Tactile Search Research Articles

Searching for a tactile target: the impact of set-size on the N140cc.

The time needed to find a visual target amongst distractors (search task) can increase as a function of the distractors' number (set-size) in the search-array (inefficient search). While the allocation of attention in search tasks has been extensively investigated and debated in the visual domain, little is known about these mechanisms in touch. Initial behavioral evidence shows inefficient search behavior when participants have to distinguish between target and distractors defined by their vibro-tactile frequencies. In the present study, to investigate the allocation of attention to items of the search-array we measured the N140cc during a tactile task in which the set-size was manipulated. The N140cc is a lateralized component of event-related brain potentials recently described as a psychophysiological marker of attentional allocation in tactile search tasks. Participants localized the target, a singleton frequency, while ignoring one, three or five homogeneous distractors. Results showed that error rates linearly increased as a function of set-size, while response times were not affected. Reliable N140cc components were observed for all set-sizes. Crucially, the N140cc amplitude decreased as the number of distractors increased. We argue that the presence of additional distractors hindered the preattentive analysis of the search array resulting in increased uncertainty about the target location (inefficient preattentive stage). This, in turn, increased the variability of the deployment of attention to the target, resulting in reduced N140cc amplitudes. Consistent with existing behavioral evidence, these findings highlight systematic differences between the visual and the tactile attentional systems.

Statistical context learning in tactile search: Crossmodally redundant, visuo-tactile contexts fail to enhance contextual cueing

In search tasks, reaction times become faster when the target is repeatedly encountered at a fixed position within a consistent spatial arrangement of distractor items, compared to random arrangements. Such “contextual cueing” is also obtained when the predictive distractor context is provided by a non-target modality. Thus, in tactile search, finding a target defined by a deviant vibro-tactile pattern (delivered to one fingertip) from the patterns at other, distractor (fingertip) locations is facilitated not only when the configuration of tactile distractors is predictive of the target location, but also when a configuration of (collocated) visual distractors is predictive—where intramodal-tactile cueing is mediated by a somatotopic and crossmodal-visuotactile cueing by a spatiotopic reference frame. This raises the question of whether redundant multisensory, tactile-plus-visual contexts would enhance contextual cueing of tactile search over and above the level attained by unisensory contexts alone. To address this, we implemented a tactile search task in which, in 50% of the trials in a “multisensory” phase, the tactile target location was predicted by both the tactile and the visual distractor context; in the other 50%, as well as a “unisensory” phase, the target location was solely predicted by the tactile context. We observed no redundancy gains by multisensory-visuotactile contexts, compared to unisensory-tactile contexts. This argues that the reference frame for contextual learning is determined by the task-critical modality (somatotopic coordinates for tactile search). And whether redundant predictive contexts from another modality (vision) can enhance contextual cueing depends on the availability of the corresponding spatial (spatiotopic-visual to somatotopic-tactile) remapping routines.

Multisensory Rather than Unisensory Representations Contribute to Statistical Context Learning in Tactile Search.

Using a combination of behavioral and EEG measures in a tactile odd-one-out search task with collocated visual items, we investigated the mechanisms underlying facilitation of search by repeated (vs. nonrepeated) spatial distractor-target configurations ("contextual cueing") when either the tactile (same-modality) or the visual array (different-modality) context was predictive of the location of the tactile singleton target. Importantly, in both conditions, the stimulation was multisensory, consisting of tactile plus visual items, although the target was singled out in the tactile modality and so the visual items were task-irrelevant. We found that when the predictive context was tactile, facilitation of search RTs by repeated configurations was accompanied by, and correlated with, enhanced lateralized ERP markers of pre-attentive (N1, N2) and, respectively focal-attentional processing (contralateral delay activity) not only over central ("somatosensory"), but also posterior ("visual") electrode sites, although the ERP effects were less marked over visual cortex. A similar pattern-of facilitated RTs and enhanced lateralized (N2 and contralateral delay activity) ERP components-was found when the predictive context was visual, although the ERP effects were less marked over somatosensory cortex. These findings indicate that both somatosensory and visual cortical regions contribute to the more efficient processing of the tactile target in repeated stimulus arrays, although their involvement is differentially weighted depending on the sensory modality that contains the predictive information.

