Fig. 1.

Discrimination task and contraction bias. (A) Schematic of the task design. The trial begins with the lowering of the mechanical stimulation probe (probe down). The monkey reacted by placing its free hand on an immovable key (key down). The time interval between these two events is known as the reaction time (RT, underlined in red). After a variable period (1.5 to 3 s), the probe oscillated for 0.5 s at the base (f₁) frequency. A 3-s delay after the first stimulus constitutes the working memory (WM) period. The delay is followed by the presentation of the second stimulus (f₂, also the “comparison period”). The offset of f₂ signals the monkey to report his decision by pressing one of the two push buttons (PBs) to indicate whether the comparison frequency was higher or lower than the base frequency. After that, the animal was rewarded for correct discriminations, or received a few seconds of delay for incorrect discriminations. (B) Stimulus set composed of frequency pairs (f₁, f₂) used in the task. Frequency values for f₁ are on the x axis, while values for f₂ are on the y axis. The diagonal dashed line represents stimulus equivalence (f₁ = f₂). The box colors indicate the percentage of correct trials, while the numbers above or below each box indicate the absolute magnitude difference (Δf) for each class. Red is for the highest correct percentages, and blue is for the lowest. (C) The upper line (green) depicts the contraction bias effect, where the base stimulus is perceived as closer to the mean value of the first frequency (<f₁>). The solid, vertical black line marks this center value. Numbers label classes, with the diagonal line marking stimulus equivalence (black, dashed). The upper diagonal (f₁ < f₂) has bias benefit (pink squares) above the range center, and bias obstruction (purple squares) is below the range center. The lower diagonal (f₁ > f₂) has bias benefit below the range center (pink), and bias obstruction above the range center (purple). (D) The numbered classes (x axis) from C by performance percentage (% of correct responses), representing the color values used in B. Pink and purple circles represent classes with bias benefit and bias obstruction, respectively. Error bars (in black) represent SDs and were obtained by computing the performance 1,000 times, resampling with replacement from the original data. (E) Percentage of trials belonging to each class number for short-RT trials (RT < median) and long-RT trials (RT > median) (black and gray circles and lines, respectively). The orange line and crosses represent the percentage of trials in each class when all trials are considered. (F, Left) Percentage of correct trials, as a function of class number, for short and long RT (dark and light blue circles and lines, respectively). Error bars are computed as in D. (F, Right) Differences in the percentage of correct trials between long- and short-RT trials in easy and difficult classes (respectively, pink and purple circles and bars). (G) Mean RT in correct (blue) and error (red) trials. Error bars represent the SE.