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Biomedical subjects

C Chubb

Publications and source records attributed to C Chubb.

At least 19 recordsLinked to original sources
PubMed2001

The size-tuning of the face-distortion after-effect.

Recently, Webster and MacLin demonstrated a face-distortion after-effect (FDAE) for both upright and inverted faces: adaptation to a distorted face makes a normal face appear distorted in the direction opposite to the adapting direction. Neurophysiological studies (e.g. Experimental Brain Research 65 (1986) 38) show that face-selective neurons in the superior temporal sulcus (STS) are remarkably size-invariant in their responses. If the site of adaptation underlying the FDAE is the homologous neuron population in human vision, then the FDAE should also be highly tolerant to changes in size between adapting and test faces. Here, we test this prediction. Observers were adapted to distorted upright/inverted faces of three different sizes (3.3 degrees x 3.7 degrees, 6.6 degrees x 7.5 degrees, and 13.1 degrees x 14.8 degrees ). For adapting faces of all three sizes, observers adjusted test faces of all three sizes until they appeared normal. Significant FDAEs were observed in all conditions. For both upright and inverted faces, FDAEs were approximately twice as strong when adapting and test faces were the same size than when they differed by even a single octave in size. The magnitudes of FDAEs were comparable for upright and inverted faces. The larger FDAEs for same-size adapting and test faces suggest that part of the FDAE derives from a neuron population with narrow size-tuning. However, the significant FDAEs obtained for adapting and test images differing by two octaves implicate a different neuron population with broad size-tuning, possibly the human homolog of the face-selective neuron population in monkey STS.

Afterimage↗
PubMed2000

DNA damage and activated caspase-3 expression in neurons and astrocytes: evidence for apoptosis in frontotemporal dementia.

Frontotemporal dementia (FTD) is a neurodegenerative disease which affects mainly the frontal and anterior temporal cortex. It is associated with neuronal loss, gliosis, and microvacuolation of lamina I to III in these brain regions. In previous studies we have described neurons with DNA damage in the absence of tangle formation and suggested this may result in tangle-independent mechanisms of neurodegeneration in the AD brain. In the present study, we sought to examine DNA fragmentation and activated caspase-3 expression in FTD brain where tangle formation is largely absent. The results demonstrate that numerous nuclei were TdT positive in all FTD brains examined. Activated caspase-3 immunoreactivity was detected in both neurons and astrocytes and was elevated in FTD cases as compared to control cases. A subset of activated caspase-3-positive cells were also TdT positive. In addition, the cell bodies of a subset of astrocytes showed enlarged, irregular shapes, and vacuolation and their processes appeared fragmented. These degenerating astrocytes were positive for activated caspase-3 and colocalized with robust TdT-labeled nuclei. These findings suggest that a subset of astrocytes exhibit degeneration and that DNA damage and activated caspase-3 may contribute to neuronal cell death and astrocyte degeneration in the FTD brain. Our results suggest that apoptosis may be a mechanism of neuronal cell death in FTD as well as in AD (228).

Aged↗
PubMed2000

Texture luminance judgments are approximately veridical.

This paper investigates intensity coding in human vision. Specifically, we address the following question: how do different luminances influence the perceived total luminance of a composite image? We investigate this question using a paradigm in which the observer attempts to judge, with feedback, which of two texture patches has higher total luminance. All patches are composed of nine luminances, ranging linearly from 0 (black) to a maximum luminance (white: 160 cd/m(2) in one condition; 20.2 cd/m(2) in another condition). Luminance histograms of the patches being compared are experimentally varied to derive, for each luminance nu, the impact exerted by texture elements (texels) of luminance nu on texture luminance judgments. We find that impact is approximately proportional to texel luminance; That is, a texture element exerts, on average, an impact on texture brightness (i.e. perceived texture luminance) that is proportional to its (the texel's) luminance. The only exception occurs for texels of maximal luminance, which surprisingly exert an impact that is slightly, but significantly, less than that exerted by texels of the next lower luminance. We conclude that visual intensity coding for purposes of assessing overall luminance of inhomogeneous patches is approximately veridical. In particular, texture luminance judgments are not mediated by a significant, compressive nonlinearity.

Contrast Sensitivity↗
PubMed2000

Variance of high contrast textures is sensed using negative half-wave rectification.