Searching on the Back: Attentional Selectivity in the Periphery of the Tactile Field

Recent evidence has identified the N140cc lateralized component of event-related potentials as a reliable index of the deployment of attention to task-relevant items in touch. However, existing ERP studies have presented the tactile search array to participants' limbs, most often to the hands. Here, we investigated distractor interference effects when the tactile search array was presented to a portion of the body that is less lateralized and peripheral compared to the hands. Participants were asked to localize a tactile target presented among distractors in a circular arrangement to their back. The N140cc was elicited contralateral to the target when the singleton distractor was absent. Its amplitude was reduced when the singleton distractor was present and contralateral to the target, suggesting that attention was directed at least in part to the distractor when the singletons are on opposite sides. However, similar N140cc were observed when the singleton distractor was ipsilateral to the target compared to distractor absent trials. We suggest that when target and singleton distractor are ipsilateral, the exact localization of the target requires the attentional processing of all items on the same side of the array, similar to distractor absent trials. Together, these observations replicate the distractor interference effects previously observed for the hands, suggesting that analogous mechanisms guide attentional selectivity across different body parts.

When visual distractors predict tactile search: The temporal profile of cross-modal spatial learning.

Contextual cueing refers to the guidance of search by associative learning of the location of task-relevant target items in relation to the consistent arrangement of distractor ("context") items in the search display. The present study investigated whether such target-distractor associations could also be formed in a cross-modal search task in which the invariant distractor context is visual and the target is tactile. Each trial display consisted of 8 vibrotactile stimuli delivered to 4 fingers of each hand, with the target singled out by a vibrotactile feature difference relative to the homogeneous nontarget vibrations. In addition, there were 4 visual (Gabor) distractors and 4 empty circles colocated (on a different depth plane) with the vibrotactile stimuli. Crucially, in half of the trials, the location of the tactile target was associated with an identical (repeated) configuration, or "context," of visual distractors. The results revealed facilitated RTs to the tactile targets when they were presented (250 to) 450 ms prior to-but not simultaneously with or after-a repeated visual distractor configuration, demonstrating a novel visuo-tactile contextual-cueing effect. Additional tests indicated that the optimal tactile preview period is broadly in line with the time required to recode the (initially) somatotopically sensed tactile stimuli into an external format shared with the visual items. We propose that, besides divided attention between the 2 modalities, the acquisition of search-guiding cross-modal context associations critically depends on the spatiotemporal colocation of the visuo-tactile stimuli. (PsycInfo Database Record (c) 2021 APA, all rights reserved).

Behavioural and electrophysiological evidence for the effect of target-distractor separation in a tactile search task

Evidence suggests that the N140cc component of event-related potentials (ERP) observed in tactile search tasks reflects the attentional selection of the target. Here, we investigated whether the target selection processes are affected by the separation between the target and an ipsilateral singleton distractor (singletons delivered to contiguous or non-contiguous fingers of the same hand). In addition, the external distance between search items was varied through posture (splayed or touching fingers). Accuracy improved when target and distractor were delivered to contiguous fingers that were also touching. Regardless of target-distractor separation, the N140cc was larger when the external distance between search-array stimuli decreased (touching fingers). Importantly, a smaller N140cc was observed at reduced target-distractor separations, suggesting a narrower attentional focus for contiguous singletons. These findings reveal that the mechanisms responsible for tactile target selection in the presence of an ipsilateral singleton distractor are fundamentally different from those emerged in vision.

Tactile active sensing in an insect plant pollinator.

