PubMed Health⌕ Search

Biomedical subjects

G J Mitchison

Publications and source records attributed to G J Mitchison.

14 recordsLinked to original sources
PubMed1999

Can Hebbian volume learning explain discontinuities in cortical maps?

It has recently been shown that orientation and retinotopic position, both of which are mapped in primary visual cortex, can show correlated jumps (Das & Gilbert, 1997). This is not consistent with maps generated by Kohonen's algorithm (Kohonen, 1982), where changes in mapped variables tend to be anticorrelated. We show that it is possible to obtain correlated jumps by introducing a Hebbian component (Hebb, 1949) into Kohonen's algorithm. This correspondents to a volume learning mechanism where synaptic facilitation depends not only on the spread of a signal from a maximally active neuron but also requires postsynaptic activity at a synapse. The maps generated by this algorithm show discontinuities across which both orientation and retinotopic position change rapidly, but these regions, which include the orientation singularities, are also aligned with the edges of ocular dominance columns, and this is not a realistic feature of cortical maps. We conclude that cortical maps are better modeled by standard, non-Hebbian volume learning, perhaps coupled with some other mechanism (e.g., that of Ernst, Pawelzik, Tsodyks, & Sejnowski, 1999) to produce receptive field shifts.

Algorithms↗
PubMed1999

A probabilistic treatment of phylogeny and sequence alignment.

Carrying out simultaneous tree-building and alignment of sequence data is a difficult computational task, and the methods currently available are either limited to a few sequences or restricted to highly simplified models of alignment and phylogeny. A method is given here for overcoming these limitations by Bayesian sampling of trees and alignments simultaneously. The method uses a standard substitution matrix model for residues together with a hidden Markov model structure that allows affine gap penalties. It escapes the heavy computational burdens of other models by using an approximation called the "*" rule, which replaces missing data by a sum over all possible values of variables. The behavior of the model is demonstrated on test sets of globins.

Bayes Theorem↗
PubMed1990

Mechanisms underlying the anisotropy of stereoscopic tilt perception.

There is a marked anisotropy in the perception of stereoscopic tilt: vertical gradients of horizontal disparity are more easily perceived than horizontal gradients. This could be explained if orientation disparity (the orientation difference in the two eyes' views of the same line) were one of the cues used to determine tilt, since orientation disparities are in general larger for vertical gradients. We show here that a marked anisotropy in tilt perception is present even with stereograms which contain equally strong orientation disparity cues for horizontal and vertical gradients. This implies that there must be other mechanisms for stereoscopic tilt perception, or further processing steps in the use of orientation disparity, which are anisotropic in their mode of action.

Depth Perception↗
PubMed1988

The role of retinal correspondence in stereoscopic matching.

On brief viewing, stereo matching of a regularly-spaced horizontal row of points is determined by the disparity of the points at the edges. Stereo matches on corresponding retinal loci are initially overridden in favor of matches located on or near the disparity plane of the edge points. Edge-based matching is observed when the inter-point spacing is as large as 15-30 min arc for crossed disparities, and as large as 1 deg for uncrossed disparities. Edge points located at a lateral distance more than 2.5 deg away from the center of the row can still determine the initial stereo matching of the center. Given longer viewing time, vergence usually changes from the fixation plane towards the initially-perceived depth plane associated with the edges. However, if the eyes are held tightly converged in the fixation plane, the edge-based matches will gradually yield to matches in the fixation plane. This shift from edge-based matching to a match determined by retinal correspondence takes 1-4 sec if the inter-point spacing is large (10-30 min arc). For smaller inter-point spacings, the edge-based matches are very stable, and a shift in depth is seldom discernible.

Convergence, Ocular↗
PubMed1987

The resolution of ambiguous stereoscopic matches by interpolation.

Matching in stereograms made of horizontal rows of points can be described as follows: characteristic features, such as edges and gaps in the rows, have unambiguous matches in the two eyes, and these features are matched first. A plane interpolated between the positions in depth assigned to these features then guides the matching in the intervening sets of regularly spaced points which have potentially ambiguous matches. The intervening points are matched so that their disparity with respect to this interpolation plane is minimized. The "nearest disparity" rule describes matching for tilted interpolation planes as well as for fronto-parallel planes. While it is possible to construct stereograms which violate these matching rules, the rules work remarkably well in describing typical matching behaviour for many patterns.

