A total of 10 participants went through 2–12 conditioning sessions, each lasting approximately 20 min. Unlike in previous studies, our subjects went through multiple training sessions to ensure that any potential lack of conditioning would not be due to too little training. Here we used an eyetracker to test whether it is possible to condition the autonomic pupillary constriction response by pairing a tone (CS) and a light (US) with a 1s CS-US interval. Moreover, measuring the pupil size used to be cumbersome compared with today when an eyetracker can continuously measure pupil size non-invasively. The apparatus for recording pupil size, the type of stimuli used and the interval between the stimuli has varied in previous attempts-which may explain the inconsistent results. While a few early studies reported successful conditioning of pupillary constriction, later studies have failed to replicate this. Several attempts have been made to condition the pupillary constriction and dilation responses, with the earliest published attempts dating back to the beginning of the 20th century. By conditioning the pupillary response, one might avoid potential volition-related confounds. In contrast, the pupillary response is an autonomic response, not under voluntary control. But the fact that eyelids are under voluntary control means it is ultimately impossible to ascertain whether a blink response is ‘conditioned’ or a timed ‘voluntary’ blink response. Eyeblink conditioning is the most popular paradigm for studying classical conditioning in humans.
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