Biomedical subjects

H C Nothdurft

Publications and source records attributed to H C Nothdurft.

At least 19 recordsLinked to original sources

PubMed1994

Common properties of visual segmentation.

Recent experiments on the psychophysics of texture segmentation have produced several interesting observations that provide a useful basis for discussing the underlying neuronal mechanisms. Segmentation is predominantly achieved by analysis of local differences across a texture border, not global similarities. This suggests the involvement of a local, orientation contrast-sensitive mechanism similar to lateral inhibition in the retina. For segmentation to occur, orientation differences at the texture border must be increased when the overall orientation variation in the background of a pattern is raised. Similar properties are found for other visual clues that provide perceptual segregation. For motion, segmentation is based on the detection of local differences between areas rather than the analysis of global motion coherence within areas. Such local differences must be increased for successful segmentation when background movement is made non-uniform. Colour, luminance and binocular disparity behave in a similar way, suggesting a general role for feature contrast in visual segmentation.

Animals↗

PubMed1993

The role of features in preattentive vision: comparison of orientation, motion and color cues.

Arrays of lines or blobs were used to investigate the role of features vs feature contrast in preattentive vision. This study continues earlier work on orientation cues and extends it into the dimensions of motion and color. Tests were performed on patterns displaying continuous feature gradients, i.e. continuous variation, from element to element, in either line orientation, direction of motion, or color. In different series of experiments, the following four aspects of visual perception were investigated: (i) detection of a salient target ("odd man out" paradigm), (ii) segmentation of texture fields, (iii) search strategies for given targets, and (iv) figure-ground discrimination by grouping. Features were, in general, not found to play an important role in these tasks and performance was instead related to feature contrast. Only in the case of color did performance also depend upon hue, i.e. feature properties themselves. Whereas in all dimensions tested pop-out ("saliency") and segmentation were obtained from target or border elements whose local feature contrast was well above the level of variation elsewhere in the pattern, performance in the two latter tasks differed between orientation, motion, and color. In search, orientation targets were detected quickly when sufficiently distinct from their neighbors, but were apparently searched for serially if feature contrast was similar to that of other elements nearby. Color targets, however, were always detected fast in these patterns, independently of local feature contrast. Also, the perceived grouping of orientation or motion defined targets depended only on local feature contrast and not on the similarity of target elements. In fact, figures of dissimilar elements were seen as easily as figures of similar elements, indicating that, in these dimensions, stimulus coherence is not essential for the discrimination of figure and ground. For color, however, figures made up of the same targets were always seen slightly better than figures composed of different colors.

Adolescent↗

PubMed1993

Absence of express saccades to texture or motion defined targets.

Subjects were asked to foveate luminance defined, texture (orientation) defined, or motion defined targets while the reaction times of their saccadic eye movements were measured. With luminance defined targets, latencies followed the known bimodal distribution of fast ("express") and normal saccades, but in five highly trained subjects neither orientation nor motion defined targets evoked express saccades. Interestingly, even luminance targets no longer generated express saccades when the contrast was adjusted so as to match the salience of orientation or motion defined targets. This suggests that the neuronal mechanism underlying the generation of express saccades is driven by localized, high-contrast stimuli at an early processing stage of the visual system. Together with the neurophysiological findings in the monkey that both orientation and motion contrast produce increased activity at early cortical levels, the negative finding of this study indicates only little, if any, direct contribution of V1 activity to the generation of express saccades.

Adult↗

PubMed1993

Faces and facial expressions do not pop out.

Subjects were asked to detect faces or facial expressions from patterns with a variable number of nonfaces or faces expressing different emotions. In most tests, reaction time was found to increase steeply with sample size, thus indicating serial-search characteristics for the patterns tested. There were, however, considerable differences in the slopes of the graphs (search time versus sample size), which could be attributed to visual (but not face) cues that are discriminated at similar speeds. Slopes did not change when patterns were presented upside down, although such a modification strongly affects the perception of faces and facial expressions.

Face↗

PubMed1993

The conspicuousness of orientation and motion contrast.

