Judgements of time to contact are affected by rate of appearance of visible texture

Abstract

More fine-grained texture becomes visible if the distance between an observer and an object or surface is reduced. This article illustrates with a schematic example that the inverse rate of the relative appearance of visible texture provides information about time to contact if the observer has a constant visual acuity and the texture has a certain scale-independent structure. An experiment is reported in which texture appearance was manipulated. Participants were asked to make forced-choice time-to-contact judgements. A small but significant effect indicates that the judgements were affected by the rate of appearance of the texture. It is concluded that observers use this type of information.

Keywords:

• Fractals
• Nesting principle
• Perception
• Tau
• Visual acuity

Acknowledgments

This material is based upon work supported by Project HUM2006–11603–C02–02 of the Spanish Ministry of Education and Science. We thank David Travieso, David Field, and two anonymous reviewers for helpful comments on earlier versions of this article.

Notes

¹ The curves shown in Figure 3 were calculated for an approaching disc, assuming this to be a reasonable approximation of the frontal part of the spheres used in the experiment. The calculations would have been more complicated for a sphere because for a sphere the line of sight cannot be assumed to be orthogonal to the surface and because fine-grained texture goes out of sight at the occluding edge of an approaching sphere. Otherwise the texture-appearance conditions were computed as in the experiment.

² The logistic curve is defined by y = 1/{1  +  exp[–(a + bx)]}, in which a is the intercept, and b the slope. We fitted the curves using a least squares method. Finite ranges of slopes and intercepts were used to fit the data of individuals. In several cases the best fits were observed for slopes and/or intercepts that lay at the extremes of these ranges. Even so, similar overall results were obtained with different ranges.

³ Let us mention that this interpretation is based on hypothetical internal constructs and hence is more representational than direct in a Gibsonian sense (Gibson, 1979/1986; cf. Calvo Garzón, 2008).