322,982 research outputs found
Conflicting strategies of encoding irregular shapes in a sentence-pictures veridication task.
No full textLatenza della risposta in funzione del contenuto e dell'ordine di presentazione degli stimoli (l'effetto sorpresa).
No full textDraughtsmen at work
No full textTo 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
Boundary completion, contrast polarity and the perception of illusory tilt
No full textWhat we perceive as a unitary object can be the result of integrative processes that
generate a whole from parts. Although this issue of visual perception has been widely explored,
recent experimental findings demonstrate that our knowledge is still incomplete. In particular,
the question whether contour binding is affected by the sign of contrast (contrast polarity) across
edges requires more in-depth examination. Here we show the effects of edge bindings that origi-
nate from the merging of laterally displaced edges with the same contrast polarity. We have
studied a particular context in which such effects may emerge: a checkerboard with a series of
alternated dark and light shapes superimposed on the corners of the squares. The phenomenal
observations and experimental findings support the theories according to which boundary
completions are originated by phenomena of edge propagation within a `field of completion'
(eg Shipley and Kellman, 2003 Perception 32 985 ^ 999) adjacent to an edge ending. Our findings
conform to the Shipley and Kellman theory that boundary completion results from the inter-
action of edges as well as from edges and shapes lacking in oriented contours, the latter serving
as `receiving units', anchoring the paths of activations generated by oriented edges. We propose
to integrate this theory with the hypothesis that interactions sensitive to the contrast sign gener-
ate conjunction paths of edges that alter their perceived orientation. Based on this perspective
we propose an alternative account for the Cafe ̈ Wall illusion that can be extended to other
phenomena of orientation misperception and to a Cafe ̈ Wall inversion effect that has not been
observed previous
Features of the selectivity for contrast polarity in contour integration revealed by a novel tilt illusion
No full textWe studied a novel illusion of tilt inside checkerboards due to the role of contrast
polarity in contour integration. The preference for binding of oriented contours having same
contrast polarity, over binding of opposite polarity ones (CP rule), has been used to explain
several visual illusions. In three experiments we investigated how the binding effect is influenced
by luminance contrast value, relatability of contour elements, and distance among them. Experi-
ment 1 showed that the effect was indeed present only when the CP rule was satisfied, and found
it to be stronger when the luminance contrast values of the elements are more similar. In experi-
ment 2 the illusion was reported only with relatable edges, and its strength was modulated by
the degree of relatability. The CP-rule effectiveness, thus, seems to depend on good continuation.
The intensity of contrast polarity signals propagating from an oriented contour might be the less
intense, the more its direction deviates from linearity. In experiment 3 we estimated the distance
threshold and found it to be smaller than the one found for other illusions, arising with collinear
fragments. This seems to show that the reach of the contrast polarity signal inside the association
field of a contour unit is shorter along non-collinear orientations than along collinear on
Effects of contextual and local factors on Ponzo illusion magnitude.
No full textPonzo illusion has been explained by considering either just the inducing elements present in a restricted area of the visual field, the same area in which the test elements are located, or the stimulus configuration as a whole in which even the most distal figural elements - i.e., the external converging lines, here called "Ponzo wedge" - play a crucial role. The two studies reported here aimed at showing that both global configurational characteristics and inducing elements locally interacting with the test stimuli can independently affect the illusory effect. This hypothesis was tested using stimuli in which graphic-inducing elements giving rise to a herringbone pattern (Coren & Girgus, 1978) were drawn in the same area of the test segments. Results of Exp. 1 confirmed the effect of the two factors. In particular, both factors proved to determine the illusion, since they induced illusory effects either in isolation or in the same/opposite direction. In Exp. 2 the relative weight of these two factors was evaluated in relation to the width of the angle of the inducing elements and to the distance of the test segments from the vertex. Results showed no linear relationships between the distance of the test segments from the external inducing elements and the weight of the Ponzo wedge factor
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