Subjects were asked to compare the salience of different targets in a texture array. Elements that popped out from local differences in orientation or motion were compared with texture elements that popped out from luminance because they were brighter than all other elements in the pattern. The luminance of a comparison target that produced an equal preference rating for both targets was taken as a direct measure of the salience of the target under test. Saliency matches were made for targets in different background conditions; orientation and motion defined targets were tested in separate experiments. The data show that the salience of pop-out targets does not linearly increase with increasing orientation or motion contrast but reveals nonlinear properties from threshold and saturation effects. With increasing variation of background elements, target salience decreases continuously. Already small variations in background texture make a given target appear less compelling than when presented on a homogeneous background. The data also show that orientation and motion properties behave very similarly in these aspects and suggest that a similar neuronal mechanism may underly pop-out in both visual dimensions.

Adult↗

PubMed1992

Feature analysis and the role of similarity in preattentive vision.

Texture arrays of line elements at various orientations were used to study three phenomena of preattentive vision. Subjects were asked (1) to discriminate texture areas and to distinguish their form (experiments on texture segmentation); (2) to detect salient or vertical line elements (experiments on pop-out); and (3) to identify configurations of similar or or dissimilar targets (experiments on grouping). Within the patterns, line orientation was systematically varied to distinguish the effect of differences between areas from the effect of similarity within areas. In all of the experiments, performance was found to depend on local orientation contrast at texture borders rather than on the analysis of line orientation itself. Texture areas were correctly identified only when the orientation contrast at the border well exceeded the overall variation of line orientation in the pattern. Similarly, only target elements with high local orientation contrast were detected fast and "in parallel". Targets with an orientation contrast lower than background variation required serial search. Preattentive grouping was found to depend on saliency, as defined by local orientation contrast, but not on the similarity of line elements. In addition to local orientation contrast, which played an important role in all of the visual phenomena studied, influences from the alignment of line elements with the outline of a figure were also seen.

Adult↗

PubMed1991

Different effects from spatial frequency masking in texture segregation and texton detection tasks.

The paper reports psychophysical experiments set up to study the visual cues used in texture discrimination. In particular, the special role of "textons", i.e. distinct visual features such as blobs of different size, lines at different orientation, line intersections ("crossings") and line ends ("terminators") which have been proposed to provide the basis of perceptual segregation of texture areas, has been investigated. Texture pairs were briefly presented and simultaneously masked with two-dimensional visual noise at various spatial frequency bands. In different tasks on similar patterns, observers had to estimate the orientation of globally dissecting texture areas ("texture segregation") and to identify and distinguish the texture elements themselves ("texton detection"). Differential masking effects between these tasks indicate that texture segregation is often based on visual cues different from the supposed texton features. The segregation of crossing or terminator differences is also achieved from associated differences in the spatial frequency composition, that of differences in blob size from associated differences in mean luminance. Only differences in line orientation revealed similar masking curves in texture segregation and texton detection tasks.

Adult↗

PubMed1991

Texture segmentation and pop-out from orientation contrast.

In arrays of oriented lines, a target at a different orientation is effortlessly detected; it "pops out" from the pattern. Similarly, textures with line arrays at different orientations seem to dissect into separate areas with the spontaneous percept of distinct borders between them. In recent models, these perceptual phenomena were linked to the pre-attentive detection of certain features and of first-order differences in their spatial distribution. In contrast, however, psychophysical experiments show that texture segmentation and visual pop-out arise from orientation differences rather than from the orientation features themselves, a view which is supported by neurophysiological data from the monkey visual cortex.

Attention↗

PubMed1990

Texton segregation by associated differences in global and local luminance distribution.

Perceptual segregation of visual textures has been attributed to certain features ('textons') such as (elongated) blobs of given size and orientation, line crossings, and line ends. Differences in the spatial distribution of these features were assumed to be detected pre-attentively and to provide the instantaneous impression of segregating texture areas and of borders between them. This paper questions the validity of this general view and, in particular, the role of some of these features in texture discrimination. It is demonstrated that for some textons, perceptual segregation is independent of detection and discrimination of the texton itself. In addition, segregation can be strongly affected by positional or luminance jitter of texture elements or by other modifications that change the luminance distribution in the pattern but do not affect the supposed texton differences. From the textons reported in the literature, only differences in orientation were found to be fairly robust against such modifications.

Humans↗

PubMed1990

Texture discrimination by cells in the cat lateral geniculate nucleus.