The interaction between insects and the flowers they pollinate has driven the evolutionary diversity of both insects and flowering plants, two groups with the most numerous species on Earth. Insects use vision and olfaction to localize host plants, but we know relatively little about how they find the tiny nectary opening in the flower, which can be well beyond their visual resolution. Especially when vision is limited, touch becomes crucial in successful insect-plant pollination interactions. Here, we studied the remarkable feeding behavior of crepuscular hawkmoths Manduca sexta, which use their long, actively controlled, proboscis to expertly explore flower-like surfaces. Using machine vision and 3D-printed artificial flower-like feeders, we revealed a novel behavior that shows moths actively probe surfaces, sweeping their proboscis from the feeder edge to its center repeatedly until they locate the nectary opening. Moreover, naive moths rapidly learn to exploit these flowers, and they adopt a tactile search strategy to more directly locate the nectary opening in as few as three to five consecutive visits. Our results highlight the proboscis as a unique active sensory structure and emphasize the central role of touch in nectar foraging insect-plant pollinator interactions.

Online sensory feedback during active search improves tactile localization.

Natural motor behavior is usually refined by ongoing sensory input in closed feedback loops. Research has suggested that humans make systematic errors when localizing touch on the skin, and that perceptual body representations underlying these behaviors are distorted. However, experimental procedures usually prevent participants from touching the target limb, interrupting the natural action-perception loop. It is currently unknown how such experimental strategies affect localization and systematic perceptual distortions. Here, participants received a brief touch on their left forearm and, with closed eyes, searched for the target location by moving the right index finger across the left arm. Tactile search significantly reduced the localization error present at touchdown of the searching finger on the target arm. Localization improvement was largely absent when a barrier above the target arm prevented online tactile feedback of the target region. Vision of the arms while reaching to, and searching on, the skin, greatly reduced the localization error at touchdown, but tactile search further improved localization slightly. Thus, both tactile and visual feedback help matching the positions of reaching and target limbs during localization. Yet, even if small, the unique improvement through tactile information confirms the importance of target-related, closed-loop tactile feedback for tactile localization. (PsycInfo Database Record (c) 2020 APA, all rights reserved).

Electrophysiological correlates of attentional selection in tactile search tasks: The impact of singleton distractors on target selection.

Recent event-related potential (ERP) studies have suggested that the N140cc component reflects target selection mechanisms in tactile search tasks in which the target is presented simultaneously with homogeneous distractors. To investigate if and how the attentional selection of the tactile target is affected by the presence of a singleton distractor, we presented a four-item search array (two stimuli to the middle and index fingers of the left and right hand) which included a singleton distractor in addition to the target and two other homogenous distractors. Participants had to localize the target (top or bottom finger on either hand), while ignoring all distractors. Behavioral results revealed that the target localization was hindered by the presence of the singleton distractor as demonstrated by distractor-presence costs for both speed and accuracy. ERP results confirmed that attention was directed to the singleton distractor when this was the only singleton item in the array as suggested by the presence of a distractor-related N140cc component on distractor only trials. Furthermore, when target and singleton distractor were presented to opposite hands (contralateral distractor trials) the target-related N140cc amplitude was reduced as compared to target only trials, suggesting reduced attentional resources to the target. However, when target and singleton distractor were presented to the same hand (ipsilateral distractor trials), the N140cc amplitude was comparable to that observed on target only trials. These findings suggest that the N140cc reflects the attentional selection of the target side rather than the competition between stimuli presented to the same hand.

Principles of tactile search over the body.

