Figure 8. Experiment 4: compound conditioning.

(A) During acquisition, a tone and light were presented simultaneously (compound conditioned stimulus [CS]), serving as the imperative signal for the reaching movement. They were paired with a 15° clamp on all acquisition trials (white background). During the probe phase (gray background), no feedback was provided, and the CSs were presented either together (compound CS) or alone (single CS; tone or light). (B) Mean heading angle (N=22) as a function of trial number. Shaded region represents SEM. (C) Experimental results for the Δ heading angle (median ± SEM) during the probe phase, showing a positive relative change for the compound CS on the current trial n (filled bar) and a negative relative change for each of its elements (empty bars). Friedman test: χ²[2]=9.82, p=0.007. (D) Predictions from the Rescorla-Wagner model for trial-by-trial Δ heading angle during the probe phase. The two elements were assumed to have equal weight in the simulation. (E) Scatter plot showing the between-participant trade-off in terms of the associative strength of the two CSs (the dotted black line represents the unity line). (F) The Δ heading angle (mean ± SEM) after pooling the two single-CS conditions, and the results of a two-way repeated-measures ANOVA for the effects of the previous CS type (compound vs singleton, dark and light pink, respectively) and current CS (filled and empty bars). Thin lines represent individual participants.