Scientists have discovered new insights into how the human brain makes perceptual judgements of the external world.
The study, published in PLOS One, explored how we perceive the size of objects around us. Led by Professor Tim Meese, of the school of optometry at Aston University, and Dr Daniel Baker, of the department of psychology at University of York, it reveals how our visual system exploits defocus blur to infer scale. It is important for fields such as robotics and computer vision.
To judge the size of objects from retinal images, our visual system needs to estimate the distance to the object. The retinal image contains cues, such as linear perspective. To derive absolute size, our brain needs to know about spatial scale. It takes account of defocus blur – similar to the blurry parts of an image outside the depth of focus of a camera – to achieve this.
The research team gave people pairs of photographs of full-scale railway scenes subjected to various artificial blur treatments, as well as small-scale models of railway scenes taken with a long exposure and small aperture to diminish defocus blur. The task was to detect which photograph in each pair was the real full-scale scene.
Participants were fooled and believed the small models to be the full-scale scenes when the artificial blur was appropriately oriented with the ground plane – the horizontal plane representing the ground on which the viewer is standing – in the full-scale scenes. Simple uniform bands of blur at the top and bottom of the photographs achieved almost equivalent miniaturisation effects to realistic gradients of blur.
Daniel Baker, senior lecturer in psychology at the University of York, said: “These findings demonstrate that our perception of size is not perfect and can be influenced by other properties of a scene. It also highlights the remarkable adaptability of the visual system.
“This might have relevance for understanding the computational principles underlying our perception of the world. For example, when judging the size and distance of hazards when driving.”
Tim Meese, professor of vision science at Aston University, said: “Our results indicate that human vision can exploit defocus blur to infer perceptual scale but that it does this crudely – more a heuristic than a metrical analysis.
“Overall, our findings provide new insights into the computational mechanisms used by the human brain in perceptual judgments about the relation between ourselves and the external world.”