Abstract
A key challenge for the visual system is to rapidly process the continuous stream of incoming information, segregating separate events over time. Visual temporal acuity has been measured using the two-flash fusion (2FF) paradigm, in which the ability to correctly report the presence of both flashes depends on the interstimulus interval (ISI). When the ISI is near threshold, participants sometimes report seeing two flashes but often only see one flash. Several studies have linked this fluctuation to the momentary brain state, as measured by EEG. In a new series of online (N= 97; N= 80) and laboratory (N=80; N=50) experiments, we have measured 2FF thresholds as a function of both the “state” and “trait” of the participant. Within participants, we found that 2FF performance was predicted by resting state EEG measures, and also by changes in emotional arousal (but not valence) both before and after a happy/sad/anxious/calm mood induction procedure. Performance varied along the circadian rhythm (assessed with the MEQ). Between participants, average 2FF thresholds varied by age. Differences in state-anxiety (assessed with the STAI) and depression (BDI) scores also played a role in processing speed thresholds. These results suggest that visual temporal acuity, as measured by 2FF, fluctuates between fast visual segregation and slower, more integrative processing. Such fluctuations might depend on the efficiency of the visual system in rapidly responding to sensory input, linked to alterations in arousal, or to a strategic need to balance between temporal integration and segmentation. These results point to the usefulness of the 2FF task to measure temporal acuity/resolution (as compared to, for example, flicker fusion), in order to track reactive changes in visual temporal acuity and to estimate state and trait changes in neural speed of processing.