The spontaneous segregation of texture areas is an impressive perceptual phenomenon, the neural basis of which is not yet understood. In the texton concept (Julesz and Bergen 1983; Julesz 1984, 1986) it is assumed that the visual system analyzes a stimulus for certain features ('textons') the spatial distribution of which is pre-attentively registered and may provide the percept of dissected texture areas. Supposed textons are blobs of a given size, oriented lines, line intersections and line terminators, suggesting that texture analysis is exclusively mediated by form-specific filters at higher, e.g. cortical, processing levels. This paper investigates the contribution of cells in the cat lateral geniculate nucleus (LGN) to segregation of typical texton differences. The results indicate that LGN cells, though not resembling the supposed texton filters, often distinguished textured arrangements of such features on the basis of a variety of other visual cues, such as global or local variations in mean luminance or differences in spatial frequency composition. Thus, cells responded to texture borders between areas differing in the size or the density of texture elements and often revealed differential firing rates to textures differing by the crossing or the terminator feature. For textures with differences in line orientation, however, only small variations of the firing rate were seen. In summary, the observations suggest a means of texture representation in the cat LGN which is different from recent concepts of texture segregation in man. For a given pair of textures, cells with receptive fields larger than, or similar to the texture raster respond to global and local luminance variations between areas and, in particular, to differences in their spatial frequency composition. These cells, hence, may signal the global texture difference without encoding spatial details of the pattern from which texton features could be identified. Cells with receptive fields small in comparison to texture elements transfer all the information necessary for analyzing these elements in detail, but themselves are relatively insensitive to global texture differences.

Animals↗

PubMed1987

Some observations on dynamic properties of receptive field organization of complex cells in cat visual cortex.

The dynamic properties of discharge fields (DFs) of the complex cells in areas 17 and 18 of cat's brain were observed. The sequence of activation for different areas in the DF indicated a concentric arrangement of latency distribution. The DE organization was proved to be dependent closely on velocity of stimulus movement. Twin- or triple-DFs were found for some cells, each responded selectively to one of the opposite directions of movement.

Animals↗

PubMed1985

Sensitivity for structure gradient in texture discrimination tasks.

Recent experiments indicate that the segregation of visual structures ("texture discrimination") depends not only on the form of texture elements but also on their spacing. Structures with discriminable elements in close proximity can be segregated more easily than patterns in which the same texture elements are more widely spaced. In dot arrays with areas of different dot luminance, segregation was found to depend on both the luminance difference and dot spacing; discrimination of texture areas in coarse dot rasters required greater differences in luminance than in fine rasters. Also, in regular arrays of iso-luminant line patterns, the maximal spacing between neighbouring lines for which different texture areas could still be discriminated was found to be influenced by the degree of dissimilarity between elements. For lines of a given length, texture areas with small differences in orientation became indiscriminable at smaller spacings than texture areas with orthogonal line orientations. Line length additionally had a strong effect on texture discrimination; increasing the line length for a given spacing provided easier segregation of texture areas. However, over a range of raster widths, discrimination of texture areas with a given difference in line orientation varied not with absolute values of line length but with the ratio of line length to interline spacing. Overall, the data suggest that texture discrimination in man is based on the evaluation of variation in structure over space (defined as the "texture gradient"). If local variation of structure is too small, texture areas cannot be discriminated, though differences between texture elements themselves may be apparent. As far as the dependence on variation over space is concerned, discrimination of iso-luminant textures resembles the limited sensitivity of the visual system for differences in texture luminance.

Adult↗

PubMed1985

Texture discrimination: representation of orientation and luminance differences in cells of the cat striate cortex.

Neuronal texture discrimination in the cat striate cortex was investigated by measuring the responses of single cells to different pattern structures. The representation of two independent features, texture orientation and texture luminance, was analysed in detail and the sensitivity of neurones to either feature was studied at different levels of structure density. Texture patterns were systematically moved across the receptive field. From the cell response to various parts of the pattern, "response patterns" were generated which displayed the cell transform of the textured stimulus pattern. Only when texture structures were coarse, were cells able to encode the texture orientation of an area. Differences in texture luminance, on the other hand, were detected only in fine texture structures. Further, these textural features were processed in a different manner: Cells responded to differences in texture luminance but continuously to areas of similar texture orientation. Thus, responses of striate cells reveal an ambiguous representation of texture features and a failure to uniquely encode texture borders.

Animals↗

PubMed1985

Orientation sensitivity and texture segmentation in patterns with different line orientation.

