A fast, efficient version of the EN paradigm, adapted for use with clinical populations, was used to assess local and global processing limits. In the EN tasks, participants judged whether a number of signal elements presented for a brief duration were, on average, drifting clockwise or anticlockwise of vertical-upward motion (motion task;
Fig. 1B), tilted to the left or right of vertical (orientation task;
Supplementary Fig. S2A), or smaller or larger than a reference (size task;
Supplementary Fig. S2B). The reference direction, orientation, and size were defined by the fixation guide itself, which consisted of a small white circle bisected by a vertical line (identical in all tasks).
Two independent staircases were randomly interleaved: a “zero noise” and a “high noise” condition (
Fig. 1C). In the zero noise condition, external noise was set to zero, and the staircase tracked the minimum orientation offset from vertical (orientation task), directional offset from vertical (motion task), or size offset from reference (size task) that could be reliably classified (
Supplementary Fig. S1B). In the high noise condition, the staircase tracked the maximum level of external noise that could be tolerated for a large (fixed) signal offset (
Supplementary Fig. S1C). In this condition, the signal level was fixed at ±22.5° for the orientation, ±45° for the motion, and ±0.5 octaves for the size task. These values were selected on the basis of previous studies and pilot data.
15,19,20 Both staircases terminated after 75 trials each. As per the coherence task, the staircases were under the control of QUEST and converged on 82% correct thresholds. For each participant and task a two-parameter EN function was fit to their data, providing estimates of internal noise (a measure of local processing) and sampling (global processing; see
Fig. 1C and
Supplementary Material). To accustom participants to the nature of the task, all test blocks were preceded by 15 practice trials. In addition, for a subset of observers (10 participants with migraine and 8 without), 15 catch trials were randomly interleaved into each EN paradigm. On each catch trial, the stimulus was presented at a large signal level in the absence of external noise (±22.5°, ±45°, and ±0.5 octaves for orientation, motion, and size tasks, respectively).