Abstract: Estimating the time-to-collision (TTC) of approaching objects is crucial for organism survival. Researchers have proposed the constructivist approaches, the ecological optics theory and the tau hypothesis to explain how humans estimate TTC and which factors may affect the estimation. Recently, a few studies examined how the emotional content of stimuli impacts TTC estimation, by comparing TTC judgements between threatening and nonthreatening stimuli. Their findings suggest that natural threatening stimuli (e.g., images of snakes) lead to underestimation of TTC compared to natural nonthreatening stimuli (e.g., images of rabbits). However, other findings suggest that TTC underestimation of social threatening stimuli (e.g. pictures of angry faces) is smaller or absent. Underestimated TTC of threatening stimuli may be due to 1) a specific response to threatening stimuli, 2) high emotional arousal of threatening stimuli, and 3) a perceptual bias causing threatening stimuli to appear closer and move faster than typical. We suggest that future studies should (1) further investigate the reasons why TTC underestimation of social threatening stimuli is smaller or absent, (2) explore the autonomic physiological response patterns and neural correlates of TTC estimation of threatening stimuli, (3) examine TTC estimation of threatening stimuli in virtual reality (VR) environments, and (4) experimentally test the effects of individual differences (e.g., gender and personality traits) on TTC estimation.

Key words: time-to-collision estimation, threat-specific response, emotional arousal, psychological distance, speed perception