Detection of low spatial frequencies: a single filter or multiple filters?

Ever since it was first reported that the contrast sensitivity for low spatial frequency sinusoidal gratings falls off with a slope approximating unity it has been suggested that this may represent gradient detection by a single spatial filter tuned to higher spatial frequencies. We have further investigated this possibility using aperiodic as well as periodic stimuli. Experiments are presented that involve manipulations of luminance, field size and spatial adaptation, and the results are consistent with a multiple filter model accounting for the detection of low spatial frequency (0.2-2 c/deg) periodic and aperiodic stimuli.

The effect of minimal occlusion therapy on binocular visual functions in amblyopia.

The binocular visual functions of amblyopic children were studied during treatment involving brief weekly periods of occlusion of the unaffected eye while the child performed demanding visuomotor tasks against either a background of rotating gratings or a stationary uniform gray stimulus. The gains in stereoacuity were quite significant and in most cases more obvious than the rather small gains in letter visual acuity. On initial presentation only 21 of the 60 patients showed evidence of stereopsis and of these only seven possessed a stereoacuity of 100 secs or better. Following six treatment sessions the number of patients that demonstrated stereopsis increased to 36 of whom 17 possessed reasonably good stereoacuity (100 secs or better). However, there was no difference in the degree of improvement exhibited by those patients that viewed rotating grating patterns during treatment and others from the control group that viewed the uniform gray stimulus. Thus, there was no evidence that any of the visual gains were enhanced or promoted by active visual stimulation of the amblyopic eye with rotating gratings during the brief periods of occlusion of the unaffected eye. Finally, a comparison of the scores of the children on various stereo-tests suggest that tests comprised of small figure elements that are present in high density may be best for screening purposes. On the other hand, for quantifying the stereoacuity of children known to possess abnormal binocular vision it may be more appropriate to employ tests that use large figure elements that provide strong fusion cues.

Contrast thresholds for sine gratings of children with amblyopia.

Contrast threshold functions were measured on 26 amblyopic children before and after minimal occlusion therapy. On initial presentation the relative contrast sensitivity deficit the amblyopic eye was in every case much less than that predicted from the deficit in visual acuity for letters. In fact, in seven of the children the contrast sensitivity functions for the amblyopic and fellow nonamblyopic eye were indistinguishable despite the expression of substantial amblyopia on letter charts. Only four children exhibited a substantial contrast sensitive loss in the amblyopic eye with a cut-off spatial frequency below 30 cycles/degree. The majority of children who showed deficits in contrast sensitivity did so only at medium and high spatial frequencies. On the basis of these findings it appears that measurements of contrast thresholds for single sinusoidal gratings do not probe fully the deficits of spatial resolution in amblyopia. Finally, among the few children who exhibited a sizeable initial deficit, only two showed obvious improvement in contrast sensitivity in response to minimal occlusion therapy.

A comparison of threshold and suprathreshold appearance of gratings with components in the low and high spatial frequency range.

1. The appearance of square gratings with some of their Fourier components missing has been investigated for both threshold and suprathreshold contrasts.2. If high frequency components are removed from a square grating there is only a very small effect on the detection threshold, or suprathreshold appearance, unless the components are visible by themselves.3. If the fundamental frequency is removed from a square-wave grating which has a spatial frequency lower than 1 cycle per degree (c/d) the contrast sensitivity is not altered. This is a generalisation of the Craik-Cornsweet illusion. If the contrast is raised above the detection threshold the grating is indistinguishable from a square grating, unless the contrast is high enough to see the fundamental when it is presented alone.4. If the fundamental is removed from a square grating which has a spatial frequency higher than 1 c/d the contrast threshold and the appearance at all contrasts are changed. At threshold it appears as a sinusoidal grating of three times the fundamental frequency. The threshold is dictated solely by the amplitude of the third harmonic. If the contrast is further raised, so that the fifth harmonic also reaches threshold, the periodictiy of the fundamental is seen.5. Therefore, gratings of many different luminance profiles (including the Craik-Cornsweet profile) all produce the perception of a square grating simply because those missing components which would be required in each case to produce a perfect square are by themselves undetectable. The visual system responds as though hardwired to detect square gratings and edges by means of quasi-Fourier analysis.6. These results are analagous to the missing fundamental, or residue, effect in hearing.