Behavior: Basic Psychology of the User - Foundations for Designing User-Centered Systems - page 99

Table 4.3 Perceptual depth cues

Size The larger of two otherwise identical objects appears to be closer than

the smaller one

Interposition If one object partially occludes a second object then the blocked object

is perceived to be behind and beyond the blocking object

Contrast, clarity and
brightness

Sharper and more distinct objects appear to be nearer, and duller objects
appear to be farther away

Shadow Shadows cast by an object provide some cues about the relative position

of objects

Texture As the apparent distance increases, the texture of a detailed surface

becomes less grainy

Motion parallax When moving one's head from side to side the objects one sees are

displaced at different rates

Stereoscopic depth Two images of the same object from slightly different angles are

presented separately to each eye. Perceived depth is induced through
the fusion of the two images. This is often used in virtual reality

Fig. 4.14 The time to count
objects as the number of
objects varies from one to
eight. Counts from one to
three are substantially faster
per item than for five and
greater (reprinted with
permission from Peterson and
Simon 2000 )

4.5.3 Subitizing

Figure 4.14 shows that the curve for counting objects presented visually in a psy-
chology experiment (not that appear one at a time like train cars or a haptic situ-
ation like counting coins in your pocket) has a bend on it at about between 3 and 4.
Up to three objects, you recognize the number effectively, with about 50 ms dif-
ference per object, above four and certainly at five, you have to count the objects, so
the time to respond increases by about 250-300 ms/object. The first part of the
curve is called subitizing, and is thought to be an effect of the architecture of the
perceptual system.
Given the effect of subitizing, if you are passing sets of information to a user, or
giving them things to count, understand that smaller numbers are much faster to
recognize and count than larger numbers. In a related effect, if you are counting