Perceptual asymmetries in the somatosensory system: a dichhaptic experiment and critical review of the literature from 1929 to 1986

Haptic recognition memory and lateralisation for verbal and nonverbal shapes

Laterality effects generally refer to an advantage for verbal processing in the left hemisphere and for non-verbal processing in the right hemisphere, and are often demonstrated in memory tasks in vision and audition. In contrast, their role in haptic memory is less understood. In this study, we examined haptic recognition memory and laterality for letters and nonsense shapes. We used both upper and lower case letters, with the latter designed as more complex in shape. Participants performed a recognition memory task with the left and right hand separately. Recognition memory performance (capacity and bias-free d') was higher and response times were faster for upper case letters than for lower case letters and nonsense shapes. The right hand performed best for upper case letters when it performed the task after the left hand. This right hand/left hemisphere advantage appeared for upper case letters, but not lower case letters, which also had a lower memory capacity, probably due to their more complex spatial shape. These findings suggest that verbal laterality effects in haptic memory are not very prominent, which may be due to the haptic verbal stimuli being processed mainly as spatial objects without reaching robust verbal coding into memory.

Laterality effects in the haptic discrimination of verbal and non-verbal shapes

The left hemisphere is known to be generally predominant in verbal processing and the right hemisphere in non-verbal processing. We studied whether verbal and non-verbal lateralization is present in haptics by comparing discrimination performance between letters and nonsense shapes. We addressed stimulus complexity by introducing lower case letters, which are verbally identical with upper case letters but have a more complex shape. The participants performed a same-different haptic discrimination task for upper and lower case letters and nonsense shapes with the left and right hand separately. We used signal detection theory to determine discriminability (d'), criterion (c) and we measured reaction times. Discrimination was better for the left hand for nonsense shapes, close to significantly better for the right hand for upper case letters and with no difference between the hands for lower case letters. For lower case letters, right hand showed a strong bias to respond "different", while the left hand showed faster reaction times. Our results are in agreement with the right lateralization for non-verbal material. Complexity of the verbal shape is important in haptics as the lower case letters seem to be processed as less verbal and more as spatial shapes than the upper case letters.

Subjective Evaluation of the Spectral Temporal SIMilarity (ST-SIM) Measure for Vibrotactile Quality Assessment

Recent standardization efforts for Tactile Internet (TI) and haptic codecs have paved the route for delivering tactile experiences in synchrony with audio and visual interaction components. Since humans are the ultimate consumers of tactile interactions, it is utmost important to develop objective quality assessment measures that are in close agreements with human perception. In this article, we present the results of a large-scale subjective study of a recently proposed objective quality assessment approach for vibrotactile signals called ST-SIM (Spectral Temporal SIMilarity). ST-SIM encompasses two components: perceptual spectral and temporal similarity measures. Two subjective experiments were conducted to validate ST-SIM, and elicited subjective ratings are used to create a VibroTactile Quality Assessment (VTQA) database. The VTQA database together with ST-SIM provide viable means to the development of vibrotactile compression and transmission applications. Our experimental results show that the ST-SIM highly correlates with human opinions in both experiments and significantly outperforms commonly used measures. The VTQA database is made publicly available at https://www.raniahassen.com/RESEARCH/.

Exploring laterality and memory effects in the haptic discrimination of verbal and non-verbal shapes

The brain's left hemisphere often displays advantages in processing verbal information, while the right hemisphere favours processing non-verbal information. In the haptic domain due to contra-lateral innervations, this functional lateralization is reflected in a hand advantage during certain functions. Findings regarding the hand-hemisphere advantage for haptic information remain contradictory, however. This study addressed these laterality effects and their interaction with memory retention times in the haptic modality. Participants performed haptic discrimination of letters, geometric shapes and nonsense shapes at memory retention times of 5, 15 and 30 s with the left and right hand separately, and we measured the discriminability index d'. The d' values were significantly higher for letters and geometric shapes than for nonsense shapes. This might result from dual coding (naming + spatial) or/and from a low stimulus complexity. There was no stimulus-specific laterality effect. However, we found a time-dependent laterality effect, which revealed that the performance of the left hand-right hemisphere was sustained up to 15 s, while the performance of the right-hand-left hemisphere decreased progressively throughout all retention times. This suggests that haptic memory traces are more robust to decay when they are processed by the left hand-right hemisphere.

