systematically measured spatial acuity in four areas of the back for both horizontal and vertical orientations. The results show no significant differences in spatial acuity for the back areas that were tested. Spatial acuity was, however, higher in the horizontal direction than in the vertical direction by roughly a factor of two. This means that when designing vibrotactile displays for the back the tactor density can be lower in the vertical direction than in the horizontal direction and density should be constant for different areas of the back.

Integrating force and position: testing model predictions.

In this study, we investigated the integration of force and position information in a task in which participants were asked to estimate the center of a weak force field. Two hypotheses, describing how participants solved this task, were tested: (1) by only using the position(s) where the force reaches the detection threshold, and (2) by extrapolating the force field based on perceived stiffness. Both hypotheses were also described formally, assuming a psychophysical function obeying a power law with an exponent smaller than one. The hypotheses were tested in two psychophysical experiments, in which 12 participants took part. In Experiment 1, an asymmetric force field was used and the presence of visual feedback about hand position was varied. In Experiment 2, a unilateral force field was used. For both experiments, hypothesis 1 predicts biases between (Experiment 1) or at (Experiment 2) the position(s) of the force detection threshold, while hypothesis 2 predicts smaller biases. The measured data show significant biases in both experiments that coincide with the biases predicted by using force detection thresholds from the literature. The average measured responses and their variabilities also fitted very well with the mathematical model of hypothesis 1. These results underline the validity of hypothesis 1. So, participants did not use a percept of the stiffness of the force field, but based their estimation of the center of the force field on the position(s) where the force reached the detection threshold. This shows that force and position information were not integrated in this task.

Bimanual and unimanual length perception.

From previous studies, it is unclear how bimanual length discrimination differs from unimanual length discrimination. To investigate the difference, we designed an experiment with four conditions. In the first two conditions, unimanual and bimanual discrimination thresholds are determined. In the third and fourth conditions, length is explored with the two index fingers like in the bimanual condition, but the reference is either internal, by clasping the hands together, or external, by grasping handles connected to the table. We find that thresholds for the unimanual condition (7.0%) and the clasping condition (9.2%) are both lower than for the bimanual condition (16%) and the grasping handles condition (15%). We conclude that when discriminating length unimanually and bimanually while clasping the hands together, the internal reference within the hand can be used and that explains the lower discrimination thresholds.

Hands-Free Haptic Navigation Devices for Actual Walking.

In this survey, we give an overview of hands-free haptic devices specifically designed for navigation guidance while walking. We present and discuss the devices by body part, namely devices for the arm, foot and leg, back, belly and shoulders, waist and finally the head. Although the majority of the experimental tests were successful in terms of reaching the target while being guided by the device, the experimental requirements were wide-ranging. The distances to be covered ranged from just a few meters to more than a kilometer, and while some of the devices worked autonomously, others required the experimenter to act as Wizard of Oz. To compare the usefulness and potential of these devices, we created a table in which we rated several relevant aspects such as autonomy, conspicuity and compactness. Major conclusions are that outdoor devices have the highest technology readiness level, because these allow autonomous navigation through GPS, and that the most compact devices still require the action of an experimenter. Unfortunately, none of the hands-free devices are at a level of readiness where they could be useful to people with visual impairments. The most important factor that should be improved is localization accuracy, which should be high and available at all times.

Influence of Back Length on Vibrotactile Acuity in Vertical Direction.

In this study, we were interested in the question whether vibrotactile thresholds on the back, in terms of the Just Noticeable Difference (JND), scale with back length. Although there exists only indirect physiological evidence, it could be assumed that the density of mechanoreceptors is lower if back size is larger. As a consequence, the JND would increase with back length. We measured psychophysical curves for 40 male participants with back lengths in the range of 36 to 55 cm. Nine equally spaced vibration motors were placed in vertical direction on their back, 2 cm left of the spine. For each stimulus pair, participants had to decide whether the second vibration was above or below the first vibration. It was found that the slope of the fit of the JND as a function of back length was not significantly different from zero, so contrary to our expectation, we did not find an influence of back length on JND. This means that when customizing a wearable haptic device for the back, measuring back length is not the way to go.

