Early research on time-to-contact (TTC) focused primarily on the monocular information provided by an approaching object's fractional rate of change of retinal image size (i.e. τ). More recently, it has been shown that binocular information provided by an approaching object's rate of change of retinal disparity is also used. Furthermore, it has been demonstrated that in some common everyday situations (e.g. catching a tumbling rugby ball) binocular information about TTC can compensate for the ineffectiveness of τ. In this chapter we discuss psychophysical studies on the use of binocular TTC information. We review how the physics of geometrical optics combines with the physiological dynamic properties of the mechanisms sensitive to changing-size and changing-disparity to differentially weight the effectiveness of monocular and binocular information about TTC in different viewing situations. Finally, we discuss anecdotal evidence that supports the idea that binocular information about TTC is important in everyday life for actions such as hitting and catching.
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