Figure 6.

The height of the blue histogram indicates the number of target + flanker combinations (from a sample of 128) that induced a particular level of performance (x-axis). Results are based on the pooled responses of two observers each conducting 20 identical runs. The heavy red line shows the expected number of stimuli that should elicit given levels of proportion correct, if agreement across runs is based solely on binomial probability. The red shaded regions show the 99.9% (i.e. 0.00005 & 0.00095) confidence interval on this estimate. The green shaded region indicate 15 stimuli that consistently produce performance lower than 45% across all 40 runs, an extraordinarily unlikely eventuality if crowding simply made subjects’ responses more variable. The two inset stimuli both contain targets generated to contain stimuli with an average clockwise tilt of 3.12°; subjects correctly judged the orientation of the target elements in the upper stimulus on 40/40 presentations, but in the lower stimulus on only 1/40 presentations (i.e. subjects consistently saw the target elements of the lower stimulus as being anti-clockwise of vertical).