thomas papathomas - Academia.edu (original) (raw)

Papers by thomas papathomas

Journal of The Optical Society of America A-optics Image Science and Vision, 1993

We present a series of experiments in texture discrimination with textures whose local elements a... more We present a series of experiments in texture discrimination with textures whose local elements are defined by their color (red andlor green), luminance polarity, and orientation (vertical and/or horizontal). The 23 distinct texture configurations were designed for testing and parameterizing a model based on the generalization of the concept of double opponency. The double-opponent mechanisms of the model are classified into four domains: the luminance-and color-oriented (LO and CO) domains and the luminance-and color-nonoriented (LnO and CnO) domains. Texture edge strength is extracted from the weighted, half-wave rectified outputs of each double-opponent mechanism. These signals are then combined through probability summation within each domain. Overall sensitivity to a given texture pair is predicted by the probability summation of all the domain outputs. Good fits of the experimental data are obtained when the contribution of the CO domain is the smallest and the contribution of the CnO domain is the largest. We suggest that the double-opponency principle is a natural way of implementing spatial interactions in higher-order domains and that it could be generalized to other dimensions such as spatial frequency, motion, and stereopsis.

IEEE Transactions on Systems, Man, and Cybernetics, 1997

We have developed a computational model for texture perception which has physiological relevance ... more We have developed a computational model for texture perception which has physiological relevance and correlates well with human performance. The model attempts to simulate the visual processing characteristics by incorporating mechanisms tuned to detect luminance-polarity, orientation, spatial frequency and color, which are characteristic features of any textural image. We obtained a very good correlation between the model's simulation results and data from psychophysical experiments with a systematically selected set of visual stimuli with texture patterns defined by spatial variations in color, luminance, and orientation. In addition, the model predicts correctly texture segregation performance with key benchmarks and natural textures. This represents a first effort to incorporate chromatic signals in texture segregation models of psychophysical relevance, most of which have treated grey-level images so far. Another novel feature of the model is the extension or the concept of spatial double opponency to domains beyond color, such as orientation and spatial frequency. The model has potential applications in the areas of image processing, machine vision and pattern recognition, and scientific visualization

Journal of The Optical Society of America A-optics Image Science and Vision, 1989

We describe a family of stimuli consisting of colored bars of different orientations, which, when... more We describe a family of stimuli consisting of colored bars of different orientations, which, when presented in rapid succession, may elicit unambiguous motion perception. These stimuli permitted the isolation of directional spatiotemporal information extracted from oriented luminance clues, from nonoriented chromatic-plus-luminance clues, or, when the stimuli were presented under equiluminant conditions, from pure chromatic clues. As a general rule, matching of orientation induces weaker motion-detection performances than does matching of color. When the orientation clues are in competition with the chromatic ones, motion perception based on the former is always overridden by motion perception based on the latter. We indirectly isolated an oriented chromatic mechanism that also contributes to motion perception. We finally showed that, under equiluminant conditions, matching of orientation across different colors is inefficient in eliciting motion perception, either because motion information is extracted poorly across different chromatic channels or because such channels show little orientational selectivity. Because motion strength determined by each of the manipulated attributes follows different functions with the displacement (or velocity) of the stimuli, we propose the existence of three underlying mechanisms, a luminance mechanism, a chromatic-plus-luminance mechanism, and a pure chromatic mechanism, each of which provides motion information.

Trends in Cognitive Sciences, 2002

Trends in Cognitive Sciences is indispensable reading for anyone interested in this multi-discipl... more Trends in Cognitive Sciences is indispensable reading for anyone interested in this multi-disciplinary, diverse field. At the heart of the journal there are authoritative Reviews and Opinions written in an accessible style by leading authors, which summarize exciting developments in all of the cognitive sciences. The majority of Reviews and Opinions are commissioned by the editor, but some originate from proposals. All of them are thoroughly peer reviewed.

