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Fig. 8. Peak saccade probability occurs at a critical stimulus size regardless of the time course of stimulation. (A) The time of peak saccade probability (t_peak) for each stimulus was determined using a 5 Hz low-pass filter on the saccade probability histograms from Fig. 5 (not all shown here; black traces). The ratio l/v is a single metric determining the apparent size of a looming visual stimulus over time (red traces). (B) An iterative best fit to the constant-velocity (black circles), accelerating (dark red circle), and decelerating (dark blue circle) stimuli shows a strong linear relationship on this plot of t_peak versus l/v (regression coefficient r²=0.91). The lighter circles behind the accelerating and decelerating stimuli show how those points would move for different values of t_peak; the values for the other stimuli would shift only vertically because l/v is constant. The threshold stimulus size, _crit, is derived from the slope of the black best-fit line, and equals 62° in this case. , the delay between the time of the critical stimulus (t_crit) and t_peak, is the y-intercept of this line, and evaluates to 49 ms. The orange circles are the partial (horizontal, vertical and diagonal) stimuli, and the green circle is the concentric square stimulus. The broken line was fit to the three full, constant-velocity stimuli (black circles) only (r²=1.00). From this broken line, _crit=71° and =22 ms.

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