Evaluating vibrotactile dimensions for the design of tactons

Vibrotactile stimuli are defined in terms of their amplitude, frequency, waveform and temporal profile all of which have been varied to create tactons. A number of approaches have been adopted to design tactons including multidimensional scaling, iterative empirical methods and using perceptual processing models. The objective of the present set of experiments was to create sets of tactons based on the properties of the dimensions of vibrotactile stimuli. An absolute identification paradigm was used in which each of nine tactons was presented eight times using a tactor mounted on either the index finger or forearm. It was found that tactons created by varying the frequency, amplitude and temporal profile of the vibrotactile stimuli were correctly identified on 73-83 percent of the trials, with a mean information transfer of 2.41 bits. The latter metric indicates that for these sets of nine tactons between five and six could be reliably identified. The vibrotactile stimuli delivered in the experiments were identified as consistently on the forearm as the hand and the IT values were similar at the two locations. This suggests that sites other than the hand can be used effectively in tactile communication systems and that it is channel capacity that ultimately determines performance on this type of task.

A new perceptual paradigm to investigate the visual remapping of others' tactile sensations onto one's own body shows "mirror touch" for the hands

The last two decades have seen a multitude of publications showing the activation of an observer’s somatosensory cortical system during the observation of touch on another person. Behavioral demonstrations of “mirror touch,” however, have been slow in coming forward, and have so far primarily been shown as “visual remapping of touch” on the face. The present study uses a new paradigm to investigate the mirroring of others’ tactile sensations: a 2-AFC task of intensity judgment for touch on the observer’s left and right index finger pads. Observers viewed a left and right hand in an egocentric position, which were either touched passively (pencil moving to touch index finger pad) or actively sought touch (index finger moving to touch pencil). Touch and no-touch events for the two viewed hands were designed to eliminate confounding effects of spatial attention. Felt touches were either concurrent with viewed touch or no-touch events, or were delayed in time to assess potential response bias. The findings demonstrate visual remapping of touch for touch on the hands. If touch was shown on one of the hands only (e.g., left), observers were more likely to perceive touch on the same hand (i.e., their own left hand) as more intense than touch on the other hand even if tactile intensities did not differ, compared to touch shown on both or neither hand. These remapping effects occurred only when viewed and felt touches were concurrent, they were strongly modulated by the way in which viewed touch was incurred, and they were more reliable for touch on the left hand. A second, control experiment, in which touch observation was replaced by bright dots shown on or next to the finger pads, confirmed that these effects were largely due to genuine tactile mirroring rather than to somatotopic cueing. This 2-AFC tactile intensity judgment task may be a useful paradigm to investigate the remapping of others’ tactile sensations onto an observer’s own body.

Termite fishing laterality in the Fongoli savanna chimpanzees (Pan troglodytes verus): further evidence of a left hand preference

Whether nonhuman primates show population-level handedness is a topic of much scientific debate. A previous study of handedness for termite fishing reported population-level left handedness in the chimpanzees from Gombe National Park, Tanzania. In the current study, we examined whether similar hand preferences were evident in a savanna-dwelling chimpanzee population with regards to termite fishing. Hand preference data were collected for 27 chimpanzees from February 2007 through July 2008 and November 2011 through January 2012 in southeastern Senegal. Overall, the Fongoli chimpanzees demonstrate a trend toward population-level handedness, though the results did not reach conventional levels of statistical significance likely due to the limited sample size. Fongoli chimpanzees showed the same pattern of left hand preference as reported at Gombe and the two populations did not differ significantly. When the data were combined across all studies, wild chimpanzees showed a population-level left hand preference for termite fishing.

