Cognitive Vision

Hemispheric asymmetries in hierarchical stimulus processing are modulated by stimulus categories and their predictability

by Nathalie Bedoin

Published in:
Kéïta, L., & Bedoin, N. (2011). Hemispheric asymmetries in hierarchical stimuli processing are modulated by stimulus categories and their predictability. Laterality, 16(3), 333-355.

Hemispheric dominance has been behaviourally documented for the local (left
hemisphere, LH) or global (right... more Hemispheric dominance has been behaviourally documented for the local (left
hemisphere, LH) or global (right hemisphere, RH) processing of hierarchical letters.
However, Fink et al. (1997) indicated that stimulus category modulates this hemispheric
asymmetry. The purpose of this study was to investigate the influence of the category (letters
versus objects) on hemispheric specialisation for global and local processing using a visual
half-field presentation in a task where participants ignored whether the target appeared at the
global or local level. In Experiment 1, we replicated the classic hemispheric asymmetry for
global/local processing of hierarchical letters. In Experiment 2, which consisted of
hierarchical objects processing, a RH dominance for the local level was observed. In
Experiment 3, a within-subject task was used where the anticipation about the stimulus
category was precluded, resulting in the classic RH and LH specialisations for global and
local processing for both letter-based and object-based stimuli. Taken together, these results
suggest that the highly-demanding local processing stage engages one hemisphere more than
the other, according to the lateralisation of cerebral networks specialised for this level, in
addition to stimulus category. Therefore hemispheric specialisation for global/local levels
depends at least in part, on the category of the stimuli.

Gaze direction determination in schizophrenia

by Guy Tiberghien

Sensitivity of cerebral hemispheres to the local and global components of verbal and non-verbal stimuli.

by Guy Tiberghien

A spatial perturbation technique for the investigation of discrete internal representations of visual patterns

by David Foster

A technique is proposed for the investigation of discrete internal representations assumed to be formed by the visual... more A technique is proposed for the investigation of discrete internal representations assumed to be formed by the visual system in response to pattern stimuli. The proposed technique involves applying a local 1-parameter group of spatial transformations to a pattern to generate a "continuum" of patterns. The visual discriminability of pairs of perturbed patterns corresponding to small fixed increments in the transformation parameter is determined at various points in the parameter range. By means of a simple rule, the characteristics of this discrimination performance are then related to the probability density functions assumed to underlie the hypothesized discrete internal representations. Two experimental applications of the proposed technique are described. The first is concerned with a discrete internal representation involving the specification of the collinearity or noncollinearity of the points in a pattern; the second is concerned with a discrete internal representation involving the specification of the acuteness or obtuseness of the angle between two lines in a pattern.

Discrete and continuous modes of curved-line discrimination controlled by effective stimulus duration

by David Foster

In previous experiments two extreme modes of visual discrimination performance have been investigated by measuring... more In previous experiments two extreme modes of visual discrimination performance have been investigated by measuring small differences in pattern shape at points along a continuum of pattern shapes. These two modes, associated with discrete and continuous encoding processes, were obtained by simultaneously manipulating the number of pattern components in the display and the effective duration of the display. In this experiment, discrimination performance was measured for a fixed number of pattern components, namely three, and variable display time course. The stimuli used were curved lines drawn from a continuum with curvature parameter s. There were three stimulus time courses: (1) 2-s stimulus display, (2) 100-ms stimulus display, and (3) 100-ms stimulus display followed by a post-stimulus mask. Discrimination performance declined smoothly and monotonically with s for (1), but varied non-monotonically with s revealing a central peak for (3). Performance for (2) was intermediate between that for (1) and that for (3). A reduction in effective stimulus duration alone was thus sufficient to cause a transition from continuous to discrete modes of discrimination performance, a result which may be compatible with an explanation of variable discrimination modes based on a method of successive internal approximations of the stimulus.

El proceso visoperceptivo en la enfermedad de Parkinson: de la retina a la corteza frontal

by José María Ruiz Sánchez de León

Visoperceptual Processing in Parkinson's disease. From the retina to the prefrontal cortex [In Spanish]. Revista de Neurología 2005, 40(9), 557-562.

