Suppressed SSVEP strength in stroke patients with unilateral spatial neglect

• ¹ Meiji University, Department of Electronics and Bioinformatics, Japan
• ² Murata Hospital, Department of Rehabilitation, Japan

Introduction: Previous studies demonstrated that attention to the visual stimulus enhances the strength of the SSVEP signals in healthy adults, suggesting that the attention-related increase in the SSVEP response may represent the strength of attention oriented to the visual stimulus. We therefore compared the attention-related increase in the SSVEP response among healthy controls, stroke patients with unilateral spatial neglect (USN), and those without USN to better understand the mechanism between attention and primary visual processing in patients with USN. Methods:Ten healthy young adults and 14 stroke patients participated in the experiment. Four out of 14 stroke patients were with USN. EEG was measured from PO7, O1, O2, and PO8 based on the international 10% system. A fixation point was placed at the center position in front of the participants and a flicker stimulus (white LED, 6Hz) was placed on their right or left visual field (45 and 15 degrees in healthy controls and patients, respectively). First, participants watched the fixation point without paying attention to the flicker stimulus. EEG was measured for 30 s for 3 times with each visual field. Second, we conducted the same experiment again except we placed a red LED in the vicinity of the white LED, which blinked randomly 5 or 6 times during the measurement. They were instructed to report the number of the blink of the red LED at the end of the measurement, yielding spontaneous attention of participants to the specific visual field. Results and Discussion: Healthy controls and patients without USN reported correct number (accuracy: 96-100%) of the red LED blinking located at either visual hemifield. Patients with USN reported the number of blinking as correct as other participants (98%) when the red LED was presented to the right visual hemifield but they failed to count more than half number of blinking when it was presented to the left visual hemifield (46%). Wavelet-based time-frequency analysis showed that attention to the visual hemifield robustly enhanced SSVEP strength in the healthy controls. Although patients without USN showed enhanced SSVEP strength by paying attention, the increase in SSVEP response was is smaller than that of healthy controls. The increase of patients with USN in SSVEP response was further smaller than those in patients without USN. Patients with USN showed decreased SSVEP response in case of paying attention to their right side. These results suggest that patients were not able to pay attention than healthy controls. Also, patients with USN could not pay attention to both visual hemifields than those without USN. The pathology of USN even affects the primary visual response of attention-oriented change in SSVEP. Further research would be conducted to study whether the recovery of USN is associated with the recovered strength of SSVEP response in stroke patients.