A rectifying transformation is required to sense variations in texture contrast. Various theoretical and practical considerations have inclined researchers to suppose that this rectification is full-wave, rather than half-wave. In the studies reported here, observers are asked to judge which of two texture patches has higher texture variance. Textures are composed of small squares, with each square being painted with one of nine different luminances. Different texture variances are achieved by manipulating the histograms of the texture patches to be compared. When the nine luminances range linearly from 0 to 160 cd/m(2), the transformation mediating judgments of texture variance takes the form of a negative half-wave rectifier: texture variance judgments are determined exclusively by the frequencies of luminances below mean luminance in the textures being compared. However, when the nine luminances range linearly from 60 to 100 cd/m(2), two of three observers use a full-wave rectifying transformation in making texture variance judgments; the third observer continued to use a negative half-wave rectifier. The unexpectedly asymmetric roles played by low versus high luminances in texture variance judgments suggest that the off-center system may play a dominant role in human perception of texture contrast.

Contrast Sensitivity↗
PubMed2000

Every discrete, finite image is uniquely determined by its dipole histogram.

A finite image I is a function assigning colors to a finite, rectangular array of discrete pixels. Thus, the information directly encoded by I is purely locational. Such locational information is of little visual use in itself: perception of visual structure requires extraction of relational image information. A very elementary form of relational information about I is provided by its dipole histogram DI. A dipole is a triple, ((dx, dy), alpha, beta), with dx and dy horizontal and vertical, integer-valued displacements, and alpha and beta colors. For any such dipole, DI((dx, dy), alpha, beta) gives the number of pixel pairs ((x1, y1), (x2, y2)) of I such that I[x1, y1] = alpha, I[x2, y2] = beta, and, (x2, y2) - (x1, y1) = (dx, dy). Note that DI explicitly encodes no locational information. Although DI is uniquely determined by (and easily constructed from) I, it is not obvious that I is uniquely determined by DI. Here we prove that any finite image I is uniquely determined by its dipole histogram, DI. Two proofs are given; both are constructive, i.e. provide algorithms for reconstructing I from DI. In addition, a proof is given that any finite, two-dimensional image I can be constructed using only the shorter dipoles of I: those dipoles ((dx, dy), alpha, beta) that have magnitude of dx < or = ceil((# columns in I)/2) and magnitude of dy < or = ceil((# rows in I)/2), where ceil(x) denotes the greatest integer < or = x.

Algorithms↗
PubMed1999

Texture-Based Methods for Analyzing Elementary Visual Substances.

Human vision is generally assumed to embody a limited number of low-level, spatially parallel image transformations. These transformations define the basic properties vision can sense. Because texture discrimination tasks require rapid, spatially parallel processing, they are ideally suited to investigate these basic transformations. Here it is shown how to use paired comparison texture tasks to functionally analyze such transformations. Tasks are considered in which the observer is presented with two patches of texture, A and B, tiled with micropatterns drawn from a set Omega, and is asked to judge which is greater, M(A) or M(B), for some specific texture property M (e.g., M(A) might be the intensity variance of patch A). Performance is modeled by supposing that the observer approximates M(A) and M(B) by noisy, subjective quantities,;M(A) and;M(B), synthesized from the basic transformations resident in human vision. Psychophysical methods are provided for determining the differential impact on;M of the various micropatterns in Omega. Specifically, an occurrence in patch A (similarly for patch B) of a given micropattern omegainOmega is assumed to contribute an independent, additive random variable X(omega) to;M(A). The mean m(omega) of X(omega) gives the average impact on;M(A) of an occurrence of omega. It is assumed that the observer judges M(A)>M(B) on a given trial if;M(A)-;M(B)+Y>0, for Y a normal random variable with mean zero and unspecified variance. Simple, efficient techniques are provided for accurately estimating the shape of the function m: Omega-->&Ropf;, which defines the impacts of different micropatterns on M-judgments. These methods yield results that are invariant with respect to all unmeasured model parameters. MATLAB code is supplied for data analysis. Copyright 1999 Academic Press.

Journal Article↗
PubMed1999

Contrast facilitation in motion detection: evidence for a Reichardt detector in human vision.

We measured contrast thresholds for detecting the direction of movement of a grating in a two-frame sequence with a quadrature shift between frames. The threshold contrast for the first frame was determined for a range of contrasts of the second frame. As the contrast of the second frame was increased, the threshold contrast of the first frame initially fell, and then remained constant at a value that was approximately 0.3 log units below the threshold contrast when the two frames had the same contrast (the yoked threshold). We show that this motion facilitation effect is a specific prediction of a Reichardt detector, with additive noise before the multiplication site. When the order of the two frames was reversed, the motion facilitation effect was found for contrast of the second frame, but at sufficiently high contrasts of the first frame, masking was found. The temporal order asymmetry is not found at near yoked-threshold contrasts of the first frame. The Motion Facilitation Effect was smaller than the contrast facilitation in a classical contrast discrimination experiment.

