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Biomedical subjects

A V Popple

Publications and source records attributed to A V Popple.

6 recordsLinked to original sources
PubMed2000

Amblyopes see true alignment where normal observers see illusory tilt.

Amblyopia ("lazy eye") is an impairment in visual acuity resulting from abnormal neural development in the visual cortex. We tested the responses of ten amblyopic and six normal observers to illusions of perceived orientation in textures of Gabor patches: the "Fraser illusion," the "phase illusion," and a "tilted chain" illusion. The illusory tilt of the stimulus rows was matched by actual tilt in the opposite direction by using the method of constant stimuli. Amblyopes showed a significant increase in the Fraser illusion, a decrease in the phase illusion, and a reversal of the tilted chain illusion. Amblyopic performance could be simulated in normal observers by reducing the length of the rows. These results can be modeled by a theory which places the neural abnormality in amblyopia at the level of second stage grouping processes. Additionally, the illusions might be useful in the early diagnosis of amblyopia without the need for prior refractive correction.

Amblyopia↗
PubMed2000

A Fraser illusion without local cues?

In the well-known Fraser illusion, a line composed of tilted elements itself appears tilted. The standard explanation of this illusion has been that the global orientation of the line is in some way influenced by the local orientation of the elements. The illusion was recreated using a texture composed of collinear Gabor stimuli, which were vertical. There was no local tilt. The illusory tilt was produced by gradually shifting the phase of the successive Gabors along each line. Although the 2D Fourier transform of this global pattern peaks at off-vertical orientations, the local energy of the patches is predominantly vertical. How does the visual system nevertheless pick up this global information? This can be explained by elongated linear filters, or a phase-tuned second-stage mechanism. We examined the first theory using a stereoscopic demonstration. When lines of opposite tilt are presented in the two eyes, they combine binocularly to produce stereoscopic slant. We tested whether the illusory tilts in the phase-shifted Gabors texture give stereoscopic slant, when opposite tilts are presented to the two eyes. They do not. Instead, stereoscopic depth is dominated by the local phase-disparity of the individual patches. This indicates that the illusion is not present at the stage of linear filters, which are input to stereo, but must involve second-stage interactions or collators.

Cues↗
PubMed2000

A new illusion demonstrates long-range processing.

In the Fraser phase-illusion [Popple & Sagi. Vision Research, 40 (2000) 873-878] rows of Gabor patches appear tilted because successive elements are shifted in phase. We measured this bias in global orientation judgment, while varying the number of patches in each row, and their separation. We found that illusory tilt increases with the number of patches, at least up to ten patches for a separation of four carrier periods. This finding implies that the visual system is able to integrate information over large (>10 degrees ) strips of the central visual field. Our model shows that the tilt illusion might be the result of averaging the activity of oriented filters.

Humans↗
PubMed1999

'Coarse-to-fine' cyclopean processing.

Previously (Popple et al, 1998 Vision Research 38 319-326) we found, using random-dot stereograms, that initial vergence increases with the size of a cyclopean disc. A corresponding improvement in stereoacuity within the disc was predicted, because disparities in the disc would be brought closer to the plane of current fixation. In the present experiment, we looked at the effect of the spatial extent of a briefly presented (< or = 500 ms) cyclopean depth pedestal on stereoacuity thresholds. Observers were required to judge the depth of a small, 1.7 deg, central disc relative to a larger surrounding disc in a random-pattern stereogram. The larger disc was set, initially, at a pedestal disparity of +/- 24 min of arc against a fixation-plane surround. The size of the larger disc was varied from 2.6 to 8.0 deg. As predicted, stereoacuity thresholds fell significantly with increasing pedestal disc size. Next, the disparity of the pedestal disc was varied. When pedestal disparity was reduced to +/- 2.4 min of arc, a disparity too small to demand vergence, the size effect disappeared except when the pedestal boundary was within 30 min of arc of the test disc boundary. We argue from this result that the effect was largely due to vergence and not cyclopean integration alone. However, the effect of pedestal size was found to persist with stimuli too brief to permit vergence (< or = 100 ms) suggesting that factors other than vergence may also play a role.

Depth Perception↗
PubMed1998

The area of spatial integration for initial horizontal disparity vergence.

We investigated over what central area disparity in a random dot stereogram is integrated to stimulate an initial vergence response. Vergence was measured subjectively, with a forced choice dichoptic nonius vernier task following a brief (230 msec) stimulus presentation. Stimuli were random-dot stereograms showing a central circular disc of 12.5 min arc crossed retinal disparity in front of, and occluding, a same density fixation plane surround. The size of the disc was varied. All ten observers responded to the brief stimulus. Initial vergence increased with increasing disc diameter and, for nine out of ten subjects, reached a maximum with the disc ca 6 deg, suggesting this is the extent of the spatial integration region. Below 6 deg diameter, surround and target disparities were averaged together. Initial horizontal vergence responds automatically to a cyclopean target presented in the centre of gaze by pooling disparities within a limited but surprisingly large area.

Adult↗