Figure 4. Three observers’ results for both tasks.

Increasing wiggle from 0 to 60 deg makes letter identification (solid symbols) harder and phase discrimination (open symbols, including the crossed squares) easier. In particular, contrast threshold for letter identification (solid squares) increases with wiggle, while contrast threshold for phase discrimination (open and crossed squares) of a single gabor embedded among other gabors decreases with wiggle. Phase discrimination data are split (post hoc) into same-target-phase (open squares) and opposite-target-phase (crossed squares) trials. Phase affects both wiggle and threshold in a consistent way, so that data from both phases trace out the same curve. Phase discrimination threshold for an unflanked gabor (small open circle, on the far right of each graph) serves as a baseline for the flanked phase thresholds. Letter identification is done with a 10-letter alphabet (solid squares), but analyzing just the letter-I trials yields very similar contrast thresholds (small solid triangles), showing that the letter differences (“I” vs. others) do not affect the measured thresholds. Contrast thresholds for snake letters rotated 45 deg clockwise from vertical (small diamonds) are similar to those for upright letters (squares). Error bars are mean ± standard error of log thresholds. All points, except thresholds for letter identification in letter-I trials, have error bars, but the error bars are invisible when smaller than the symbol. The effect of wiggle up to 60 deg is monotonic. Beyond 60 deg, it varies among observers. In every observer, however, the effect of wiggle on each of the two tasks is a mirror image of the other. In the above plots, the data points in the 0 to 60 deg range of wiggle are left unconnected for unobstructed view of the quadratic fits (dashed). The data points beyond that range are connected by straight lines (solid).