Genotype and phenotype of 101 dutch patients with congenital stationary night blindness.

OBJECTIVE: To investigate the relative frequency of the genetic causes of the Schubert-Bornschein type of congenital stationary night blindness (CSNB) and to determine the genotype-phenotype correlations in CSNB1 and CSNB2. DESIGN: Clinic-based, longitudinal, multicenter study. PARTICIPANTS: A total of 39 patients with CSNB1 from 29 families and 62 patients with CSNB2 from 43 families. METHODS: Patients underwent full ophthalmologic and electrophysiologic examinations. On the basis of standard electroretinograms (ERGs), patients were diagnosed with CSNB1 or CSNB2. Molecular analysis was performed by direct Sanger sequencing of the entire coding regions in NYX, TRPM1, GRM6, and GPR179 in patients with CSNB1 and CACNA1F and CABP4 in patients with CSNB2. MAIN OUTCOME MEASURES: Data included genetic cause of CSNB, refractive error, visual acuity, nystagmus, strabismus, night blindness, photophobia, color vision, dark adaptation (DA) curve, and standard ERGs. RESULTS: A diagnosis of CSNB1 or CSNB2 was based on standard ERGs. The photopic ERG was the most specific criterion to distinguish between CSNB1 and CSNB2 because it showed a "square-wave" appearance in CSNB1 and a decreased b-wave in CSNB2. Mutations causing CSNB1 were found in NYX (20 patients, 13 families), TRPM1 (10 patients, 9 families), GRM6 (4 patients, 3 families), and GPR179 (2 patients, 1 family). Congenital stationary night blindness 2 was primarily caused by mutations in CACNA1F (55 patients, 37 families). Only 3 patients had causative mutations in CABP4 (2 families). Patients with CSNB1 mainly had rod-related problems, and patients with CSNB2 had rod- and cone-related problems. The visual acuity on average was better in CSNB1 (0.30 logarithm of the minimum angle of resolution [logMAR]) than in CSNB2 (0.52 logMAR). All patients with CSNB1 and only 54% of the patients with CSNB2 reported night blindness. The dark-adapted threshold was on average more elevated in CSNB1 (3.0 log) than in CSNB2 (1.8 log). The 3 patients with CABP4 had a relative low visual acuity, were hyperopic, had severe nonspecific color vision defects, and had only 1.0 log elevated DA threshold. CONCLUSIONS: Congenital stationary night blindness 1, despite different causative mutations, shows 1 unique CSNB1 phenotype. Congenital stationary night blindness 2 caused by mutations in CABP4 merely shows cone-related problems and therefore appears to be distinct from CSNB2 caused by mutations in CACNA1F. FINANCIAL DISCLOSURE(S): The author(s) have no proprietary or commercial interest in any materials discussed in this article.

Illusory rotation in the haptic perception of a moving bar.

Haptic matching of the orientation of bars separated by a horizontal distance leads to large systematic deviations. This finding leads to the following intriguing question which we investigated in this study: How will a bar moving from left to right in a fixed orientation be perceived by blindfolded observers? Interestingly, this previous finding predicts that the translating bar will cause the illusory perception of a rotation. In our experiment, we used psychophysical methods to determine the rotation needed to perceive the bar as non-rotating, for both rightward and leftward translations. From our results, it can be estimated that, on average, a bar translating in parallel over 60 cm is perceived as rotating 18°, so we established that the predicted illusory rotation indeed exists. This implies that static and dynamic signals are processed in a similar way.

Superior spatial touch: improved haptic orientation processing in deaf individuals.

The present study investigated haptic spatial orientation processing in deaf signers, hearing sign language interpreters, and hearing controls. Blindfolded participants had to set two bars parallel in the horizontal plane, with either a 2-s or a 10-s delay between inspection of the reference bar and the setting of the test bar. The deaf group outperformed the other two groups which did not differ from each other. Together these results indicate that deaf individuals can better identify the allocentric spatial coordinates of haptically inspected orientations. These results are discussed in terms of the possible neurocognitive consequences of auditory deprivation.