Resumen:
Introducción. El creciente interés por los déficit cognitivos que presentan los pacientes con enfermedad... more Resumen:
Introducción. El creciente interés por los déficit cognitivos que presentan los pacientes con enfermedad de Parkinson ha dado lugar a la proliferación de trabajos sobre este tema durante los últimos años. En el presente artículo se revisa la alteración visuoespacial en dichos pacientes, que habitualmente se relaciona con otros trastornos como los que afectan a la planificación, la secuenciación, la atención o los procesos de la memoria. Desarrollo. Se exponen los hallazgos más relevantes que apuntan a que la existencia de esas
alteraciones visuoespaciales que muestran los pacientes con enfermedad de Parkinson no son siempre secundarias a otros déficit cognitivos de tipo frontal, como se ha afirmado en los últimos años, sino que pueden deberse a trastornos en otros puntos anteriores del proceso perceptivo (como consecuencia de la deficiencia dopaminérgica en el circuito basal-talámico-cortical). Para ello, se clasifican los trastornos visuoperceptivos en función de su localización cerebral: desde la retina hasta el núcleo geniculado lateral, la corteza visual y la corteza extraestriada, y, finalmente, la corteza frontal y prefrontal. Conclusiones. Se propone la presente clasificación de los trastornos según la localización de los mismos en aras de una selección objetiva de la muestra y de las pruebas neuropsicológicas utilizadas en los estudios. Es deseable, a este respecto, un mayor consenso entre los investigadores a la hora de diseñar las investigaciones que versen sobre alteraciones visuoespaciales en los pacientes con enfermedad de Parkinson.
Palabras clave:
Corteza visual. Enfermedad de Parkinson. Neuropsicología.
Percepción visual. Trastornos cognitivos. Trastornos
perceptivos.

Abstract:
Introduction. The growing interest in the cognitive impairment shown by patients with Parkinson’s disease has led to a wealth of research in this line over recent years. In this paper we review the visuospatial alterations in these patients, which are usually linked to other disorders such as those affecting planning, sequencing, attention or mnemonic processes. Development. We report the most relevant findings, which suggest that the existence of these visuospatial disorders shown by patients with Parkinson’s disease are not always secondary to other frontal-type cognitive impairments, as has been claimed in recent years. Instead, they may be due to disorders in other anterior points of the perceptive process (as a result of dopaminergic deficits in basal-thalamic-cortical circuits). Thus, visuoperceptual disorders are classified according to their location in the brain: from the retina to the lateral geniculate nucleus, the visual cortex and the extrastriate cortex and, finally, the frontal and prefrontal cortex. Conclusions. We propose this classification of the disorders according to their location to aid in achieving an objective selection of the sample and of the neuropsychological tests used in studies. In this regard, we consider that there should be a higher degree of agreement among researchers when it comes to designing research projects that deal with visuospatial disorders in patients with Parkinson’s disease.
Key words:
Cognitive disorders. Neuropsychology. Parkinson’s disease. Perceptive disorders. Visual cortex. Visual perception.

Scene articulation: dependence of illuminant estimates on number of surfaces

by David Foster

The ability of observers to detect changes in illuminant over two scenes containing different random samples of... more The ability of observers to detect changes in illuminant over two scenes containing different random samples of reflecting surfaces was determined in an experiment with Mondrian-like patterns containing different numbers of coloured patches. Performance was found to improve as the number of patches increased from 9 to 49. In principle, observers could have used space-average scene colour as the cue ('grey-world' hypothesis) or the colour of the brightest surface in the scene ('bright-is-white' hypothesis), as the two cues generally covary. In a second experiment, observers matched illuminants across different patterns in which the space-average cue and the brightest-patch cue were independently manipulated. The articulation of the patterns was varied: the number of patches increased from 49 (patch width 1 deg visual angle) to over 50000 (patch width 0.03 deg), while the gamut of colours was held constant. Space-average colour was found to be the dominant cue with all patterns except for those with the largest patches.

