Sandrine Delord

The Computation of Contour Information in Complex Objects

Perceptual organisation, and especially the computation of contour information, has been the object of considerable interest in the last few years. In the first part of the paper we review recent accounts on the mechanisms involved in the processing of contour. In the second part we report an experiment designed to examine (1) how physical parameters such as spatial proximity and collinearity of elements affect the integration of global contour in objects and (2) whether the activation of stored representations of objects facilitates the computation of contour. Incomplete forms varying in the spacing and the alignment of line segments on their contour were used as stimuli in a matching task. Subjects were asked to decide which of two laterally displayed figures matched a reference form presented previously. The matching target and the distractor were physically identical but differed in their orientation. In one condition the reference object was always an outline drawing of an object. In a second condition the reference object was either a complete object or a more or less identifiable incomplete form. Little variation in performance was found for forms having continuous and discontinuous contour up to a spacing of 5 pixels (10.8 min) between elements. Response times and errors increased abruptly beyond this limit. This effect occurred in the two conditions of reference stimulus, suggesting that the computation of contour information is more affected by physical constraints at early processes than by high-level processes involving activation of stored structural representations of objects.

Psychophysical assessment of magno- and parvocellular function in schizophrenia.

Recently developed psychophysical techniques permit the biasing of the processing of the stimulus by early visual channels so that responses reflect characteristics of either magno- or parvocellular pathways (Pokorny & Smith, 1997). We used such techniques to test psychophysically whether the global magnocellular dysfunction reported in schizophrenia also affects early processes. Seven schizophrenic patients and 19 normal controls participated. The task was a four-alternative forced-choice luminance discrimination, using a 2 x 2 configuration of four 1-deg squares. Target luminance threshold was determined in three conditions: the stimulus, including the target, was pulsed for 17 ms (pulse paradigm); the target was presented on a steady background of four squares (steady paradigm), or the target was presented alone (no background paradigm). We replicated previous results demonstrating magnocellular and parvocellular signatures in control participants. No evidence for an early magnocellular deficit could be detected as the thresholds of all schizophrenic observers were higher both in the steady paradigm (presumed magnocellular mediation) and in the pulse paradigm (presumed parvocellular mediation). Magnocellular dysfunction, if present in schizophrenia, must concern more integrated processes, possibly at levels at which parvocellular and magnocellular paths interact.

Alteration of Attentional Blink in High Functioning Autism: A Pilot Study

Autism is characterized by deficits in attention. However, no study has investigated the dynamics of attentional processes in autistic patients yet. The attentional blink (AB) paradigm provides information about the temporal dynamics of attention in particular about the allocation and the duration of an attentional episode. We compared 11 high functioning autistic patients with 15 control participants on a classical AB task. Autistic patients exhibited a deficit in rapidly allocated attentional resources. Further investigations such as coupling AB evaluation with neuroimaging data and/or increasing the size of groups, would allow for investigating the neurobiological substrates of these AB alterations in autistic patients.

Influence of Visual Deficits on Object Categorization in Normal Aging

Background/Study Context: The authors assessed whether age-related changes in low-level vision affects higher-level processes involved in object categorization. Methods: Thirty young and 30 older observers were asked to categorize gray levels photographs of natural and artifactual objects. The authors manipulated contrast (8% vs. 30%) and eccentricity (central vs. 21° peripheral presentations). Results: Older people were slower and less accurate than young people but this impairment varied with contrast and eccentricity. The pattern of performance was equivalent for young and for old people when pictures were presented centrally with a 30% contrast. Performance was impaired for older people when pictures were presented peripherally with a low contrast. Moreover, a category-specific deficit was found in the old group, specifically for peripheral presentations. Discussion: The results are consistent with an age-related deficit in the ability to categorize objects but the deficit was specifically observed under low-contrast condition and peripheral vision, suggesting a reduced response in the magnocellular pathway. The results are interpreted in the framework of age-related deficits in the two main visual streams.

Vieillissement de la catégorisation visuelle d’objet : interaction entre un déficit « fréquence spatiale-spécifique » et un déficit « catégorie-spécifique »

The study investigated the aging of object categorization manipulating the spatial frequency (SF) content in photographs of object and the object category. Thirty young (m=22 years old) and 24 mature adults (m=57 years old) categorized 120 items (animals/tools) presented for 200 ms each, in one of three versions: a normal version (no filter), a band-pass filtered version (medium to high SF) and a low-pass filtered version (low SF). Results showed that this categorization task relied mainly on the medium to high SF band and that the mature group had a large impairment on that band. This impairment resulted for this group in a category-specific deficit toward the tools, for which the weak intra-category similarity in the items requires that SF range to be processed. An impairment of performance with increasing age was also found for the low SF band, specifically for animals. This interaction between the SF-specific deficit with age and the category is discussed according to the relevance of SF band for the task and to the characteristics of the two categories.

Does a yellow filter improve visual object categorization in normal aging

ABSTRACT Previous studies have shown that a yellow filter (CPF450) can increase contrast, motion sensitivity, vergence, and accommodation. We investigated whether a yellow filter can reduce age-related visual deficits. We tested two groups of 60 observers (mean age 24 vs. 72) in an object categorization task. Grayscale photographs of natural objects and artifacts were displayed either centrally or peripherally (21°) at low (8%) or medium (30%) contrast. There were three filter conditions (no filter, placebo filter, and yellow filter). Both groups of observers performed similarly on central and medium-contrast pictures. The deleterious effects of reduced contrast and eccentricity were stronger in elderly individuals. Moreover, the yellow filter globally improved the speed of categorization for the elderly participants. The decrease in response time in the yellow filter condition was larger when the stimuli were displayed peripherally in both groups. A yellow filter should be considered as a potential means for visual improvement in normal aging.