The gap between rod and frame influences the rod-and-frame effect with small and large inducing displays

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Abstract
The role of the spatial separation between the ends of a rod and a frame’s inner edge (gap) in modulating the rod-and-frame effect (RFE) has been studied here with frames subtending either large or small retinal angles. With a large frame, rod settings were always in the direction of frame tilt (direct effects) and varied inversely with gap size. With a small frame, rod settings were in the direction of frame tilt for tilts between 7.5° and 15°; with larger frame tilts, rod settings in the direction opposite that of the frame were observed (indirect effects). Increasing gap size produced a tendency toward negativity (away from frame tilt). Consequently, direct effects were larger for small gaps, while the opposite was true for indirect effects. Overall, these results point to the importance of gap size in modulating the RFE, for both large and small displays.