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Lines that induce transparency
No full textThree neighbouring opaque surfaces may appear split into two layers, one transparent
and one opaque beneath, if an outline contour is drawn that encompasses two of them.
The phenomenon was originally observed by Kanizsa [1955 Rivista di Psicologia 69 3 ^ 19; 1979
Organization in Vision: Essays on Gestalt Psychology (New York: Praeger)], for the case where
an outline contour is drawn to encompass one of the two parts of a bicoloured figure and a
portion of a background of lightest (or darkest) luminance. Preliminary observations revealed
that the outline contour yields different effects: in addition to the stratification into layers
described by Kanizsa, a second split, opposite in depth order, may occur when the outline
contour is close in luminance to one of the three surfaces. An initial experiment was designed
to investigate what conditions give rise to the two phenomenal transparencies: this led to the
conclusion that an outline contour superimposed on an opaque surface causes this surface to
emerge as a transparent layer when the luminances of the contour and the surface differ,
in absolute value, by no more than 13.2 cd mÿ2. We have named this phenomenon `transparency
of the intercepted surface', to distinguish it from the phenomenal transparency arising when the
contour and surface are very different in luminance. When such a difference exists, the contour
acts as a factor of surface definition and grouping: the portion of the homogeneous surface
it bounds emerges as a fourth surface and groups with a nearby surface if there is one close in
luminance. The transparency phenomena (`transparency of the contoured surface') perceived in this
context conform to the constraints of Metelli's model, as demonstrated by a second experiment,
designed to gather `opacity' ratings of stimuli. The observer judgments conformed to the values
predicted by Metelli's formula for perceived degree of transparency, a. The role of the outline
contour in conveying figural and intensity information is discussed
Texture segregation on the basis of contrast polarity of odd-symmetric filters
No full textThis 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
Saliency from orthogonal velocity component in texture segregation.
No full textWe found that a moving target line, more-vertical than 45 deg-oriented background lines, pops-out (d'=1.2) although it moves at the same speed of background elements and although it is invisible in static presentation (d'=.7). We suggest that the moving more-vertical target is more salient because the motion system responds to the orthogonal-velocity-component (V(perpendicular)=Delta d/Delta t sin theta) that is larger for the more-vertical target than for distracters. However, motion does not produce high d' when the target is more horizontal than background (d'=.6). This result is not expected if saliency resulted from the sum of saliency of orientation and motion independently coded but is instead predicted by visual search asymmetry. A line length effect on the moving target saliency also suggests that V(perpendicular) is extracted on the whole line and this operation is facilitated by line length in the same way for more-vertical and more-horizontal targets. Altogether, these results demonstrate that speed-based segmentation operating on V(perpendicular) not only affects speed and direction of motion discrimination, as previously demonstrated, but accounts for high saliency of image features that would otherwise prov
Mask interrupts texture segregation but not grouping processes
No full textAlthough texture segregation models confine border extraction to high-level processes, recent
findings suggest the involvement of low-level mechanisms (Grieco et al, 2006 Vision Research 46
3526 ^ 3536). To investigate whether texture segregation is restricted to early-filtering mechanisms,
or whether they involve later-levels of analysis, we measured how a backward-masking stimulus,
known to interrupt re-entrant signals from high-level areas (Lamme et al, 2002 Journal of Cogni-
tive Neuroscience 14 1044 ^ 1053), affects subject's accuracy in target-present (requiring texture
segmentation) and target-absent (requiring distractors grouping) trials in a detection task, after a
learning session without backward-masking. Here, we show that, although learning effects in
texture segregation were specific to contrast polarity, indicating low-level filtering mechanisms,
the improvements were abolished by a backward-mask presentation, suggesting the involvement
of re-entrant signals from high-level areas. Instead, learning of distractor grouping was not
specific to contrast polarity, nor was it affected by backward masking, indicating a high-level
learning process. This finding raises an important challenge that must be faced in understanding
the mechanisms underlying texture segregation and grouping by similarity
The effect of preview eccentricity in a lexical decision task
No full textWe report lexical decision experiments in which eye movements and lexical decision time were analysed. The results show that nonwords produced more fixation than words, and that the time to make a lexical decision was also greater for nonwords. Further, a preview of the stimulus was presented on some occasions either at fixation or peripherally. When the presentation was peripheral the number of refixations and lexical decision times were reduced. Parafoveal previews of words also reduced word length effects on refixations and lexical decision time. These effects decreased (for nonwords) when the case of the letters was changed in the preview. These results are compatible with the idea that the peripheral preview benefit derives from word visual structure in addition to information about word length, word envelope, and single letter identitie
Long-term effects of MDMA (ecstasy) on the human central nervous system revealed by visual evoked potentials
No full textSeveral studies indicate long-term cognitive impairment of MDMA (ecstasy) users. In the present study we attempted to establish whether electrophysiological correlates of low-level cognitive processes present a long-term alteration, dependent on the level of use of ecstasy. We addressed this issue by investigating amplitude and latency of VEPs related to a very simple discrimination task involving sustained attention (arousal). Eight heavy-MDMA users, eight moderate-MDMA users and 18 drug-free control subjects were asked to discriminate whether the digit at the centre of the screen was 1 or 2. None of the subjects (except one) had used MDMA in the 6 months previous testing. We measured psychophysical performance and EEG, recorded in Oz and Fz during task execution. The heavy-MDMA users made significantly more errors than the other two groups (p < .05). Moreover, they presented reduced amplitude but not latency of VEPs in both Oz and Fz. The effect in Oz is present in P200 (for heavy users only, p < .05) and in P300 components (for both MDMA groups; heavy users: p < .001, moderate users: p < .0.5). In Fz, the amplitude effect is present in N250 (for heavy users only, p < .05) and in P300 components (for both MDMA groups; heavy users: p < .05, moderate users: p < .05). The three groups do not differ in early components, reflecting low-level processing. These results provide evidence of long-term electrophysiological abnormality displayed by ecstasy users and agree with the suggestion that even typical recreational doses of ecstasy are sufficient to cause long-term altered cortical activity in humans
Experience enhances texture saliency by reducing behavioural and cortical responses to irrelevant texture features
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