Although we routinely experience complex tactile patterns over our entire body, how we selectively experience multisite touch over our bodies remains poorly understood. Here, we characterized tactile search behavior over the full body using a tactile analog of the classic visual search task. On each trial, participants judged whether a target stimulus (e.g., 10-Hz vibration) was present or absent anywhere on the body. When present, the target stimulus could occur alone or simultaneously with distractor stimuli (e.g., 30-Hz vibrations) on other body locations. We systematically varied the number and spatial configurations of the distractors as well as the target and distractor frequencies and measured the impact of these factors on tactile search response times. First, we found that response times were faster on target-present trials compared with target-absent trials. Second, response times increased with the number of stimulated sites, suggesting a serial search process. Third, search performance differed depending on stimulus frequencies. This frequency-dependent behavior may be related to perceptual grouping effects based on timing cues. We constructed linear models to explore how the locations of the target and distractor cues influenced tactile search behavior. Our modeling results reveal that, in isolation, cues on the index fingers make relatively greater contributions to search performance compared with stimulation experienced on other body sites. Additionally, costimulation of sites within the same limb or simply on the same body side preferentially influence search behavior. Our collective findings identify some principles of attentional search that are common to vision and touch, but others that highlight key differences that may be unique to body-based spatial perception.NEW & NOTEWORTHY Little is known about how we selectively experience multisite touch patterns over the body. Using a tactile analog of the classic visual target search paradigm, we show that tactile search behavior for flutter cues is generally consistent with a serial search process. Modeling results reveal the preferential contributions of index finger stimulation and two-site stimulus interactions involving ipsilateral patterns and within-limb patterns. Our results offer initial evidence for spatial and temporal principles underlying tactile search behavior over the body.

Foraging for Handholds: Attentional Scanning Varies by Expertise in Rock Climbing

Efficiency of search often comes with experience. We explored search processes during a highly dynamic and complex task: rock climbing. In general, we found similarities between expert and non-expert climbers regarding their visual search and differences in their tactile search. Analyses determined that experts and non-experts did not differ in their visual fixations toward areas of interest (AOIs), but differed in their mean fixation times depending on the terrain. Experts performed fewer investigative touches than non-experts, suggesting that experts might have climbed faster due to their reliance on visual rather than tactile cues. These findings support the theory of information foraging in a dynamic environment by suggesting that non-experts used tactile search to acquire information rather than relying on visual search.

Contextual cueing of tactile search is coded in an anatomical reference frame.

This work investigates the reference frame(s) underlying tactile context memory, a form of statistical learning in a tactile (finger) search task. In this task, if a searched-for target object is repeatedly encountered within a stable spatial arrangement of task-irrelevant distractors, detecting the target becomes more efficient over time (relative to nonrepeated arrangements), as learned target-distractor spatial associations come to guide tactile search, thus cueing attention to the target location. Since tactile search displays can be represented in several reference frames, including multiple external and an anatomical frame, in Experiment 1 we asked whether repeated search displays are represented in tactile memory with reference to an environment-centered or anatomical reference frame. In Experiment 2, we went on examining a hand-centered versus anatomical reference frame of tactile context memory. Observers performed a tactile search task, divided into a learning and test session. At the transition between the two sessions, we introduced postural manipulations of the hands (crossed ↔ uncrossed in Expt. 1; palm-up ↔ palm-down in Expt. 2) to determine the reference frame of tactile contextual cueing. In both experiments, target-distractor associations acquired during learning transferred to the test session when the placement of the target and distractors was held constant in anatomical, but not external, coordinates. In the latter, RTs were even slower for repeated displays. We conclude that tactile contextual learning is coded in an anatomical reference frame. (PsycINFO Database Record

Hand distance modulates the electrophysiological correlates of target selection during a tactile search task.

This study investigated whether the N140cc ERP component, described as a possible electrophysiological marker of target selection in touch, was modulated by body posture. Participants performed a tactile search task in which they had to localize a tactile target, presented to the left or right hand, while a simultaneous distractor was delivered to the opposite hand. Importantly, the distance between target and distractor (hands separation) was manipulated in different experimental conditions (near vs. far hands). Results showed reduced errors and enhanced amplitudes of the late N140cc when the hands were far apart than in close proximity. This suggests that the competition between target and distractor is stronger when the hands are close together in the near condition, resulting in a degraded selection process. These findings confirm that the N140cc reflects target selection during the simultaneous presentation of competing stimuli and demonstrate for the first time that the attentional mechanisms indexed by this ERP component are based at least in part on postural representations of the body.

