Individual variation of the dark-background-contingent upshift of gaze: effect of past habituation

We studied the shift of gaze direction induced by dark background in monkeys. Faced with large inter-individual variability, we asked how common the upshift is, and how the upshift size is distributed. Furthermore, we sought to reckon processes influencing the variability. Approaching these questions necessitates a large sample. Here we report data from 10 rhesus monkeys recorded in Tübingen, together with reported data from 4 cynomolgus monkeys studied in Rehovot. In all 14 monkeys, dark background induced upshift – but no systematic horizontal shift. The upshift might be thought of as a simple sensorimotor response; nevertheless, surprisingly, the monkeys’ previous experience appeared to have a decisive role in influencing the upshift’s size. All the monkeys were previously trained in tasks that involved vision and eye movements; by their previous training, the monkeys were naturally divided into two groups. Monkeys of the first, ‘bright-habituated’ group, previously trained in photopic, bright ambient-light conditions; monkeys of the second, ‘dark-habituated’ group previously trained mostly with isolated dots of light appearing in dim ambient lighting or in full darkness. The dark-habituated monkeys had a larger upshift than the bright-habituated: the groups were separated by a border-value such that 6/7 of the dark-habituated monkeys had upshift larger than the border, and 5/7 of the bright-habituated monkeys had upshift smaller than the border. Thus, the size of the dark-background-induced upshift largely reflects the extent to which a monkey is habituated to work in the dark. Though the upshift is reflex-like sensorimotor behavior, its amplitude largely reflects cumulative experience.