Dj. Tolhurst et Y. Tadmor, BAND-LIMITED CONTRAST IN NATURAL IMAGES EXPLAINS THE DETECTABILITY OFCHANGES IN THE AMPLITUDE SPECTRA, Vision research, 37(23), 1997, pp. 3203-3215
The psychophysical task of discriminating changes in the slopes of the
amplitude spectra of complex images has been used in the past to test
,whether the human visual system might be optimised for coding the spa
tial structure in natural images (e.g. Knill et al., 1990; Tadmor & To
lhurst, 1994). We have reported that the dependency of these discrimin
ation thresholds on the reference slope has the same overall general f
orm, regardless of the particular digitised photographs that are used
for generating the stimuli, The actual discrimination thresholds, howe
ver, differ markedly in magnitude for stimuli that are derived from di
fferent digitised photographs, Here, we describe a model that aims at
explaining this diversity of threshold magnitudes: we suppose that the
observer is detecting small changes in image contrast estimated,withi
n limited spatial-frequency bands of about 1 octave bandwidth. This lo
cal-contrast analysis reveals that contrast changes in only one freque
ncy band are of comparable magnitudes to the changes that observers ne
ed for detecting differences in the Michelson contrast of simple sinus
oidal gratings, The success of this band-limited contrast model is fur
ther shown in experiments where the slopes of the amplitude spectra of
stimuli were changed only within restricted frequency bands, We show
that when the slope is changed outside the limited frequency band impl
icated by the contrast model, the observer's thresholds are greatly el
evated, Thresholds remain unchanged when slope changes are made within
the implicated band, We also find that the exact bandwidth of the con
trast operator is not critical, provided that it is in the range of ab
out 0.6-1.5, which is the characteristic bandwidth range of V1 neurons
. (C) 1997 Elsevier Science Ltd.