Fig. 1. Experimental design. (A) Sample scene images used in the experiment. Natural scene images from continuously changing seven categories served as conditioned stimuli (CS+), unpaired conditioned stimuli (CS−), and generalization stimuli (GSs). The seven categories were sandy desert, gravel desert, vegetated desert, grassland, mountain, forest, and swamp. Participants were counterbalanced into two groups. For half of the participants (Group 1), scenes of the sandy desert category were paired with US, while scenes of the swamp category were paired with US for the other half (Group 2). (B) The experiment was conducted for two consecutive days. On the first day (Day 1), participants were introduced to the familiarization, pre-learning test, and learning sessions, and they performed post-learning test on the next day (Day 2). During the familiarization, scene images of all seven categories and US were presented in random order. During the learning, only scenes from the CS+ and CS− categories were presented, and half of the CS+ presentations were co-terminated with US. During the pre-learning or post-learning test, participants rated the subjective feeling or US-expectancy for each scene on a 1~5 scale while each scene image was presented. ITI=intertrial interval.
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