Why Is the Size of an Object Unchanged Regardless of Changes in Viewing Distance?
Material below summarizes the article Computation of Object Size in Visual Cortical Area V4 as a Neural Basis for Size Constancy, published on August 26, 2015, in JNeurosci and authored by Shingo Tanaka and Ichiro Fujita.
Perceiving the sizes of visual objects appears to be a simple perceptual experience in which no big scientific conundrum exists. Contrary to this intuitive impression, size perception involves complex neural processes and is not determined solely by the size of object images projected onto the retina.
A notable example is that 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. For example, when a car is moving away from us, we perceive this car as getting gradually further away, not gradually shrinking in size, although the retinal image size of the car changes in a similar way between the two cases. This phenomenon, called size constancy, is accomplished by combining retinal image size and distance information in our brain.
The importance of distance information in size perception has been known for 2,000 years since the time of Ptolemaeus in ancient Rome. Yet, where and how the computation for size constancy is performed in the brain remains poorly understood.
Neurons in various visual cortical areas respond preferentially to a particular range of sizes of visual stimuli. As a stimulus becomes larger, their responses typically become stronger up to a peak, then decline and stabilize. It has been believed that the responses in many visual cortical areas are tuned to retinal image size of an object, rather than to the size of the object itself. However, the neurons that represent retinal image size are not sufficient to fulfill the size constancy. The visual system has to combine image size and distance information to calculate the object size.
If neurons encode the size of an object, their preference for retinal image size should systematically vary with the observer-to-object distance. They should prefer a larger image when an object is located at a nearer position and a smaller image when it is located at a more distant position.
We searched for such neurons by recording visual responses of single neurons in area V4, a mid-tier visual cortical area along the object vision pathway, of behaving macaque monkeys. We used binocular disparity embedded in random dot stereograms (RDSs) to manipulate the distance of a target object. This allowed us to change the size and the perceived distance of a target object without changing any monocular visual cues. Therefore, any effects on the neuronal response to the image size by changing binocular disparity can be taken as evidence for effects of changing (perceived) distance.
We found that most V4 neurons change their size tuning depending on the perceived distance of a visual stimulus. They prefer larger retinal image sizes when stimuli are stereoscopically presented nearer and smaller retinal image sizes when stimuli are presented farther away. This is exactly the property of neurons that express object size should have. These neurons can provide a possible mechanism for perceptual size constancy by transmitting information about the actual object size, not simply the retinal image size.
Computation of Object Size in Visual Cortical Area V4 as a Neural Basis for Size Constancy. Shingo Tanaka, Ichiro Fujita. The Journal of Neuroscience August 2015, 35(34): 12033-12046; DOI: 10.1523/JNEUROSCI.2665-14.2015