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.

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.

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.

Size, weight, and expectations.

The size-weight illusion is well-known: if two equally heavy objects differ in size, the large one feels lighter than the small one. Most explanations for this illusion assume that because the information about the relevant attribute (weight itself) is unreliable, information about an irrelevant but correlated attribute (size) is used as well. If such reasoning is correct, one would expect that the illusion can be inverted: if size information is unreliable, weight information will be used to judge size. We explored whether such a weight-size illusion exists by asking participants to lift Styrofoam balls that were coated with glow in the dark paint. The balls (2 sizes, 3 weights) were lifted using a pulley system in complete darkness at 2 distances. Participants reported the size using free magnitude estimation. The visual size information was indeed unreliable: balls that were presented at a 20% larger distance were judged 15% smaller. Nevertheless, the judgments of size were not systematically affected by the 20% weight change (differences < 0.5%). We conclude that because the weight-size illusion does not exist, the mechanism behind the size-weight illusion is specific for judging heaviness.

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.

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.

Perception of vibrotactile distance on the back.

Vibrotactile displays worn on the back can be used as sensory substitution device. Often vibrotactile stimulation is chosen because vibration motors are easy to incorporate and relatively cheap. When designing such displays knowledge about vibrotactile perception on the back is crucial. In the current study we investigated distance perception. Biases in distance perception can explain spatial distortions that occur when, for instance, tracing a shape using vibration. We investigated the effect of orientation (horizontal vs vertical), the effect of positioning with respect to the spine and the effect of switching vibration motors on sequentially versus simultaneously. Our study includes four conditions. The condition which had a horizontal orientation with both vibration motors switching on sequentially on the same side of the spine was chosen is the baseline condition. The other three conditions were compared to this baseline condition. We found that distances felt longer in the vertical direction than in the horizontal direction. Furthermore, distances were perceived to be longer when vibration motors were distributed on both sides of the spine compared to when they were on the same side. Finally, distances felt shorter when vibration motors were switched on simultaneously compared to sequentially. In the simultaneous case a distance of 4 cm was not clearly perceived differently than a distance of 12 cm. When designing vibrotactile displays these anisotropies in perceived distance need to be taken into account because otherwise the intended shape will not match the perceived shape. Also, dynamically presented distances are more clearly perceived than static distances. This finding supports recommendations made in previous studies that dynamic patterns are easier to perceive than static patterns.

No need to touch this: Bimanual haptic slant adaptation does not require touch.

In our daily life, we often interact with objects using both hands raising the question the question to what extent information between the hands is shared. It has, for instance, been shown that curvature adaptation aftereffects can transfer from the adapted hand to the non-adapted hand. However, this transfer only occurred for dynamic exploration, e.g. by moving a single finger over a surface, but not for static exploration when keeping static contact with the surface and combining the information from different parts of the hand. This raises the question to what extent adaptation to object shape is shared between the hands when both hands are used in static fashion simultaneously and the object shape estimates require information from both hands. Here we addressed this question in three experiments using a slant adaptation paradigm. In Experiment 1 we investigated whether an aftereffect of static bimanual adaptation occurs at all and whether it transfers to conditions in which one hand was moving. In Experiment 2 participants adapted either to a felt slanted surface or simply be holding their hands in mid-air at similar positions, to investigate to what extent the effects of static bimanual adaptation are posture-based rather than object based. Experiment 3 further explored the idea that bimanual adaptation is largely posture based. We found that bimanual adaptation using static touch did lead to aftereffects when using the same static exploration mode for testing. However, the aftereffect did not transfer to any exploration mode that included a dynamic component. Moreover, we found similar aftereffects both with and without a haptic surface. Thus, we conclude that static bimanual adaptation is of proprioceptive nature and does not occur at the level at which the object is represented.

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.

Thermal Perception and Thermal Devices Used on Body Parts Other Than Hand or Face.

Most fundamental research on thermal perception focuses on the fingers or the hand. Also most existing and proposed thermal devices are meant to be applied to hand or fingers. However, if the hands are needed for other tasks, application of thermal stimulation to other body regions should be considered. This paper surveys the literature on thermal perception and thermal devices relevant to such other body regions. It starts with a short description of the experimental methods used in the various studies, such as the methods of limits, the two-alternative forced choice method, and magnitude estimation. This is followed by thermal psychophysical studies on detection, adaptation, spatial summation, and resolution. Next some striking thermal illusions are presented, such as a thermal grill and a seemingly continuously warming or cooling stimulus. Finally, the few studies on thermal communication and applications are summarized. These latter studies mainly focus on communicating emotions or playing computer games. The overall conclusion of this survey is that thermal devices should not focus on conveying complex messages, but especially in the areas of gaming or communication there seem to be interesting possibilities for further developments.

When Does One Decide How Heavy an Object Feels While Picking It Up?

When lifting an object, it takes time to decide how heavy it is. How does this weight judgment develop? To answer this question, we examined when visual size information has to be present to induce a size-weight illusion. We found that a short glimpse (200 ms) of size information is sufficient to induce a size-weight illusion. The illusion occurred not only when the glimpse was before the onset of lifting but also when the object's weight could already be felt. Only glimpses more than 300 ms after the onset of lifting did not influence the judged weight. This suggests that it takes about 300 ms to reach a perceptual decision about the weight.

Reliance on Haptic Assistance Reflected in Haptic Cue Weighting.

