Sergio Roncato

Constructional apraxia : an information processing analysis

Abstract Constructional apraxia is a disorder resulting from neurological damage which is manifested by the inability to copy and/or construct to command two- and three-dimensional stimuli. Three neurological patients presenting with these clinical symptoms were administered a battery of tests that were developed on the basis of a model describing the processing stages involved in copying and construction to command. The patients investigated differed in the pattern of performance across the tests in the battery. It was suggested that constructional apraxia is a syndrome that may be caused by different processing impairments, although this conclusion was tempered by a difficulty in separating out some of the processing components.

Transformation processes upon the visual code

Seven experiments investigated whether orientation-dependent latency functions for the visual code resemble those observed in studies of mental rotations of visual images. The subjects were required to perform “same-different” classifications of two simultaneously presented letters. The dependent variables considered were reaction time IRT and accuracy. Experiments 1, 2, 4, 5, and 6 showed that subjects could correctly classify two different letters on the basis of the visual code without preceding transformations. Experiments 1, 2, and 7 showed orientationdependent effects for “same“ responses. It appeared, however, that orientation functions for the visual cede were clearly different from those previously observed for visual images. In addition, the findings of Experiments 4, 5, and 6 indicated that a frame that jointly rotated with the disoriented letters could eliminate the orientation-dependent effects for “same” responses. Experiment 7 showed that the results of Experiments 4, 5, and 6 must be attributed to the structural characteristics of the frame and not to a directional cue. The results of Experiment 3 seemed to demonstrate that transformations did not occur when the subjects used the phonetic code to classify the letters. Overall, the results of the seven experiments were considered to provide a demonstration of the importance of the distinction between the operations on visual images and those on the visual code.

The influence of contrast and spatial factors in the perceived shape of boundaries

When an edge can be perceived to continue either with a collinear edge of the opposite contrast polarity or with a noncollinear edge of the same contrast polarity, observers perceive an alignment between the edges of the same contrast polarity, even though they are noncollinear. Using several stimulus configurations and both free and tachistoscopic viewing, we studied the luminance and spatial factors affecting the perceived distortion and binding. The results showed that the two noncollinear edges tended to align when they had the same contrast polarity (Experiment 1A) and to misalign when they had opposite contrast polarity (Experiment 2), providing that (1) they were separated by a distance larger than 1 arcmin and smaller than 3–4 arcmin (for all configurations) and (2) they laterally overlapped for about 7 arcmin (Experiment 1B). The results also showed that the direction of apparent distortion depended on the direction of overlapping. The results of Experiment 3 ruled out the localattraction/repulsion explanation but, instead, supported the suggestion that the interactionconcerned the global edges, or part of them, and produced aninward tilt, which made the edges of the same contrast polarity perceptually to align, or anoutward tilt, so that the edges of opposite contrast polarity were perceived to be more misaligned. From the overlap and distance limits found, it can be inferred that for two noncollinear contours to join perceptually, the tilt must not exceed 18°, a limit compatible with the orientation bandwidth of contrast-sensitive early cortical mechanisms.

The effect of luminance variation on the apparent position of an edge

A gray outline against a white (or black) ground appears to deviate when one of the divided regions turns into black (white). The direction of shift is not predictable on the basis of luminance profile and polarity contrast of this part of contour, called gray edge (to indicate a stepwise gradient from black to gray and from gray to white). Rather, it appears to depend on the luminance profiles of the collinear regions: A gray edge shifts toward the dark side whenever collinear with a gray line traversing a white ground. The same gray edge takes the opposite direction whenever it extends against a black ground. This rule proved to be successful in predicting the illusory convergence of the sides of a square that formed the stimuli of the first experiment, but the magnitude of the phenomenon was affected by luminance ratios and polarity contrasts of the gray edges, in agreement with the findings of the experiments on gray or blurred edge misalignment. A second experiment tested some hypotheses predicting the combined effects of two or more distorting sources. These hypotheses, suggested by the physical theory of vector sum, were partially disproved. A new model is proposed that assumes different ways of integrating local distortions. The third experiment tested predictions of how distorting pulses in opposite directions combine. The illusory misplacement of edge studied in this experiment is proposed as the underlying phenomena of the café wall illusion, the hollow square illusion, and other illusory phenomena observed with blurred areas. A connection with the induction grid phenomena is hypothesized.

