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M H Brill

Publications and source records attributed to M H Brill.

17 recordsLinked to original sources
PubMed1990

Image segmentation by object color: a unifying framework and connection to color constancy.

A unifying framework is presented for algorithms that use the bands of a multispectral image to segment the image at material (i.e., reflectance) boundaries while ignoring spatial inhomogeneities incurred by accidents of lighting and viewing geometry. The framework assumes that the visual stimulus (image field) from a uniformly colored object is the sum of a small number of terms, each term being the product of a spatial and a spectral part. Based on this assumption, several quantities depending on the reflected light can be computed that are spatially invariant within object boundaries. For an image field either from two light sources on a matte surface or from a single light source on a dielectric surface with highlights, the invariants are the components of the unit normal to the plane in color space spanned by the pixels from the object. In some limited cases the normal to the plane can be used to estimate spectral-reflectance parameters of the object. However, in general the connection of color-constancy theories with image segmentation by object color is a difficult problem. The concomitant constraints on segmentation and color-constancy algorithms are discussed in light of this fact.

Algorithms↗
PubMed1990

Mesopic color matching: some theoretical issues.

In this paper I address some theoretical issues concerning the additivity laws of Grassmann as they might apply to color matches in mesopic vision (in which light intensities are such that both rods and cones are active). It is first shown that rods and cones have linearly independent spectral responses, and hence rod and cone spectral responses span a four-dimensional matching space. Next, the iterative tetrachromatic-matching experiment of Trezona [Vision Res. 13, 9 (1973)] is examined for consistency with a four-dimensional Grassmann structure; convergence conditions for the technique are derived and shown to be consistent with the actual conditions of the experiment. Finally, a discussion is presented of the conditions under which mesopic color matching would require only three primaries, even when color matching obeys four-dimensional (but not three-dimensional) Grassmann laws. A two-dimensional rod-cone diagram gives significant but not complete insight into the conditions for trichromacy in a four-dimensional color-matching space.

Color Perception↗
PubMed1986

Heuristic analysis of von Kries color constancy.

The properties of constancy models based on the proportionality rule of von Kries are examined in a series of simplified examples. It is found that the breadth of receptor-sensitivity functions causes metamerism, thwarting color constancy. Overlap of these functions limits the accuracy of von Kries adaptation for a more subtle reason: it causes nonzero off-diagonal elements in the transformation matrix relating object reflectance to receptor stimulations. Such off-diagonal elements make von Kries adaptation an inexact color-constancy scheme, even when the illuminant is restricted to prevent metamerism.

Color Perception↗
PubMed1986

Analysis of human color mechanisms using sinusoidal spectral power distributions.

We examined the effects of probing human color mechanisms using sinusoidal spectral power distributions (SPD's) varying in frequency (i.e., from 0.1 to 5.0 cycles/300 nm for a constant starting phase) and phase (i.e., from 0 to 360 deg for a fixed frequency of 1 cycle/300 nm) through computer simulation using several color models. Predicted modulation sensitivity functions (MSF's) in spectral frequency and phase differ among the models and indicate that measurements of the minimum amplitudes necessary to detect sinusoidal SPD's would be useful for distinguishing among theories of color vision. MSF's obtained from similar analyses of dichromats' color mechanisms reveal characteristic patterns of modulation sensitivities and suggest that such measures could serve to distinguish type and degree of color-vision defect. Some implications based on sinusoidal approximations to illuminant and reflectance spectra are discussed along with more general considerations regarding sine-wave SPD's as a probe for mechanisms of color vision.

Color Perception↗
PubMed1985

Remarks on signal-processing explanations of the trichromacy of vision.

We present criticism of signal-processing arguments recently used to explain the trichromacy of color vision. In particular, we note that illuminant spectral power distributions (SPD's) can be metameric even when the signal-processing arguments state that trichromatic vision represents such SPD's without ambiguity. Being statistical in nature, these arguments need not apply to each individual SPD; however, further use of the arguments calls for attributing them to some ensemble of SPD's and then testing whether the ensemble satisfies the underlying statistical assumptions.

Color Perception↗
PubMed1985

Decomposition of Cohen's matrix R into simpler color invariants.

Matrix R of linear-group tristimulus invariants, presented by Cohen and Friden (1976) and Cohen and Kappauf (1982), was decomposed into functions of simpler tristimulus volume ratios (also linear-group invariants). Such volume ratios were applied to the dual problems of color constancy and illuminant-invariant object-color recognition by an artificial trichromatic photosensor. The proof of the decomposition theorem relies on a lemma used in previous work elucidating the conditions for illuminant-invariance of clockwise/counterclockwise ordering of triads of object colors in chromaticity space.

