Electrotactile Stimuli Research Articles

Electrotactile BCI for Top-Down Somatosensory Training: Clinical Feasibility Trial of Online BCI Control in Subacute Stroke Patients.

This study investigates the feasibility of a novel brain-computer interface (BCI) device designed for sensory training following stroke. The BCI system administers electrotactile stimuli to the user's forearm, mirroring classical sensory training interventions. Concurrently, selective attention tasks are employed to modulate electrophysiological brain responses (somatosensory event-related potentials-sERPs), reflecting cortical excitability in related sensorimotor areas. The BCI identifies attention-induced changes in the brain's reactions to stimulation in an online manner. The study protocol assesses the feasibility of online binary classification of selective attention focus in ten subacute stroke patients. Each experimental session includes a BCI training phase for data collection and classifier training, followed by a BCI test phase to evaluate online classification of selective tactile attention based on sERP. During online classification tests, patients complete 20 repetitions of selective attention tasks with feedback on attention focus recognition. Using a single electroencephalographic channel, attention classification accuracy ranges from 70% to 100% across all patients. The significance of this novel BCI paradigm lies in its ability to quantitatively measure selective tactile attention resources throughout the therapy session, introducing a top-down approach to classical sensory training interventions based on repeated neuromuscular electrical stimulation.

A TETRA-Based System for Remote Health Monitoring of First Responders: Peak AoI Assessment in Direct and Trunked Mode

In this article, we study peak age of information (PAoI) performance of a novel Internet of Things (IoT) solution for remote health monitoring of first responders over terrestrial trunked radio (TETRA) links. The solution features a set of sensors embedded in a smart garment that periodically records and sends physiological parameters of first responders to a remote agent. The received data are analyzed by the remote agent, which feeds back notifications and warnings to the first responders in the form of electrotactile stimuli. The communication in the system is performed over the TETRA short data service (SDS), which is the default option for the development of third-party applications and which has rather limited capabilities. The choice of PAoI as the parameter of interest is motivated by its suitability to measure data freshness in IoT applications with periodic monitoring. We derive closed-form expressions of PAoI for different packet-management schemes allowed by the TETRA standard and verify the analytical results through extensive simulations under varying message generation rates. Our results provide important insights on the expected PAoI performance, which can be used for the system design guidelines. To the best of our knowledge, this is the first work that analyzes AoI performance of the TETRA networks.

Novel electrotactile brain-computer interface with somatosensory event-related potential based control.

A brain computer interface (BCI) allows users to control external devices using non-invasive brain recordings, such as electroencephalography (EEG). We developed and tested a novel electrotactile BCI prototype based on somatosensory event-related potentials (sERP) as control signals, paired with a tactile attention task as a control paradigm. A novel electrotactile BCI comprises commercial EEG device, an electrical stimulator and custom software for EEG recordings, electrical stimulation control, synchronization between devices, signal processing, feature extraction, selection, and classification. We tested a novel BCI control paradigm based on tactile attention on a sensation at a target stimulation location on the forearm. Tactile stimuli were electrical pulses delivered at two proximal locations on the user's forearm for stimulating branches of radial and median nerves, with equal probability of the target and distractor stimuli occurrence, unlike in any other ERP-based BCI design. We proposed a compact electrical stimulation electrodes configuration for delivering electrotactile stimuli (target and distractor) using 2 stimulation channels and 3 stimulation electrodes. We tested the feasibility of a single EEG channel BCI control, to determine pseudo-online BCI performance, in ten healthy subjects. For optimizing the BCI performance we compared the results for two classifiers, sERP averaging approaches, and novel dedicated feature extraction/selection methods via cross-validation procedures. We achieved a single EEG channel BCI classification accuracy in the range of 75.1 to 88.1% for all subjects. We have established an optimal combination of: single trial averaging to obtain sERP, feature extraction/selection methods and classification approach. The obtained results demonstrate that a novel electrotactile BCI paradigm with equal probability of attended (target) and unattended (distractor) stimuli and proximal stimulation sites is feasible. This method may be used to drive restorative BCIs for sensory retraining in stroke or brain injury, or assistive BCIs for communication in severely disabled users.