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.

Manual exploratory procedures and asymmetries for a haptic search task: a comparison between capuchins (Cebus apella) and humans

Left-hand preferences in monkeys (capuchins, macaques) for haptic discriminations have been taken to indicate that the right hemisphere is more involved than the left hemisphere in solving these tasks. We confirmed this left-hand bias in a group of 21 capuchins haptically searching for sunflower seeds located in crevices in 12 different objects (Lacreuse & Fragaszy, 1996). In an attempt to specify the relations of the left-hand bias to hand performance and hemispheric lateralisation, we analysed the manual exploratory procedures used by the subjects when exploring the objects, and compared them to those of humans confronted with the same task. All the hand-movement patterns displayed by humans were also observed in capuchins, but humans performed exhaustive explorations, whereas capuchins restricted their haptic investigations to limited portions of the objects. Both species adopted several ''exploratory procedures'' to investigate particular objects. Although capuchins showed a left-hand preference to perform the task, finer analyses of haptic exploratory procedures revealed no difference in the way the left and right hands explored the objects, nor in the efficiency with which each hand solved the task. Nor was any measurable difference in manual exploratory procedure and efficiency found in humans. The discussion emphasises the need to complement measures of manual preference with finer indices of hemispheric lateralisation, for a better understanding of functional asymmetries in primates.

Performance on figure ground perception following stroke induced hemiplegia: a compairson of pre-and post-rehabilitation with the neurologically unimpaired

This article compares performance times on the Figure Ground Perception (FGP) of the Sensory Integration and Praxis Tests (SIPT) in persons with stroke-induced hemiplegia and unimpaired older persons. Two hundred and twelve right dominant persons with hemiplegia on admission and discharge from comprehensive rehabilitation and 321 neurologically unimpaired persons in the community were evaluated on the FGP subtests of the SIPT. There were significant differences in performance between unimpaired persons and those with hemiplegia following a stroke. Among the persons with stroke-induced hemiplegia, performance on discharge was significantly better than on admission across all five FGP subtests. Age, sex, and severity of hemiplegia also affected performance on specific subtests; the influence of laterality was significant in persons with right hemispheric lesions; and persons with Wernicke's aphasia performed more poorly. Although further investigation is clearly needed, the association of increasing age (particularly males 75 years and over), severity of hemiplegia, and presence of aphasia on FGP performance suggest that these factors should be considered in the development of rehabilitation programs for stroke patients.

Tactile hyperacuity thresholds correlate with finger maps in primary somatosensory cortex (S1)

Behavioral tactile discrimination thresholds were compared with functional magnetic resonance imaging measurements of cortical finger representations within primary somatosensory cortex (S1) for 10 human subjects to determine whether cortical magnification in S1 could account for the variation in tactile hyperacuity thresholds of the fingers. Across 10 subjects, the increase in tactile thresholds from the index finger to the little finger correlated with the decrease in cortical representation across fingers in S1. Additionally, representations of the fingers within S1, in Brodmann areas 3b and 1, were also correlated with the thresholds. These results suggest that tactile hyperacuity is largely determined by the cortical representation of the fingers in S1.

Haptic face identification activates ventral occipital and temporal areas: An fMRI study

Many studies in visual face recognition have supported a special role for the right fusiform gyrus. Despite the fact that faces can also be recognized haptically, little is known about the neural correlates of haptic face recognition. In the current fMRI study, neurologically intact participants were intensively trained to identify specific facemasks (molded from live faces) and specific control objects. When these stimuli were presented in the scanner, facemasks activated left fusiform and right hippocampal/parahippocampal areas (and other regions) more than control objects, whereas the latter produced no activity greater than the facemasks. We conclude that these ventral occipital and temporal areas may play an important role in the haptic identification of faces at the subordinate level. We further speculate that left fusiform gyrus may be recruited more for facemasks than for control objects because of the increased need for sequential processing by the haptic system.

