Billock, Cunningham & Tsou (2008) 6 APPROVED FOR PUBLIC RELEASEfeedback on the accuracy of their response. If the observer correctly identified the reference image three times in a row, the difference between the two images' exponents Δβ was decreased. In contrast, Δβ was increased after each incorrect response. Each staircase continued for 8 reversals, with the mean of the last 6 reversals being used as a measure of the threshold. Two presentation conditions (Sequential and Simultaneous) were used. In the Simultaneous condition, the reference and comparison stimuli were presented side by side (1. 1 mm apart) for 2. 133 seconds. The location of the reference image (left versus right) was randomized across trials. In the Sequential condition, the two stimulus images were sequentially presented in the center of the screen for 2. 133 seconds each. The screen was blanked for 500 ms between the two images to prevent masking effects. The order of presentation of the reference and comparison images was randomized across trials. Combining two viewing distances with two presentation modes yielded four experimental conditions. Two observers were presented with the Near conditions first, and two with the Far conditions first. For all observers, the presentation style (simultaneous vs. sequential) alternated after each threshold. The order of presentation of the 10 exponents was randomized for each of the 4 conditions. Each threshold was measured 3 times, with the thresholds in all 4 conditions being completed once before being re-measured. This required approximately 20 hours of data collection per subject, which was generally completed in 2 one-hour sessions each day, over a two-week period. Figure 3. Group averages for all 4 conditions. Results and Analysis General Findings Discrimination thresholds (d ȕ) are generally in the range of 0. 05-0. 20 for ȕ values of 0. 4-2. 2 (see Figs. 3, 4). The discrimination function is not flat; it has higher (worse) discrimination thresholds for both low and high values of ȕ, and lower (better) discrimination thresholds for in-between