Design, evaluation and calibration of wearable electrotactile interfaces for enhancing contact information in virtual reality

Electrotactile feedback is a convenient tactile rendering method thanks to reduced form factor and power consumption. Yet, its usage in immersive virtual reality has been scarcely addressed. This paper explores how electrotactile feedback could be used to enhance contact information for mid-air interactions in virtual reality. We propose an electrotactile rendering method which modulates the perceived intensity of the electrotactile stimuli according to the interpenetration distance between the user’s finger and the virtual surface. In a first user study (N=21), we assessed the performance of our method against visual interpenetration feedback and no-feedback. Contact precision and accuracy were significantly improved when using interpenetration feedback. The results also showed that the calibration of the electrotactile stimuli was key, which motivated a second study exploring how the calibration procedure could be improved. In a second study (N=16), we compared two calibration techniques: a non-VR keyboard and a VR direct interaction method. While the two methods provided comparable usability and calibration accuracy, the VR method was significantly faster. These results pave the way for the usage of electrotactile feedback as an efficient alternative to visual feedback for enhancing contact information in virtual reality.

Continuous Transition Impairs Discrimination of Electrotactile Frequencies.

Just-noticeable difference (JND), indicating the ability to accurately identify small differences in stimulation parameters, can be used to choose more sensitive stimulation methods as well as to calibrate tactile feedback in closed-loop human-machine interfacing. The JND is typically estimated using a forced-choice-discrimination task, in which two stimuli with different intensities are delivered separated by a brief pause. In the applications of tactile feedback, however, the stimulation parameters are typically modulated continuously. It is unclear if the discriminability of stimuli separated in time characterizes the ability to distinguish continuous changes in stimulation intensity. The present study compared the JND when pairs of frequency-modulated electrotactile stimuli were separated in time and presented continuously at two different baseline frequencies (20 and 60 Hz). The results showed that the JND was significantly smaller with time-separation between stimuli, but that the JND obtained with different types of transitions were in most cases linearly associated. In conclusion, the discriminability of time-separated stimuli is systematically better compared to that of the stimuli presented continuously. This can have an impact when calibrating the tactile feedback where the conventional method of the JND assessment might lead to an overly optimistic estimate of detectable changes.

Somatosensory perception\u2013action binding in Tourette syndrome

It is a common phenomenon that somatosensory sensations can trigger actions to alleviate experienced tension. Such “urges” are particularly relevant in patients with Gilles de la Tourette (GTS) syndrome since they often precede tics, the cardinal feature of this common neurodevelopmental disorder. Altered sensorimotor integration processes in GTS as well as evidence for increased binding of stimulus- and response-related features (“hyper-binding”) in the visual domain suggest enhanced perception–action binding also in the somatosensory modality. In the current study, the Theory of Event Coding (TEC) was used as an overarching cognitive framework to examine somatosensory-motor binding. For this purpose, a somatosensory-motor version of a task measuring stimulus–response binding (S-R task) was tested using electro-tactile stimuli. Contrary to the main hypothesis, there were no group differences in binding effects between GTS patients and healthy controls in the somatosensory-motor paradigm. Behavioral data did not indicate differences in binding between examined groups. These data can be interpreted such that a compensatory “downregulation” of increased somatosensory stimulus saliency, e.g., due to the occurrence of somatosensory urges and hypersensitivity to external stimuli, results in reduced binding with associated motor output, which brings binding to a “normal” level. Therefore, “hyper-binding” in GTS seems to be modality-specific.

Lingual electrotactile discrimination ability is associated with the presence of specific connective tissue structures (papillae) on the tongue surface.

