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Objective. We used EEG to investigate cortical oscillatory activities during the tactile discrimination task and characterize the correlation between the EEG features and subjects’ discrimination performance. Approach. Transcutaneous electrical nerve stimulation (TENS) was applied on two finger areas (thumb and index for healthy hands, thumb and index-projected areas for disabled hands) to evoke two kinds of tactile sensations (vibration and pressure) with three levels of intensities (low, medium and high). Four forearm amputees and thirteen able-bodied subjects were recruited to discriminate the specific intensity and area of the applied stimulation. We assessed the discrimination performance [discrimination accuracy rate (AR) and response time (RT)] to quantify the tactile acuity. During the stimulation, EEG signals were recorded and the evoked cortical oscillatory activities were analyzed. Linear regression analyses were performed between EEG features and tactile discrimination performance. Main results. Spectral analysis revealed that alpha ERD over somatosensory regions persisted the whole task period and was related to the sensory information processing. Alpha ERD over prefrontal regions was only found during the stimulation judgement period and might reflect advanced cognitive process. There was no linear correlation between prefrontal alpha ERD and tactile discrimination performance. While contralateral somatosensory alpha ERDs exhibited significantly negative correlations with ARs () and positive correlations with RTs (). Specifically, the fitting results of higher alpha band (10–13 Hz) were superior to lower alpha band (8–10 Hz). Significance. Alpha ERD over contralateral somatosensory cortex could be used as an objective index for the evaluation of tactile acuity and might have the potential to be applied in sensory rehabilitation for amputees.

Objective. The somatotopic interface (SI) and non-somatotopic interface (NI) are commonly used to provide non-invasive sensory feedback. Nevertheless, differences between SI and NI are rarely reported, and objective evaluations of the corresponding brain response are missing as well. Few studies have reported how to design the stimulation encoding based on the two interfaces. The objective of this study was to investigate the difference in sensory characteristics between SI and NI, and propose an optimal encoding method for non-invasive feedback interfaces. Approach. We recruited seven amputees and compared the tactile sensitivity to stimulated positions and intensities between SI (phantom finger area) and NI (upper arm) in a tactile discrimination task. Electroencephalography (EEG) evaluation task was subsequently conducted to objectively evaluate the stimulus-evoked brain response. Finally, the two kinds of tactile information (stimulated position and intensity) was applied to an object recognition task. Specifically, the object size was reflected by the prosthetic finger position through stimulated position encoding, and the object stiffness was reflected by the contact force of prosthetic fingers through stimulated intensity encoding. We compared the performance under four feedback conditions (combinations between two kinds of tactile information and two interfaces). Results. Behavioral results showed that NI was more sensitive to position information while SI was more sensitive to intensity information. EEG results were consistent with behavioral results, showing a higher sensitivity of sensory alpha ERD for NI in the position discrimination, while the trend was opposite in the intensity discrimination. The feedback encoding allowed amputees to discriminate the size and stiffness of nine objects with the best performance of 62% overall accuracy (84% for size discrimination, 71% for stiffness discrimination) when position and intensity information was delivered on the NI and SI, respectively. Signicance. Our results provided an instructive strategy for sensory feedback via non-invasive solutions.

Objective. The objective of this study is to propose an objective index to evaluate the difference of tactile acuity between the left and right hand based on steady-state somatosensory evoked potential (SSSEP). Approach. Two kinds of tactile sensations (vibration and pressure) with three levels of intensities (low/medium/high) were evoked on two finger areas of the left or right hand (thumb and index for healthy hands, thumb and index-projected areas for disabled hands) via transcutaneous electrical nerve stimulation (TENS). Three forearm amputees and 13 able-bodied subjects were recruited to discriminate the specific level and area of the applied stimulation. Electroencephalography was adopted to simultaneously record the somatosensory cortex response to TENS. We assessed the discrimination performance (discrimination accuracy rate (AR) and response time (RT)) to quantify the tactile acuity, while the evoked SSSEP was synchronously analyzed. Linear regression analyses were performed between the difference of SSSEP amplitudes and the difference of discrimination performance for the left and right hand stimulation. Main results. Frequency domain analysis revealed that SSSEP amplitude increased with the increase of the stimulation intensity. There were positive correlations between the difference of SSSEP amplitudes and the difference of ARs for the left and right hand stimulation in the sensations of vibration (R2  =  0.6389 for able-bodied subjects, R2  =  0.5328 for amputees) and pressure (R2  =  0.6102 for able-bodied subjects, R2  =  0.5452 for amputees), respectively. Significance The SSSEP amplitude could be used as an objective index to evaluate the difference of the tactile acuity between the left and right hand and has the potential to be applied in sensory rehabilitation for amputees or stroke patients.

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