Intermanual and intramanual tactual grating discrimination

For intermanual tactual discrimination to occur, it is thought that neural signals have to cross the corpus callosum in some way at least once. In this note we address the question of whether this interhemispheric transfer affects grating discrimination in active touch. Eight right-handed participants made intermanual and intramanual tactual discriminations of sinusoidal gratings that had slightly different spatial periods. Intramanual discrimination included comparisons in which the gratings were felt with the same finger, as well as comparisons made with two different fingers of the same hand. During intermanual discrimination the gratings were felt with corresponding fingers or with non-corresponding fingers of different hands. We found that thresholds for intramanual conditions were lower than for intermanual conditions in active dynamic touch. This suggests that there is a component of the task that is unilateral, as might be mediated by a somatosensory region that has predominantly or exclusively contralateral receptive fields.

Somatoperceptual asymmetry for cognitive stimuli: support from experimental and clinical observations

Two studies, one experimental with intact-brain subjects (Study I), and one clinical with left or right hemisphere-damaged and non-neurological general medical patients (Study II), were conducted to examine somatolateral asymmetry in the recognition of verbal (letter) and visuospatial (nonsense shapes) stimuli. Blindfolded subjects were required to explore two stimuli dichhaptically, and were instructed to haptically identify the target stimulus in a set of test stimuli. The findings of Study I indicated that fewer trials were required to recognise nonsense shapes when they were presented to the left hand, whereas fewer trials were required for letters when they were presented to the right hand. The findings of Study II indicated that performance of the hand ipsilateral to the damaged left hemisphere was better for nonsense shapes than for letters, and performance of the hand ipsilateral to the damaged right hemisphere was better for letters than for nonsense shapes.

Hemispheric differences in dichaptic scanning of verbal and spatial material by adult males and females

An experimental design based on the combination of dichaptic presentation associated to the Posner's paradigm was adopted to investigate laterality effects for verbal and spatial non-linguistic stimuli in male and female adult normal subjects. In a grapheme similarity judgment task based on "Name Identity" a right hand/left hemisphere advantage was found. Conversely, laterality effects were neither observed when the task involved "Perceptually Identical" or "Different" letter pairs. In a further experiment, the same methodology was adopted to verify hemispheric effects with spatial non-linguistic material, and a significant advantage for the left hand/right hemisphere was observed. Contrary to many previous studies, no gender or gender x task effects have been detected in both experiments. The present results suggest the existence, also in the tactile domain, of a direct link between input type and the linguistic or non-linguistic processing to which the two hemispheres are devoted. The overall pattern of data seriously hampers Witelson's original hypothesis that letter stimuli presented in the tactile modality are primarily processed as spatial stimuli, and are therefore dependent on the right hemisphere functioning.

Vibro-tactile and visual asynchronies: sensitivity and consistency

We investigated the consistency between tactually and visually designated empty time intervals. In a forced-choice discrimination task, participants judged whether the second of two intervals was shorter or longer than the first interval. Two pulses defined the intervals. The pulse was either a vibro-tactile burst presented to the fingertip, or a foveally presented white square. The comparisons were made for uni-modal and cross-modal intervals. We used four levels of standard interval durations in the range of 100- 800 ms. The results showed that tactile empty intervals must be 8.5% shorter to be perceived as long as visual intervals. This cross-modal bias is larger for small intervals and decreases with increasing standard intervals. The Weber fractions (the threshold divided by the standard interval) are 20% and are constant over the standard intervals. This indicates that the Weber law holds for the range of interval lengths tested. Furthermore, the Weber fractions are consistent over uni-modal and cross-modal comparisons, which indicates that there is no additional noise involved in the cross-modal comparisons.

Does men’s advantage in mental rotation persist when real three-dimensional objects are either felt or seen?