The attentive homunculus: ERP evidence for somatotopic allocation of attention in tactile search

Our brain constantly receives tactile information from the body’s surface. We often only become aware of this information when directing our attention towards the body. Here, we report a study investigating the behavioural and neural response when selecting a target amongst distractor vibrations presented simultaneously to several locations either across the hands or body. Comparable visual search studies have revealed the N2pc as the neural correlate of visual selective attention. Analogously, we describe an enhanced negativity contralateral to the tactile target side. This effect is strongest over somatosensory areas and lasts approximately 200ms from the onset of the somatosensory N140 ERP component. Based on these characteristics we named this electrophysiological signature of attentional tactile target selection during tactile search the N140-central-contralateral (N140cc). Furthermore, we present supporting evince that the N140cc reflects attentional enhancement of target rather than suppression of distractor locations; the component was not reliably altered by distractor but rather by target location changes. Taken together, our findings present a novel electrophysiological marker of tactile search and show how attentional selection of touch operates by mainly enhancing task relevant locations within the somatosensory homunculus.

Poorer resolution for audiotactile than for audiovisual synchrony detection in cluttered displays.

The brain integrates signals from multiple modalities to provide a reliable estimate of environmental events. A temporally cluttered environment presents a challenge for sensory integration because of the risk of misbinding, yet it also provides scope for cross-modal binding to greatly enhance performance by highlighting multimodal events. We present a tactile search task in which fingertips received pulsed vibrations and participants identified which finger was stimulated in synchrony with an auditory signal. Results showed that performance for identifying the target finger was impaired when other fingers were stimulated, even though all fingers were stimulated sequentially. When the number of fingers vibrated was fixed, we found that both spatial and temporal factors constrained performance, because events occurring close to the target vibration in either space or time reduced accuracy. When tactile search was compared with visual search, we found overall performance was lower in touch than in vision, although the cost of reducing temporal separation between stimuli or increasing the presentation rate was similar for both target modalities. Audiotactile performance benefitted from increasing spatial separation between target and distractors, with a particularly strong benefit for locating the target on a different hand to the distractors, whereas the spatial manipulations did not affect audiovisual performance. The similar trends in performance for temporal manipulations across vision and touch suggest a common supramodal binding mechanism that, when combining audition and touch, is limited by the poor resolution of the underlying unisensory representation of touch in cluttered settings. (PsycINFO Database Record

Contextual cueing: implicit memory of tactile context facilitates tactile search.

In visual search, participants detect and subsequently discriminate targets more rapidly when these are embedded in repeatedly encountered distractor arrangements, an effect termed contextual cueing (Chun & Jiang Cognitive Psychology, 36, 28–71, 1998). However, whereas previous studies had explored contextual cueing exclusively in visual search, in the present study we examined the effect in tactile search using a novel tactile search paradigm. Participants were equipped with vibrotactile stimulators attached to four fingers on each hand. A given search array consisted of four stimuli (i.e., two items presented to each hand), with the target being an odd-one-out feature singleton that differed in frequency (Exps. 1 and 2) or waveform (Exp. 3) from the distractor elements. Participants performed a localization (Exps. 1 and 2) or discrimination (Exp. 3) task, delivering their responses via foot pedals. In all three experiments, reaction times were faster when the arrangement of distractor fingers predicted the target finger. Furthermore, participants were unable to explicitly discriminate repeated from nonrepeated tactile configurations (Exps. 2 and 3). This indicates that the tactile modality can mediate the formation of configural representations and use these representations to guide tactile search.

Tactile search for change has less memory than visual search for change.