Behavior Research Methods, 1988

This paper describes a novel class of visual stimuli that have been created to study stroboscopic... more This paper describes a novel class of visual stimuli that have been created to study stroboscopic motion perception along multiple attributes. The stimuli are composed of discrete elements, each of which can be characterized by an arbitrary number of attributes (color, luminance, spatial frequency, binocular disparity, etc.). There are two choices for the spatiotemporal arrangement of each attribute: to elicit either unambiguous (unidirectional) motion perception or ambiguous (bidirectional) motion perception. This affords the unique ability to elicit simultaneous motion perception in opposing directions from identical stimuli. The prevailing direction depends on the relative strength of the attributes’ contribution to the motion mechanisms (for short frame duration), or on the particular attribute being attended to (for long frame duration). Thus, this class of stimuli opens up the possibility of isolating specific motion mechanisms in the visual system.

International Journal of Imaging Systems and Technology, 1996

We present a series of experiments whose results show strong similarities between textural groupi... more We present a series of experiments whose results show strong similarities between textural grouping and motion experiments. A family of stimuli consisting of elements of different colors, luminancepolarity, and orientation are used in experiments in both eliciting textural grouping and detection of apparent motion. Among the similarities are that the orientation attribute is a weaker attribute than either color or luminance polarity in eliciting both textural grouping and in detection of apparent motion. However, if the orientation elements are collinear they become salient and contribute toward grouping and apparent motion. The results also indicate that chromatic mechanisms play a significant role in both texture and motion perception. The similarity suggests that perceptual rules governing spatial grouping are analogous to those governing spatiotemporal grouping. The results of these experiments could be used in the areas of image segmentation, pattern recognition, and scientific visualization.

European Journal of Neuroscience, 2005

At what stages of the human visual hierarchy different features are bound together, and whether t... more At what stages of the human visual hierarchy different features are bound together, and whether this binding requires attention, is still highly debated. We used a colour-contingent motion after-effect (CCMAE) to study the binding of colour and motion signals. The logic of our approach was as follows: if CCMAEs can be evoked by targeted adaptation of early motion processing stages, without allowing for feedback from higher motion integration stages, then this would support our hypothesis that colour and motion are bound automatically on the basis of spatiotemporally local information. Our results show for the first time that CCMAE's can be evoked by adaptation to a locally paired opposite-motion dot display, a stimulus that, importantly, is known to trigger direction-specific responses in the primary visual cortex yet results in strong inhibition of the directional responses in area MT of macaques as well as in area MT+ in humans and, indeed, is perceived only as motionless flicker. The magnitude of the CCMAE in the locally paired condition was not significantly different from control conditions where the different directions were spatiotemporally separated (i.e. not locally paired) and therefore perceived as two moving fields. These findings provide evidence that adaptation at an early, local motion stage, and only adaptation at this stage, underlies this CCMAE, which in turn implies that spatiotemporally coincident colour and motion signals are bound automatically, most probably as early as cortical area V1, even when the association between colour and motion is perceptually inaccessible.

Applications of computer graphics to the visualization of meteorological data

ACM Siggraph Computer Graphics, 1988

Page 1. @ Computer Graphics, Volume 22, Number 4, August 1988 APPLICATIONS OF COMPUTER GRAPHICS T... more Page 1. @ Computer Graphics, Volume 22, Number 4, August 1988 APPLICATIONS OF COMPUTER GRAPHICS TO THE VISUALIZATION OF METEOROLOGICAL DATA TV Papathomas, JA Schiavone and B. Julesz AT&T Bell Laboratories Murray Hill, New Jersey 07974 ...

