Delayed mirror visual feedback presented using a novel mirror therapy system enhances cortical activation in healthy adults

Abstract

Background: Mirror visual feedback (MVF) generated in mirror therapy (MT) with a physical mirror promotes the recovery of hemiparetic limbs in patients with stroke, but is limited in that it cannot provide an asymmetric mode for bimanual coordination training. Here, we developed a novel MT system that can manipulate the MVF to resolve this issue. The aims of this pilot study were to examine the feasibility of delayed MVF on MT and to establish its effects on cortical activation in order to understand how it can be used for clinical applications in the future.

Methods: Three conditions (no MVF, MVF, and 2-s delayed MVF) presented via our digital MT system were evaluated for their time-course effects on cortical activity by event-related desynchronization (ERD) of mu rhythm electroencephalography (EEG) during button presses in 18 healthy adults. Phasic ERD areas, defined as the areas of the relative ERD curve that were below the reference level and within -2-0 s (P0), 0-2 s (P1), and 2-4 s (P2) of the button press, were used.

Results: The overall (P0 to P2) and phasic ERD areas were higher when MVF was provided compared to when MVF was not provided for all EEG channels (C3, Cz, and C4). Phasic ERD areas in the P2 phase only increased during the delayed-MVF condition. Significant enhancement of cortical activation in the mirror neuron system and an increase in attention to the unseen limb may play major roles in the response to MVF during MT. In comparison to the no MVF condition, the higher phasic ERD areas that were observed during the P1 phase in the delayed-MVF condition indicate that the image of the still hand may have enhanced the cortical activation that occurred in response to the button press.

Conclusions: This study is the first to achieve delayed MVF for upper-limb MT. Our approach confirms previous findings regarding the effects of MVF on cortical activation and contributes additional evidence supporting the use of this method in the future for upper-limb motor training in patients with stroke.

Fig. 1

Diagram of the digital mirror therapy system. The host personal computer (PC) (a) captures images of the active hand via a high definition (HD) webcam (b) placed above the movement area of the therapy table (c). The host PC sends the processed images (vertically mirrored) to the mirror area (an HD monitor) of the therapy table

Fig. 2

Processing of the event-related desynchronization (ERD) curve, overall ERD area, and phasic ERD areas (data from the Cz channel of one subject in the no-mirror visual feedback condition). a The processed surface electromyography (EMG) signal was used to define the start point of the press movement. b Filtered electroencephalography (EEG) epochs (n = 50) for the mu rhythm (8–12 Hz). c The filtered EEG epochs were squared and averaged to form the mu rhythm ERD signal. d The relative ERD power curve ( % of the reference level) was formed by smoothing the raw ERD signal (each grey dot over the 250-ms time window). e The overall ERD area was defined as the averaged area of the entire ERD curve below the reference level. We used the area to quantify the cortical activation in response to the button press. f According to the phasic ERD areas for the P0 (-2–0 s), P1 (0–2 s), and P2 (2–4 s) phases (yellow, red, and blue areas in (e), respectively), a time-course change in cortical activation can be observed

Fig. 3

Typical examples of the time-course event-related desynchronization (ERD) waveforms of the C3, Cz, and C4 channels for all three test conditions: no-mirror visual feedback (MVF) (a), MVF (b), and delayed-MVF (c) conditions. The red circles mark the points where the mu power amplitude decreased from or increased to the relative peak and was lower than the reference level in response to the button press or MVF. a ERD mainly occurred after the button press. b ERD occurred earlier and with significantly larger amplitude in response to MVF compared to that observed with no MVF. c ERD reappeared when delayed MVF was provided around 2 s after the button press

Fig. 4

Averaged overall event-related desynchronization (ERD) areas (n = 18) with error bars (standard error) for the three test conditions (no mirror visual feedback [MVF]: green; MVF: blue, and delayed MVF: red) for the C3, Cz, and C4 channels, respectively. *Overall ERD areas of this test condition are significantly higher than the areas of the previous test condition for the same EEG channel. The overall ERD areas of the delayed-MVF condition are obviously larger than the areas for the no-MVF condition for all channels (all P < 0.001, not shown in the figure)