Asymmetries of visual perception and processing
Our visual field, the area of perceptible stimuli around a point of central fixation, isn’t uniform: there are some obvious, and other more subtle, discrepancies in our capacity for detection and response, known in the psychophysical literature as asymmetries.
The most obvious asymmetry exists between our central and peripheral vision. This is most clearly demonstrated when reading text, which requires a high degree of visual acuity, the ability to make out fine details of an image. Looking at a word directly, you can see it clearly, but shift your eyes even slightly away from it, and you’ll hardly be able to read it. You can still tell that something’s there — and maybe even make out gross details like the color and font — but your ability to discern individual letters will be lost.
On the other hand, contrast sensitivity, our ability to detect whether a stimuli such as a flash of light is presented, is worse in our central vision. One common demonstration of this occurs in low-lit environments. If you look at a dim source of light directly, you’ll find that it’s more difficult to make it out, and that if you look just off to the side of it, you’re able to perceive the light more clearly.
Another well-documented asymmetry, this one much more subtle, exists along the horizontal and vertical midlines of our visual fields. Subjects in psychological studies demonstrate higher visual acuity and contrast sensitivity along the nasal half of the horizontal midline, while the same is true for the lower half of the vertical midline.
Some researchers believe that the vertical midline asymmetry also generalizes, so that there exists a definite lower-field advantage. Others, however, believe that this advantage is only arises due to the increased sensitivity of the lower half of the vertical midline, and that the lower-field advantage falls off as you move towards the periphery.
These asymmetries in visual perception and processing can be due to three different factors: the physiology of the retina, the wiring of the brain, or experience.
In the case of central versus peripheral visual, the asymmetry is due to both retinal physiology as well as the wiring of the brain. First, the density of photoreceptors decreases as a function of distance from the fovea, that area of the retina which provides for central vision. Additionally, more areas of the visual processing cortex are devoted to inputs from the fovea.
As for the horizontal and vertical median asymmetries, differential distributions of photoreceptors are also responsible. The density of ganglion cells, which are responsible for sending signals into the brain, is slightly higher in the upper hemiretina, which corresponds to the lower visual field. At the same time, there’s evidence that the lower visual field is preferentially represented in areas of higher visual processing.
We can speculate about the possible contribution of experience on the lower-field asymmetry. Reading is typically performed in the lower visual field, and may represent a common experience which makes us more likely to perform better in psychophysical tests.
There’s an interesting hypothesis regarding the lower-field advantage. Some researchers have postulated that the lower visual field has stronger connections with the dorsal visual processing stream, which is involved in integrating visual stimuli for planned actions such as reaching and grasping, while the upper visual field has stronger connections with the ventral visual processing stream, which is involved in object recognition. This may have developed because we typically use our lower visual field to interact with objects in peripersonal space, which refers to that area immediately around our body, while in the upper visual field we encounter objects in our extrapersonal space. This hypothesis is supported by the finding that visually-guided actions are performed more successfully in the lower visual hemifield than in the upper, although the contribution of experience may represent a confounding variable to these results.
Although it isn’t an asymmetry of visual perception or processing, but rather an asymmetry of oculomotor or eye movement mechanisms, another interesting asymmetry consists of the discrepancy between saccades directed into the upper visual field versus the lower visual field. Saccades are ballistic movements of the eyes, such as those we make when switching our gaze from one object to another, or when moving from word to word along a line of text. It has repeatedly been found that saccades directed into the upper visual field are associated with shorter latencies, meaning that they’re made quicker than those directed into the lower visual field.
Combining this experimental finding, which has been called the vertical visual field asymmetry, with the previous hypothesis regarding visual processing and peripersonal and extrapersonal space, we can tentatively suggest the following. During primate evolution, saccades oriented into the upward visual field were chiefly performed in order to shift attention from peripersonal to extrapersonal space, and this action in turn was primarily done to identify distance objects (as opposed to shifts in attention towards the peripersonal space, which contains objects which have already been identified). Therefore, these saccades may be associated with the ventral visual processing pathway, which is responsible for object recognition.
The heterogeneity of the visual field is just another example of the frailty of the notion of a world as such, and the striking dependence of our perceptual abilities — and limitations — in constructing our reality.