Electrical stimulation of nerve endings in the tongue can be used to communicate information to users and has been shown to be highly effective in sensory substitution applications. The anterior tip of the tongue has very small somatosensory receptive fields, comparable to those of the finger tips, allowing for precise two-point discrimination and high tactile sensitivity. However, perception of electrotactile stimuli varies significantly between users, and across the tongue surface. Despite this, previous studies all used uniform electrode grids to stimulate a region of the dorsal-medial tongue surface. In an effort to customize electrode layouts for individual users, and thus improve efficacy for sensory substitution applications, we investigated whether specific neuroanatomical and physiological features of the tongue are associated with enhanced ability to perceive active electrodes. Specifically, the study described here was designed to test whether fungiform papillae density and/or propylthiouracil sensitivity are positively or negatively associated with perceived intensity and/or discrimination ability for lingual electrotactile stimuli. Fungiform papillae number and distribution were determined for 15 participants and they were exposed to patterns of electrotactile stimulation (ETS) and asked to report perceived intensity and perceived number of stimuli. Fungiform papillae number and distribution were then compared to ETS characteristics using comprehensive and rigorous statistical analyses. Our results indicate that fungiform papillae density is correlated with enhanced discrimination ability for electrical stimuli. In contrast, papillae density, on average, is not correlated with perceived intensity of active electrodes. However, results for at least one participant suggest that further research is warranted. Our data indicate that propylthiouracil taster status is not related to ETS perceived intensity or discrimination ability. These data indicate that individuals with higher fungiform papillae number and density in the anterior medial tongue region may be better able to use lingual ETS for sensory substitution.

A 36-Class Bimodal ERP Brain-Computer Interface Using Location-Congruent Auditory-Tactile Stimuli.

To date, traditional visual-based event-related potential brain-computer interface (ERP-BCI) systems continue to dominate the mainstream BCI research. However, these conventional BCIs are unsuitable for the individuals who have partly or completely lost their vision. Considering the poor performance of gaze independent ERP-BCIs, it is necessary to study techniques to improve the performance of these BCI systems. In this paper, we developed a novel 36-class bimodal ERP-BCI system based on tactile and auditory stimuli, in which six-virtual-direction audio files produced via head related transfer functions (HRTF) were delivered through headphones and location-congruent electro-tactile stimuli were simultaneously delivered to the corresponding position using electrodes placed on the abdomen and waist. We selected the eight best channels, trained a Bayesian linear discriminant analysis (BLDA) classifier and acquired the optimal trial number for target selection in online process. The average online information transfer rate (ITR) of the bimodal ERP-BCI reached 11.66 bit/min, improvements of 35.11% and 36.69% compared to the auditory (8.63 bit/min) and tactile approaches (8.53 bit/min), respectively. The results demonstrate the performance of the bimodal system is superior to each unimodal system. These facts indicate that the proposed bimodal system has potential utility as a gaze-independent BCI in future real-world applications.

Temporal Asynchrony but Not Total Energy Nor Duration Improves the Judgment of Numerosity in Electrotactile Stimulation.

Stroke patients suffer from impairments of both motor and somatosensory functions. The functional recovery of upper extremities is one of the primary goals of rehabilitation programs. Additional somatosensory deficits limit sensorimotor function and significantly affect its recovery after the neuromotor injury. Sensory substitution systems, providing tactile feedback, might facilitate manipulation capability, and improve patient's dexterity during grasping movements. As a first step toward this aim, we evaluated the ability of healthy subjects in exploiting electrotactile feedback on the shoulder to determine the number of perceived stimuli in numerosity judgment tasks. During the experiment, we compared four different stimulation patterns (two simultaneous: short and long, intermittent and sequential) differing in total duration, total energy, or temporal synchrony. The experiment confirmed that the subject ability to enumerate electrotactile stimuli decreased with increasing the number of active electrodes. Furthermore, we found that, in electrotactile stimulation, the temporal coding schemes, and not total energy or duration modulated the accuracy in numerosity judgment. More precisely, the sequential condition resulted in significantly better numerosity discrimination than intermittent and simultaneous stimulation. These findings, together with the fact that the shoulder appeared to be a feasible stimulation site to communicate tactile information via electrotactile feedback, can serve as a guide to deliver tactile feedback to proximal areas in stroke survivors who lack sensory integrity in distal areas of their affected arm, but retain motor skills.