When using an automated system, user trust in the automation is an important factor influencing performance. Prior studies have analyzed trust duringsupervisory control of automation, and how trust influences reliance: the behavioral correlate of trust. Here, we investigated how reliance on haptic assistance affects performance during shared control with an automated system. Subjects made reaches towards a hidden target using a visual cue and haptic cue (assistance from the automation). We sought to influence reliance by changing the variability of trial-by-trial random errors in the haptic assistance. Reliance was quantified in terms of the subject's position at the end of the reach relative to the two cues. Our results show that subjects aimed more towards the visual cue when the variability of the haptic cue errors increased, resembling cue weighting behavior. Similar behavior was observed both when subjects had explicit knowledge about the haptic cue error variability, as well as when they had only implicit knowledge (from experience). However, the group with explicit knowledge was able to more quickly adapt their reliance on the haptic assistance. The method we introduce here provides a quantitative way to study user reliance on the information provided by automated systems with shared control.

The role of connectedness in haptic object perception.

We can efficiently detect whether there is a rough object among a set of smooth objects using our sense of touch. We can also quickly determine the number of rough objects in our hand. In this study, we investigated whether the perceptual processing of rough and smooth objects is influenced if these objects are connected. In Experiment 1, participants were asked to identify whether there were exactly two rough target spheres among smooth distractor spheres, while we recorded their response times. The spheres were connected to form pairs: rough spheres were paired together and smooth spheres were paired together ('within pairs arrangement'), or a rough and a smooth sphere were connected ('between pairs arrangement'). Participants responded faster when the spheres in a pair were identical. In Experiment 2, we found that the advantage for within pairs arrangements was not driven by feature saliency. Overall our results show that haptic information is processed faster when targets were connected together compared to when targets were connected to distractors.

Haptic search for movable parts.

How do we know that we are touching 1 single object instead of 2 different ones? An important cue is movability: When different sources of input can move independently, it is likely that they belong to different objects or that the object consists of movable parts. We hypothesize that the haptic feature "movability" is used for making this differentiation and we expect movability to be detected efficiently. We investigated this hypothesis by using a haptic search task. In Experiment 1, participants were asked to press down on piano-like keys and respond whether 1 key was movable while the rest were static or the other way around (detection only). Search strategy was determined by comparing performance of 4 response time models. This showed that the search slope for the target absent and present trials was the same (detection without localization model). In Experiment 2, we asked participants to localize the target, in order to investigate whether localization is an extra processing step. In this case our localization after detection model described the data best. This suggests that the target was detected independent of localization. To our knowledge this is the first time such a search strategy has been reported in haptic search, and it highlights the special role of the detection of movability. (PsycINFO Database Record

How Heavy Is an Illusory Length?

The perception of object properties, such as size and weight, can be subject to illusions. Could a visual size illusion influence perceived weight? Here, we tested whether the size-weight illusion occurs when lifting two physically identical but perceptually different objects, by using an illusion of size. Participants judged the weight and length of 11 to 17 cm brass bars with equal density to which cardboard arrowheads were attached to create a Müller-Lyer illusion. We found that these stimuli induced an illusion in which the bar that was visually perceived as being shorter was also perceived as feeling heavier. In fact, a 5-mm increase in illusory length corresponded to a decrease in illusory weight of 15 g.

Haptic adaptation to slant: No transfer between exploration modes.

Human touch is an inherently active sense: to estimate an object's shape humans often move their hand across its surface. This way the object is sampled both in a serial (sampling different parts of the object across time) and parallel fashion (sampling using different parts of the hand simultaneously). Both the serial (moving a single finger) and parallel (static contact with the entire hand) exploration modes provide reliable and similar global shape information, suggesting the possibility that this information is shared early in the sensory cortex. In contrast, we here show the opposite. Using an adaptation-and-transfer paradigm, a change in haptic perception was induced by slant-adaptation using either the serial or parallel exploration mode. A unified shape-based coding would predict that this would equally affect perception using other exploration modes. However, we found that adaptation-induced perceptual changes did not transfer between exploration modes. Instead, serial and parallel exploration components adapted simultaneously, but to different kinaesthetic aspects of exploration behaviour rather than object-shape per se. These results indicate that a potential combination of information from different exploration modes can only occur at down-stream cortical processing stages, at which adaptation is no longer effective.

Perceptual Grouping Affects Haptic Enumeration Over the Fingers.

Spatial arrangement is known to influence enumeration times in vision. In haptic enumeration, it has been shown that dividing the total number of items over the two hands can speed up enumeration. Here we investigated how spatial arrangement of items and non-items presented to the individual fingers impacts enumeration times. More specifically, we tested whether grouping by proximity facilitates haptic serial enumeration (counting). Participants were asked to report the number of tangible items, amongst non-items, presented to the finger pads of both hands. In the first experiment, we divided the tangible items in one, two, or three groups that were defined by proximity (i.e., one nonitem in between two groups) and found that number of groups and not number of items were the critical factor in enumeration times. In a second experiment, we found that this grouping even takes place when groups extend across fingers of both hands. These results suggest that grouping by proximity affects haptic serial enumeration and that this grouping takes place on a spatial level possibly in addition to the somatotopic level. Our results support the idea that grouping by proximity, a principle introduced in vision, also greatly affects haptic processing of spatial information.

Object size can influence perceived weight independent of visual estimates of the volume of material.

The size-weight illusion is the phenomenon that the smaller of two equally heavy objects is perceived to be heavier than the larger object when lifted. One explanation for this illusion is that heaviness perception is influenced by our expectations, and larger objects are expected to be heavier than smaller ones because they contain more material. If this would be the entire explanation, the illusion should disappear if we make objects larger while keeping the volume of visible material the same (i.e. objects with visible holes). Here we tested this prediction. Our results show that perceived heaviness decreased with object size regardless of whether objects visibly contained the same volume of material or not. This indicates that object size can influence perceived heaviness, even when it can be seen that differently sized objects contain the same volume of material.