The Global Figural Characteristics in the Zöllner Illusion

The Zöllner illusion has been accounted for in terms of local interactions between the vertical lines and the crossing segments. Recently, however, some evidence supporting the importance of global figural characteristics—ie of figural elements that are not directly interacting with the test lines—in the occurrence of orientation illusions has been reported. Three experiments have been conducted with parts of the Zöllner figure to test whether this illusion is affected by the global figural characteristics. The results indicate that, similarly to what has been observed for other orientation illusions, the Zöllner illusion depends on both local and global characteristics of the stimulus configuration. In addition, results suggest a similar weight for both these figural characteristics in determining the occurrence of the illusory effect. Finally, relations among different orientation illusions are also discussed.

Orientation Misperceptions Induced by Contrast Polarity: Comment on “Contrast Polarities Determine the Direction of Café Wall Tilts” by Akiyoshi Kitaoka, Baingio Pinna, and Gavin Brelstaff (2004):

According to Kitaoka et al (2004, Perception 33 11–20), the Café Wall illusion can be reduced to misalignment effects produced locally by a large shape on a line passing nearby. I demonstrate here that the interacting units are edges and not whole shapes, and that the source of the illusion does not consist in a local tilt but in a tendency of the edges to join when they have the same contrast polarity.

Texture segregation on the basis of contrast polarity of odd-symmetric filters.

This is the first study to demonstrate the selectivity of learning for contrast polarity. The finding is the main result of an investigation into the existence of central and peripheral vision mechanisms selective for contrast polarity within the texture-segregation process, using the perceptual learning paradigm in a detection task. Energy models (Malik & Perona, 1990) exclude segregation of textures composed of elements of odd-symmetric luminance profile by contrast polarity differences. Here the target was a Gabor patch (0.8 deg) of 1 cyc/deg in sine phase (odd-symmetry) embedded in a background of mirror-image elements. Our results showed that, in fovea, segregation on the basis of contrast polarity was above threshold from the first session. After learning, the target popped-out in both central and peripheral vision for durations over 10 ms. Our major result is that learning is selective for contrast polarity; it is also selective for orientation and position, all characteristics distinctive of early processing. Since the learning effects were obtained with texture composed of odd-symmetric mirror-image elements, they indicate that the output from odd-symmetric filters was not excluded or inhibited in texture segmentation, but instead played an active role. Our data support models of texture segmentation, in which detection of texture gradient is achieved on the basis of early cortical process, before the non-linear transformation of their output.

Draughtsmen at Work

To obtain the representation of a contour, the visual system integrates fragments of a pattern. One of the ‘binding rules’ governing this process requires that a path of conjunction in which contrast polarity is preserved be followed. Here we show that this rule has a corollary: where two alternative paths compete to emerge in opposite directions, the one with greater contrast luminance is likely to prevail.

DIFFERENT LEVELS IN PROCESSING SIMPLE GEOMETRICAL FIGURES

An experiment has been conducted in order to extend to geometrical figures Posners (1969) findings of various levels of processing for alphabetical material. Pairs of simple figures (triangles and trapezoids) were tachistoscopically presented to subjects asked to judge if they were “same” or “different.” Subjects were instructed to respond “same” either if the two figures had the same shape or the same name, e.g., a scalene triangle paired with a right-angled one. The results support the theory of different levels of processing also for geometrical figures.

Perceived rigidity and nonrigidity in the kinetic depth effect

Stroboscopic simulations of three-dimensional rotating rigid structures can be perceived as highly nonrigid. To investigate this nonrigidity effect a sequence of either three (experiment 2 and 3) or thirty six frames (experiment 4) was used, each consisting of a set of dots with location on the horizontal axis corresponding to the parallel projection of a nominally defined helix. Observers were asked to judge the angle of rotation of eighty helices defined by the factorial combination of eight phase (φ) values (ie difference between the sinusoidal path of one dot and its neighbours) and ten different angular displacement values (α). When in each static frame the dots can be organized into curved dotted line (small values of φ), the perceived 3-D helices are highly nonrigid. But when shape information is not available in each static frame (high values of φ), the helices are perceived as rigid and rotation judgement is possible providing that α < 15°. It appears that at small values of φ observers fail to recover the rigid structure of the helices since the input to the structure from the motion process may be distorted.