In several spatial tasks in which men outperform women in the processing of visual input, the sex difference has been eliminated in matching contexts limited to haptic input. The present experiment tested whether such contrasting results would be reproduced in a mental rotation task. A standard visual condition involved two-dimensional illustrations of three-dimensional stimuli; in a haptic condition, three-dimensional replicas of these stimuli were only felt; in an additional visual condition, these replicas were seen. The results indicated that, irrespective of condition, men's response times were shorter than women's, although accuracy did not significantly differ according to sex. For both men and women, response times were shorter and accuracy was higher in the standard condition than in the haptic one, the best performances being recorded when full replicas were shown. Self-reported solving strategies also varied as a function of sex and condition. The discussion emphasizes the robustness of men's faster speed in mental rotation. With respect to both speed and accuracy, the demanding sequential processing called for in the haptic setting, relative to the standard condition, is underscored, as is the benefit resulting from easier access to depth cues in the visual context with real three-dimensional objects.

Tactile spatial resolution in blind braille readers

OBJECTIVE

To determine if blind people have heightened tactile spatial acuity.

BACKGROUND

Recently, studies using magnetic source imaging and somatosensory evoked potentials have shown that the cortical representation of the reading fingers of blind Braille readers is expanded compared to that of fingers of sighted subjects. Furthermore, the visual cortex is activated during certain tactile tasks in blind subjects but not sighted subjects. The authors hypothesized that the expanded cortical representation of fingers used in Braille reading may reflect an enhanced fidelity in the neural transmission of spatial details of a stimulus. If so, the quantitative limit of spatial acuity would be superior in blind people.

METHODS

The authors employed a grating orientation discrimination task in which threshold performance is accounted for by the spatial resolution limits of the neural image evoked by a stimulus. The authors quantified the psychophysical limits of spatial acuity at the middle and index fingers of 15 blind Braille readers and 15 sighted control subjects.

RESULTS

The mean grating orientation threshold was significantly (p = 0.03) lower in the blind group (1.04 mm) compared to the sighted group (1.46 mm). The self-reported dominant reading finger in blind subjects had a mean grating orientation threshold of 0.80 mm, which was significantly better than other fingers tested. Thresholds at non-Braille reading fingers in blind subjects averaged 1.12 mm, which were also superior to sighted subjects' performances.

CONCLUSION

Superior tactile spatial acuity in blind Braille readers may represent an adaptive, behavioral correlate of cortical plasticity.

Tactile perception in blind Braille readers: a psychophysical study of acuity and hyperacuity using gratings and dot patterns

It is not clear whether the blind are generally superior to the sighted on measures of tactile sensitivity or whether they excel only on certain tests owing to the specifics of their tactile experience. We compared the discrimination performance of blind Braille readers and age-matched sighted subjects on three tactile tasks using precisely specified stimuli. Initially, the blind significantly outperformed the sighted at a hyperacuity task using Braille-like dot patterns, although, with practice, both groups performed equally well. On two other tasks, hyperacute discrimination of gratings that differed in ridge width and spatial-acuity-dependent discrimination of grating orientation, the performance of the blind did not differ significantly from that of sighted subjects. These results probably reflect the specificity of perceptual learning due to Braille-reading experience.

Practice-related improvements in somatosensory interval discrimination are temporally specific but generalize across skin location, hemisphere, and modality

This paper concerns the characterization of performance and perceptual learning of somatosensory interval discrimination. The purposes of this study were to define (1) the performance characteristics for interval discrimination in the somatosensory system by naive adult humans, (2) the normal capacities for improvement in somatosensory interval discrimination, and (3) the extent of generalization of interval discrimination learning. In a two-alternative forced choice procedure, subjects were presented with two pairs of vibratory pulses. One pair was separated in time by a fixed base interval; a second pair was separated by a target interval that was always longer than the base interval. Subjects indicated which pair was separated by the target interval. The length of the target interval was varied adaptively to determine discrimination thresholds. After initial determination of naive abilities, subjects were trained for 900 trials per day at base intervals of either 75 or 125 msec for 10-15 d. Significant improvements in thresholds resulted from training. Learning at the trained base interval generalized completely across untrained skin locations on the trained hand and to the corresponding untrained skin location in the contralateral hand. The learning partially generalized to untrained base intervals similar to the trained one, but not to more distant base intervals. Learning with somatosensory stimuli generalized to auditory stimuli presented at comparable base intervals. These results demonstrate temporal specificity in somatosensory interval discrimination learning that generalizes across skin location, hemisphere, and modality.

