THE ability to estimate accurately the time interval until the juncture of an observer with an object moving with relative radial velocity towards the observer seems to be highly developed in humans and many animals. In many practical situations, of probable importance for survival, it is not the distance of the object from the observer which is of interest, nor is it the velocity of the object, but rather the ratio of the two, which gives the time taken by the object to reach the observer. The importance of this parameter has certainly been renewed in the age of the motor car. The popularity of ball games may be partly due to the exercise of this faculty otherwise dormant in a sedentary era.

• Visual Mechanism determining Flight Distance in Zebra Danios (Brachydanio rerio Pisces)

  • LAWRENCE M. DILL

  Nature New Biology (1972)

• Failure of Visual Estimation of Motion under Strobe

  • THOMAS A. CROFT

  Nature (1971)