Phase change materials and the perception of wetness.

Phase change materials (PCMs) are increasingly incorporated in textiles in order to serve as a thermal buffer when humans change from a hot to a cold environment and the reverse. Due to the absence of wetness sensors in the skin, cooling of the skin may be perceived as a sensation of wetness instead of cold. In order to investigate if this phenomenon occurs when manipulating textiles, nine subjects were asked to touch or manipulate PCM-treated and untreated fabrics. In 75% of the cases, the subjects indicated that the treated material felt wetter than the untreated material independent of the way the textiles were manipulated. We conclude that incorporating PCMs in textiles may lead to a feeling of wetness which might be uncomfortable. Therefore, we recommend investigating a change in cooling properties to minimise this feeling. PRACTITIONER SUMMARY: This article describes a psychophysical experiment into the sensation of wetness of textiles treated with phase change materials. It was found that in 75% of the cases, subjects found the treated fabric to feel wetter than the untreated. This may affect the comfort of wearing clothes made of these textiles.

The Oblique Effect in the Perception of the Direction Between Two Points of Vibration on the Back.

Vibrations on the back of a person can convey information about direction through sequentially switching on two vibration motors. For perception of direction the oblique effect can occur, meaning that perception of cardinal directions is more precise than perception of oblique directions. We investigated the role of the positioning of the vibrations with respect to the spine. In the first condition all vibration motors were placed in a circle around the spine ('Circle' condition) and direction was conveyed by switching on vibration motors on opposite sides of the circle. In the second condition the vibrations were placed in two semi-circles of which the centers were on the left and right sides of the back ('Semi-circle' condition). We found that participants showed larger deviations as well as a larger spread for oblique directions than for cardinal directions in both conditions. This indicates that the oblique effect occurred. Therefore, the oblique effect can occur irregardless of the positioning of the vibration motors with respect to the spine. Both deviations and spread were larger in the 'Semi-circle' condition than in the 'Circle' condition suggesting an advantage for centering motors around the spine, although this might have been influenced by the distance between vibrations.

Hand-Held Haptic Navigation Devices for Actual Walking.

In this survey, we give an overview of hand-held haptic navigation devices specifically designed for and tested with pedestrians. We distinguish devices for indoor use and for outdoor use as the implementation is usually quite different. Outdoor devices make use of Global Positioning Systems (GPS) tracking built-in in smartphones; indoor devices use a variety of sensors, and tracking and localization systems and these are usually restricted to a small part of a building. Overall, the high success rates reported in the studies show that vibrotactile stimulation via a hand-held user interface is suitable for navigation instructions, as in all experiments (almost) all participants reached their goal. An issue for several of the indoor devices is that walking speeds were (much) lower than normal walking speeds and path efficiency was relatively low. However, these issues might be overcome with some training as in most studies there was hardly any practice time. Several of the outdoor devices seem quite close to taking the last step before commercial use. In the Discussion, we evaluate the suitability of the devices for persons with visual and/or hearing impairments. Especially devices that provide very specific instructions, such as, 'go straight' or 'go right,' seem valuable for this population.

Comparison of the haptic and visual deviations in a parallelity task.

Deviations in both haptic and visual spatial experiments are thought to be caused by a biasing influence of an egocentric reference frame. The strength of this influence is strongly participant-dependent. By using a parallelity test, it is studied whether this strength is modality-independent. In both haptic and visual conditions, large, systematic and participant-dependent deviations were found. However, although the correlation between the haptic and visual deviations was significant, the explained variance due to a common factor was only 20%. Therefore, the degree to which a participant is "egocentric" depends on modality and possibly even more generally, on experimental condition.

The effect of feature saliency on haptic subitizing.