Haptic perception of a 2D image is thought to make heavy demands on working memory. During active exploration, humans need to store the latest local sensory information and integrate it with kinesthetic information from hand and finger locations in order to generate a coherent perception. This tactile integration has not been studied as extensively as visual shape integration. In the current study, we compared working-memory capacity for tactile exploration to that of visual exploration as measured in change-detection tasks. We found smaller memory capacity during tactile exploration (approximately 1 item) compared with visual exploration (2-10 items). These differences generalized to position memory and could not be attributed to insufficient stimulus-exposure durations, acuity differences between modalities, or uncertainty over the position of items. This low capacity for tactile memory suggests that the haptic system is almost amnesic when outside the fingertips and that there is little or no cross-position integration.

High-Accuracy Patient-to-Image Registration for the Facilitation of Image-Guided Robotic Microsurgery on the Head

Image-guided microsurgery requires accuracies an order of magnitude higher than today's navigation systems provide. A critical step toward the achievement of such low-error requirements is a highly accurate and verified patient-to-image registration. With the aim of reducing target registration error to a level that would facilitate the use of image-guided robotic microsurgery on the rigid anatomy of the head, we have developed a semiautomatic fiducial detection technique. Automatic force-controlled localization of fiducials on the patient is achieved through the implementation of a robotic-controlled tactile search within the head of a standard surgical screw. Precise detection of the corresponding fiducials in the image data is realized using an automated model-based matching algorithm on high-resolution, isometric cone beam CT images. Verification of the registration technique on phantoms demonstrated that through the elimination of user variability, clinically relevant target registration errors of approximately 0.1 mm could be achieved.

Impairments in tactile search following superior parietal damage

The superior parietal cortex is critical for the control of visually guided actions. Research suggests that visual stimuli relevant to actions are preferentially processed when they are in peripersonal space. One recent study demonstrated that visually guided movements towards the body were more impaired in a patient with damage to superior parietal cortex. Whereas past studies have explored disordered movement in optic ataxic patients, there has been less exploration of space perception in terms of search capacity in this population. In addition, there is some debate concerning the relationship between deficits of visuomotor control and impaired attention/perception in optic ataxia. Given that the dorsal stream has been implicated in the spatial processing of stimuli in peripersonal space, and damage to this region is known to cause optic ataxia, we felt that further investigation was warranted. We examined tactile search behavior in the fronto-parallel and radial planes in a patient with right superior parietal damage and optic ataxia. We used a pegboard with removable cylindrical pegs that allowed for the reorganization of targets between trials. To better characterize three-dimensional search behavior, we included both horizontal and vertical search conditions. Results showed that the patient spent more time searching, was more accurate and revisited more targets in right versus left space. Interestingly, the patient spent the majority of her time specifically searching the lower right quadrant of the stimulus array. Further analysis revealed lower target detection rates along the outer borders of the pegboard on all sides. The search pattern observed here is unusual considering that all targets were within arm’s reach. The present experiment demonstrates that damage to superior parietal cortex impairs tactile search and biases exploration towards lower right peripersonal space.

A critical speed for gating of tactile detection during voluntary movement

This study addressed the paradoxical observation that movement is essential for tactile exploration, and yet is accompanied by movement-related gating or suppression of tactile detection. Knowing that tactile gating covaries with the speed of movement (faster movements, more gating), we hypothesized that there would be no tactile gating at slower speeds of movement, corresponding to speeds commonly used during tactile exploration (<200mm/s). Subjects (n=21) detected the presence or absence of a weak electrical stimulus applied to the skin of the right middle finger during two conditions: rest and active elbow extension. Movement speed was systematically varied from 50 to ~1,000mm/s. No subject showed evidence of tactile gating at the slowest speed tested, 50mm/s (rest versus movement), but all subjects showed decreased detection at one or more higher speeds. For each subject, we calculated the critical speed, corresponding to the speed at which detection fell to 0.5 (chance). The mean critical speed was 472 mm/s and >200mm/s in almost all subjects (19/21). This result is consistent with our hypothesis that subjects optimize the speed of movement during tactile exploration to avoid speeds associated with tactile gating. This strategy thus maximizes the quality of the tactile feedback generated during tactile search and improves perception.