Neuroreport, 2005

Feature-based attention a¡ects the processing of the selected feature throughout the visual ¢eld.... more Feature-based attention a¡ects the processing of the selected feature throughout the visual ¢eld. Here, we show that such global attentional modulation is not restricted to the attended feature but spreads to task-irrelevant features that are bound to the attended one. Attention to a color in one of the visual hemi¢elds affected the processing of task-irrelevant motion in the other hemi¢eld when it was associated with a stimulus that shared the attended color. This cross-feature global attentional selection in-creased the duration of the motion aftere¡ect and the strength of functional magnetic resonance imaging responses in the motion-sensitive area MT + , evoked by the task-irrelevant motion. These ¢ndings imply that features belonging to the same object are bound and selected jointly even outside the focus of attention. NeuroReport 16:1389^1393 c 2005 Lippincott Williams & Wilkins.

Implicit Attentional Selection of Bound Visual Features

Neuron, 2005

Traditionally, research on visual attention has been focused on the processes involved in conscio... more Traditionally, research on visual attention has been focused on the processes involved in conscious, explicit selection of task-relevant sensory input. Recently, however, it has been shown that attending to a specific feature of an object automatically increases neural sensitivity to this feature throughout the visual field. Here we show that directing attention to a specific color of an object results in attentional modulation of the processing of task-irrelevant and not consciously perceived motion signals that are spatiotemporally associated with this color throughout the visual field. Such implicit cross-feature spreading of attention takes place according to the veridical physical associations between the color and motion signals, even under special circumstances when they are perceptually misbound. These results imply that the units of implicit attentional selection are spatiotemporally colocalized feature clusters that are automatically bound throughout the visual field.

Stereoscopic illusion based on the proximity principle

Perception, 1989

A class of ambiguous random-dot stereograms were created that share the following interesting pro... more A class of ambiguous random-dot stereograms were created that share the following interesting property: Although the binocular disparity forms a periodic 'sawtooth' waveform as a function of row number (the disparity is constant for a given row), these stimuli yield a monotonically increasing depth percept along the rows. The random-dot pattern of each row is periodic along the horizontal direction for the purpose of producing an ambiguous depth percept. It is this ambiguity that makes it possible for the periodic stimulus to give rise to a monotonic percept. This monotonic percept is substantially enhanced when the rows are shown in temporal sequence instead of all being displayed together. Experiments are reported which indicate that this illusion is due to the proximity, or pulling, effect in stereopsis.

Vision Research, 2001

Contextual inhibition of neural activity in the primary visual cortex begins immediately and is m... more Contextual inhibition of neural activity in the primary visual cortex begins immediately and is most pronounced in the early transient response component. Using backward masking to control available processing time, we investigated whether the interaction between perceptual contextual modulation and processing time reflects the neural dynamics of contextual inhibition. We found that the threshold elevation due to contextual inhibition in an orientation-discrimination task is essentially independent of the available processing time and that it is closely related to contextual inhibition of the early transient response component of orientation-selective neurons in the primary visual cortex.

Vision Research, 2004

Object-based theories of visual attention predict that attempting to direct attention to a partic... more Object-based theories of visual attention predict that attempting to direct attention to a particular attribute of a visual object will result in an automatic selection of the whole object, including all of its features. It has been assumed, but not critically tested, that the spreading of attention from one feature to another in this manner, i.e. cross-feature attentional (CFA) effects, takes place at object-level stages of processing as opposed to early, local stages. In the present study we disambiguated these options for color-tomotion CFA by contrasting attention's effect on bivectorial transparent versus bivectorial locally paired motion displays. We found that association between features at the global, but not at the local, stage of motion processing leads to cross-feature attentional effects. These findings provide strong psychophysical evidence that such effects are indeed object-based.