Color Constancy

by David Foster

A quarter of a century ago, the first systematic behavioral experiments were performed to clarify the nature of color... more A quarter of a century ago, the first systematic behavioral experiments were performed to clarify the nature of color constancy—the effect whereby the perceived color of a surface remains constant despite changes in the spectrum of the illumination. At about the same time, new models of color constancy appeared, along with physiological data on cortical mechanisms and photographic colorimetric measurements of natural scenes. Since then, as this review shows, there have been many advances. The theoretical requirements for constancy have been better delineated and the range of experimental techniques has been greatly expanded; novel invariant properties of images and a variety of neural mechanisms have been identified; and increasing recognition has been given to the relevance of natural surfaces and scenes as laboratory stimuli. Even so, there remain many theoretical and experimental challenges, not least to develop an account of color constancy that goes beyond deterministic and relatively simple laboratory stimuli and instead deals with the intrinsically variable nature of surfaces and illuminations present in the natural world.

Parallel detection of violations of color constancy

by David Foster

The perceived colors of reflecting surfaces generally remain stable despite changes in the spectrum of the... more The perceived colors of reflecting surfaces generally remain stable despite changes in the spectrum of the illuminating light. This color constancy can be measured operationally by asking observers to distinguish illuminant changes on a scene from changes in the reflecting properties of the surfaces comprising it. It is shown here that during fast illuminant changes, simultaneous changes in spectral reflectance of one or more surfaces in an array of other surfaces can be readily detected almost independent of the numbers of surfaces, suggesting a preattentive, spatially parallel process. This process, which is perfect over a spatial window delimited by the anatomical fovea, may form an early input to a multistage analysis of surface color, providing the visual system with information about a rapidly changing world in advance of the generation of a more elaborate and stable perceptual representation.

Horizontal-Vertical Filters in Early Vision Predict Anomalous Line-Orientation Identification Frequencies

by David Foster

A characteristic of early visual processing is a reduction in the effective number of filter mechanisms acting in... more A characteristic of early visual processing is a reduction in the effective number of filter mechanisms acting in parallel over the visual field. In the detection of a line target differing in orientation from a background of lines, performance with brief displays appears to be determined by just two classes of orientation-sensitive filter, with preferred orientations close to the vertical and horizontal. An orientation signal represented as a linear combination of responses from such filters is shown to provide a quantitative prediction of the probability density function for identifying the perceived orientation of a target line. This prediction was confirmed in an orientation-matching experiment, which showed that the precision of orientation estimates was worst near the vertical and horizontal and best at about 30-degrees each side of the vertical, a result that contrasts with the classical oblique effect in vision, when scrutiny of the image is allowed. A comparison of predicted and observed frequency distributions showed that the hypothesized orientation signal was formed as an opponent combination and horizontal and vertical filter responses.

Orientation contrast vs orientation in line-target detection

by David Foster

This study concerns the roles of absolute and relative orientation in determining detectability of a line-element... more This study concerns the roles of absolute and relative orientation in determining detectability of a line-element target in a background field of uniformly oriented line elements. Target detectability was determined as a function of background-field orientation, sampled at 5 deg intervals, for three levels of orientation contrast-the difference between target and background orientations-sampled at 10, 20 and 30 deg. Stimulus displays were presented briefly and followed by a mask, There were 10 observers, whose detection performance was quantified by the discrimination index d' from signal detection theory, Target detectability was found to depend both on absolute orientation, represented by background-field orientation, and on orientation contrast. At each level of orientation contrast, performance was best when the background field, not the target element, was vertical or horizontal. These data are difficult to explain by general models of orientation discrimination based on simple orientation opponency between local Line-sensitive filter units; three other models specifically concerned with target detection are briefly considered.

Multiple groups of orientation-selective visual mechanisms underlying rapid orientated-line detection

by David Foster

Visual search for an edge or line element differing in orientation from a background of other edge or line elements... more Visual search for an edge or line element differing in orientation from a background of other edge or line elements can be performed rapidly and effortlessly. In this study, based on psychophysical measurements with ten human observers, threshold values of the angle between a target and background line elements were obtained as functions of background-element orientation, in brief masked displays. A repeated-loess analysis of the threshold functions suggested the existence of several groups of orientation-selective mechanisms contributing to rapid orientated-line detection; specifically, coarse, intermediate and fine mechanisms with preferred orientations spaced at angles of approximately 90 deg, 35-50 deg and 10-25 deg, respectively. The preferred orientations of coarse and some intermediate mechanisms coincided with the vertical or horizontal of the frontoparallel plane, but the preferred orientations of fine mechanisms varied randomly from observer to observer, possibly reflecting individual variations in neuronal sampling characteristics.