Tactile Orientation Discrimination Research Articles

Less might be more: 1 mA but not 1.5 mA of tDCS improves tactile orientation discrimination

BackgroundTranscranial direct current stimulation (tDCS) is a frequently used brain stimulation method; however, studies on tactile perception using tDCS are inconsistent, which might be explained by the variations in endogenous and exogenous parameters that influence tDCS. ObjectivesWe aimed to investigate the effect of one of these endogenous parameters—the tDCS amplitude—on tactile perception. MethodsWe conducted this experiment on 28 undergraduates/graduates aged 18–36 years. In separate sessions, participants received 20 min of 1 mA or 1.5 mA current tDCS in a counterbalanced order. Half of the participants received anodal tDCS of the left SI coupled with cathodal tDCS of the right SI, and this montage was reversed for the other half. Pre- and post-tDCS tactile discrimination performance was assessed using the Grating Orientation Task (GOT). In this task, plastic domes with gratings of different widths cut into their surfaces are placed on the fingertip, and participants have to rate the orientation of the gratings. ResultsLinear modeling with amplitude, dome, and session as within factors and montage as between factors revealed the following: significant main effects of grating width, montage, and session and a marginally significant interaction effect of session and amplitude. Posthoc t-tests indicated that performance in GOT improved after 1 mA but not 1.5 mA tDCS independent of the montage pattern of the electrodes. ConclusionIncreasing the stimulation amplitude from 1 mA to 1.5 mA does not facilitate the tDCS effect on GOT performance. On the contrary, the effect seemed more robust for the lower-current amplitude.

Grating orientation task trial numbers for short- and long-term tactile discrimination learning

Perceptual learning generally improves with training intensity, but the number of training trials sufficient for transient and long-term improvement in tactile grating orientation task (GOT) discrimination has not been systematically studied. To define reliable trial numbers for tactile discrimination learning, we compared tactile orientation discrimination performance of the right-finger following 200 and 400 training trials. Fifty-one neurologically healthy subjects were recruited. Tactile spatial acuity for orientation (parallel or orthogonal to the long axis of the finger) across different grating frequencies was assessed before, immediately after, 30 min after, and 24 h after sessions consisting of 200 training trials (50/block × 4 blocks), 400 training trials (50/block × 8 blocks), or no training (sham control). Both the 200- and 400-trial training conditions reduced the grating orientation discrimination threshold at 24 h after training. In contrast, the control condition had no effect on the grating orientation discrimination threshold. There was a negative correlation between the baseline grating orientation discrimination threshold and training-induced change in threshold (improvement) following both 200 and 400 trials. Fewer GOT trials (200) substantially prolong tactile discrimination learning, presumably by promoting the consolidation of the underlying neuroplastic mechanisms. In this widely used perceptual learning paradigm, 200 and 400 training trials appear effective for inducing short-term and long-term perceptual memory.

Perceptual learning evidence for supramodal representation of stimulus orientation at a conceptual level

When stimulus inputs from different senses are integrated to form a coherent percept, inputs from a more precise sense are typically more dominant than those from a less precise sense. Furthermore, we hypothesized that some basic stimulus features, such as orientation, can be supramodal-represented at a conceptual level that is independent of the original modality precision. This hypothesis was tested with perceptual learning experiments. Specifically, participants practiced coarser tactile orientation discrimination, which initially had little impact on finer visual orientation discrimination (tactile vs. visual orientation thresholds = 3:1). However, if participants also practiced a functionally orthogonal visual contrast discrimination task in a double training design, their visual orientation performance was improved at both tactile-trained and untrained orientations, as much as through direct visual orientation training. The complete tactile-to-visual learning transfer is consistent with a conceptual supramodal representation of orientation unconstrained by original modality precision, likely through certain forms of input standardization. Moreover, this conceptual supramodal representation, when improved through perceptual learning in one sense, can in turn facilitate orientation discrimination in an untrained sense.

α-tACS over the somatosensory cortex enhances tactile spatial discrimination in healthy subjects with low alpha activity.

