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C Christou

Publications and source records attributed to C Christou.

9 recordsLinked to original sources
PubMed2000

Perception, representation and recognition: a holistic view of recognition.

It is clear that humans have mental representations of their spatial environments and that these representations are useful, if not essential, in a wide variety of cognitive tasks such as identification of landmarks and objects, guiding actions and navigation and in directing spatial awareness and attention. Determining the properties of mental representation has long been a contentious issue (see Pinker, 1984). One method of probing the nature of human representation is by studying the extent to which representation can surpass or go beyond the visual (or sensory) experience from which it derives. From a strictly empiricist standpoint what is not sensed cannot be represented; except as a combination of things that have been experienced. But perceptual experience is always limited by our view of the world and the properties of our visual system. It is therefore not surprising when human representation is found to be highly dependent on the initial viewpoint of the observer and on any shortcomings thereof. However, representation is not a static entity; it evolves with experience. The debate as to whether human representation of objects is view-dependent or view-invariant that has dominated research journals recently may simply be a discussion concerning how much information is available in the retinal image during experimental tests and whether this information is sufficient for the task at hand. Here we review an approach to the study of the development of human spatial representation under realistic problem solving scenarios. This is facilitated by the use of realistic virtual environments, exploratory learning and redundancy in visual detail.

Computer Simulation↗
PubMed1996

Shape constancy in pictorial relief.

Pictorial relief was measured for a series of pictures of a smooth solid object. The scene was geometrically identical (ie the perspective of the three-dimensional scene remained the same) for all pictures, the rendering different. Some of the pictures were monochrome full-scale photographs taken under different illumination of the scene. Also included were a silhouette (uniform black on uniform white) and a 'cartoon'-style rendering (visual contour and key linear features rendered in thin black line on a uniform white ground). Two subjects were naive and started with the silhouette, saw the cartoon next, and finally the full-scale photographs. Another subject had seen the object and did the experiment in the opposite sequence. The silhouette rendering is impoverished, but has considerable relief with much of the basic shape. The cartoon rendering yields well-developed pictorial relief, even for the naive subjects. Shading adds only small local details, but different illumination produces significant alterations of relief. It is concluded that shape constancy under changes in illumination is dominant throughout, but that the (small) deviations from true constancy reveal the effect of cues such as shading in a natural setting. Such a ¿perturbation analysis' appears more promising than either stimulus-reduction or cue-conflict paradigms.

Attention↗
PubMed1996

Surface gradients, contours and the perception of surface attitude in images of complex scenes.

Sophisticated computer graphics were used to generate images of three-dimensional blocks-world scenes to investigate the perception of surface attitude. Three types of surface primitive (planar blocks, cylinders, and ellipsoids) were combined to form structured settings. The experiments were designed to investigate whether surface-based information such as gradients in shading and texture provide any significant advantage in attitude judgments over information derived from object contours. Images of shaded, textured, and line-drawn surfaces formed the stimulus set. The subjects' task consisted of setting an attitude probe on different parts of the scene so that the probe appeared to be locally coplanar with the perceived surfaces. Analysis of settings according to attitude components, slant and tilt, shows remarkable agreement in slant settings for the shaded and line-drawn scenes but poor correlation between shaded and textured scenes. Similarly, tilt was also easily judged in shaded and line-drawn scenes and the experiments indicate that explicit surface boundaries are important for stable tilt perception. In general, the results suggest that, for the simple surfaces employed here, surface cues provide little extra information beyond that which is derived from contours.

Cues↗
PubMed1996

Perturbation study of shading in pictures.

Pictorial relief was measured for a series of pictures of a smooth solid object. The scene was geometrically identical for all pictures, but the rendering was different. Whereas all pictures were monochrome full-scale photographs, they were taken under different illuminations of the scene, the source being frontal and displaced towards either the upper left, the upper right, the lower right, or the lower left. It was found that different illuminations led to significantly different, systematic alterations of pictorial relief. It is concluded that though shape constancy under changes in illumination might be said to rule in the first rough approximation, the deviations from true constancy are indeed both significant and systematic. Different from either stimulus-reduction or cue-conflict paradigms, this 'perturbation analysis' shows that shading is used as an important source of information even if the particular illumination appears to be ignored at first blush. For all subjects, brighter parts in the stimulus were consistently interpreted as being nearer in pictorial space, both for the global layout and for the subsidiary relief.

Anisotropy↗
PubMed1991

The triggering role of physical injury in the onset of peripheral arthritis in seronegative spondyloarthropathy.

Three more cases of B27-positive patients who developed peripheral arthritis immediately after trauma are reported. The first had an exacerbation of arthritis in the right hip after falling from her motor-bike. The second had arthritis of the distal interphalangeal (DIP) joint of the right forefinger after shutting his finger in the door of his car. The third had arthritis of the right sternoclavicular joint after a road-accident while fastening her safety belt.

Adult↗
PubMed1990

Differential diagnosis between osteitis condensans ilii and sacroiliitis.

Sacroiliitis of seronegative spondyloarthropathy may sometimes show on pelvis plain films findings indistinguishable from those of osteitis condensans ilii. Computed tomography (CT) can differentiate earlier than plain radiography between the 2 conditions; furthermore, it should also be possible to make this differentiation clinically. The aim of our study was to verify whether the criteria recently proposed by the European Spondylarthropathy Study Group (EESG) for the classification of spondyloarthropathy are useful. CT scans through the synovial part of the sacroiliac joints of 7 consecutive patients meeting the ESSG criteria and showing typical findings of osteitis condensans ilii on plain films were mixed with those of 15 consecutive patients with osteitis condensans ilii not meeting the ESSG criteria. Scans were examined for joint space and surface abnormalities blindly and independently by 2 observers. Six patients in the spondyloarthropathy group and one in the osteiitis condensans ilii group showed clear erosions and/or joint space narrowing of less than 2 mm in at least one joint. The difference was statistically significant (p less than 0.001). Our results suggest that by using criteria valid for the whole group of seronegative spondyloarthropathies, it is possible to differentiate clinically between seronegative spondyloarthropathies with sacroiliitis mimicking osteitis condensans ilii and "true" osteitis condensans ilii.

Adult↗
PubMedDate not supplied

Structure and coherence of reasoning ability in Down Syndrome adults and typically developing children.

The present study investigates the ability of Down Syndrome (DS) adults to reason: (a) deductively with transitivity (linear and reverse relations) and categorical syllogisms (all-some relations); (b) inductively with classical verbal analogies and non-verbal analogical reasoning (Raven's Coloured Progressive Matrices); and (c) to retain information in short-term memory. The results have shown that: (i) The Down Syndrome adults did not differ from typically developing children, matched on expressive and verbal ability, in transitivity and non-verbal analogical thinking; (ii) they differed in categorical reasoning, classical verbal analogies and short-term memory. Application of a structural model demonstrated that, despite differences in slope means in the three measures, the structure of functioning within-and-across all domains of cognition tests and its growth pattern, equally reliable and coherent, goes in parallel for the Down Syndrome adults and the typically developing children. The results are discussed within the context of the two-group developmental and difference approach.

Adult↗