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.
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 ×
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.
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