Convergence, Ocular↗
PubMed1987

Interpolation and the detection of fine structure in stereoscopic matching.

Under some conditions, the perceived depth of a stereogram made of regularly-spaced points depends on the disparity of the edges of the stereogram rather than on discrete stereo matching of the points themselves. These depth percepts are seen only if the viewing time is brief, less than 2-3 sec, and the spacing between the points is small, less than 5-7 min of arc. In this paper, we examine the hypothesis that depth interpolation reflects a failure of the stereo matching apparatus to resolve the fine structure of the stereogram. We show that fine structure can be resolved during brief presentations (160 msec). It seems likely that interpolation represents an intermediate stage in the stereo matching process.

Convergence, Ocular↗
PubMed1987

Limit to the detection of Glass patterns in the presence of noise.

A method is developed for quantifying the strength of the moiré effects known as Glass patterns. Unpaired randomly placed dots are added to the pattern while the discriminability d' of the degraded pattern is determined in a yes-no test. For a given discriminability the number of pairs required increases in direct proportion to the number of random dots. A model is developed based on the ideal discrimination of an excess of oriented pairs. Results conforming to Weber's law are predicted; the dependence of d' on the amount of noise and the number of point pairs is also predicted. A numerical constant derived from the model provides a measure of the strength of the moiré effect of a chosen pattern. Note that, for this task, statistical considerations predict Weber's law, not the square-root law, as a limit, and this result holds whenever second-order structure is detected in the image.

Form Perception↗
PubMed1984

The perception of depth in simple figures.

When subjects with good stereoscopic acuity are given the task of judging which of two vertical lines lies nearer, the presence of other features nearby alters the perceived depth within the test pair. In the presence of a single flanking line shown with disparity, the test line pair is seen as fronto-parallel when it has disparity in the direction which tends to align it in depth with the flanking line. The notion of "salience" is introduced. This is the summed disparity--weighted approximately inversely with distance--between test objects and their neighbours. We make the hypothesis that objects appear at equal depths when they have equal salience. The salience hypothesis accounts for a variety of depth interaction effects between test lines and adjoining features, such as one or more other lines and a lattice of dots with a disparity gradient. Whether features other than nearest neighbours influence depth judgments depends on the individual. For five good stereo subjects, in two a single line completely masked all effects beyond the nearest neighbour, two others had partial masking, and one had none. If the visual system is interested in corners between planes in depth and in objects protruding from such planes, then salience constitutes a useful indicator for this purpose.

Depth Perception↗
PubMed1976

Measurement of an inhibitory zone.

An inhibitory zone mechanism generates the heterocyst pattern in Anabaena. These inhibitory zones can be measured; we find that they extend about five cells on either side of a heterocyst. We can use our observations to predict with reasonable accuracy which cells in the filaments will differentiate into heterocysts.

Cell Differentiation↗
PubMed1975

Mutants of Anabaena cylindrica altered in heterocyst spacing.

Nitrosoguanidine induced mutants of Anabaena cylindrica have been obtained, which are altered in heterocyst spacing. In the wild type organism the pattern is composed of single intercalary heterocysts. The mutant patterns fall into several classes: those with only terminal heterocysts, with both terminal and intercalary heterocysts, with groups of heterocysts and those totally lacking heterocysts. The mutants are described in detail, and the various pattern modications are interpreted in terms of a model we have proposed.

Ammonia↗
PubMedDate not supplied

Interpolation in stereoscopic matching.

Anyone who has stared at a repeating wallpaper pattern, or a periodic pattern of tiles, has probably experienced the phenomenon of a false stereoscopic depth percept. This arises because of a mismatching in the two eyes of repeating elements in the pattern. The phenomenon is less likely to occur if an edge of the textured region is in view; the edge seems to fix the registration of elements. We describe here a stereogram which exemplifies this principle; it has a central, periodic region bounded on either side by edges with pre-assigned disparities. We find that the perceived depth of the central region is controlled by the edges. In certain conditions (when the period is spatially large), the edges simply impose one of the expected discrete matchings. In other conditions, however, we observe a striking phenomenon: interpolation in depth occurs between the edges, violating any possible feature-by-feature matching.

Depth Perception↗