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Glare effect animation: a peculiar case of causality
No full textIn this experiments we have found a particular case of perceptual causality in Glare effect animated (Zavagno & Bressanelli, 2006). In animated Glare effect a white centre T (which luminance was kept constant) of a black cross F, was seen to increase its luminosity in time and to cause the physical changes in F, i.e. the modification of ramp of luminance gradients from black-black to black-white. The present study is a contribution to enlarge the catalog of perceptual causality to qualitative changes. In previous experiment we accelerated end decelerated the animation found an effect of time on the perception of causality. For longer animation (5fr/sec) participants’ descriptions seemed to be consistent to the presence of a luminosity threshold, which was found in static Glare effect (Zavagno e Caputo, 2001,2005). Moreover, it seemed that this threshold would have a role on perception of causality
The glare effect and the perception of luminosity
Full text linkThe impression of self-luminosity in the glare effect was studied in two experiments. In experiment 1 the target (CS) was set to the highest luminance of the field and subjects were asked to adjust the luminance ramp of the inducers (R) against five backgrounds (B) to the point where they began to see CS as self-luminous. It was found that there is a linear relationship between background and luminance ramp. Another group of subjects carried out the same task in experiment 2, but this time CS and R were linked together so that CS would always have the same luminance as the highest luminance level of R, as adjustments were performed. It was found that: (i) adjustments were always lower than the highest luminance available; (ii) the linear relationship between background and luminance ramp was confirmed; (iii) observers reported a compelling impression of self-luminous grays. Data are discussed in relation to Bonato and Gilchrist's model for the perception of luminosity. The authors advance the hypothesis that luminance ramps are used at an early stage of encoding for the perception of luminosity
Glowing greys and surface-white: the photo-geometric factors of luminosity perception
Full text linkThe perception of luminosity is thought to depend upon the intensity of the stimulus: a surface begins to appear self-luminous when it emits or reflects a certain amount of light. This is known as the luminosity threshold. It is a common opinion among vision scientists that such a threshold is correlated to the intensity of a perceptually white surface, in the sense that only an area of the visual field with luminance higher than perceived surface-white will appear lumi-
nous. Here we show grey colours that appear luminous in virtue of surrounding luminance ramps. These ramps are intended to mimic halos seen around light sources in natural environments. The results of three experiments indicate that the phenomenon is in direct contradiction to the aforementioned assumptions and suggest the existence of separate perceptual pathways for self-luminosity perception and for surface-colour perception
The problem of being white: testing the highest luminance rule.
No full textIt is a common understanding that white serves as an “anchor” for the visual system for lightness scaling
purposes. By lightness we are referring to surface color perception in the achromatic domain. The
importance of “surface white” is stated also in the literature about luminosity perception, where it is often
claimed that in order for a region of the visual field to appear as self-luminous, its luminance must be
somewhat higher than the luminance of a surface perceived as white under the same conditions of
illumination. Implicit in this assumption is that the visual system is able to determine what is to be seen as
white instead of luminous, glowing or light gray. A “highest luminance rule”, eventually corrected by an
area factor, seems so far to be the best candidate. This approach has been applied to several lines of data
with apparent success. However, below we will describe two experiments, one concerning lightness and
the other perceived luminosity, that show the severe limitations of the highest luminance rule hypothesis
Somiglianze cross-modali fra stimoli visivi e stimoli uditivi
No full textParise C, Bricolo E, Vezzani S, Zavagno D. Somiglianze cross-modali fra stimoli visivi e stimoli uditivi. Presented at the Congresso Nazionale della Sezione Sperimentale dell’Associazione Italiana di Psicologia, Rovereto, IT
Top-down and bottom-up neuromodulation over two different visual illusions
No full textFor decades visual illusions have been used as a tool to investigate the properties of the visual system. In parallel, the neural bases of visual illusions have been investigated. Two principal methods have been used: the modification of perception in brain damage patients and the recording of brain activity induced by illusion perception. In general terms, the neuropsychological and imaging techniques suggest
that the perception of illusions takes place at low level in the human brain. Here we use a non-invasive brain stimulation technique (tDCS, transcranial Direct Current Stimulation) to verify the bottom-up arising and the top-down modulation of a visuo-spatial illusion (Brentano illusion, BI) and of a brightness illusion (Glare effect, GE). Two groups of healthy subjects were tested with BI and GE while receiving cathodal or sham tDCS to the right or left posterior parietal cortex (PPC) or to the right or left occipital cortex (OC). Results show different effects for the two illusions. For BI, there is an increase of the illusory effect after cathodal tDCS of the right PPC and a decrease after the stimulation of the left PPC. No changes were found for OC stimulations. On the contrary GE
was not modulated neither by PPC nor by OC stimulations. These evidences uphold the involvement of attention and its hemispheric asymmetry in modulating the perceptual processing underlying the BI, mediated by PPC and not by OC. GE as a pure luminance gradient illusion does not seem to be modulated by tDCS
Lightness effects in Delboeuf and Ebbinghaus size-contrast illusions
No full textWe examined lightness effects observed in Delboeuf and Ebbinghaus size-contrast illusions. Results of four experiments are reported. Experiment 1 was conducted with Delboeuf-
like stimuli and shows that the disk that appears bigger appears either lighter or darker than
the disk that appears smaller, depending on the contrast polarity between disks and background.
Experiment 2 shows that the direction of these lightness effects is not influenced by the lumi-
nance of the size-contrast inducers. Experiment 3 shows that a similar lightness effect is also
observed in modified Ebbinghaus size-contrast displays. Experiment 4 tested the presence of the size-contrast illusion in the stimuli used in experiments 2 and 3
Measuring the meter: on the constancy of lightness scales seen against different backgrounds.
Full text linkThe constancy of a 16-step achromatic Munsell scale was tested with regards to background variations in two experiments. In experiment 1 three groups of observers were asked to find lightness matches for targets in simultaneous lightness displays by using a 16-step achromatic Munsell scale placed on a white, black, or white-black checkered background. In experiment 2, a yellow-blue checkered background and a green-red checkered background replaced Munsell scales on the black and on the white backgrounds. Significant effects of scale background on matches were found only in experiment 1, suggesting that background luminance is a crucial factor in the overall appearance of the scale. The lack of significant differences in experiment 2, however, may stand for an overall robustness of the scale with respect to background luminance changes occurring within certain luminance ranges
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