Action, arousal, and subjective time.

Saccadic chronostasis refers to the subjective temporal lengthening of the first visual stimulus perceived after an eye movement. It has been quantified using a duration discrimination task. Most models of human duration discrimination hypothesise an internal clock. These models could explain chronostasis as a transient increase in internal clock speed due to arousal following a saccade, leading to temporal overestimation. Two experiments are described which addressed this hypothesis by parametrically varying the duration of the stimuli that are being judged. Changes in internal clock speed predict chronostasis effects proportional to stimulus duration. No evidence for proportionality was found. Two further experiments assessed the appropriateness of the control conditions employed. Results indicated that the chronostasis effect is constant across a wide range of stimulus durations and does not reflect the pattern of visual stimulation experienced during a saccade, suggesting that arousal is not critical. Instead, alternative processes, such as one affecting the onset of timing (i.e., the time of internal clock switch closure) are implicated. Further research is required to select between these alternatives.