MIT Encyclopedia of the Cognitive Sciences - Cryptome

472 Limbic System
Figure 2. Lightness could not be based on local edge ratios unless
the edges were first classified. (After Adelson 1993.)
account of our perception of the illumination, not just of
surface lightness.
3. Computing absolute or specific shades of gray
requires an anchoring rule, a rule that ties some locus on the
scale of perceived grays to some feature of the retinal
image. One candidate rule, endorsed by Wallach (1948) and
by Land and McCann (1971), says that the highest luminance
is white, with lower luminances scaled relative to this
standard. An alternative rule, implicit in concepts like the
gray world assumption and Helson’s adaptation-level theory
(1964), says that the average luminance is middle gray, with
higher and lower values scaled relative to the average. When
these rules are tested by presenting a display consisting of a
very restricted range of grays, it is found that the highest
luminance appears white, but the average does not appear
middle gray (Cataliotti and Gilchrist 1995; Li and Gilchrist
in press). But relative area also plays an important role. An
increase in the area of the darker regions at the expense of
the lighter causes the darker regions to lighten in gray value
and the lightest region to appear self-luminous.
These rules of anchoring by highest luminance and relative
area apply to both simple visual displays and to frameworks
or groups embedded within complex images. In
complex images, however, perceived lightness can be predicted
by a compromise between lightness values computed
within these relatively local frameworks and lightness values
computed across the entire visual field. The weighting
in this compromise increasingly shifts to the local framework
as that becomes larger and more highly articulated. If
the study of anchoring has undermined the portrait of a
highly ratiomorphic lightness computation recovering veridical
values of reflectance and illumination, it has nevertheless
provided a remarkable account of perceptual errors.
Emerging anchoring models portray a more rough-andready
system (see MID-LEVEL VISION) that, while subject to
apparently unnecessary errors, is nevertheless quite robust
in the face of a wide variety of challenges to perceptual veridicality.
See also COLOR VISION; DEPTH PERCEPTION; GESTALT
PERCEPTION; STEREO AND MOTION PERCEPTION; TEXTURE;
TRANSPARENCY
—Alan Gilchrist
References
Adelson, E. (1993). Perceptual organization and the judgment of
brightness. Science 262: 2042–2044.
Arend, L. E., J. N. Buehler, and G. R. Lockhead. (1971). Difference
information in brightness perception. Perception and Psychophysics
9: 367–370.
Bergström, S. S. (1977). Common and relative components of
reflected light as information about the illumination, colour,
and three-dimensional form of objects. Scandinavian Journal
of Psychology 18: 180–186.
Cataliotti, J., and A. L. Gilchrist. (1995). Local and global processes
in lightness perception. Perception and Psychophysics
57(2): 125–135.
Cornsweet, T. N. (1970). Visual Perception. New York: Academic
Press.
Gilchrist, A. (1979). The perception of surface blacks and whites.
Scientific American 240: 112–123.
Gilchrist, A. (1988). Lightness contrast and failures of constancy: a
common explanation. Perception and Psychophysics 43(5):
415–424.
Helmholtz, H. von. (1866/1924). Helmholtz’s Treatise on Physiological
Optics. New York: Optical Society of America.
Helson, H. (1964). Adaptation-Level Theory. New York: Harper
and Row.
Hurvich, L., and D. Jameson. (1966). The Perception of Brightness
and Darkness. Boston: Allyn and Bacon.
Land, E. H., and J. J. McCann. (1971). Lightness and retinex theory.
Journal of the Optical Society of America 61: 1–11.
Li, X., and A. Gilchrist. (Forthcoming). Relative area and relative
luminance combine to anchor surface lightness values. Perception
and Psychophysics.
Wallach, H. (1948). Brightness constancy and the nature of achromatic
colors. Journal of Experimental Psychology 38: 310–324.
Whittle, P., and P. D. C. Challands. (1969). The effect of background
luminance on the brightness of flashes. Vision Research
9: 1095–1110.
Further Readings
Gilchrist, A., Ed. (1994). Lightness, Brightness, and Transparency.
Hillsdale, NJ: Erlbaum.
Gilchrist, A., C. Kossyfidis, F. Bonato, T. Agostini, J. Cataliotti, X.
Li, B. Spehar, V. Annan, and E. Economou. (Forthcoming). An
anchoring theory of lightness perception. Psychological
Review.
Hurlbert, A. (1986). Formal connections between lightness algorithms.
Journal of the Optical Society of America A. Optics and
Image Science 3: 1684–1693.
Koffka, K. (1935). Principles of Gestalt Psychology. New York:
Harcourt, Brace, and World, pp. 240–264.
MacLeod, R. B. (1932). An experimental investigation of brightness
constancy. Archives of Psychology 135: 5–102.
Wallach, H. (1976). On Perception. New York: Quadrangle/The
New York Times Book Co.
Limbic System
Much as other systems with a historic origin (e.g., the reticular
system), the limbic system (LS) is difficult to define as
it has gone through numerous modifications, adaptations,
refinements, and expansions during the more than 100 years
of its existence. Furthermore, problems with its description
arise from the facts that it is frequently composed of only