The centre of gaze provides the highest visual detail. As stimuli move farther from where you are looking, spatial resolution and some forms of discrimination become more difficult.
Peripheral vision is still extremely important for detecting motion, broad scene changes and events outside your direct point of focus.
Keep your eyes on the central fixation marker. When the peripheral target appears, respond without deliberately moving your gaze to search for it first. If you chase every target with your eyes, the task becomes closer to a normal visual-search test.
Use a comfortable viewing distance and avoid making the browser window so large that targets fall outside your practical field of view.
Practice can make you better at a particular visual-attention task, especially as you learn where targets can appear and how to hold fixation. That does not mean a game expands the anatomical boundaries of your visual field.
For gaming, the practical benefit is learning to maintain attention near the centre while staying responsive to peripheral changes.
Sudden changes in peripheral vision, blind spots or other visual symptoms require appropriate medical assessment. A browser game cannot map visual fields with the calibration, controlled luminance, geometry and clinical interpretation used by eye-care professionals.
No. Keep your gaze on the centre if you want the task to challenge peripheral detection.
No. It is an entertainment/training task, not a clinical visual-field examination.
Peripheral detection becomes more demanding as targets move farther from fixation, and target size, contrast and attention also matter.