Article of the Month, August 2017 (Zohary's lab)

August 9, 2017


Size constancy following long-term visual deprivation

Authors: Andres et al. (Udi Zohary's lab)

Published in Current Biology, July 2017.


One key feature of visual perception is object constancy: The ability to perceive an object as it is, despite radical changes in its physical appearance due to variation in the level of illumination, shading, viewpoint, physical distance, etc. For example: we do not perceive an object as getting smaller when we move away from it, even though its projection on our retina (e.g. its retinal size) is becoming smaller. Apparently, our brain makes use of the pictorial information (such as perspective and other 3D cues) to derive the true size of the object. This capability is termed Size Constancy. But what if you've had very poor vision from birth. Are you able to acquire such an ability later in life?

We studied 23 Ethiopian children suffering from bilateral, early-onset cataract, who were surgically treated only years after birth. To study size constancy in the newly-sighted we placed two balls of various sizes in two distances (close and far) from the subjects and asked them to estimate which ball was bigger. If the visual system of the newly-sighted cannot take the distance of the object into account one would expect subjects to estimate the further ball as being smaller than the closer one, even if it’s actually physically bigger. This was clearly not the case in our newly-sighted group. Closer examination indicated that, although before surgery the patients were diagnosed as having a full, mature bilateral cataract, they nevertheless had some residual form vision, typically limited to very close range. We therefore suggest that the brain mechanisms supporting size constancy can develop even in patients with highly blurred vision and very poor shape recognition.  In hindsight, this may not be that surprising since a highly blurred retinal image is still inversely scaled with distance, and object size can be confirmed, at least for objects within hand range. 

Full article at: https://authors.elsevier.com/a/1VRE73QW8Rih43


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