International Journal of Infectious Diseases, 2008

Inflammatory pseudotumor is a relatively rare entity; originally identified in the lung, it has b... more Inflammatory pseudotumor is a relatively rare entity; originally identified in the lung, it has been described in multiple extrapulmonary anatomic locations.We report on the unusual case of an inflammatory pseudotumor associated with Mycobacterium tuberculosis infection, which was initially mistaken for a renal malignancy both in clinical and radiological settings. We additionally present three brief reviews concerning: (1) infectious agents postulated to induce morphological changes of an inflammatory pseudotumor; (2) mycobacterial pseudotumors; and (3) distinction from inflammatory myofibroblastic tumors of the renal pelvis.The present case highlights the diagnostic importance of PCR-based detection of mycobacterial DNA in granulomatous tissue responses. It is of crucial importance that clinicians are aware of this unusual manifestation of mycobacterial infection to ensure that pertinent laboratory evaluation is employed and appropriate treatment is administered in order to avoid potential clinical implications.

European Journal of Endocrinology, 2006

Objective: The pathological distinction between parathyroid neoplasms and hyperplasias remains di... more Objective: The pathological distinction between parathyroid neoplasms and hyperplasias remains difficult in several cases. Endoglin (CD105) is a proliferation-associated and hypoxia-inducible protein abundantly expressed in angiogenic endothelial cells. Vascular endothelial growth factor (VEGF) induces angiogenesis and VEGF-R2 is a tyrosine kinase receptor expressed early in development by endothelial cell precursors. We attempted to examine whether immunohistochemical expression of CD105, VEGF and VEGF-R2 may be useful in distinguishing between parathyroid hyperplasia and neoplasia as well as to elucidate, to some extent, the mechanism of neovascularization in proliferative lesions of the parathyroid gland. Design: Tissue specimens were taken from 38 patients with primary hyperparathyroidism (HPT) (17 adenomas and 21 primary hyperplasias) and from 30 patients with secondary HPT. Normal glands served as controls. Methods: In a standard immunohistochemical procedure, monoclonal antibodies to endoglin, VEGF and VEGF-R2 were applied to detect angiogenic endothelial cells. Immunostaining was estimated by image analysis and statistical analysis was subsequently performed. Results: Positive CD105 immunoreaction was significantly increased in parathyroid adenomas by comparison with primary hyperplasias (PZ0.033) and with secondary hyperplasias (PZ0.033). When parathyroid adenomas, primary hyperplasia and secondary hyperplasia specimens were comparatively evaluated, VEGF immunoreaction was much more common in adenomas (PZ0.018). In addition, in samples with secondary hyperplasia, VEGF-R2 immunoreactivity was positively linked with VEGF expression as well as with the apoptotic index of parathyroid cells (PZ0.038 and 0.010 respectively). In secondary hyperplasia specimens, an inverse correlation between cyclin D1 immunoexpression and angiogenic indexes, such as CD105 and VEGF, was noticed (PZ0.007 and 0.0017 respectively). Conclusions: This study shows increased angiogenesis in parathyroid adenomas compared with parathyroid proliferative lesions. In secondarily hyperplastic glands increased angiogenesis and increased apoptosis occur simultaneously; in the latter glands, the overexpression of cyclin D1 does not appear to be the genetic abnormality responsible for increased angiogenesis.

Diagnostic Pathology, 2008

Background: Pseudosarcomatous myofibroblastic lesions of the urinary bladder are relatively rare ... more Background: Pseudosarcomatous myofibroblastic lesions of the urinary bladder are relatively rare entities of an uncertain pathogenesis and benign indolent nature. We present an extremely rare case of an ALK-1-positive pseudosarcomatous myofibroblastic lesion of the urinary bladder, which was initially misinterpreted as a low-grade leiomyosarcoma of myxoid subtype on histologic examination owing to prominent atypia, high mitotic activity, abnormal mitotic figures and infiltration of the bladder wall. Although the histologic features were suggestive of a sarcoma, the correct diagnosis was finally established and radical surgical treatment was subsequently avoided. The patient is currently free of disease without any evidence of tumor recurrence or metastasis at 3 years post-operatively. The key differentiating point rests in distinguishing the aforementioned mass forming lesion from the myxoid subtype of low-grade leiomyosarcoma in order to avoid unnecessary radical therapy.