EPV 11. Subliminal Tactile Stimuli Modulate Perception Rhythmically

Perception of near-threshold stimuli varies rhythmically over trials depending on the time point of attentional cues ( Landau and Fries, 2012 , Landau et al., 2015 ). Neuronal oscillatory phase in sensory cortices predicts the perception of visual ( Busch et al., 2009 ) and tactile ( Baumgarten et al., 2015 ) stimuli. This gives rise to the question if oscillatory phase accounts for rhythmic variations in perception. Sensory stimulation can reset ongoing oscillatory phase in sensory cortices ( Lakatos et al., 2007 ), thereby allowing to externally influence perception. Here, we investigated if perception of suprathreshold electrotactile stimuli is influenced by a preceding subliminal electrotactile stimulus. We hypothesized that the time interval between the preceding stimulus and the target stimuli rhythmically determines the perception of the target stimuli. 18 right-handed subjects (6 male, 28 ± 5 y) were measured in a behavioral paradigm assessing the perception of electrotactile stimuli applied to the left index finger. First, one subliminal stimuli with subthreshold amplitude was presented. Subsequently, either zero, one or two suprathreshold target stimuli separated by specific stimulus onset asynchronies (SOAs) were presented. SOAs included an individually determined SOA (32 ± 17.6 ms) intended to achieve a 1:1 response distribution for perceived one vs. two stimuli. The temporal interval between subliminal stimulus and target stimuli varied from 20 to 600 ms in steps of 20 ms. Subjects were asked to report if they perceived zero, one or two stimuli. Perception rates were analyzed as a function of the temporal interval between subliminal stimulus and target stimuli. To determine rhythmical dependencies, Fourier-analysis was performed on behavioral data. We found that temporal perceptual discrimination rates varied rhythmically with the temporal interval between subliminal stimulus and target stimuli. Fourier analysis revealed significant rhythms in the delta/theta ( ∼ 3 Hz) and beta band ( ∼ 15 Hz). The present results demonstrate a rhythmic relationship between the perception of two subsequently presented electrotactile stimuli and the temporal interval between subliminal stimulus and target stimuli. This relationship was most prominent in the delta/theta and beta band. These results illustrate that subliminal stimulation which is not consciously perceived can influence the perception of suprathreshold stimuli. Perceptual fluctuations at 3 Hz can be interpreted as a result of attentional sampling ( Landau and Fries, 2012 , Landau et al., 2015 ). The 15 Hz component can be explained by discrete perceptual sampling in the somatosensory domain ( Baumgarten et al., 2015 ). Presumably, subliminal stimulation resulted in a phase reset within the somatosensory system. This determined the position of the target stimuli relative to ongoing perceptual cycles, consequently, modulating stimulus perception.

EP 55. Perception of the Double-Flash-Illusion in patients with Parkinson’s disease and its modulation by dopamine compared to matched healthy controls

When a brief single visual stimulus is presented interposed between two brief tactile stimuli in a time window (SOA; Stimulus Onset Asynchrony) of 70–100 ms healthy subjects frequently perceive an illusory second visual stimulus. This phenomenon is known as “Double-Flash-Illusion” (DFI) ( Shams et al., 2000 ). Hypothesis Studies have shown that patients with Parkinson’s disease (PD) need a longer time interval to perceive two unimodal stimuli correctly as two ( Artieda et al., 1992 ). Here we investigated whether in PD patients the time window to perceive the DFI is also prolonged compared to healthy controls. We hypothesized that the SOA to perceive the DFI is prolonged in PD patients and that dopamine medication (ON) shortens the SOA to perceive the DFI relative to medication withdrawal (OFF). Methods Up to now 20 PD patients (6 female; mean age 58, 5 y) and 16 matched controls took part. Only patients with a hypokinetc-rigid subtype in state I–III by Hoehn & Yahr, who received medical treatment only and did not have any other neurological disorders took part. Participants received visuo-electrotactile stimulation to the left index finger. Either one or two visual stimuli were presented with either one or two electrotactile stimuli (combinations: v, vv, tv, tvt, tvtv, vtv; with v standing for the visual and t for the tactile stimulus). The stimuli in each combination were separated by the following SOAs: 67, 84, 100, 135, 170, 200, 270, 300, 400, 500 and 600 ms. The condition with two electrotactile stimuli interposed with one visual stimulus (tvt) was supposed to induce the DFI. Participants reported the number of perceived visual stimuli. The experiment was first done without medication (OFF) and than repeated with dopamine medication (ON) with PD patients. The control subjects performed the experiment twice without any medication. The UPDRS (Unified Parkinson’s Disease Rating Score) was measured in OFF and ON for the PD group. Findings Among the PD patients, we found two groups: one group (9 subjects) who did not report the DFI (NOSEEer; one female), while the other group (11 subjects) frequently reported the DFI (SEEer; 5 female). The SEEer-Group shows significantly increased DFI perception in the ON condition compared to OFF for the SOAs 64, 100 and 135 ms. Additionally, we found that the SOA for which we observed maximum illusory perception shifted from 270 ms to 200 ms from OFF to ON. In the control group we found 12 SEEer and 4 NOSEEer. There was no significant difference between first measurements in PD patients (OFF) and controls with respect to perception of illusion. When patients’ ON condition was compared to the controls’ second measurement the patients perceived the stimulation significantly more often as two visual stimuli for the SOAs 64 and 84 ms. Conclusion In summary, we find a modulation of DFI perception by dopamine: With medication, patients perceived the illusion more often for short SOAs than without medication. Interestingly, we did not find a significant difference between patients in the OFF state and healthy controls’ first measurement. This finding indicates that other factors than disease or dopamine may additionally influence the perception of the DFI. Future studies analysing brain activity of both groups might help to understand the present findings as well as the differences between SEEer group and NOSEEer group.

