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Description
Paper
Link: https://www.nature.com/articles/s41598-019-41158-5
Year: 2019
Summary
- findings provide a basis for determining stimulus parameters for neural engineering studies
- proposed experimental paradigm could also provide a precise framework for future SSVEP-related studies
Methods
- 46 flickers covering a visual field over an angle of 11.5° were successively delivered to evoke SSVEP responses in a random sequence
- periodic flicker (a white circle) whose appearance and disappearance alternated on a black background
- diameter of the flicker was set to a visual angle of 3° (3.14 cm)
- distance between the center of the two adjacent layers corresponded to a visual angle of 2°, and all the flickers covered a visual field with an angle of 11.5°
- gaze points of the participants were displayed as a white cross in real time, and a green ring with a radius of 1.5° around the center of the monitor was employed to restrict the gaze points. trials were removed if any of the gaze points were beyond the green ring
- EEG data were collected at a sampling rate of 500 Hz via a BrainAmp DC amplifier
- 31 active electrodes were placed according to the extended international 10–20 system standard, grounded to FCz and referenced to TP10
- Gaze data were recorded at a sampling rate of 60 Hz via an Eye-Trac6 eye tracker (ASL, USA). The eye tracker could track gaze over a vertical visual angle of approximately 30–35° and a horizontal visual angle of approxi-mately 40–45°. Moreover, a nine-point calibration procedure was performed before data collection.
- seated 60 cm in front of a 19 inch LED monitor with a refresh rate of 60 Hz
Results
Distribution of ssVep responses in the visual field
- average SSVEP response presented circular areas and exhibited a downward trend with increasing distance from the central visual field
- average SSVEP response declined dramatically within the first three layers (0–4°) but declined slowly within the last three layers (6–10°)
- evaluate the competitive effect between central and peripheral stimuli, percentage of SSVEP responses to the peripheral stimuli (stimuli in layers 2–6) that were higher than SSVEP responses to the central stimulus, which was defined as the error rate. the average error rate was 11.55% (p> 0.001) at 2° (layer 2), decreased to 5.30% (p< 0.001) at 4° (layer 3), and ultimately reached 2.08% (p< 0.001) at 6°
Effects of stimulus position on SSVEP responses
- occipital region had the highest contribution to SSVEP detection compared with other brain regions
- stimuli that were closer to the visual field center evoked stronger SSVEP responses, and the stimuli below the horizontal midline of the visual field evoked stronger SSVEP responses than above the horizontal midline of the visual field
- found the largest SSVEPs were recorded from the occipital scalp sites, contralateral to the stimulus position in the horizontal direction, and these SSVEPs remained generally positive for all stimulus positions
Stimulus shape
- circular stimulus may be more efficient for evoking SSVEPs than an equal-area stimulus with other shapes
- experimental results revealed that circular stimuli outperformed square stimuli in four out of the five stimulus frequencies
Spatial Distance
- multiple stimuli delivered simultaneously to the visual field will influence the performance of SSVEP-BCI systems due to the limited visual neural resources
- there may be no obvious difference between the SSVEP responses of the peripheral stimuli and that of central stimulus, as well as the EEG background activities can also lead the identification errors. Therefore, the spatial distance between the central stimulus (attended stimulus) and the peripheral stimulus (competing stimulus) becomes an important factor in SSVEP-BCI designs
- the average SSVEP responses decreased dramatically within 4° but decreased less rapidly between 6°-and 10°
- conclude that the center-to-center spatial distance between visual stimuli of SSVEP-BCIs should be more than 4° to avoid significant stimulus competition
- superior SSVEP-BCI accuracy could be attained when stimuli are placed spatially more than 6°
- to achieve the tradeoff between the number of items filled in certain area and detection accuracy, the optimal center-to-center spatial distance between visual stimuli should be within 4–6°
Stimulus position
- characters on the visual stimuli should be positioned slightly higher to increase the stimulus area below the character. Therefore, when the participants automatically gaze on the characters, a stronger SSVEP response could be evoked.
