Attention facilitates three-dimensional shape from shading

The visual system utilises several monocular cues to perceive the three-dimensional environment. One critical cue is shading, whereby the three-dimensional shape of an object can be inferred by shading patterns in a process termed “shape from shading”. Shape from shading has been considered an early and pre-attentive process, however recent evidence has suggested that there exists a later stage of processing for shape from shading that requires top-down attention. Here, we investigated this proposal using event-related potentials to record brain activity with great temporal resolution, whilst participants passively and actively viewed two-dimensional or three-dimensional shaded stimuli. We found an early stage of shape from shading around 150-200 milliseconds post stimulus onset that occurred during passive viewing, demonstrating processing for shape from shading without guided attention to the stimuli. Further, we found evidence for a right hemispheric lateralization of this early processing only during active viewing, suggesting that guided attention facilitated right hemispheric processing for shape from shading. Additionally, we found a later stage of processing for shape from shading around 290-350 milliseconds post stimulus onset that occurred only when attention was guided to the stimuli. Therefore, this indicates that guided attention is necessary for later-stage processing related to shape from shading. Finally, differences in processing between convex and concave three-dimensional shape were observed at both early and later stages of processing, highlighting differences in the processing of specific three-dimensional shapes. This research presents evidence for two separate processes involved in three-dimensional shape from shading. Moreover, it suggests that guided attention facilitates early processing for shape from shading but is necessary for later-stage processing related to three-dimensional perception.