A direct comparison of visual discrimination of shape and size on a large range of aspect ratios

•A direct comparison of visual discrimination of shape and size on a large range of aspect ratio.•The size discrimination threshold decreases with an increase in the circularity of the ellipses.•Size discrimination thresholds tend to be lower than aspect ratio thresholds.•Effect of size on aspect ratio discrimination such that larger stimuli appear more circular.

Participants viewed pairs of ellipses differing in size and aspect ratio (short axis divided by long axis length). In separate experiments with identical stimuli participants were asked to indicate the larger or the more circular ellipse of the pair. First, the size discrimination thresholds decreased with an increase in the circularity of the ellipses. Second, size discrimination thresholds were lower than aspect ratio thresholds, except for the circle and more elongated ellipses where both were similar. Third, there was also an effect of size on aspect ratio discrimination such that larger stimuli appeared more circular.