Biomedical subjects

Piers D L Howe

Publications and source records attributed to Piers D L Howe.

5 recordsLinked to original sources

PubMed2006

Testing the coplanar ratio hypothesis of lightness perception.

What determines an object's lightness remains unclear, but it is generally thought that the ratios of its luminance to the luminance of other objects in a scene play a crucial role because these ratios allow the relative reflectance of each object to be estimated, providing all the objects are under the same illumination. Because objects that lie in the same plane are typically illuminated equally, it has been suggested that it is the luminance ratios between coplanar objects that primarily determine lightness (Gilchrist, 1977 Science 195 185-187; Gilchrist et al, 1999 Psychological Review 106 795-834). An alternative hypothesis is that perceived illumination differences can affect lightness directly. As the studies that provided evidence for the coplanar ratio hypothesis always varied the illumination and the coplanar relationships simultaneously, it is unclear which hypothesis is correct. I measured the influence of each factor separately and found that the perceived illumination differences have a greater effect on lightness.

Contrast Sensitivity↗

PubMed2005

V1 partially solves the stereo aperture problem.

If a bar stimulus extends beyond a cell's receptive field, then alterations in binocular disparity parallel to the bar's orientation leave the portion of the stimulus within the cell's receptive field unchanged. This makes it hard for the cell to respond correctly to the bar's disparity. Likening the cell's receptive field to an aperture through which the cell views the world, this issue has been called the "aperture problem" and is a specific form of the more general stereo correspondence problem. We found no cells in macaque primary visual cortex (V1) that, when faced with this situation, were sensitive to the disparity of the bar. However, we did find a number of cells that showed sensitivity to parallel disparity shifts, but these cells responded only to the ends of the long bar. The ability to respond selectively to such tracking features could be the first step towards solving the stereo aperture problem. The second step would require either that the disparity information that in V1 is associated only with the ends of the stimulus be associated with the rest of the stimulus or that subsequent stages of visual processing respond preferentially to the end-selective cells. As this second step does not appear to occur in V1, we conclude that V1 only partially solves the stereo aperture problem.

Animals↗

PubMed2005

White's effect: removing the junctions but preserving the strength of the illusion.

White's effect (also known as the Munker White effect) is a lightness illusion in which, contrary to expectations based on simultaneous contrast and Wallach's rule, a gray rectangle predominantly surrounded by white appears lighter than an identical rectangle that is mainly surrounded by black. The illusion is often explained in terms of T-junctions that are formed by the three-way intersection of the gray rectangle, a black stripe, and a white stripe. I present a circular variant of White's effect in which all the junctions have been removed without significantly affecting the strength of the illusion, suggesting that junctions are not an important consideration in all versions of White's effect.

Contrast Sensitivity↗

PubMed2003

A laminar cortical model of stereopsis and three-dimensional surface perception.

A laminar cortical model of stereopsis and later stages of 3D surface perception is developed and simulated. The model describes how initial stages of monocular and binocular oriented filtering interact with later stages of 3D boundary formation and surface filling-in in the lateral geniculate nucleus and cortical areas V1, V2, and V4. In particular, it details how interactions between layers 4, 3B, and 2/3A in V1 and V2 contribute to stereopsis, and clarifies how binocular and monocular information combine to form 3D boundary and surface representations. Along the way, the model modifies and significantly extends the disparity energy model. Neural explanations are given for psychophysical data concerning: contrast variations of dichoptic masking and the correspondence problem, the effect of interocular contrast differences on stereoacuity, Panum's limiting case, the Venetian blind illusion, stereopsis with polarity-reversed stereograms, da Vinci stereopsis, and various lightness illusions. By relating physiology to psychophysics, the model provides new functional insights and predictions about laminar cortical architecture.

Contrast Sensitivity↗

PubMed2003

Measuring the depth induced by an opposite-luminance (but not anticorrelated) stereogram.

The same-sign hypothesis suggests that only those edges in the two retinal images whose luminance gradients have the same sign, known as same-sign edges, can be stereoscopically fused to generate a perception of depth. If true, one would expect that the magnitude of the depth induced by an opposite-luminance stereogram (eg one where the figure in one stereo half-image is black and the figure in the other is white) should be determined by the disparity of the same-sign edges. Despite the considerable work on the same-sign hypothesis this prediction has yet to be verified. Here we confirm this prediction for a particular opposite-luminance stereogram and discuss possible reasons why it is not true for opposite-luminance stereograms that are presented briefly or where each stereo half-image contains many elements.

Depth Perception↗