1,720,973 research outputs found
Search for motion direction: Pop-out and set-size dependencies explained by stimulus and intrinsic uncertainty
No full textWe measured motion coherence and contrast thresholds for determining the direction of motion of a random dot pattern. Dots within a "target" region falling in one of 8 positions equally spaced around a circle moved either leftwards or rightwards (limited life 2-4 frames) for 150 ms. Subjects were required to identify the direction of motion, not the position of the patch. The target was cued by a partial cueing technique, where 1, 2, 4 or 8 spokes pointed to the target, as well as to a variable number of "distractors". The 8 potential-target regions could either be contiguous with the background random dots or be windowed as distinct stimuli behind a grey background. Coherence thresholds were lowest when the target was cued by a single spoke, and increased with the square-root of the number of spokes. We also measured contrast sensitivity for motion direction and found no set-size effect; but if we added a field of high-contrast noise dots only on the target and distractor regions, there was a square-root set-size effect for contrast thresholds. Interestingly, in conditions where a set-size effect was observed, the slope of the psychometric function also increased with set-size; when set-size had no effect on threshold, the slope of the psychometric function was always steep. A model based on the largest response of the pool of monitored detectors predicts that the slope of psychometric function increases with set-size (uncertainty), and that the presence or absence of a set size effect depends on stimulus and intrinsic uncertainty. The high intrinsic uncertainty for contrast thresholds may reflect an underlying large neuronal population (such as V1), while the low uncertainty associated with coherence thresholds may reflect a smaller population with large receptive fields (such as MT)
Methods to Reproduce In-Plane Deformability of Orthotropic Floors in the Finite Element Models of Buildings
Full text linkIn the modelling of reinforced concrete (RC) buildings, the rigid diaphragm hypothesis to represent the in-plane behavior of floors was and still is very commonly adopted because of its simplicity and computational cheapness. However, since excessive floor in-plane deformability can cause a very different redistribution of lateral forces on vertical resisting elements, it may be necessary to consider floor deformability. This paper investigates the classical yet intriguing question of modeling orthotropic RC floor systems endowed with lightening elements by means of a uniform orthotropic slab in order to describe accurately the building response under seismic loads. The simplified method, commonly adopted by engineers and based on the equivalence between the transverse stiffness of the RC elements of the real floor and those of the orthotropic slab, is presented. A case study in which this simplified method is used is also provided. Then, an advanced finite element (FE)-based method to determine the elastic properties of the equivalent homogenized orthotropic slab is proposed. The novel aspect of this method is that it takes into account the interaction of shell elements with frame elements in the 3D FE model of the building. Based on the results obtained from the application of this method to a case study, a discussion on the adequacy of the simplified method is also provided
Pooling and segmenting motion signals
No full textHumans are extremely sensitive to visual motion, largely because local motion signals can be integrated over a large spatial region. On the other hand, summation is often not advantageous, for example when segmenting a moving stimulus against a stationary or oppositely moving background. In this study we show that the spatial extent of motion integration is not compulsory, but is subject to voluntary attentional control. Measurements of motion coherence sensitivity with summation and search paradigms showed that human observers can combine motion signals from cued regions or patches in an optimal manner, even when the regions are quite distinct and remote from each other. Further measurements of contrast sensitivity reinforce previous studies showing that motion integration is preceded by a local analysis akin to contrast thresholding (or intrinsic uncertainty). The results were well modelled by two standard signal-detection-theory models. (C) 2008 Elsevier Ltd. All rights reserved
Spatiotemporal mechanisms of perisaccadic vision revealed by psychophysical reverse correlation
No full textSpatial frequency selectivity during saccadic eye movements revealed by masking
No full textPurpose: Low but not high spatial frequencies are suppressed during saccades (Burr et al., J. Physiol. 1982), probably because of selective suppression of the magnocellular pathway (Burr et al., Nature 1994). Here we investigate further the mechanisms for the suppression, using masking techniques. Methods: Contrast sensitivity for horizontally oriented test grating patches of low spatial frequency (0.07 c/deg) was measured in the presence of continuously displayed, randomly jittering mask gratings of variable contrast and spatial frequency, using a forced-choice procedure. The test gratings were presented briefly (8 ms) during normal vision and during large horizontal saccades. Results: Sensitivity decreased with mask contrast, with a similar (near linear) dependency during normal viewing and during saccades. For both normal and saccadic viewing, maximum reduction in sensitivity occurred when the mask had the same spatial frequency as that of the test (0.07 c/deg), at all levels of mask contrast. Conclusions: That maximum masking occurs at the same spatial frequency of the test implies that mechanisms selective to low spatial frequencies are not totally suppressed during saccades, but continue to operate with reduced sensitivity. This result fits well with the acceleration of the impulse response for luminance but not for chromatic stimuli during saccades (Burr and Morrone, Vision Res. 1996), that may result from gain changes in the magnocellular pathway during saccades
Spatiotemporal dynamics of perisaccadic remapping in humans revealed by classification images
No full textWe actively scan our environment with fast ballistic movements called saccades, which create large and rapid displacements of the image on the retina. At the time of saccades, vision becomes transiently distorted in many ways: Briefly flashed stimuli are displaced in space and in time, and spatial and temporal intervals appear compressed. Here we apply the psychophysical technique of classification images to study the spatiotemporal dynamics of visual mechanisms during saccades. We show that saccades cause gross distortions of the classification images. Before the onset of saccadic eye movements, the positive lobes of the images become enlarged in both space and in time and also shifted in a systematic manner toward the pre-saccadic fixation (in space) and anticipated in time by about 50 ms. The transient reorganization creates a spatiotemporal organization oriented in the direction of saccadic-induced motion at the time of saccades, providing a potential mechanism for integrating stimuli across saccades, facilitating stable and continuous vision in the face of constant eye movements
Methods to Reproduce In-Plane Deformability of Orthotropic Floors in the Finite Element Models of Buildings
Full text linkIn the modelling of reinforced concrete (RC) buildings, the rigid diaphragm hypothesis to represent the in-plane behavior of floors was and still is very commonly adopted because of its simplicity and computational cheapness. However, since excessive floor in-plane deformability can cause a very different redistribution of lateral forces on vertical resisting elements, it may be necessary to consider floor deformability. This paper investigates the classical yet intriguing question of modeling orthotropic RC floor systems endowed with lightening elements by means of a uniform orthotropic slab in order to describe accurately the building response under seismic loads. The simplified method, commonly adopted by engineers and based on the equivalence between the transverse stiffness of the RC elements of the real floor and those of the orthotropic slab, is presented. A case study in which this simplified method is used is also provided. Then, an advanced finite element (FE)-based method to determine the elastic properties of the equivalent homogenized orthotropic slab is proposed. The novel aspect of this method is that it takes into account the interaction of shell elements with frame elements in the 3D FE model of the building. Based on the results obtained from the application of this method to a case study, a discussion on the adequacy of the simplified method is also provided
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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