Spatial ability and lateralization in the haptic modality

Two experiments examined the relation between spatial visualization ability and spatial lateralization using a unimanual haptic task with random shapes as stimuli. High spatial ability was not associated with enhanced right hemisphere functioning. However, spatial ability considered in the context of sex differences (Experiment 1) and level of verbal ability (Experiment 2) did yield interactions with the laterality (i.e., hand) variable, at least for first-list data. In Experiment 2, verbal ability also interacted with the gender and hand variables and in the same crossed ability-sex manner as was found for spatial ability in Experiment 1. It is concluded that cognitive abilities, including the relation between abilities, may be associated with the extent to which the right cerebral hemisphere mediates tactuospatial performance and that this association may differ for males and females.

The tactual horizontal-vertical illusion depends on radial motion of the entire arm

We sought to clarify the causes of the tactual horizontal-vertical illusion, where vertical lines are overestimated as compared with horizontals in L and inverted-T figures. Experiment 1 did not use L or inverted-T figures, but examined continuous or bisected horizontal and vertical lines. It was expected that bisected lines would be perceived as shorter than continuous lines, as in the inverted-T figure in the horizontal-vertical illusion. Experiment 1 showed that the illusion could not be explained solely by bisection, since illusory effects were similar for continuous and bisected vertical and horizontal lines. Experiments 2 and 3 showed that the illusory effects were dependent upon stimulus size and scanning strategy. Overestimation of the vertical was minimal or absent for the smallest patterns, where it was proposed that stimuli were explored by finger movement, with flexion at the wrist. Larger stimuli induce whole-arm motions, and illusory effects were found in conditions requiring radial arm motion. The illusion was weakened or eliminated in Experiment 4 when subjects were forced to examine stimuli with finger-and-hand motion alone, that is, their elbows were kept down on the table surface, and they were prevented from making radial arm motions. Whole-arm motion damaged performance and induced perceptual error. The experiments support the hypothesis that overestimation of the vertical in the tactual horizontal-vertical illusion derives from radial scanning by the entire arm.

What does the haptic modality do during cognitive activities on letter shapes? A study with left- and right-handers

This study was undertaken to analyze intermanual (interhemispheric) transfer in left and right handed subject and to assess how information was extracted during finger scanning of letter shape at the different levels of letter processing: shape recognition during a physical matching task, letter recognition in a verbal "meaning" matching task and letter naming. The dichhaptic procedure was used to study interhemispheric relations. It was hypothesized that cognitive activities have a feed-forward effect on the exploration of shapes, and that the performance is related to the nature of the task and to handedness. The exploratory strategies of the two types of handedness were also analyzed. The results showed that response latencies were generally similar for left- and right-handed subjects, but accuracy was better for left than right handers in "verbal" matching with the same overall exploratory strategies. In physical matching, left- and right-handed subjects performed equally but used different exploratory strategies. The naming task was very difficult for both groups but failed to discriminate their on accuracy, response latency, and exploratory strategy. The results are discussed with reference to the different exploratory strategies used and the interhemispheric interaction at work in different cognitive processes.

Spatial field advantages for tactile line bisection as a function of hemispheric specialisation inferred from dichotic listening

Thirty left-ear advantaged (LEA) and 30 right-ear advantaged (REA) right-handed subjects performed tactile line bisection (TLB) in left and right spatial fields, and at midline. REA subjects were found to perform significantly better in the right spatial field than in the left and LEA subjects significantly better in the left than in the right. Significant directional deviations were found only at midline with the left hand deviating to the right and the right hand to the left. These findings suggest that TLB is primarily a spatial task subserved by feedforward motor processes and that directional deviations are due to the differential scaling of egocentric space.