Contrast Sensitivity↗
PubMed1998

Analysis of differential protein expression in normal and neoplastic human breast epithelial cell lines.

High-resolution two-dimensional gel electrophoresis (2-DE) and database analysis was used to establish protein expression patterns for cultured normal human mammary epithelial cells and thirteen breast cancer cell lines. The Human Breast Epithelial Cell database contains the 2-DE protein patterns, including relative protein abundances, for each cell line, plus a composite pattern that contains all the common and specifically expressed proteins from all the cell lines. Significant differences in protein expression, both qualitative and quantitative, were observed not only between normal cells and tumor cells, but also among the tumor cell lines. Eight percent (56/727) of the consistently detected proteins were found in significantly (P< 0.001) variable levels among the cell lines. Eight proteins present in normal cultured breast epithelial cells were not detected in any of the tumor cell lines. We identified a subset of the differentially expressed proteins using a combination of immunostaining, protein sequencing, comigration, and subcellular fractionation. These identified proteins include the intermediate filament components vimentin and cytokeratins. The cell lines can be classified into four distinct groups based on their intermediate filament protein profile. We also identified heat shock proteins; hsp27 and hsp60 varied in abundance and in some cases in the relative phosphorylation levels among the cell lines. Many of the differentially expressed proteins we identified have roles in cellular proliferation and differentiation, including annexin V, elongation initiation factor 5A, Rho GDP dissociation inhibitor, and prohibitin. We identified inosine-5-monophosphate dehydrogenase in each of the cell lines, and found the levels of this enzyme in the tumor cell lines elevated 2- to 20-fold relative to the levels in normal cells. These results expand the human breast epithelial cell protein database (http:// www.anl.gov/CMB/PMG) which is being built to assist researchers with the identification of abnormal patterns of expression and pathways associated with malignancy.

Amino Acid Sequence↗
PubMed1998

Saccadic localization of random dot targets.

The targets for saccadic eye movements in natural visual scenes are spatially extended objects, yet saccades land at a single position within them. To characterize the spatial transformation that determines the saccadic goal position within attended objects, we studied saccadic localization of large patterns of random dots. Saccades landed with a high degree of precision near the center-of-gravity of the patterns (average error < 10%; SDs around the center-of-gravity = 7-11% of target eccentricity). Predictions of landing position were improved by using a weighted center-of-gravity, in which the weight assigned to each dot was reduced by the presence of neighboring dots. Weighting based either on the eccentricity of dots or their position relative to the boundary of the pattern had no effect. The results can be accounted for by a spatial transformation in which the "local signs" of an initial array of detectors, weighted by the activity of each, are averaged to yield the saccadic goal. This model can account for accurate and precise saccadic localization of large targets, while preserving sensitivity to local pattern characteristics. Unlike models of recognition, the boundary of the object has the same status as the internal details.

Female↗
PubMed1997

Structure detection: a statistically certified unsupervised learning procedure.

We present a class of structure detection procedures (SDPs) that can extract the characteristic structures in an arbitrary population of images. An SDP adaptively augments the power of a novel, statistical, structure test to reject the null hypothesis that a randomly chosen image is devoid of structure. The core of the structure test consists of an orthonormal basis B of receptive fields that is refined into an increasingly sensitive detector of characteristic image structures. Adaptive refinement is accomplished as follows: for each image x in a random training sequence, B is updated by a planar rotation that decreases the p-value of a statistical structure test for x. This image-by-image refinement procedure is very efficient, obeying time and space constraints similar to those that limit processes of perceptual organization in real organisms. SDPs' capabilities are demonstrated in three test populations: natural images, faulty random number generators, and artificial images composed of mixtures of basis functions. (1) An SDP succeeds in rejecting the null hypothesis that the UNIX random number generator rand() is truly random. (2) When images are composed by adding arbitrary pairs of orthogonal component images, an SDP extracts the components. (3) For a large set of natural image patches, an SDP yields a basis B1 that detects structure with p-value < 0.005 in 88% of a new set of patches. B1's elements resemble the receptive fields of V1 simple cells. (4) Of special interest are biconvergent SDPs that derive in parallel a basis B, as well as a pointwise transformation f, specifically sensitized to evaluate the response values that result from applying B to images in the target population. A biconvergent SDP applied to natural image patches yields a basis B2 similar to B1, as well as a pointwise transformation f with vastly heightened sensitivity to extreme response values. We conjecture that sensory neurons have evolved cooperatively to maximize their collective power to reject the null hypothesis that their input is devoid of structure, thereby evolving receptive fields that efficiently represent characteristic input structures.

