The first thing that can be said about vision is that we have two-dimensional images impressed on our retinas. Beyond this, we are hard-wired to perceive depth through stereoscopy and apparent size. But when we stand erect and look straight ahead, we see things arranged from low to high, from the earth to the sky -- or from floor to ceiling. When we look down at our feet we see things arranged on the ground or floor in what we call a plan view or map view, as pieces might be arranged on a chess board.
In the world around us, the horizontal and the vertical are profoundly present. Gravity determines the uprightness of trees and of our stance and controls the paths in which rocks and water falls. A puddle, lake or ocean are all flat and horizontal as gravity dictates. As for light and shadow, the primary light in the entire world's existence is the sun, starting on the horizon, moving to the zenith and then setting. It always comes from "above". We are hard-wired to interpret shadowed scenes in terms of light from above . The figure below easily illustrates this.
Psychologists have long appreciated humans' propensity to see and feel and organize things in the vertical and the horizontal. Roger Shepard in particular has written eloquently about these topics [1,2,3]. In the issue of Ref. 3, many take issue with his views, but on the whole I think they are fundamentally sound.
Space is well reflected in language, including the language of quantity as in saying that "the temperature is higher today". These topics are discussed in other essays on this site.
What does this have to do with diagrams? A great deal. Diagrams, in the main, reflect the two views of the world, the vertical and the horizontal. Data graphs are typically organized vertically, with large values at the top and smaller ones at the bottom. Cross-sectional views are typically sections that are cut vertically, showing a structure, "from the side". Cutting a cake with a knife is a common example from life. Maps are the ultimate horizontally organized diagrams. Scatter plots and other visualizations lie somewhere in between. All this suggests that well-designed and readily understandable diagrams will adhere to a clear vertical or horizontal organization.
This said, we are capable of dealing with rotated views, so that it is not unusual for bars in bar charts to be horizontally oriented. It is common practice in science when arranging values from left to right for the larger values to be toward the right. Descartes and Fermat arranged their coordinate systems in this manner and this organization has remained with us for the intervening 300 years. It would be interesting to know if ancient carvings reflecting the passage of events depicted time from left to right. It has been my thesis that laterality in humans is the primal determinant of such things. When creating an inscription with the right hand, one can drag the instrument easily to the right and in addition see the inscription up to that point. The journal Laterality (Taylor & Francis) may hold the answers to some of these puzzles. Archaelogical evidence has been used to investigate handedness of the early hominids , but directionality is another topic.
1. Shepard, R. N. (1984). Ecological constraints on internal representation: Resonant kinematics of perceiving, imagining, thinking, and dreaming. Psychological Review, 91, 417-447.
2. Shepard, R. N. (1994). Perceptual-cognitive universals as reflections of the world. Psychonomic Bulletin & Review, 1(1), 2-28.
3. Shepard, R. N. (2001). Perceptual-Cognitive Universals As Reflections Of The World. Behavioral and Brain Sciences, 24(4), 581-601.
4. Mamassian, P., Goutcher, R. (2001). Prior knowledge on the illumination position. Cognition, 81, B1-B9.
5. Pobiner, B. L. (1999). The use of stone tools to determine handedness in hominids. Current Anthropology, 40(1), 90-92.
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