Why is an object’s size perceived the same regardless of changes in distance?

The neural mechanism of size constancy clarified

Aug 27, 2015

A group of researchers at Osaka University found that neurons in the monkey visual cortical area V4*1, one of the areas in the visual cortex, calculate the size of an object based on information on its 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 (Figure 1) cannot be achieved by such cells alone.

Ichiro Fujita (Professor) and Shingo Tanaka (then student in the doctoral course) of the Graduate School of Frontier Biosciences, Osaka University, examined if there were cells that express 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 (in Figure 2) 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 expresses object size should have. This group's experiment 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.

This group’s achievements 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 results were published in the Journal of Neuroscience website on August 26, 2015.

* 1 Visual cortical area V4

There are more than 30 areas of different functions in the visual cortex in humans and monkeys. The visual cortical area V4 is one of them and deals with visual information used for recognizing the object such as shape, color, pattern, binocular disparity, and size.

Even when we view an object from different distances, so that the size of its projection onto the retina varies, we perceive its size to be relatively unchanged. In this perceptual phenomenon known as size constancy, the brain uses both distance and retinal image size to estimate the size of an object. Given that binocular disparity, the small positional difference between the retinal images in the two eyes, is a powerful visual cue for distance, we examined how it affects neuronal tuning to retinal image size in visual cortical area V4 of macaque monkeys. Depending on the imposed binocular disparity of a circular patch embedded in random dot stereograms, most neurons adjusted their preferred size in a manner consistent with size constancy. They preferred larger retinal image sizes when stimuli were stereoscopically presented nearer and preferred smaller retinal image sizes when stimuli were presented farther away. This disparity-dependent shift of preferred image size was not affected by the vergence angle, a cue for the fixation distance, suggesting that different V4 neurons compute object size for different fixation distances rather than that individual neurons adjust the shift based on vergence. This interpretation was supported by a simple circuit model which could simulate the shift of preferred image size without any information about the fixation distance. We suggest that a population of V4 neurons encodes the actual size of objects, rather than simply the size of their retinal images, and that these neurons thereby contribute to size constancy.

Significant statement
We perceive the size of an object to be relatively stable despite changes in the size of its retinal image that accompany changes in viewing distance. This phenomenon, called size constancy, is accomplished by combining retinal image size and distance information in our brain. We demonstrate that a large population of V4 neurons changes their size tuning depending on the perceived distance of a visual stimulus derived from binocular disparity. They prefer larger or smaller retinal image sizes when stimuli are stereoscopically presented nearer or farther away, respectively. This property makes V4 neurons suitable for encoding the actual size of objects, not simply the retinal image sizes, and providing a possible mechanism for perceptual size constancy.

Figure 1. A distance cue affects perception of the object size.

Figure 2. Visual cortical area V4 in humans (left) and monkeys (right)

To learn more about this research, please view the full research report entitled " Computation of object size in visual cortical area V4 as a neural basis for size constancy " at this page of the Journal of Neuroscience website.

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