Form Perception↗
PubMed1996

Precise assessment of the mean effective luminance of texture patches--an approach based on reverse-phi motion.

In studying the response of mechanisms to contrast-defined texture stimuli, it is critical that the average effective luminance of these textures be equal to that of the background, to minimize net luminance-based signals. We present an efficient and accurate technique for constructing such equiluminant textures to isolate contrast-sensitive mechanisms for investigating their properties. The technique is based on the reverse-phi motion phenomenon, and the resulting settings agree closely with those obtained by photometric means for the class of textures studied. The method also allows one to explore the properties of contrast- and luminance-driven motion mechanisms and, in particular, to evaluate the contribution of putative second-order mechanisms to the motion percept. Results of applying the method are presented, and its advantages over the minimum-flicker and minimum-motion techniques are discussed.

Humans↗
PubMed1995

Analysis of proteins from human breast epithelial cells using two-dimensional gel electrophoresis.

The human breast is a highly specialized, complex tissue comprised of a heterogeneous population of cells with varying functions. Interactions between the different cell types, changes in their relative abundance, state of differentiation and function in response to stimuli, as well as the alterations that lead to the aberrant growth associated with malignancy are poorly understood. Two-dimensional gel electrophoresis is being used to compare the proteins found in different breast cells in order to identify the gene products that are common or specific to particular cell types so as to provide markers that will be useful in studies of normal breast cell differentiation and the dedifferentiation or blocked differentiation characteristic of cancer. Protein patterns have been obtained from cells prepared for electrophoresis immediately after isolation from human milk, from cells cultured for fewer than ten passages after isolation from healthy breast tissue removed during reduction mammoplasty, and from cells maintained in long-term tissue culture after isolation from the pleural effusions of patients with breast carcinomas. Differential expression of cytokeratins 8, 18, and 19, shown previously to be predominantly expressed by epithelial cells in the luminal layer of breast tissue, was observed among the cells analyzed. Other non-cytokeratin proteins were also found to be differentially expressed in subsets of both the normal and tumor cells. A composite human breast cell protein pattern was created which includes all the commonly and specifically expressed proteins found in this study. This pattern will be the basis for continuing studies of proteins in the human breast.

Amino Acid Sequence↗
PubMed1995

Measuring the spatial frequency selectivity of second-order texture mechanisms.

Recent investigations of texture and motion perception suggest two early filtering stages: an initial stage of selective linear filtering followed by rectification and a second stage of linear filtering. Here we demonstrate that there are differently scaled second-stage filters, and we measure their contrast modulation sensitivity as a function of spatial frequency. Our stimuli are Gabor modulations of a suprathreshold, bandlimited, isotropic carrier noise. The subjects' task is to discriminate between two possible orientations of the Gabor. Carrier noises are filtered into four octave-wide bands, centered at m = 2, 4, 8, and 16 c/deg. The Gabor test signals are w = 0.5, 1, 2, 4 and 8 c/deg. The threshold modulation of the test signal is measured for all 20 combinations of m and w. For each carrier frequency m, the Gabor test frequency w to which subjects are maximally sensitive appears to be approximately 3-4 octaves below m. The consistent m x w interaction suggests that each second-stage spatial filter may be differentially tuned to a particular first-stage spatial frequency. The most sensitive combination is a second-stage filter of 1 c/deg with first-stage inputs of 8-16 c/deg. We conclude that second-order texture perception appears to utilize multiple channels tuned to spatial frequency and orientation, with channels tuned to low modulation frequencies appearing to be best served by carrier frequencies 8 to 16 times higher than the modulations they are tuned to detect.

Contrast Sensitivity↗
PubMed1994

Non-Fourier motion analysis.

It has been realized for some time that the visual system performs at least two general sorts of motion processing. First-order motion processing applies some variant of standard motion analysis (i.e. spatiotemporal Fourier energy analysis) directly to stimulus luminance, whereas second-order motion processing applies standard motion analysis to one or another grossly non-linear transformation of stimulus luminance. We have developed a method for disentangling the different sorts of mechanisms that may operate in human vision to detect second-order motion. This method hinges on an empirical condition called transition invariance that may or may not be satisfied by a family psi of textures. Any failure of this condition indicates that more than one mechanism is involved in detecting the motion of stimuli composed of the textures in psi. We have shown that the family of sinusoidal gratings oriented orthogonally to the direction of motion and varying in contrast and spatial frequency is transition invariant. We modelled the results in terms of a single-channel motion computation. We have new results indicating that a specific class of textures differing in texture element density and texture element contrast decisively fails the test of transition invariance. These findings suggest that in addition to the single second-order motion channel required by our earlier results there exists at least one other second-order motion channel. We argue that the preprocessing transformation used by this channel is a pointwise non-linearity that maps stimulus contrasts of absolute value less than some relatively high threshold tau onto 0, but increases with magnitude of c-tau for contrasts. c of absolute value greater than tau.