Orientation sensitivity (OS) and the ability of human subjects to discriminate structured areas of different texture orientation (DOT) were investigated using line arrays of varying line length. In general, OS was mediated by shorter lines than was DOT; lines can be distinguished by their orientation before they give the impression of a texture border between adjacent regions with lines differing in orientation. This difference was found to hold over a range of retinal eccentricities from 5 degrees nasal to 30 degrees temporal. Decreasing visual acuity, associated with increasing distance from the fovea, cannot, however, account for the higher threshold of DOT, even taking into account that DOT requires a larger area for analysis than OS. When angle of orientation between adjacent texture areas was varied instead of line length, DOT thresholds at different retinal locations were reached at similar values. The difference between OS and DOT, found consistently at all retinal positions, suggests that they are mediated by distinct neural mechanisms.

Adolescent↗

PubMed1985

Texture discrimination does not occur at the cyclopean retina.

The ability to segregate texture patterns at the cyclopean retina was tested with random-dot stereograms. When fused, patterns displayed arrays of texture elements which varied either in their form or in apparent depth. If elements of different form appeared at similar disparity in the random-dot stereograms, they did not provide the visual impression of distinct texture areas, although individually they could be easily discriminated. When texture elements differed in apparent depth rather than in form, segregation of different areas was readily achieved. These results restrict the possible site in the visual system for texture discrimination.

Depth Perception↗

PubMed1985

Discrimination of higher-order textures.

Arrays of figural elements differing in certain features ('textons') may be visually segregated to yield the impression of a global figure of different texture. This fact was used to construct texture patterns of a higher level of complexity. In microstructure, these patterns reveal regular arrays of distinguishable figural elements, the segregation of which can be predicted from previous studies of human texture sensitivity. In macrostructure, clusters of such elements form new figural elements which, when repeated over space, themselves give the impression of texture at a perceptually higher level. Discrimination of such macrostructure textures was found to place similar restrictions on the form of figural elements as those of texture discrimination at the microstructure level.

Humans↗

PubMed1984

Representation of spatial details in textured patterns by cells of the cat striate cortex.

The ability of single cells to represent the spatial details of textured stimuli was investigated. Two complementary aspects of cell response were considered, the ability to discriminate fine stimulus details and the property of integration over wider areas of a structure to encode differences in mean luminance. Responses of simple and complex cells were distinct in some respects. Spatial discrimination: Simple cells would encode orientation of line arrays as long as individual line elements could be spatially resolved. By contrast, complex cells were able to distinguish the orientation of texture areas even when the individual lines of the stimulus were not resolved in their response. Threshold sensitivity for texture orientation was of the same order in both cell classes despite differences in receptive field size. Spatial integration: Complex cells responded to texture luminance differences of much coarser patterns than did simple cells. These responses, however, were not biased for contour orientation unless finer patterns were used. Only with very fine textures did responses become indistinguishable from those to uniform stimuli for both simple and complex cells. For complex cells, there was a smooth transition from resolution to fusion of spatial details with increasing structural density. Simple cells were insensitive to both detailed and global properties of a stimulus pattern over a wide range of texture density. Implications for alternative measures of visual acuity of single cells are discussed.

Animals↗

PubMed1982

Responses to coloured patterns in the macaque lateral geniculate nucleus: pattern processing in single neurones.

Cell responses to complex visual patterns such as compositions of broad-band (non-monochromatic) colour areas are presented. Patterns were scanned over the receptive field, and cell response at each point was recorded. "Response patterns" were constructed which display the cell transform of the stimulus pattern. Parvocellular layer (PCL) cells in the lateral geniculate nucleus, in a very sustained fashion, signal the spectral composition of areas in the pattern, cells of different classes showing different spectral responsiveness. Little or no edge enhancement was present. Magnocellular layer (MCL) cells mark luminance steps in a pattern; they are not colour-specific. Responses to monochromatic stimuli provided a reliable guide to cell responsiveness with mixed colours. However, integration by spectrally opponent mechanisms is present with broad-band colour stimulation and partly accounts for a high variability of response patterns within the same cell class. With some cells, stimulus patterns were either shown through narrow-band colour filters, or a monochromatic background was added. Response patterns to isolated wavelength components compared with those to full colour patterns revealed an almost linear additivity of individual spectral components. Adaptation to a chromatic background, on the other hand, strongly modified the effectiveness of excitatory and suppressive components in the stimulus pattern and markedly changed the structure of a response pattern compared with the non-adapted situation.

Animals↗