Two-point tactile discrimination ability is influenced by temporal features of stimulation

Two-point discrimination threshold is commonly used for assessing tactile spatial resolution. Since the effect of temporal features of cutaneous test stimulation on spatial discrimination ability is not yet well known, we determined whether the ability to discriminate between two stimulus locations varies with the interstimulus interval (ISI) of sequentially presented tactile stimuli or the length of the stimulus train. Electrotactile stimuli were applied to one or two locations on the skin of the thenar eminence of the hand in healthy human subjects. Tactile discrimination ability was determined using methods based on the signal detection theory allowing the assessment of sensory performance, independent of the subject's response criterion. With stimulus pairs, the ability to discriminate spatial features of stimulation (one location vs. two stimulus locations 4 cm apart) was improved when the ISI was equal to or longer than that required for tactile temporal discrimination. With stimulus trains, the ability to discriminate spatial features of stimulation was significantly improved with an increase in the stimulus train (from 3 to 11 pulses corresponding to train lengths from 40 to 200 ms). These results indicate that temporal features of tactile stimulation significantly influence sensory performance in a tactile spatial discrimination task. Precise control of temporal stimulus parameters should help to reduce variations in results on the two-point discrimination threshold.

Electro-tactile device for material texture simulation

This paper deals with the simulation of material texture by means of electro-tactile stimuli, which directly derive from real material properties. The research presents a novel tactile display by its hardware and software system architecture and the elaboration procedure to generate the stimulating signals, and validates the adopted simulation strategy by experimental testing. The tactile system elaborates data from scans of real material samples and generates electrical stimuli to reproduce roughness and texture coarseness sensations. It also adds a coherent sound feedback to improve the realism of the simulation. The research defines also an experimental protocol based on the theory of Psychophysics to carry out system calibration and tests with users. The scope is to validate the proposed tactile system as a new tool for material simulation, which can be adopted for material virtual prototyping in several fields (product design, textile and clothing, gaming and entertainment, virtual museum, rehabilitation, etc.). Experimentations have been carried out to measure the users’ response to our different material classes (wood, paper, rubber and textile fabric). Experimental results concern how good the adopted simulation approach is and the analysis of the human tactile perception simulated by the system. Main findings relate to the system performance and the users’ response in terms of signal recognition and material class discrimination.

Discrimination is not impaired when more cortical space between two electro-tactile markers increases perceived duration

The purpose of the present study was to examine how duration processing is affected by space between two electro-tactile stimuli marking inter-stimulus time intervals. The results of two experiments, where the method of constant stimuli was used, indicated that discrimination remained at the same level when delivering two markers to different fingers (of the same hand) resulted in longer perceived duration than delivering them to the same finger. Indeed, in Experiment 1, intervals were overestimated while discrimination remained at the same level when the leading and tailing markers were delivered to the index and ring fingers, respectively, compared with when both markers were delivered to the index finger. In Experiment 2, while there were individual differences in spatial effect on perceived duration when the leading and tailing markers were delivered to the middle and little fingers, respectively, discrimination remained at the same level even with participants overestimating intervals. This indicates that variability in duration processing is constant within the same cortical hemisphere when more space between two stimuli marking time results in longer perceived duration.

