Is One Eye Better Than Two When Viewing Pictorial Art?
- Is One Eye Better Than Two When Viewing Pictorial Art?
During viewing of most objects in one's everyday environment, the binocular and monocular relative depth cues interact in a harmonious, concordant and reinforcing manner to provide perceptual stability. However, when one views pictorial art, these binocular and monocular cues are discordant, and thus a perceptual "cue conflict" arises. This acts to reduce the relative apparent perceived distance of objects in a painting, thus producing overall perceptual depth "flattening." The theory and physiology underlying this phenomenon are discussed.
You enter a prestigious art museum and immediately begin admiring paintings by the "masters"—the richness of colors, the perfection of form and detail, the balance of composition and other acknowledged attributes of such fine art. A few moments later, you casually rub one eye for a few seconds. While doing so, you continue to maintain your gaze with the fellow eye on a large landscape painting depicting an old country road leading to a distant village. To your amazement, the scene suddenly appears to "come to life," to "jump out" at you—its initial sense of three-dimensionality becomes markedly enhanced. Can this be true? Is one eye better than two when viewing pictorial art?
Practical Advantages Of Binocular Vision
The possession of two eyes rather than one has numerous distinct advantages. The most often cited advantage is stereopsis, which refers to the ability to detect exceedingly fine relative depth differences binocularly as compared to depth perception and discrimination obtained monocularly. Such judgments of relative depth have particular importance at near distances when performing fine manipulative tasks, such as threading a needle. We will get back to this point later, as it is critical to the overall discussion.
However, there are other important advantages to having two eyes and normal binocular vision. First, it increases the horizontal field-of-view [1]. We can see more in the periphery. This clearly is of benefit to such basic tasks as ambulation and motor vehicle operation. Second, it provides for a binocular sensory enhancement summation effect, which occurs independent of any changes in field size [2]—that is, the ability to detect an object is improved by about 40% in dark environments, such as under a moonlit sky in the wilderness. This clearly had evolutionary-based survival value to our cave-dwelling ancestors. Additionally, the ability to discriminate the fine detail of an object under normal daylight conditions is increased nearly 10%, which represents a noticeable improvement in our visual resolution capability. Third, eye-hand coordination is more accurate and dynamically time-optimal with the two eyes functioning in an integrated manner [3,4]. Fourth, the coordinated binocular inward and outward horizontal movements of the eyes used to track targets moving in depth (i.e. disparity or fusional vergence eye movements) also provide to the visual perceptual system relatively crude information about the relative depth of objects [5] (Fig. 1a). That is, the neurological control signal sent to the eye tracking system is also coded for distance; the larger the signal, the shorter the apparent object distance (Fig. 1b). Lastly, the presence of a second eye functions as a "spare eye" in case of serious injury to the fellow eye [6]. Again, this has obvious important survival aspects from an evolutionary point of view.
Binocular Cues
The most relevant of the aforementioned factors to our discussion are stereopsis and the depth-tracking eye movements. These are the primary binocular sources of visual information related to the relative depth of objects in our world.
Let us take a closer look at what is meant by stereopsis. This exceedingly fine depth-discrimination capability is based on the neurophysiology of horizontal retinal-image disparity, which refers to the depth-wise angular difference subtended at the eyes between the target that one is binocularly gazing upon and some other object in the visual field [7,8,9] (Fig. 2). Specific cells in the visual cortex of the brain then translate this spatial disparity information into relative depth. It is this basic principle of binocular vision that is exploited in such diverse devices as the parlor stereoscope, View MasterTM, 3D...
