73 research outputs found
Renormalisation group improvement of the early universe dynamics
No full textSelected applications of the Functional Renormalisation Group Equation technique to the early universe dynamics
Material characterisation in phase contrast imaging: The basis decomposition method revisited
No full textA method for basis decomposition of materials, based on hybrid phase-attenuation X-ray imaging, is presented. The effective composition of the imaged object in terms of two basis materials is reconstructed from X-ray images formed by coherent plane waves. The range of effectiveness of the technique is evaluated by means of a simplified model of image formation and a numerical simulation
From spectral decomposition through SVD to quantitative description of monochromatic CT images: a phantom study
No full textIn this work, we applied the singular value decomposition (SVD) method to a set of monochromatic images to extract the dominant physical contributions to image formation. We showed that the first two principal components can be related to an arbitrary pair of basis material in mathematically enclosed expression. The later
principal components are assumed to carry mostly sub-leading image formation effects, noise, and reconstruction artifact contribution. The proof of concept is shown on numerical (linear) images and later confirmed on
physical spectral CT phantom images obtained with monochromatic x-ray radiation at Elettra synchrotron in
Trieste, Italy. Following material decomposition, we also performed a quantitative description of tissue-equivalent
phantom materials in terms of material density and effective atomic number
Modelling fast forms of visual neural plasticity using a modified second-order motion energy model
No full textContains fulltext :
132717.pdf (Publisher’s version ) (Closed access
Tilt aftereffect following adaptation to translational Glass patterns
No full textGlass patterns (GPs) consist of randomly distributed dot pairs (dipoles) whose orientations are determined by specific geometric transforms. We assessed whether adaptation to stationary oriented translational GPs suppresses the activity of orientation selective detectors producing a tilt aftereffect (TAE). The results showed that adaptation to GPs produces a TAE similar to that reported in previous studies, though reduced in amplitude. This suggests the involvement of orientation selective mechanisms. We also measured the interocular transfer (IOT) of the GP-induced TAE and found an almost complete IOT, indicating the involvement of orientation selective and binocularly driven units. In additional experiments, we assessed the role of attention in TAE from GPs. The results showed that distraction during adaptation similarly modulates the TAE after adapting to both GPs and gratings. Moreover, in the case of GPs, distraction is likely to interfere with the adaptation process rather than with the spatial summation of local dipoles. We conclude that TAE from GPs possibly relies on visual processing levels in which the global orientation of GPs has been encoded by neurons that are mostly binocularly driven, orientation selective and whose adaptation-related neural activity is strongly modulated by attention
Sparks fade with distance: The effect of electric field distribution on global motion perception using different tES techniques
No full textPrevious evidence has shown that high-frequency transcranial random noise stimulation (hf-tRNS) reduces motion coherence thresholds when applied with a cephalic montage (i.e., return electrode over Cz). Extracephalic montages, which avoid stimulating regions under the return electrode, have also been used to modulate behavioral performance. In this study, we investigated the effects of different transcranial electrical stimulation (tES) protocols on visual motion discrimination, placing the return electrode on the ipsilateral arm. We assessed the impact of electrode positioning using hf-tRNS, anodal, cathodal transcranial direct current stimulation (tDCS), and Sham stimulation over hMT+, a brain region involved in global motion perception. Motion direction discrimination was measured using random dot kinematograms (RDKs). Given the increased distance between the stimulation and return electrodes in this montage, we expected a smaller reduction in motion discrimination thresholds compared to our previous study. Our results suggest that increasing interelectrode distance alters current flow characteristics - such as current distribution and focality - within the cortical areas under the target electrode, producing different effects. Additionally, no significant effects were observed with the other tES protocols tested. Our findings suggest that change in the interelectrode distance influences current flow characteristics, such as current distribution and focality, within the cortical areas under the target electrode, resulting in differential neuromodulatory effects. These results highlight the importance of stimulation configuration on performance, particularly a potential electric field shift due to the change in the interelectrode distance. Given the widespread application of brain stimulation techniques in clinical and cognitive research, our results can guide future studies carefully considering this further aspect of stimulation montage configurations
A novel approach to background subtraction in contrast‐enhanced dual‐energy digital mammography with commercially available mammography devices: Polychromaticity correction
No full textPurpose:
Contrast-enhanced digital mammography is an image subtraction technique that is able to improve the detectability of lesions in dense breasts. One of the main sources of error, when the technique is performed by means of commercial mammography devices, is represented by the intrinsic polychromaticity of the x-ray beams. The aim of the work is to propose an iterative procedure, which only assumes the knowledge of a small set of universal quantities, to take into account the polychromaticity and correct the subtraction results accordingly.
