Myrthe A. Plaisier

Salient features in 3-D haptic shape perception

Shape is an important cue for recognizing an object by touch. Several features, such as edges, curvature, surface area, and aspect ratio, are associated with 3-D shape. To investigate the saliency of 3-D shape features, we developed a haptic search task. The target and distractor items consisted of shapes (cube, sphere, tetrahedron, cylinder, and ellipsoid) that differed in several of these features. Exploratory movements were left as unconstrained as possible. Our results show that this type of haptic search task can be performed very efficiently (25 msec/item) and that edges and vertices are the most salient features. Furthermore, very salient local features, such as edges, can also be perceived through enclosure, an exploratory procedure usually associated with global shape. Since the subjects had to answer as quickly as possible, this suggests that speed may be a factor in selecting the appropriate exploratory procedure.

Haptic pop-out in a hand sweep

Visually, a red item is easily detected among green items, whereas a mirrored S among normal Ss is not. In visual search, the former is known as the pop-out effect. In daily life, people often also conduct haptic (tactual) searches, for instance, when trying to find keys in their pocket. The aim of the present research was to determine whether there is a haptic version of the pop-out effect. Blindfolded subjects had to search for a target item which differed in roughness from the surrounding distractor items. We report reaction time slopes as low as 20 ms/item. When target and distractor identities were interchanged the slopes increased indicating a search asymmetry. Furthermore, we show that differences in search slope were accompanied by search strategy differences. In some conditions a single-hand sweep over the display was sufficient, while in others a more detailed search strategy was used. By relating haptic search slopes to parallel and serial search strategies we show, for the first time, that pop-out effects occur under free manual exploration.

Visually Guided Haptic Search

In this study, we investigate the influence of visual feedback on haptic exploration. A haptic search task was designed in which subjects had to haptically explore a virtual display using a force-feedback device and to determine whether a target was present among distractor items. Although the target was recognizable only haptically, visual feedback of finger position or possible target positions could be given. Our results show that subjects could use visual feedback on possible target positions even in the absence of feedback on finger position. When there was no feedback on possible target locations, subjects scanned the whole display systematically. When feedback on finger position was present, subjects could make well-directed movements back to areas of interest. This was not the case without feedback on finger position, indicating that showing finger position helps to form a spatial representation of the display. In addition, we show that response time models of visual serial search do not generally apply for haptic serial search. Consequently, in teleoperation systems, for instance, it is helpful to show the position of the probe even if visual information on the scene is poor.

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

Abstract‘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.

Mass Is All That Matters in the Size–Weight Illusion

An object in outer space is weightless due to the absence of gravity, but astronauts can still judge whether one object is heavier than another one by accelerating the object. How heavy an object feels depends on the exploration mode: an object is perceived as heavier when holding it against the pull of gravity than when accelerating it. At the same time, perceiving an object’s size influences the percept: small objects feel heavier than large objects with the same mass (size–weight illusion). Does this effect depend on perception of the pull of gravity? To answer this question, objects were suspended from a long wire and participants were asked to push an object and rate its heaviness. This way the contribution of gravitational forces on the percept was minimised. Our results show that weight is not at all necessary for the illusion because the size–weight illusion occurred without perception of weight. The magnitude of the illusion was independent of whether inertial or gravitational forces were perceived. We conclude that the size–weight illusion does not depend on prior knowledge about weights of object, but instead on a more general knowledge about the mass of objects, independent of the contribution of gravity. Consequently, the size–weight illusion will have the same magnitude on Earth as it should have on the Moon or even under conditions of weightlessness.

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.

Haptic subitizing across the fingers

Numerosity judgments of small sets of items (≤ 3) are generally fast and errorfree, while response times and error rates increase rapidly for larger numbers of items. We investigated an efficient process used for judging small numbers of items (known as subitizing) in active touch. We hypothesized that this efficient process for numerosity judgment might be related to stimulus properties that allow for efficient (parallel) search. Our results showed that subitizing was not possible forraised lines among flat surfaces, whereas this type of stimulus could be detected in parallel over the fingers. However, subitizing was possible when the number of fingers touching a surface had to be judged while the other fingers were lowered in mid-air. In the latter case, the lack of tactile input is essential, since subitizing was not enabled by differences in proprioceptive information from the fingers. Our results show that subitizing using haptic information from the fingers is possible only whensome fingers receive tactile information while other fingers do not.

Cold objects pop out

We can haptically extract thermal properties of different material, but we can also sense object temperature. It has been shown that thermal properties of materials are not very salient features. In this study, we investigate saliency of actual temperature differences. To this end we let subjects grasp varying numbers of spheres in the hand. These spheres were warmer (38°C) than the hand temperature, but in half of the trials there was one sphere colder (22°C) than the hand temperature. Subjects had to indicate whether the cold sphere was present and response times were measured as a function of the number of spheres. This yielded a target present slope as small as 32 ms/item. This is comparable to slopes found earlier for search for a tetrahedron among spheres and indicates that there is pop-out effect for a cold sphere among warm spheres.

Haptic Search for Spheres and Cubes

If you have multiple objects in your pocket, some are easy to find among the other ones, for instance, when they differ much in material properties or shape. Information on which haptic features stand out among others is valuable for research into the haptic system in general, but also for haptic interface design. In this research we focussed on saliency of shape, by letting subjects search for cubes or spheres. Response times were measured as a function of the number of items. We found that search for a cube among spheres is more efficient than search for a sphere among cubes and that the dynamics of the sliding of the shapes along each other play an important role in haptic search.

Two hands perceive better than one

We often handle an object with both of our hands. The information from the two hands has to be combined to arrive at a single percept of the object. Research on multi-sensory perception has shown that redundant information between the senses is integrated such that the combined percept is more precise than either of the two individual inputs. However, while bimanual information can be redundant, it is not necessarily the case because both hands are usually touching different parts of the same object. To investigate whether there is a difference in precision of unimanual and bimanual information, we asked participants to discriminate stiffness unimanually as well as bimanually. Our results clearly show that bimanual perception of stiffness was more precise than unimanual perception. The precision of the bimanual percept was in agreement with the precision predicted from combining the two unimanual inputs in a statistical optimal fashion.