'Subitizing' refers to fast and error-free numerosity judgment for small (<4) sets of items. For larger sets, the slower process of 'counting' is used. Counting has a serial character, whereas subitizing is believed to have a parallel character. While subitizing was initially found in vision, it has been shown to exist in touch as well. In vision, it has been demonstrated that adding distractor items to a set of target items influences numerosity judgment of the target items. Subitizing was in this case only possible if the distractor item is highly salient among the targets. In the present study, we investigated the effect of adding a distractor item on haptic judgement of a set of target items. To this end, we asked subjects to judge the number of spheres grasped in their hand. Either a cube or an ellipsoid could be added to the set. A cube among spheres has been shown to be highly salient, while an ellipsoid among spheres is not. Our results show that adding a distractor item led to an increase in the response time slopes regardless of the distractor shape. Subitizing was, however, only possible in the case of a salient distractor. This is in agreement with results from vision.

An extended 15 Hz ERG protocol (2): data of normal subjects and patients with achromatopsia, CSNB1, and CSNB2.

The amplitude versus flash strength curve of 15 Hz electroretinograms (ERGs) shows two minima. The minima are caused by interactions between the primary and the secondary rod pathways (first minimum), and the secondary rod pathway and the cone-driven pathway (second minimum). Furthermore, cone pathway contributions cause higher-order harmonics to occur in the responses. We measured 15 Hz ERGs in 20 healthy subjects to determine normal ranges and in patients to verify our hypotheses on the contributions of the different pathways and to investigate the clinical application. We analyzed the amplitudes and phases of the 15, 30, and 45 Hz components in the ERGs. The overall shape of the 15 Hz amplitude curves was similar in all normal subjects and showed two minima. The 30 and 45 Hz amplitude curves increased for stimuli of high flash strengths indicating cone pathway contributions. The 15 Hz amplitude curve of the responses of an achromat was similar to that of the normal subjects for low flash strengths and showed a minimum, indicating normal primary and secondary rod pathway function. There was no second minimum, and there were no higher-order harmonics, consistent with absent cone pathway function. The 15 Hz ERGs in CSNB1 and CSNB2 patients were similar and of low amplitude for flash strengths just above where the first minimum normally occurs. We could determine that in the CSNB1 patients, the responses originate from the cone pathway, while in the CSNB2 patients, the responses originate from the secondary rod pathway.

An extended 15 Hz ERG protocol (1): the contributions of primary and secondary rod pathways and the cone pathway.

The minimum in the amplitude versus flash strength curve of dark-adapted 15 Hz electroretinograms (ERGs) has been attributed to interactions between the primary and secondary rod pathways. The 15 Hz ERGs can be used to examine the two rod pathways in patients. However, previous studies suggested that the cone-driven pathway also contributes to the 15 Hz ERGs for flash strengths just above that of the minimum. We investigated cone pathway contributions to improve upon the interpretation of (abnormal) 15 Hz ERGs measured in patients. We recorded 15 Hz ERGs in five healthy volunteers, using a range of flash strengths that we extended to high values. The stimuli were varied in both colour (blue, green, amber, and red) and flash duration (short flash and square wave) in order to stimulate rods and cones in various ways. The differences in the responses to the four colours could be fully explained by the spectral sensitivity of rods for flash strengths up to approximately 12.5 log quanta·deg(-2). At higher flash strengths, higher-order harmonics appeared in the responses which could be attributed to cones being more sensitive than rods to higher frequencies. Furthermore, the amplitude curves of the blue and green responses showed a second minimum suggesting rod to cone interactions. We present a descriptive model of the contributions of the rod and cone pathways. In clinical application, we would advise using the short flash flicker instead of the square wave flicker, as the responses are of larger amplitude, and cone pathway contributions can be recognized from large higher-order harmonics.

Hands-Free Devices for Displaying Speech and Language in the Tactile Modality - Methods and Approaches.

