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Biomedical subjects

J P Frisby

Publications and source records attributed to J P Frisby.

At least 19 recordsLinked to original sources
PubMed1990

Transparency and the uniqueness constraint in human and computer stereo vision.

The sensation of depth that is obtained with human binocular vision results from the differences in the projection of the world onto the two retinae. The process entails solving the problem of stereo correspondence, which involves choosing the correct matches between left and right image features. Many computational models of stereo vision assume a uniqueness constraint on stereo matching-that is, each feature identified in one image should eventually be matched with only one feature in the other image. This constraint would seem to be justified, as allowing non-unique matches would be tantamount to supposing that the scene entities to which matches relate are in two places at once. The value of the uniqueness constraint for eliminating false matches has been demonstrated in a variety of stereo algorithms. Yet on the basis of psychophysical results Weinshall concluded that it was not used by humans in dealing with certain types of ambiguous random-dot stereograms. We have now tested how Weinshall's stereograms are dealt with by PMF, a stereo algorithm which uses a unique-matches selection procedure in conjunction with a purely local similar-disparity support scheme. We found that PMF produces results that are closely analogous to the psychophysical results. This suggests that Weinshall's experiments should not be interpreted as evidence that the human stereo mechanism establishes non-unique matches.

Algorithms↗
PubMed1989

Integration of stereo and texture cues in the formation of discontinuities during three-dimensional surface interpolation.

A series of stereograms are presented which demonstrate that texture boundaries can strongly influence the perception of discontinuities between neighbouring three-dimensional (3-D) surfaces portrayed by means of stereo cues. In these demonstration figures, no stereo information is available in the immediate vicinity of the boundary between the two 3-D stereo surfaces because all texture in that region is removed in one eye's view. On the other hand, various forms of texture boundary information are provided in the resulting monocular region. This stimulus paradigm is used to explore the question: what influence does texture boundary information have on the nature of the perceived 3-D surface that is interpolated between two stimulus regions which carry stereo cues? It is shown that if a clear-cut texture boundary is present in the monocular region then this is used by the human visual system to fix the perceived location of 3-D crease and step surface discontinuities between the stereo regions. Collett (1985) explored this issue with a similar methodology and reported weak and unreliable assistance from monocular texture boundaries in helping shape 3-D stereo surface discontinuities. The strong and robust phenomena demonstrated here seem to rely on two main differences between the present stimuli and those of Collett. In the present stimuli, figurally continuous textures containing strong texture boundaries are used, together with a technique for minimising the complications, including binocular rivalry, that arise from the borders of the stimulus regions present in only one half of each stereogram.

Cues↗
PubMed1979

Does visual texture discrimination precede binocular fusion?

Various stereoscopic demonstrations are presented which indicate that visual texture discrimination is based on processes which occur after, or at the same time as, the binocular combination of images from the two eyes. Monocularly invisible texture regions can become apparent, and monocularly visible regions can be hidden, by the processes of binocular fusion.

Depth Perception↗
PubMed1979

Surfaces with steep variations in depth pose difficulties for orientationally tuned disparity filters.

Surfaces possessing steep variations in depth present severe difficulties for orientationally tuned filter models of stereopsis. These difficulties are discussed in connection with a random-dot stereogram depicting a surface with steep horizontal corrugations. As expected on theoretical grounds, we find that a vertical +/- 45 degrees orientationally filtered version of this stereogram cannot be fused. Moreover, it is demonstrated that a horizontal +/- 45 degrees filtered version can be fused only with difficulty and its stereo percept is poor compared to that of the unfiltered original. It is concluded that orientated filters seem ill-designed to mediate the extraction of disparity cues, at least in the cases under consideration.

Depth Perception↗
PubMed1978

Contrast sensitivity function for stereopsis.

Contrast thresholds for stereopsis from narrow-band-filtered random-dot stereograms were compared with contrast thresholds for simple detection of similar narrow-band noise. Centre frequencies of filters were in the range 2.5--15 cycles deg(-1). It was found that the contrast sensitivity function for stereopsis is similar in shape to that for detection, suggesting that as far as contrast requirements are concerned the mechanisms of global stereopsis do not show a bias in sensitivity to any particular spatial frequency but instead require a constant level of suprathreshold contrast regardless of spatial frequency.

Depth Perception↗
PubMed1978

Stereopsis masking in humans is not orientationally tuned.

A stereopsis signal carried by an oriented random texture and masked by a similar noise texture is not unmasked when the orientation of the noise is rotated. This result is discussed in connection with the orientational tuning of local and global stereopsis processes.

Depth Perception↗
PubMed1978

Contrast summation effects and stereopsis.

Contrast thresholds for stereopsis were measured for a variety of bandpass-filtered random-dot stereograms in a series of experiments. The principal finding was that contrast thresholds for stereopsis from 'complex' stereograms composed of mixtures of (a) two widely different spatial frequencies or (b) two or more widely different oriented random textures, are considerably lower than would be expected if stereopsis from such stimuli is mediated by the first component to rise above its own stereopsis contrast threshold. Instead, it appears that stereopsis comes about whenever the supradetection-threshold contrast of a stereogram exceeds a certain level, regardless of whether this contrast is provided by a single component or by a mix of two different ones. The implications of these findings for models of stereopsis are discussed.

Depth Perception↗
PubMed1978

The relationship between apparent depth and disparity in rivalrous-texture stereograms.

A series of experiments is reported on rivalrous-texture stereograms composed of narrowband-filtered random noise. Experiment 1 found that the apparent deth-disparity function for such stereograms was different from that observed with similar but nonrivalrous stimuli. In particular, rivalrous divergent disparities produced the same depth as rivalrous zero disparity and this latter disparity itself produced a significant degree of protruding (i.e. 'convergent') depth in a certain type of rivalrous-texture stereogram. Free inspection was permitted and disparities were in the range 16 min convergent to 16 min divergent. Experiment 2 found no convincing evidence for reliable qualitative depth discriminations from tachistoscopic presentations of rivalrous-texture stereograms, using a forced-choice task requiring a discrimination between 16 min convergent and 16 min divergent conditions. This task was solved easily for equivalent nonrivalrous stimuli. Experiment 3 measured a hitherto unreported binocular depth effect, termed 'paradepth', which is produced by presenting a target in one field only. This effect appears to be a genuine biocular depth effect and not just the result of an ordinary monocular masking depth cue. The size of the depth effect was found to be a function of the width of the target. The overall conclusion derived from the series of experiments is that rivalrous-texture stereograms are complex stimuli capable of yielding curious and unexpected depth effects which are not readily explained in detail within any existing theoretical framework.

Cues↗
PubMed1977

Global processes in stereopsis: some comments on Ramachandran and Nelson (1976).

The use of the term 'global in the context of stereopsis is discussed. It is concluded that different meanings of this term need to be kept carefully distinguished at all times. The discussion centres around a series of demonstrations introduced by Ramachandran and Nelson, and interpretations are offered for these demonstrations in terms of spatial-frequency-tuned stereopsis channels.

Depth Perception↗