When time matters: generalization gradients in delay and trace conditioning procedures

Previous studies have documented that overshadowing weakens when a CS-US trace is inserted, and we have interpreted such effect in terms of differential generalization in trace versus delay procedures, but direct evidence in humans is lacking. Two online experiments investigated whether the weakening of cue-outcome temporal contiguity impacts generalization gradients. Participants experienced strong temporal contiguity (the offset of a cue coincides with the onset an outcome) or weak temporal contiguity (there is a temporal gap between the offset of a cue and the onset of an outcome) in a continuous predictive learning task. Initial training involved a double negative discrimination, in which there was one reinforced cue (aqua-color rectangle) and two non-reinforced cues (one greener and one bluer than the reinforced cue), and participants had to make a response whenever they predicted the outcome. After training, in the generalization test phase, outcome expectancy in presence of several stimuli along the full green-blue dimension was tested. Outcome expectancy was assessed with discrete ratings in Experiment 1 (n=252) and a behavioral dependent measure (button presses, similar to the training phase) in Experiment 2 (n=250). Regardless the type of measure, when participants experienced weak cue-outcome temporal contiguity showed a broader generalization response, although differences were more prominent in the green side of the color dimension. Furthermore, no significant differences emerged between two groups experiencing strong temporal contiguity but differing in cue duration (controlling stimulus onset asynchrony, SOA), suggesting that cue-outcome temporal contiguity plays a more critical role than cue duration in shaping generalization. Overall, this result advances our understanding of the role of temporal contiguity at a basic level, while opening the possibility to study it as a critical factor in more applied settings.