Methods:
In order to verify the procedure, it has been applied to an analytical simulation of a target containing a contrast medium and to actual radiographs of a breast phantom containing cavities filled with a solution of the same medium.
Results:
The reconstructed densities of contrast medium were compared, showing very good agreement between the theoretical predictions and the experimental results already after the first iteration. Furthermore, the convergence of the iterative procedure was studied, showing that only a small number of iterations is necessary to reach limiting values.
Conclusions:
The proposed procedure represents an efficient solution to the polychromaticity issue, qualifying therefore as a viable alternative to inverse-map functions
A novel approach to background subtraction in contrast‐enhanced dual‐energy digital mammography with commercially available mammography devices: Noise minimization
No full textPurpose: Dual-energy image subtraction represents a useful tool to improve the detectability of small lesions, especially in dense breasts. A feature it shares with all x-ray imaging techniques is the appearance of fluctuations in the texture of the background, which can obscure the visibility of interesting details. The aim of the work is to investigate the main noise sources, in order to create a better performing subtraction mechanism. In particular, the structural noise cancellation was achieved by means of a suitable extension of the dual-energy algorithm. Methods: The effect of the cancellation procedure was tested on an analytical simulation of a target with varying structural composition. Subsequently, the subtraction algorithm was also applied to a set of actual radiographs of a breast phantom exhibiting a nonuniform background pattern. The background power spectra of the outcomes were computed and compared to the ones obtained from a standard subtraction algorithm. Results: The comparison between the standard and the proposed cancellations showed an overall suppression of the magnitudes of the spectra, as well as a flattening of the frequency dependence of the structural component of the noise. Conclusions: The proposed subtraction procedure provides an effective cancellation of the residual background fluctuations. When combined with the polychromatic correction already described in a companion publication, it results in a high performing dual-energy subtraction scheme for commercial mammography units
Modelling adaptation to directional motion using the Adelson-Bergen energy sensor
No full textThe motion energy sensor has been shown to account for a wide range of physiological and psychophysical results in motion detection and discrimination studies. It has become established as the standard computational model for retinal movement sensing in the human visual system. Adaptation effects have been extensively studied in the psychophysical literature on motion perception, and play a crucial role in theoretical debates, but the current implementation of the energy sensor does not provide directly for modelling adaptation-induced changes in output. We describe an extension of the model to incorporate changes in output due to adaptation. The extended model first computes a space-time representation of the output to a given stimulus, and then a RC gain-control circuit ("leaky integrator") is applied to the time-dependent output. The output of the extended model shows effects which mirror those observed in psychophysical studies of motion adaptation: a decline in sensor output during stimulation, and changes in the relative of outputs of different sensors following this adaptation
A comparison of equivalent noise methods in investigating local and global form and motion integration
No full textStatic and dynamic cues within certain spatiotemporal proximity are used to evoke respective global percepts of form and motion. The limiting factors in this process are, first, internal noise, which indexes local orientation/direction detection, and, second, sampling efficiency, which relates to the processing and the representation of global orientation/direction. These parameters are quantified using the equivalent noise (EN) paradigm. EN has been implemented with just two levels: high and low noise. However, when using this simplified version, one must assume the shape of the overall noise dependence, as the intermediate points are missing. Here, we investigated whether two distinct EN methods, the 8-point and the simplified 2-point version, reveal comparable parameter estimates. This was performed for three different types of stimuli: random dot kinematograms, and static and dynamic translational Glass patterns, to investigate how constant internal noise estimates are, and how sampling efficiency might vary over tasks. The results indicated substantial compatibility between estimates over a wide range of external noise levels sampled with eight data points, and a simplified version producing two highly informative data points. Our findings support the use of a simplified procedure to estimate essential form-motion integration parameters, paving the way for rapid and critical applications to populations that cannot tolerate protracted measurements.This work was carried out within the scope of the project “Use-inspired basic research,” for which the Department of General Psychology of the University of Padova has been recognized as “Dipartimento di Eccellenza” by theMinistry ofUniversity and Research. RD was supported by University of Padova, Department of Psychology and by the Human Inspired Centre. This work was also supported in part by the BAGEP Award of the Science Academy, Turkey
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