EFFECTS OF STIMULUS WIDTH AND LENGTHON THE DETECTION THRESHOLDS

FOR II-nd ORDER GRATINGS

D. Mitov, Ts. Totev, K. Racheva, I. HristovInstitute 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.

METHODS1. 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 II-nd 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.