Biomedical subjects

M Boucart

Publications and source records attributed to M Boucart.

4 recordsLinked to original sources

PubMed1992

The computation of perceptual structure from collinearity and closure: normality and pathology.

Three experiments tested how structural information affects the processing of fragmented forms, using evidence from both normal subjects and an agnosic patient. The stimuli were either (a) outline drawings of objects; (b) "well" or "poorly" structured fragmented forms derived from outline drawings (Experiments 1 and 2) or (c) "well" or "poorly" structured fragmented forms derived from outline drawings of meaningless forms (Experiment 3). "Well" and "poorly" structured forms varied on the properties of collinearity and closure of the elements. Subjects decided if two simultaneously presented forms had the same orientation or were mirror-reversed. Three levels of discriminability were examined: (a) for "symmetrical" forms the decision had to be based on the detection of a small feature appearing on the same or the opposite side in the two forms; (b) for "asymmetrical" forms the decision had to be based on a large part located on the same or the opposite side of each object; and (c) for "oblique" forms, matching could be based on the global orientation of the shapes. Normal subjects performed equivalently on outline drawings and "well" structured fragmented forms and worse on "poorly" structured forms. This effect was strongest on "asymmetrical" stimuli, both with meaningful (Experiments 1 and 2) and meaningless forms (Experiment 3). This indicates that the segmentation of forms into parts, for judgements with "asymmetrical" stimuli, is affected by the structural properties of collinearity and closure. For the agnosic patient, performance was similar for "well" and "poorly" structured forms, and there was better performance in the "oblique" condition. This suggests that the patient could use global shape information, but that this coexists with a selective deficit in computing local perceptual structure, based on collinearity and closure between form elements. The implications of the results for understanding form perception and the clinical use of fragmented forms, are discussed.

Adult↗

PubMed1992

Global shape cannot be attended without object identification.

Whether the global shape of objects can be processed without accessing semantic or identity information was tested. Ss judged which of 2 fragmented forms had the same global shape as a reference stimulus. Matching stimuli could be physically identical, semantically related, or unrelated. The reference stimulus and nonmatching (distractor) form could be semantically related or unrelated. Similarity effects in the related condition were unconfounded with matches nameable and nonnameable forms. For nameable forms, related matching forms facilitated performance; a related distractor disrupted performance. Semantic interference was eliminated when nameable distractors were replaced with nonnameable partners; semantic similarity effects on matching were eliminated with a nonnameable reference stimulus and with inverted targets and distractors. Access to information concerning global shape does not normally occur without object identification.

Adult↗

PubMed1991

Influence of physical and semantic information in a categorisation task of fragmented forms.

Two experiments were carried out to investigate the influence of structural and semantic information in the categorisation of visual forms. In experiment 1 the stimuli were pairs of 'well structured' or 'poorly structured' fragmented forms differing in structural properties including convexity, collinearity, and closure of the elements, and in their 'nameability'. The influence of structural and semantic information was tested by means of a task involving the presentation to both visual hemifields separately of stimuli having within-category physical similarity. For well structured forms, subjects were asked to judge if two sequentially presented forms belonged to the same semantic category. For poorly structured forms, subjects were required to decide if two stimuli belonged to the same previously learnt category of forms. For the two types of stimuli, one category was composed of physically similar items, and the other was composed of physically dissimilar items. The results show a marked advantage for the category composed of physically similar forms when the stimuli are well structured but no differences between the two categories when the stimuli are poorly structured. This suggests a facilitation in the computation of global shape information for forms having collinear and closed elements. The only effect of semantic information was a tendency towards a right visual field advantage for same-category pairs of well structured forms. The pattern of results for well structured fragmented forms (experiment 1) and forms in which fragments had been joined in order to complete the contour (experiment 2) were similar, suggesting that structured incomplete forms can be processed in the same way as forms having a continuous contour. Hypotheses about the organisational processes of forms are proposed in the discussion.

Adult↗

PubMed1991

A study of the effect of structural information and familiarity in form perception.

Two experiments were carried out to investigate the influence of structural information and familiarity on the processing of visual forms. Pairs of "well" structured and nameable and "poorly" structured and non-nameable fragmented forms were employed as stimuli. The effects of structure and familiarity were assessed by manipulating the visual hemifield of presentation and the task. In Experiment 1 stimuli were judged as being either in the same orientation or mirror-reversed, a task that does not require high-level semantic information to be processed. Experiment 2 required physically identical forms to be matched, which may use either physical or name information. In Experiment 1 "same" judgements were equivalent for both types of stimuli, and "different" judgements were longer for the "poorly" structured (non-nameable) forms. In Experiment 2 there was little overall difference between "well" and "poorly" structured forms, though response times to "well" structured (nameable) forms were slowed for right-visual-field presentations. It is suggested that familiarity may not be sufficient to provide a perceptual advantage for nameable forms, as the advantage for nameable stimuli was confined to "same" judgements in Experiment 1 and response times were shorter for non-nameable stimuli in Experiment 2. Rather, performance depends upon factors such as the computation of global shape (due to structural properties of collinearity and closure) and on the use of different kinds or representations (physical versus name) in matching.

Adult↗