Color Perception↗
PubMed1982

Necessary and sufficient conditions for Von Kries chromatic adaptation to give color constancy.

Necessary and sufficient spectral conditions are presented for Von Kries chromatic adaptation to give color constancy. Von-Kries-invariant reflectance spectra are computed for illuminant spectral power distributions that are arbitrary linear combinations of the first three daylight phases. Experiments are suggested to test models of color constancy using computed spectra (either exact or approximate) within the illuminant-invariant framework.

Color↗
PubMed1979

Dependence of refractory period measurements on conduction distance: a computer simulation analysis.

Computer simulations were used to study the recovery of conduction velocity in the temporal and spatial domains during the relative refractory period. The impulse exhibits increased duration and reduced amplitude during the relative refractory period as well as a reduced conduction velocity. Changes in conduction velocity are reflected by a deviation from linearity in the plots of latency variation versus interstimulus interval and of interval-between-impulses versus intersimulus interval. The latency increase for the second impulse is monotonically related to interstimulus interval, and is maximal at the beginning of relative refractory period. The magnitude of the latency increase is also related monotonically, but not in a linear manner, to conduction distance. The distance-dependence of latency variation during the relative refractory period is most marked close to the site of impulse initiation. These findings may be relevant to relative refractory period measurements in clinical neurophysiology.

Action Potentials↗
PubMed1978

Simulations of conduction in uniform myelinated fibers. Relative sensitivity to changes in nodal and internodal parameters.

Conduction of impulses in myelinated axons has been studied by a new method of computer simulation. The contributions of nodal and internodal characteristics and parameters were examined. Surprisingly, the conduction velocity, theta, was found to be quite insensitive to the nodal area or the exact description of its excitable processes. The conduction velocity also is relatively insensitive to the internodal length but much more sensitive to the myelin capacitance and axoplasm conductance. Qualitative change in theta with temperature depended on which temperature-sensitive parameters were included in the simulation.

Axons↗
PubMed1978

Conduction through demyelinated plaques in multiple sclerosis: computer simulations of facilitation by short internodes.

Clinical and laboratory observations both suggest that it may be possible for action potentials to traverse, in a continuous manner and without interruption, demyelinated zones along some axons. This continuous mode of conduction requires the presence of sufficient numbers of sodium channels in the demyelinated region. One of the factors which will tend to prevent such conduction is the impedance mismatch at sites of focal demyelination, which may result in a reduction in current density sufficient to cause conduction failure. As part of an effort to examine the conditions which would promote conduction into, and beyond, the demyelinated region, we examined, using computer simulations, the effects of reduction in length of the proximal internodes closest to the demyelinated region. Our results indicate that reduction in length of the two internodes closest to the demyelinated region. to approximately one-third of normal length or less, will facilitate conduction beyond the plaque. The results suggest that reductions in internode length, which have been histologically observed along some demyelinated fibres, may have functional significance in terms of facilitating conduction past focally demyelinated zones.

Action Potentials↗
PubMed1977

Conduction velocity and spike configuration in myelinated fibres: computed dependence on internode distance.

It has been argued theoretically and confirmed experimentally that conduction velocity (theta) should be proportional to nerve fibre diameter for myelinated fibre tracts, such as normal peripheral nerve, exhibiting 'structural' similarity'. In some axons, however, the nodes of Ranvier are more closely spaced than in normal peripheral nerve. Analytic arguments have suggested that when internodal distance (L) alone is changed, the plot of theta versus L should have a relatively flat maximum. This was confirmed by several previous computer simulations of myelinated axons, but internode lengths of less than half the normal case were not examined. In order to gain insight into impulse propagation in myelinated and remyelinated fibres with short internodal lengths, the present study examines the conduction velocity and spike configuration for a wide range of internodal lengths. As L becomes large, theta falls and finally propagation is blocked; as L becomes small, theta decreases more and more steeply. From this, it is predicted that for fibres with very short internodal lengths, small local changes in L should affect substantially the conduction velocity.

Action Potentials↗
PubMed1976

Probability of conduction deficit as related to fiber length in random-distribution models of peripheral neuropathies.

This paper presents a set of probabilistic models which reproduce the proximodistal gradient of sensory deficit in peripheral neuropathies, on the basis of the occurrence of axonal dysfunction as a result of randomly distributed abnormalities. The models, which are based on conduction block, loss of temporal coherence, and weak interactions between nerve fibers, demonstrate that randomly distributed axonal dysfunction provides a sufficient condition for distal sensory deficit. The models predict a marked reduction in the length for normal sensory conduction with small increases in the probability of axomal dysfunction, providing a possible correlate for the rapid clinical progression of some neuropathies. The hypothesis that weak interactions between fibers result in paresthesiae in peripheral neuropathies is also discussed.

Humans↗