Rapid size scaling in visual search

In order to compute a representation of an object's size within a 3D scene, the visual system must scale retinal size by an estimate of the distance to the object. Evidence from size discrimination and visual search studies suggests that we have no access to the representation of retinal size when performing such tasks. In this study we investigate whether observers have early access to retinal size prior to scene size. Observer performance was assessed in a visual search task (requiring search within a 3D scene) in which processing was interrupted at a range of short presentation times. If observers have access to retinal size then we might expect to find a presentation time before which observers behave as if using retinal size and after which they behave as if using scene size. Observers searched for a larger or smaller target object within a group of objects viewed against a textured plane slanted at 0°or 60°. Stimuli were presented for 100, 200, 400 or 800 ms and immediately followed by a mask. We measured the effect of target location within a stimulus (near vs. far) on task performance and how this was influenced by the background slant. The results of experiments 1 and 2 suggest that background slant had a significant influence on performance at all presentation times consistent with the use of scene size and not retinal size. Experiment 3 shows that this finding cannot be explained by a 2D texture contrast effect. Experiment 4 indicates that contextual information learned across a block of trials could be an important factor in such visual search experiments. In spite of this finding, our results suggest that distance scaling may occur prior to 100 ms and we find no clear evidence for explicit access to a retinal representation of size. © 2008 Elsevier Ltd. All rights reserved.