IntroductionSpontaneous oscillations in the somatosensory cortex, especially of the alpha (8 − 14 Hz) and gamma (60 − 80 Hz) frequencies, affect tactile perception; moreover, these oscillations can be selectively modulated by frequency‐matched transcranial alternating current stimulation (tACS) on the basis of ongoing oscillatory brain activity. To examine whether tACS can actually improve tactile perception via alpha and gamma modulation, we measured the effects of 10‐Hz and 70‐Hz tACS (α‐ and γ‐tACS) on the left somatosensory cortex on right‐finger tactile spatial orientation discrimination, and the associations between performance changes and individual alpha and gamma activities.MethodsFifteen neurologically healthy subjects were recruited into this study. Electroencephalography (EEG) was performed before the first day, to assess the normal alpha‐ and gamma‐activity levels. A grating orientation discrimination task was performed before and during 10‐Hz and 70‐Hz tACS.ResultsThe 10‐Hz tACS protocol decreased the grating orientation discrimination threshold, primarily in subjects with low alpha event‐related synchronization (ERS). In contrast, the 70‐Hz tACS had no effect on the grating orientation discrimination threshold.ConclusionsThis study showed that 10‐Hz tACS can improve tactile orientation discrimination in subjects with low alpha activity. Alpha‐frequency tACS may help identify the contributions of these oscillations to other neurophysiological and pathological processes.

Tactile angle discriminability improvement: roles of training time intervals and different types of training tasks.

Perceptual learning, which is not limited to sensory modalities such as vision and touch, emerges within a training session and between training sessions and is accompanied by the remodeling of neural connections in the cortex. However, limited knowledge exists regarding perceptual learning between training sessions. Although tactile studies have paid attention to between-session learning effects, there have been few studies asking fundamental questions regarding whether the time interval between training sessions affects tactile perceptual learning and generalization across tactile tasks. We investigated the effects of different training time intervals on the consecutive performance of a tactile angle discrimination (AD) task and a tactile orientation discrimination (OD) task training on tactile angle discriminability. The results indicated that in the short-interval training group, AD task performance significantly improved in the early stage of learning and nearly plateaued in the later stage, whereas in the long-interval training group, significant improvement was delayed and then also nearly plateaued in the later stage; additionally, improved OD task performance resulted in improved AD task performance. These findings suggest that training time interval affects the early stage of learning but not the later stage and that generalization occurs between different types of tactile tasks.NEW & NOTEWORTHY Perceptual learning, which constitutes important foundations of complicated cognitive processes, is learning better perception skills. We demonstrate that training time interval can affect the early stage of learning but not the later stage. Moreover, a tactile orientation discrimination training task can also improve tactile angle discrimination performance. These findings may expand the characteristics of between-session learning and help understand the mechanism of the generalization across tactile tasks.

Teaching visual orientation discrimination through tactile learning

PURPOSE: Our double training studies revealed that perceptual learning can generalize to not only new locations and orientations (Xiao et al., 2008; Zhang et al., 2010), but also new physical stimuli encoded at different cortical processing stages (Wang et al., 2016). These results suggest that perceptual learning involves concept learning that improves the abstract concept of a trained feature (e.g., the concept of orientation). Here we demonstrate that such a concept can be represented and improved across the sensory modalities. METHODS & RESULTS: In Experiment I, participants first practiced tactile orientation discrimination (Tac_ori) using their index fingers that were stimulated by 3D-printed gratings at 40o or 130o. Although training improved Tac_ori thresholds (44.90%, p = 0.036 CONCLUSIONS: These results suggest that stimulus orientation as a concept can be represented and improved across tactile and visual modalities. Tactile training of orientation discrimination thus can teach visual orientation discrimination.

Topographic generalization of tactile perceptual learning.

Perceptual learning can improve our sensory abilities. Understanding its underlying mechanisms, in particular, when perceptual learning generalizes, has become a focus of research and controversy. Specifically, there is little consensus regarding the extent to which tactile perceptual learning generalizes across fingers. We measured tactile orientation discrimination abilities on 4 fingers (index and middle fingers of both hands), using psychophysical measures, before and after 4 training sessions on 1 finger. Given the somatotopic organization of the hand representation in the somatosensory cortex, the topography of the cortical areas underlying tactile perceptual learning can be inferred from the pattern of generalization across fingers; only fingers sharing cortical representation with the trained finger ought to improve with it. Following training, performance improved not only for the trained finger but also for its adjacent and homologous fingers. Although these fingers were not exposed to training, they nevertheless demonstrated similar levels of learning as the trained finger. Conversely, the performance of the finger that was neither adjacent nor homologous to the trained finger was unaffected by training, despite the fact that our procedure was designed to enhance generalization, as described in recent visual perceptual learning research. This pattern of improved performance is compatible with previous reports of neuronal receptive fields (RFs) in the primary somatosensory cortex (SI) spanning adjacent and homologous digits. We conclude that perceptual learning rooted in low-level cortex can still generalize, and suggest potential applications for the neurorehabilitation of syndromes associated with maladaptive plasticity in SI.