Electrotactile Feedback for Handheld Devices with Touch Screen and Simulation of Roughness.

We present a novel electrotactile display that can be integrated into current handheld devices with touch screens. In this display, tactile information is presented to the fingertip of the user by transmitting small currents through electrodes. Experiments were conducted to investigate the perception of simulated textures using this electrotactile display technique. One fundamental feature of texture, which is the focus of this study, is roughness. The aim of the first experiment was to investigate the relationship between electrotactile stimulation parameters such as current and pulse frequency and the perception of roughness. An increase in the current magnitude resulted in an increase in perceived roughness. The aim of the second experiment was to investigate parameter combinations of electrotactile stimuli can be used to simulate textures. Subjects adjusted the intensity and frequency of the current stimuli until the simulated textures were perceived as being equal to reference textures such as sandpapers of varying grit numbers and grooved woods with varying groove widths. Subjects tended to find an electrotactile stimulus with a high current magnitude and a low pulse frequency more suitable to represent rough surfaces. They tended to find just-perceptible current magnitudes suitable for very smooth surfaces and did not show a preference for any frequency.

Tactile perception: Do distinct subpopulations explain differences in mislocalization rates of stimuli across fingertips?

In a previous study we were able to demonstrate that the Cutaneous Rabbit Effect (CRE) could be induced across fingertips using a form of the reduced rabbit paradigm and electrotactile stimuli. The CRE, as used here, is an illusory phenomenon where two stimuli are rapidly at a site and then a stimulus is presented to a nearby site. The perception of the second of the stimuli is not at its presented location but at a site between the first and last stimuli. In this experiment, though the overall population did perceive the mislocalized stimuli as the CRE would predict, some subjects were very infrequently observed to mislocalize stimuli due to the CRE or other effects. Here we further examine this phenomena, attempting to identify whether a subpopulation exists that rarely mislocalizes stimuli on their fingertips. To test for this subpopulation, we reexamined the collected data from the previously published experiment and other unpublished data relating to that study. By examining these data for rates of mislocalization utilizing our previous metric we identified that there is a perceptual subpopulation that very infrequently misidentifies the location of a fingertip stimulus.

Electrotactile stimuli delivered across fingertips inducing the Cutaneous Rabbit Effect

Previous studies have been unable to induce the Cutaneous Rabbit Effect (CRE) when the most likely perceived location of the illusory stimulus is on a non-continuous skin area. To determine whether the CRE could be elicited when each of the delivered stimuli were on non-continuous skin areas, we developed a new electrotactile stimulation paradigm attempting to induce the CRE across the fingertips. Though our stimulation paradigm differed from classic reduced CRE paradigms through the use of electrotactile stimuli, focusing the subject attention to a 'likely' illusory site, and the inclusion of a fourth stimulation site (two stimuli after the illusory stimulus), these factors were not the cause of the illusory effect we observed. Experiments conducted on the forearm validated that our paradigm elicited similar results to those reported in previous CRE studies that used either 3-stimulation-point mechanical or electrotactile stimuli with subject attention focused on the 'likely' illusory site. Across the fingertips, we observed an increase in stimulus mislocalization onto the middle fingertip, the 'likely' perceived location of the illusory stimuli, under Illusory Rabbit Trains compared to the Motion Bias Trains. Because the Motion Bias Trains should not induce a perceived location shift of the illusory stimulus but stimulates the adjacent digits in a similar way to the Illusory Rabbit Trains, differences observed between their mislocalization rates between these trains indicate that the CRE can be induced across the fingertips. These results provide the first evidence that the CRE can 'jump' when the stimuli occur across non-continuous skin areas.

