Peripheral vision and pattern recognition: a review - strasburger - main

Peripheral_Vision.doc
eccentricity; these are shown separately for on-center and off-center fields by the triangles and
plus signs in Figs. 11c (Fischer & May 1970, Fig. 2) (comparable results with a flatter increase
were obtained by Peichl & Wässle, 1979, Fig. 7, for cat Y cells: 0.8° @fovea; 2.3° @24°). In
modern writing (cf. Equation 7), a generalized version of Riccò’s law is thus
(
0
E2
Δ L / L)
⋅ A = c (1+
E / ) , (17)
with the same notation as before.
Receptive field sizes in the monkey and human are different from those in the cat. De
Monasterio and Gouras (1975, Fig. 16) describe the sizes of macaque retinal ganglion cells, of
which the broad-band cells are of interest here. Although their sizes vary widely, their variation
with eccentricity is quite regular on average (filled circles in Figure 12c). These cells could
represent a physiological substrate for mediating Riccò’s law as shown quantitatively by Oehler
(1985).
For that analysis, Oehler used the Westheimer paradigm as a psychophysical estimate of
receptive-field size. Since light energy of a homogeneous patch of light is proportional to its
area, the limit up to which threshold is proportional to area (i.e., up to which Riccò’s law applies)
provides an estimate of receptive field size. However, as seen in Figure 12b, the borders of the
spatial summation area are not well defined. To achieve a more precise estimate, Westheimer’s
paradigm interchanges the roles of the variables: The size of the stimulus, whose increment
threshold is sought, is kept constant, and the size of a background annulus is varied instead.
With increasing size of the latter, the threshold increases to a maximum and then decreases to
a plateau further out. This so-called Westheimer function (Westheimer, 1965, first described by
Crawford, 1940) is interpreted as showing that, as long as the annulus fits into the mean size of
a receptive field, the threshold increases from an increased adaptation level. With a larger
background, then, surrounding inhibitory areas slightly decrease the adaptation level.
Consequently, the diameter at which the function’s maximum is reached is taken as an estimate
of the (mean of the) inner, summating part of the receptive field. The beginning of the plateau
region is regarded as an, psychophysically obtained, estimate of the mean total receptive field
size including the inhibitory surround. Such estimates were called perceptive fields by Jung and
Spillmann (1970) (cf. also Spillmann, 1964).
Psychophysical data from the Westheimer paradigm had previously only been available for the
human, whereas receptive-field data existed only for cat and monkey. Oehler (1985) provided
the missing link, namely psychophysical data for the monkey which could then be compared
with neurophysiology. The open and filled circles in Figure 12c show the decisive result. The
open circles refer to Oehler’s perceptive-field sizes in the monkey and the filled circles depict de
Monasterio and Gouras’ receptive field sizes for the broad band cells. It is striking how well the
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