Limited Persistence of Tactile Working Memory Resources During Delay- Dependent Grating Orientation Discrimination

Working memory is a system that actively holds information to make it available for further processing. Compared to the working memory associated with vision and audition, the human characteristics of tactile working memory are largely unknown. In this study, we collected behavioral evidence of delay-dependent tactile grating orientation discrimination in humans. Nine healthy subjects participated in the experiment. During each trial, two of seven tactile grating domes of either similar or different orientation were presented to the subject’s right index fingertip. The interstimulus delays used in the experiment were 1, 5, 10, 15, 20, 25 and 30 seconds. The subjects were asked to discriminate whether the two orientations were the same. We found that the performance of tactile orientation discrimination improved with larger differences between the presented stimulus orientations. We also found a same/different effect, in which the accuracy and reaction time (RT) resulting from the same-orientation discrimination condition differed from that of the differentorientation discrimination condition. Interestingly, the existence of delay dependence was found under the sameorientation discrimination condition, in which the performance was reduced as the delay time was increased. However, we did not find delay dependence in the different-orientation discrimination condition. These findings of non-identical tactile stimulus discrimination can be understood in the context of tactile memory processing. Keywords: Decision making, delay, passive touch, somatosensory, tactile orientation discrimination, working memory.

Seeing and feeling for self and other: Proprioceptive spatial location determines multisensory enhancement of touch

We have investigated the relation between visuo-tactile interactions and the self-other distinction. In the Visual Enhancement of Touch (VET) effect, non-informative vision of one’s own hand improves tactile spatial perception. Previous studies suggested that looking at another person’s hand could also enhance tactile perception, but did not systematically investigate the basis of this effect. In Experiment 1 we manipulated the spatial location where one’s own or another person’s hand was seen. Viewing one’s own hand enhanced tactile orientation discrimination relative to viewing a neutral object, but only when the visual image of the hand was spatially aligned with the actual location of the participant’s unseen hand, as signaled by proprioception. In contrast, viewing another person’s hand produced enhanced tactile perception irrespective of spatial location.In Experiment 2, we used a multisensory stimulation technique, known as Visual Remapping of Touch (VRT), to reduce the spatial misalignment between the visual and proprioceptive locations of the hand. Participants saw an image of their own hand being touched at the same time as the tactile stimulation, which reduces perceived misalignment. This spatial adaptation procedure caused the VET effect to return. Our results suggest that multisensory modulation of touch depends on a representation of one’s own body that is fundamentally spatial in nature. In contrast, representation of others is free from this spatial constraint.

The Involvement of Occipital Cortex in the Early Blind in Auditory and Tactile Duration Discrimination Tasks

Early blind participants outperform controls on several spatially oriented perceptual tasks such as sound localization and tactile orientation discrimination. Previous studies have suggested that the recruitment of occipital cortex in the blind is responsible for this improvement. For example, electroencephalographic studies showed an enlarged posterior negativity for the blind in these tasks compared to controls. In our study, the question was raised whether the early blind are also better at tasks in which the duration of auditory and tactile stimuli must be discriminated. The answer was affirmative. Our electroencephalographic data revealed an enlarged posterior negativity for the blind relative to controls. Source analyses showed comparable solutions in the case of auditory and tactile targets for the blind. These findings support the interpretation of these negativities in terms of a supramodal rather than a modality-specific process, although confirmation with more spatially sensitive methods seems necessary. We additionally examined whether the early blind are less affected by irrelevant tactile or auditory exogenous cues preceding auditory or tactile targets than controls. No differences in alerting and orienting effects of these cues were found between the blind and the controls. Together, our results support the view that major differences between early blind participants and sighted controls on auditory and tactile duration discrimination tasks relate to a late and likely supramodal process that takes place in occipital areas.

Persistence of visual–tactile enhancement in humans

We report two experiments in which non-informative vision of the finger enhanced tactile acuity on the fingertip. The right index finger was passively lifted to contact a grating. Twelve participants judged orientations of tactile gratings while viewing either the fingertip, or a neutral object presented via a mirror at the fingertip's location. In Expt. 1, tactile orientation discrimination for near-threshold gratings was improved when viewing the fingertip, compared to viewing the neutral object. Experiment 2 examined the temporal persistence of this effect, and found significant visual–tactile enhancement when a dark interval of up to 10 s intervened between viewing the finger and tactile stimulation. These results suggest that viewing the body modulates the neural circuitry of primary somatosensory cortex, outlasting visual inputs.