AsianScientist (Oct. 7, 2015) – A group of researchers at Osaka University have found that neurons in the monkey visual cortical area V4 are responsible for calculating the size of an object. Their research, published in Journal of Neuroscience, showed that V4 neurons use information such as retinal image size and the distance from the object.
The neural mechanism for the perceptual phenomenon in which size was perceived to be stable even if the distance from the object changed (known as size constancy) was unknown. Many neurons in the visual cortex change their activity according to the size of visual stimulus. It was believed that neurons responded to the size of the image formed on the eye (retinal image); however, size constancy cannot be achieved by such cells alone.
Ichiro Fujita and Shingo Tanaka, then-students in the doctoral course of Graduate School of Frontier Biosciences, Osaka University, examined if there were cells that encode for not the retinal image size, but the size of the object itself. The group analyzed neuron activity in the monkey’s visual cortical area V4 and found that cells in this area integrated information about retinal image size and the distance from the object to calculate the size of the object.
When an object is far away, its retinal image size becomes small, and when the object is near, its retinal image size becomes big. If neurons respond to the same retinal image size, even if the distance from the object changes, they are supposed to react to retinal image size. In contrast, if neurons convey information about object size, they are expected to react to the small retinal image when the object is far away and to react to the big retinal image when the object is near.
Fujita and Tanaka found that most cells in the visual cortical area V4 reacted well to small stimuli when the stimuli were presented far away and reacted to big stimuli when stimuli were near. This is the property of a cell that encodes for object size is expected to have.
These experiments have verified that cells in the visual cortical area V4 do not react to the retinal image size but to the size of the object. These cells are thought to contribute to constancy of the perceived size regardless of changes in distance by conveying certain information about the object’s size.
The researchers findings have clarified the neural basis of size constancy and are expected to be helpful in improving 3D image-recognition techniques and understanding causes of diseases manifesting size perception disorder.
The article can be found at: Tanaka et al. (2015) Computation of Object Size in Visual Cortical Area V4 as a Neural Basis for Size Constancy.
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Source: Osaka University.
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