Animals↗
PubMed1994

Full-wave and half-wave processes in second-order motion and texture.

A theory of human second-order motion perception is proposed and further applied to the discrimination of texture slant. The computational algorithms for deriving the direction of left-right motion from a sequence of images are equivalent to the algorithms for deriving the direction of slant (e.g. from top left to bottom right or top right to bottom left) in a single 2D image. There is a broad range of phenomena for which Fourier analysis of the image plus a few simple rules gives a good account of human perception. The problem with this first-order analysis is that there exists a broad class of 'microbalanced' stimuli in which the motion or slant is completely obvious to human subjects but is invisible to first-order analysis. Microbalanced stimuli require second-order analysis which consists of non-linear preprocessing (spatiotemporal filtering followed by rectification of the input signal) before standard motion or slant analysis. To determine whether the second-order rectification is half-wave or full-wave, we construct two special microbalanced stimulus types: 'half-wave stimuli' whose motion (or texture slant) is interpretable by a half-wave rectifying system but not by full-wave or a first-order (Fourier) analysis and 'full-wave stimuli' which are interpretable only after full-wave rectification. Such experiments show that second-order texture-slant perception utilizes both half-wave and full-wave processes, second-order motion-direction discrimination depends predominantly on full-wave rectification and second-order spatial interactions such as lateral contrast-contrast inhibition and second-order Mach bands are exclusively full-wave.

Algorithms↗
PubMed1994

Perception of apparent motion between dissimilar gratings: spatiotemporal properties.

What determines the strength of texture-defined apparent motion perception when the stimulus has no net directional energy in the Fourier domain? In a previous paper [Werkhoven, Sperling & Chubb (1993) Vision Research, 33, 463-485] we demonstrated the counterintuitive finding that the correspondence in spatial frequency and in modulation amplitude between neighboring patches of texture in a spatiotemporal motion path are irrelevant to motion strength. Instead, we found strong support for what we call a single channel or one-dimensional motion computation: a simple nonlinear transformation of the image, followed by standard motion analysis. Here, we further studied the dimensionality of the motion computation in a parameter space that includes texture orientation and stimulus display rate in addition to texture spatial frequency and modulation amplitude. We used ambiguous motion displays in which one motion path, consisting of patches of nonsimilar texture, competes with another motion path comprised entirely of similar texture patches. The data show that motion between dissimilar patches of texture that are orthogonally oriented, have a two octave difference in spatial frequency and differ 50% in modulation amplitude can easily dominate motion between similar patches of texture. A single channel accounts for more than 70% of texture-from-motion strength for the parameter space examined and this channel is invariant for stimulus display rates varying over a four-fold range.

Humans↗
PubMed1994

Histogram contrast analysis and the visual segregation of IID textures.

A new psychophysical methodology is introduced, histogram contrast analysis, that allows one to measure stimulus transformations, f, used by the visual system to draw distinctions between different image regions. The method involves the discrimination of images constructed by selecting texture micropatterns randomly and independently (across locations) on the basis of a given micropattern histogram. Different components of f are measured by use of different component functions to modulate the micropattern histogram until the resulting textures are discriminable. When no discrimination threshold can be obtained for a given modulating component function, a second titration technique may be used to measure the contribution of that component to f. The method includes several strong tests of its own assumptions. An example is given of the method applied to visual textures composed of small, uniform squares with randomly chosen gray levels. In particular, for a fixed mean gray level mu and a fixed gray-level variance sigma 2, histogram contrast analysis is used to establish that the class S of all textures composed of small squares with jointly independent, identically distributed gray levels with mean mu and variance sigma 2 is perceptually elementary in the following sense: there exists a single, real-valued function f S of gray level, such that two textures I and J in S are discriminable only if the average value of f S applied to the gray levels in I is significantly different from the average value of f S applied to the gray levels in J. Finally, histogram contrast analysis is used to obtain a seventh-order polynomial approximation of f S.

Contrast Sensitivity↗