Vincent and Regan (1997) raised a legitimate concern about the omission of realistic texture in visual simulations, including both experimental research and training with flight simulators. They noted that, in the real world, the retinal image of texture obeys the same geometrical laws as the object as a whole. For example, the retinal image of the texture of an approaching object undergoes similar expansion to the retinal image of the object’s size. They therefore hypothesized that a mis-match in the rate of expansion (ROE) between the simulated object size and its texture may affect TTC judgements.
Figure 3.1 Illustrates the type of texture pattern used by Vincent and Regan (1997). It shows an example for two cases: i) ROE of texture is faster than ROE of object size, ii) ROE of texture is slower than ROE of object size. The ROE of the object size was consistent with a TTC of 2s in all stimuli. Vincent and Regan (1997) found that the ROE of the texture had a significant effect on TTC perception. When the ROE of the
Illustration of stimuli used in Vincent & Regan (1997). Shows example at Figure 3.1
start of presentation (t0) and end of presentation (tn) for two cases. i) ROE of texture
faster than ROE of object size, ii) ROE of texture slower than ROE of object size.
t0
texture was faster than that of the object size (figure 3.1.i), estimates were significantly shorter than 2s, if the texture ROE was slower than the object size ROE (fig 3.1.ii), estimates were significantly longer than 2s, and when the texture and object size had consistent ROEs, estimates were not significantly different from 2s. This supports their hypothesis that information available from the texture of approaching objects can have an effect on TTC judgement.
Further investigation of the influence of textural information on TTC perception comes from Harris & Giachritsis (2000). They noted that, in the real world, as an object approaches, the expansion of its texture yields at least two different sources of MID information; individual elements of the texture pattern expand, and the relative position of elements changes (i.e. they appear to diverge away from one another). Harris and Giachritsis (2000) stated that these potential cues to TTC are of two distinct types: i) fine-grained, consisting of transformations of individual elements, usually across a small spatial scale (see figure 3.2a), ii) coarse-grained, consisting of
Illustration of stimuli used in Harris & Giachritsis (2000). Fine-grain and Figure 3.2
course-grain information from MID simulation over time (t1, t2, t3). a) Fine-grain
information stimulus with looming of texture elements. b) Course-grain information from divergence of texture elements.
t3
t2
t1
b) Course-grain a) Fine-grain
changes in the relative position of elements, and usually across a large spatial scale (see figure 3.2b).
Vincent and Regan (1997) used stimuli in which the ROE of the texture was consistent with both looming of individual texture elements and the radial divergence of texture elements (see figures 3.1). Therefore, fine- and coarse-grained information were consistent in their stimuli (as in real-world situations). In order to isolate the potential effects of these variables on responses, Harris and Giachritsis (2000) employed stimuli similar to that in figure 3.2, a random-dot pattern, in which fine- and coarse- grained information could be manipulated independently (see figures 3.2a and 3.2b respectively).
They found that TTC estimates increased with the TTC indicated by coarse-grained information (i.e. the rate of element divergence). However, variation of fine-grained information (i.e. the rate of expansion of individual elements relative to the rate of positional divergence) had little influence on TTC judgements. To test whether these results were an effect of information type (e.g. element expansion and spatial divergence) or of spatial scale (e.g. fine- and coarse-grained), Harris and Giachritsis (2000) replaced individual texture elements with clusters of dots. These stimuli produced similar results to the previous tests, in that the coarse-grained divergence between clusters appeared to dominate responses, and the fine-grained information from the divergence of dots within each cluster had little effect on TTC judgements. This suggests that it is the spatial scale, and not the information type, that is responsible for these differences in responses between conditions.
Unfortunately, there was not a full comparison of TTC judgements made with expansion and divergence information on the larger spatial scale. It would be interesting to note whether a large expanding circle would lead to similar TTC estimates as a (similar size) pattern of radially diverging dots, which do not change in size. This would further our understanding of the contribution of relative positional information from texture. When presented with a single small (0.2º or 0.4º) cluster of dots, which diverged, but did not expand, over time, observers appeared to be able to make use of the divergence information to judge TTC. This Indicates that fine- grained information could be utilised when presented without conflicting coarse- grained information. Yet this was shown to be a relatively weak cue to TTC, yielding inaccurate and unreliable responses. The findings suggest that the visual system is capable of utilising fine-grained divergence information, but that this information may play only a small role in TTC estimation when placed in conflict with coarse- grained TTC information.
Both Vincent and Regan (1997) and Harris and Giachritsis (2000) concede that the influence of texture information on TTC perception is likely to be relatively small, depending on viewing conditions, in accordance with previous literature (eg. Regan & Beverley, 1980; Beverley & Regan, 1983). Although TTC information from the texture pattern of an object may have little overall contribution to everyday TTC perception in the real world, the above research demonstrates that the visual system is able to make use of this information to influence TTC judgements. Therefore, a full explanation of TTC perception should be able to account for the potential use of information from the retinal image of object texture.