Women's deficiency in water-level representation: present in visual conditions yet absent in haptic contexts

The present experiment studied horizontality representation among men and women performing the water-level task either in visual conditions (the subjects saw outlines of tilted containers in which they respectively drew or set the water line) or in partially and totally haptic conditions (respectively, the subjects both saw and felt or merely felt the contours of the containers cut out of metal plates, and positioned a rod on the underside of the plates to indicate the water line). It was expected that, as the visual components of the setting were replaced by haptic ones, a reduction of the typical gender difference in proficiency would ensue. It was found that men surpassed women under visual conditions, whereas both genders were equivalent in haptic conditions. There were no gender or condition differences in a control task in which a line had to be placed horizontally in tilted containers. Forced reliance on proprioceptive cues among both men and women under the totally haptic condition was contrasted with the critical role played in visual conditions by visual references; those used by men were correct, whereas the ones used by women were incorrect.

Hand-movement profiles in a tactual-tactual matching task: effects of spatial factors and laterality

We examined the effect of spatial factors and hemispheric lateralization upon hand-scanning strategies in 14 right-handed men tested in a tactual-tactual matching task. The experiment involved comparisons (judgments of same or different) between two objects sequentially touched by the fingertips of the left or right hand. Stimuli were made of smoothly joined cubes whose junctions were not haptically discernible. Exploratory strategies were inferred from the durations and locations of hand contacts with any of the cubes composing the stimuli. Accuracy was greater when the same stimulus was touched twice by the same hand than when different hands were used to feel it. With regard to strategies, both hands touched the upper parts of the object longer than the lower parts. Subjects also inspected more portions of the objects ipsilateral to the hand used. Overall differences in time spent touching cubes were greater for the right hand than for the left hand, showing that touch times were less evenly distributed on object parts for the former than for the latter. In this study, the process of information gathering by touch appears to be determined by the intertwining integration of contextual factors (e.g., stimulus position in space), biomechanical constraints on hand movements, and such cognitive factors as hemispheric differences on the ability to encode spatial pattern information.

Laterality effects for tactile patterns

A series of studies was carried out to investigate laterality effects in the discrimination of tactile patterns presented to the fingertips of right-handed subjects. When both hands were stimulated simultaneously, a small right-hand advantage (RHA) was observed for a sequential task and a small left-hand advantage (LHA) was found for a spatial task, but only in women and only when there was a delay or mask between presentation and response. This may indicate different attentional strategies for men and women, or better shielding against interhemispheric interference in women. When only one hand was stimulated at a time (unimanual tasks), no laterality effects were found with sequential tasks, but a substantial RHA for a spatial task was observed. Because the unimanual spatial task involves more possible alternatives than the unimanual sequential ones, this RHA may be a consequence of increased information load rather than a the spatial/sequential distinction. While laterality effects in tactile laterality studies are relatively inconsistent, there are clear differences between bimanual and unimanual presentation. Effects related to hemispheric specialization are much more evident with bilateral presentation.

Hand movements and hemispheric specialization in dichhaptic explorations

Dichhaptic testing has been widely used to assess lateralization in tactile processing. The rationale of dichhaptic testing is that simultaneous exploration of two objects enhances competition between relevant cortical areas in the right and left hemispheres. The synchronization of hand movements in a dichhaptic situation was investigated to determine whether both hands explore the two shapes simultaneously. Fourteen men were tested with the aid of a dichhaptic intermodal task. Tactile stimuli were composite shapes and the activity of each hand was assessed through analysis of hand contacts on each part of the shape. Only 20% of the total exploration time was devoted to simultaneous investigation of the two shapes. In addition, it was found that (i) the recognition accuracy was greater when the target shape was explored by the left hand compared with the right, and (ii) the left hand touched a greater number of parts of the stimuli than the right. Overall, comparison of the present data with those from a previous, monohaptic, task with the same stimuli suggests an advantage of dichhaptic over monohaptic testing to demonstrate laterality differences in accuracy of recognition. However, it is suggested that this advantage is due to cognitive factors rather than to competition between homologous cortical areas.