EFFECTS OF STIMULUS WIDTH AND LENGTH
ON THE DETECTION THRESHOLDS
FOR II-nd ORDER GRATINGS
D. Mitov, Ts. Totev, K. Racheva, I. Hristov
Institute of Neurobiology, Bulgarian Academy of Sciencies
Acad. G. Bonchev str. block 23, 1330 Sofia, Bulgaria,
e-mail: mitov@bio.bas.bg
It is known that at the places of abrupt illumination changes in the visual field
contours are seen. This might be well illustrated by this slide, where the illumination
in the centre is abruptly increased in comparison with the illumination in the
periphery. This type of contours, determined by the illumination changes are also
known as I-st order contours.
However, humans are able to perceive contours not only at the places of abrupt
illumination changes, but also at the places of abrupt contrast changes. In this case,
there is a texture in the visual field (for instance a random visual noise) and the
contrast of this texture is abruptly changed at some places. The mean luminance is
kept constant over all of the visual field. This type of contours, determined by the
contrast changes, are known as II-nd order contours.
It is also known that both the I-st and the II-nd order stimuli are processed by the
same mechanisms at the first stages of the visual information processing. By definition,
second order modulations have to be carried out on a first order signal. The first-stage
filters “process” the carrier (dividing it into frequency bands), the rectifier demodulates the
II-nd order signal and the second-stage filters “detect” the II-nd order modulation while
rejecting the carrier signal.
In this work, we studied the spatial properties of second stage filters. To this aim, we
studied the effect of stimulus length and width on the detection of II-nd order Gabor
patterns.
METHODS
1. Procedure. Contrast thresholds of vertical II-nd order Gabor patterns were measured
as a function of their width, length and spatial frequency (SF), using 2AFC method.
2. Stimuli. The carrier was a binary noise, which SFs below 0.5 c/deg and higher than 8
c/deg were rejected by filters with bandwidth of 1.5 octaves. The peak-to-peak contrast of
the noise was 0.5. The Gabor patterns were 1.45 and 2.90 c/deg centered in fovea and
presented for 1000 ms. Depending on SF, the stimulus standard deviation range along
the width and the length varied from 0.35 to 5.6 wavelengths for the lower SF - 1.45 c/deg
and from 0.35 to 11.2 wavelengths for the higher SF - 2.9 c/deg.
3. Apparatus. The stimuli were generated using equipment of our own design controlled
by a computer. The stimuli were presented on the face of a black & white monitor
(phosphor P4) with a frame frequency of 60 Hz as the spatial resolution was 640 x 480
pxls. The mean luminance was 70 cd/m2 and it was not changed by stimulus onset and
offset. The viewing distance was 114 cm and at this distance the screen subtended 11.6 x
8.7 deg. Viewing was binocular with natural pupils.
The next four slides are examples of the stimuli used in this study.
II-nd order Gabor pattern with higher values of both σx and σy
II-nd order Gabor pattern with higher value of σx and smaller value of σy
II-nd order Gabor pattern with higher value of σy and smaller value of σx
II-nd order Gabor pattern with smaller values of both σx and σy
It was found that the
contrast sensitivity increased
as both the stimulus width
(σx) and the length (σy)
increased.
However,
the
effect of the stimulus length
on the contrast sensitivity was
slightly
stronger
in
comparison with the effect of
the stimulus width, especially
at higher SFs. Data obtained
with observer K.R.
Contrast
sensitivity
for
detection of II-nd order Gabor
patterns as a function of their
length (σy) and width (σx).
Data obtained with observer
I.H.
Contrast
sensitivity
for
detection of II-nd order Gabor
patterns as a function of their
length (σy) and width (σx).
Data obtained with observer
C.T.
Contrast
sensitivity
for
detection of I-st order Gabor
patterns as a function of their
length (σy) and width (σx).
Data obtained with observer
C.T. It might be seen that the
spatial summation properties
of the vision for the I-st and IInd order stimuli are similar, as
the contrast sensitivity for II-nd
order stimuli is lower.
Contrast
sensitivity
for
detection of II-nd order Gabor
patterns as a function of their
length (σy) – the left column,
and their width (σx) – the right
column, at different fixed
values of the other size – the
width
and
the
length,
respectively. Stimulus SF is
1.45 c/deg.
The same as the previous
slide but the stimulus SF is 2.9
c/deg. It might be seen on
these two slides that the
different curves presented on
each panel are parallel in most
of the cases. This allows
suggesting that the stimulus
length and the width are
independent
variables
affecting contrast sensitivity.
However, in some cases,
especially at small lengths and
widths, the separate curves
are not parallel which means
that these two variables are
not independent at all.
Contrast sensitivity as a
function of the stimulus length
(σy) for the two SFs used. The
length
is
expressed
in
absolute angular units (deg).
Stimulus width is a parameter
shown on each panel.
Contrast sensitivity as a
function of the stimulus width
(σx) for the two SFs used.
Stimulus length is a parameter
shown on each panel. The
contrast sensitivity versus length
(CSvL)
and
the
contrast
sensitivity versus width (CSvW)
functions, obtained at different
SFs, coincide at small stimulus
size and do not coincide at
greater size, when the length
and the width are expressed in
absolute angular units. In this
case, the higher is the SF, the
smaller are the corresponding
crucial values of the length and
the width, up to which these two
stimulus
variables
affect
substantially
the
threshold.
However, with the third observer
- I.H., the summation curves
obtained
at
different
SFs
coincide at all sizes studied.
Contrast sensitivity as a
function of the stimulus length
(σy) for the two SFs used. The
length is expressed as a number
of the wavelengths of the
corresponding
SF.
Stimulus
width is a parameter shown on
each panel.
Contrast sensitivity as a
function of the stimulus width
(σx) for the two SFs used. The
width is expressed as a number
of the wavelengths of the
corresponding
SF.
Stimulus
length is a parameter shown on
each panel. The contrast
sensitivity versus length (CSvL)
and the contrast sensitivity
versus width (CSvW) functions,
obtained at different SFs, seem
to be approximately parallel with
the same crucial values for the
observers
K.R.
and
C.T.,
especially at higher size. This is
not the case with observer I.H.
CONCLUSIONS
1. The stronger effect of the grating length than the effect of the width on the
contrast sensitivity allows suggesting that the underlying mechanisms should be
an array of slightly elongated receptive fields.
2. Comparison of summation curves obtained at the two SFs, used with observers
K.R. and C.T., gives reason to suggest that the length and the width of the
receptive fields tuned to different II-nd order SFs should be approximately the
same in relative spatial units (number of wavelengths). However, the data
obtained with one of the observers in this study – I.H., do not support such a
suggestion. It was found that the length and the width of the corresponding
receptive fields are rather the same in absolute angular than in relative units.
3. It should be noted that in experiments with I-st order Gabor patterns we also
found smaller (in relative units) zone of summation for the lowest SF used –
1.45 c/deg. Thus, additional experiments within wider SF-range and with a
greater number of observers would help to establish which metrics (the relative
one or the absolute one) is more proper to estimate the summation properties of
II-nd order underlying mechanisms.
4. The data obtained until now allow suggesting that mechanisms processing II-nd
order stimuli after nonlinear stage should have spatial properties similar to those
of the mechanisms processing I-st order stimuli before the nonlinear stage.