1,721,024 research outputs found
Perceived kinetics in launching and triggering events
No full textIn launching and triggering effects [Michotte, 1946 La Perception de la Causalite ̈ (Louvain: Publi-
cations Universitaires)] object A moves towards object B and, when the two objects make contact, B starts to move. Phenomenally, the movement of A appears to cause the movement of B:
either mechanically, in launching, or psychologically, in triggering. Perceived kinetics of these
events have been explored in four experiments. We measured P50, the point of subjective
equality between experimental stimulus and standard stimulus, of the perceived velocity of B. We
found that P50 in control condition, obtained on adding a pause before the motion of B
(P50 = -0.005 deg s-1 ), is different from P50 of experimental conditions: in launching (i) the
P50 is 0.56 deg s-1; in triggering (ii) the P50 is 0.42 deg s-1. Moreover, in launching para-
digm, P50 is also affected by kinetic variations of the objects before and after the collision,
related to the impression of `force' of the impact. In the decreased-force condition (iii) a small
effect is found: P50 is 0.34 deg s-1; in the increased-force condition (iv) the P50 is 0.96 deg s-1.
These results suggest that kinetic properties of causal and triggering displays are based on the global
organisation of the events
Temporal integration of speed in perceived causality
No full textCausal relation between two movements is evident in the launching paradigm [Michotte, 1946/1963 The Perception of Causality (London: Methuen)], in which the first moving object (S1) appears to cause the motion of the second object (S2). The causal relation requires a short delay between S1 and S2 (40 ms, launching) and fades when it is long (1040 ms, non-launching). We recently showed that in the launching condition the speed of S2 is overestimated by 34% with respect to the non-launching condition (Parovel and Casco Vision Research submitted). Here, we demon- strated the general properties of the spatio-temporal integration mechanism underlying speed overestimation in the causality phenomenon. We manipulated the trajectory-to-trajectory alignment [4 experiments], the spatio-temporal coincidence between S1 and S2 [3 experiments], the duration of the whole event [1 experiment], and the speed ratio [2 experiments]. A two-interval forced-choice task was used to measure the point of subjective equality between S2 speeds in launching versus non-launching conditions. Data support an integrative mechanism with different properties than motion averaging, motion trajectory integration, and sequential recruitment; indeed, S2 speed overestimation also occurs when S1 is slower than S2, independently from the trajectory-to- trajectory alignment or the spatial coincidence. The mechanism underlying perceived causality specifically relies on two temporal factors: it requires a short interval between S1 and S2 movements, and also it increases with their short durations. Moreover, we found that S2 speed overestimation in launching determines a displacement of psychometric functions rather than a change in slope, demonstrating a perceptual rather than a decisional effect, in agreement with Michotte's interpretation that the relationship of causality in launching events is directly perceived, without the mediation of high-level processes
The integration of straight contours (snakes and ladders): The role of spatial arrangement, spatial frequency and spatial phase
No full textIn the present study we addressed the issue of whether the Gestalt principle of grouping by similarity (iso-orientation) subtends extraction of straight contours made up of disconnected, iso-oriented Gabor elements, whether collinear (snakes) or parallel (ladders). To prevent the use of the most obvious grouping principle of good continuation, which allows us to perceive the relation between local and global orientation along the contour, we manipulated the spatial arrangement of randomly oriented Gabors in the background: they were positioned on an ordered grid, and grouped on the basis of good continuation, or randomly positioned and not grouped. Grid-positioned backgrounds exert a suppressive contextual influence on detection of good continuation along the contour path. Results obtained in a two-interval forced choice task showed that the orderly-positioned background did not completely prevent detection of snakes and ladders. Detection of snakes was hampered at low spatial frequency whereas detection of ladders was improved by the randomly-positioned background at high spatial frequency. These contextual influences support the suggestion that both iso-orientation and good continuation rules are employed by the association field underlying the binding of straight contours. In addition, they are not compatible with integration of snakes and ladders elements within a single receptive field. In support of this suggestion we found that phase constancy within contour elements (as opposed to phase randomization) improved snake detectability at low spatial frequency, and, unexpectedly, impaired ladder detectability at high spatial frequency. This suggests that a low-level mechanism based on the balance between excitatory and inhibitory lateral interactions at a first stage may account for the detection of both straight contours. © 2012 Elsevier Ltd
Improving motion detection via anodal transcranial direct current stimulation
No full textBackground: To study motion perception, a stimulus consisting of a field of small, moving dots is often used. Generally, some of the dots coherently move in the same direction (signal) while the rest move randomly (noise). A percept of global coherent motion (CM) results when many different local motion signals are combined. CM computation is a complex process that requires the integrity of the middle-Temporal area (MT/V5) and there is evidence that increasing the number of dots presented in the stimulus makes such computation more efficient. Objective: In this study, we explored whether anodal direct current stimulation (tDCS) over MT/V5 would increase individual performance in a CM task at a low signal-To-noise ratio (SNR, i.e. low percentage of coherent dots) and with a target consisting of a large number of moving dots (high dot numerosity, e.g. >250 dots) with respect to low dot numerosity (400 dots) to promote efficient global motion processing. Conclusions: The present study suggests that tDCS may be used under appropriate stimulus conditions (low SNR and high dot numerosity) to boost the global motion processing efficiency, and may be useful to empower clinical protocols to treat visual deficits
Priming of first- and second-order motion: Mechanisms and neural substrates
No full textPriming for luminance-modulated (first-order) motion has been shown to rely on the functional integrity of visual area V5/MT [Campana, G., Cowey, A., & Walsh, V. (2002). Priming of motion direction and area V5/MT: A test of perceptual memory. Cerebral Cortex, 12, 663-669; Campana, G., Cowey, A., & Walsh, V. (2006). Visual area V5/MT remembers "what" but not "where". Cerebral Cortex, 16, 1766-1770]. The high retinotopical organization of this area would predict that direction priming is sensitive to spatial position. In order to test this hypothesis, and to see whether a similar priming mechanism also exists with second-order motion, we tested motion direction priming and its interaction with spatial position with both first- and second-order motion. Indeed, whereas a number of studies have pinpointed the specific mechanisms and neural substrates for these two kinds of motion perception that appear to be (partially) non-overlapping (i.e., Lu, Z. L., & Sperling, G. (2001). Three-systems theory of human visual motion perception: Review and update. Journal of the Optical Society of America A, 18, 2331-2370; Vaina, L. M., & Soloviev, S. (2004). First-order and second-order motion: Neurological evidence for neuroanatomically distinct systems. Progress in Brain Research, 144, 197-212), the mechanisms and neural substrates mediating implicit memory for first- and second-order motion are still unknown. Our results indicate that priming for motion direction occurs not only with first-order but also with second-order motion. Priming for motion direction is position-sensitive both with first- and second-order motion, suggesting for both processes a locus of representation where retinotopicity is still maintained, that is within the V5/MT complex but earlier than MST. Cross-order motion priming also exists but is not sensitive to spatial position, suggesting that the locus where processing of first- and second-order motion converge is situated in MST or beyond. © 2007 Elsevier Ltd. All rights reserved
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