Abstract
Recent studies suggested that eye movements are tightly linked to temporal expectations. In these studies, target’s temporal predictability was manipulated between blocks to be either completely predictable or unpredictable (varied randomly between a 4–5 options). Findings showed that, prior to target onset, oculomotor behavior was reduced in the predictable blocks relative to the unpredictable blocks. This oculomotor inhibition (OI) was interpreted as reflecting the formation of temporal expectations. However, it is still unknown whether the OI effect is a local or a global expectation effect: Does it reflect the global state of certainty during the predictable blocks? or the local trial-by-trial level of certainty regarding the time of specific targets? We examined this question by modulating the degree of certainty regarding the target time, in an orientation-discrimination task. Trials consisted of a cue, which was followed, after 1 or 2 seconds (“foreperiod”), by the target - a slightly-titled Gabor patch. The degree of certainty was manipulated between blocks by changing the distribution of foreperiods to: (1) Highly-certain (100% fixed foreperiod); (2) Partially-certain (one foreperiod in 80% of the trials and another in 20%); and (3) Highly-uncertain (20% for five different foreperiods). Local certainty was examined by comparing trials with 100%, 80% and 20% foreperiod-probabilities. Global certainty was examined by comparing targets of 20% foreperiod-probability of the partially-certain condition, with those of the highly-uncertain condition. Results showed that, for the 1s foreperiods, OI was enhanced by local certainty (100% > 80% > 20%). No effect of local certainty was found for the 2s foreperiods. However, for 2s foreperiods, OI effect was enhanced by global certainty. No similar effect was found with 1s foreperiods. We conclude that in different contexts OI reflects both local and global temporal certainty levels.