Spatial localization of electrotactile stimuli on the fingertip in humans

This study was designed to determine the extent to which sensations elicited by discrete electrotactile stimulation can be spatially localized, with a qualitative comparison to mechanical stimulation, in a 2 × 2 electrode array on the fingertip. Electrotactile stimulation was delivered in two modes: (1) same current to all locations (constant) or (2) current adjusted to perceptual threshold of each location (varied). For each stimulus location, subjects were asked to identify the location of the stimulus. Mechanical stimulation of the same locations on the fingerpad was delivered through von Frey hairs (0.07, 0.2 and 0.4 g). The percentage of accurate responses was computed for all stimulation modes. We found that the accuracy of discrimination of stimulus location in both the constant (46%%) and varied (40%%) electrotactile stimulation modes was significantly higher than chance level (25%%; p < 0.01). Furthermore, subjects were significantly more accurate in discriminating electrotactile stimuli in the constant than in the varied mode (p < 0.05). We also found that the accuracy of spatial discrimination was dependent on stimulation site for mechanical, but not electrotactile stimulation. Finally, we found a significant difference in accuracy over the duration of the experiment only for mechanical modes, which may indicate that electrotactile stimuli are less biased over time. These results suggest that, although low in accuracy, human subjects are able to extract spatial information from electrotactile stimuli. Further research is needed to optimize the amount of the information that can be delivered through electrotactile stimulation.

Temporal and Spatio-Temporal Attention to Tactile Stimuli in Extinction Patients

The temporal deployment of attention to tactile stimuli delivered to the same or to a symmetrical position of the body was assessed in 6 Right-Brain Damaged (RBD) patients with left tactile extinction and 6 healthy controls. Two different tasks called Temporal (T) and Spatio-Temporal (ST) extinction were used. In the T task single or double electro tactile stimuli were delivered to the same point to the left or, in separate blocks, to the right index finger. Double stimuli were separated by different Stimulus Onset Asynchronies (SOAs). In the ST task, stimuli could be single (left or right) or double (left and right). Double stimuli were delivered to the index finger of both hands simultaneously or sequentially. In both tasks subjects were asked to report the number of the stimuli they perceived. In the ST task, subjects were also requested to report the stimuli location. Results show that in both tasks RBD patients' detection of left sided stimuli was significantly lower than of right sided stimuli detection, mainly at shortest SOAs. Moreover, detection of left sided stimuli was higher when two stimuli were delivered in sequence and in symmetrical body areas and in different sides of the space than when stimuli were delivered in sequence in the same body area. Results suggest that the interaction between spatial and temporal variables enhances the ability of tactile extinction patients to detect left sided stimuli.

Temporal Modulation of Spatial Tactile Extinction in Right-Brain-Damaged Patients

Unilateral and bilateral electrotactile stimuli were delivered to both hands of 11 right-brain-damaged (RBD) patients with left tactile extinction and 20 healthy subjects. Bimanual stimuli could be presented simultaneously or with varying stimulus onset asynchronies (SOAs). Subjects indicated their detection of unilateral or bilateral stimuli, their judgements of whether stimuli were simultaneous or successive, and, in the latter case, which side came first. In RBD patients, extinction was maximal with simultaneous presentations and decreased as SOA increased. With short SOAs, omissions of left-sided stimuli occurred with both right-side and left-side stimulus precedence, suggesting a forward and backward interference of the right stimulus on the processing of the left stimulus within a time window of at least 100 msec. In contrast, there was no interference of the left stimulus on the detection of the right stimulus. Unlike controls, extinction patients rarely expressed simultaneity judgements, but those that were produced tended to be veridical or nearly so, like in normal controls. Whereas controls expressed generally accurate judgements of right or left precedence, patients showed a bias toward a right precedence and a maximal uncertainty between left-first and right-first choices when the left stimulus had a lead between 100 and 200 msec. The results are consistent with the hypothesis that, as in other sensory modalities, tactile extinction is associated with an abnormal persistent bias of attention toward the ipsilesional side that delays the processing of contralesional stimuli. However, the finding that both extinction and explicit judgements of simultaneity tended to occur with simultaneous bilateral stimuli suggest the presence of some residual neural capacity to detect precise temporal coincidence.