Communication is an essential part of human life. In this article, we give an overview of hands-free tactual devices that have been developed and tested for conveying speech or language. We opted for "hands-free" because especially in the case of individuals with impaired vision, in many situations their hands will be occupied with other essential tasks. We start this survey with presenting the various word building blocks that have been tested. These blocks vary from units based on the actual speech signal, via patterns representing phonemes, to letters, or letters coded via Morse or Braille-like patterns. In the second part of this article, studies that use these building blocks to create words are discussed. General findings are that successful devices do not necessarily depend on underlying speech characteriscs, dynamic patterns give better results than static patterns, and more vibrators do not generally give better results. Moreover, some of the most successful devices required only limited training time. Most of the recent devices are still in a quite early state of development and are tested only with a limited number of patterns. However, many of these recent devices give promising results and are worthwhile to further investigate and develop.

Grabbing subitizing with both hands: bimanual number processing.

Visual judgment of small numerosities (<4) is generally assumed to be done through subitizing, which is a faster process than counting. Subitizing has also been shown to occur in haptic judgment of the number of spheres in the hand. Furthermore, interactions have been shown to exist between visually perceived numbers and hand motor action. In this study, we compare enumeration of a set of spheres presented to one hand (unimanual) and enumeration of the same total number of spheres presented divided over the two hands (bimanual). Our results show that, like in vision, a combination of subitizing and counting is used to process numbers in active touch. This shows that numbers are processed in a modality-independent way. This suggests that there are not only interactions between perception of numbers and hand motor action, but rather that number representation is modality-independent.

Cross-modal visuo-haptic mental rotation: comparing objects between senses.

The simple experience of a coherent percept while looking and touching an object conceals an intriguing issue: different senses encode and compare information in different modality-specific reference frames. We addressed this problem in a cross-modal visuo-haptic mental rotation task. Two objects in various orientations were presented at the same spatial location, one visually and one haptically. Participants had to identify the objects as same or different. The relative angle between viewing direction and hand orientation was manipulated (Aligned versus Orthogonal). In an additional condition (Delay), a temporal delay was introduced between haptic and visual explorations while the viewing direction and the hand orientation were orthogonal to each other. Whereas the phase shift of the response time function was close to 0 degrees in the Aligned condition, we observed a consistent phase shift in the hand's direction in the Orthogonal condition. A phase shift, although reduced, was also found in the Delay condition. Counterintuitively, these results mean that seen and touched objects do not need to be physically aligned for optimal performance to occur. The present results suggest that the information about an object is acquired in separate visual and hand-centered reference frames, which directly influence each other and which combine in a time-dependent manner.

Range dependent processing of visual numerosity: similarities across vision and haptics.

'Subitizing' refers to fast and accurate judgement of small numerosities, whereas for larger numerosities either counting or estimation are used. Counting is slow and precise, whereas estimation is fast but imprecise. In this study consisting of five experiments we investigated if and how the numerosity judgement process is affected by the relative spacing between the presented numerosities. To this end we let subjects judge the number of dots presented on a screen and recorded their response times. Our results show that subjects switch from counting to estimation if the relative differences between subsequent numerosities are large (a factor of 2), but that numerosity judgement in the subitizing range was still faster. We also show this fast performance for small numerosities only occurred when numerosity information is present. This indicates this is typical for number processing and not magnitude estimation in general. Furthermore, comparison with a previous haptic study suggests similar processing in numerosity judgement through haptics and vision.

Representing Numerosity Through Vibration Patterns.

It can be useful to display information about numerosity haptically. For instance, to display the time of day or distances when visual or auditory feedback is not possible or desirable. Here, we investigated the possibility of displaying numerosity information by means of a sequence of vibration pulses. From previous studies on numerosity perception in vision, haptics and audition it is known that numerosity judgment can be facilitated by grouping. Therefore, we investigated whether perception of the number of vibration pulses in a sequence can be facilitated by temporally grouping the pulses. We found that indeed temporal grouping can lead to considerably smaller errors and lower error rates indicating that this facilitated the task, but only when participants knew in advance whether the pulses would be temporally grouped. When grouped and ungrouped series of pulses were presented randomly interleaved, there was no difference in performance. This means that temporally grouping vibration sequences can allow the sequence to be displayed at a faster rate while it remains possible to perceive the number of vibration pulses accurately if the users is aware of the temporal grouping.