From pixels to letters : discrimination of orientation contrasts during visual word recognition

Tese de mestrado, Psicologia (Secção de Cognição Social Aplicada), Universidade de Lisboa, Faculdade de Psicologia, 2018To effectively discriminate mirrored letters (e.g., b and d), when learning to read one must overcome mirror invariance (an original property of the visual system that treats lateral reflected images as equivalent percepts). Previous studies suggested that mirror invariance might still occur during letter identification, and that in contrast with nonreversible letters (which differ from other letters of the script by shape, being orientation an irrelevant feature: f, R), discrimination of reversible letters (for which orientation is a diagnostic feature: d, p, b: N, Z) relies on a specific mechanism of mirror-image suppression. We explored how orientation contrasts influence discrimination of reversible (b; d; p) and nonreversible (f; r, t) letters during word recognition. In Experiment 1, we adopted a lexical decision task with a sandwich priming paradigm, to reduce lexical influences. Lowercase primes differed from uppercase targets (e.g., IDEA) on the critical letter only: identical prime (lowercase version of the target: idea); mirrored prime (mirror-image of the critical letter: ibea); rotated prime (180º plane rotation of the critical letter: ipea); control prime (critical letter replaced by a mask: ). In Experiment 2, we adopted a same-different task with masked priming, using the same prime-target conditions. The pattern of results was similar in both experiments. Target decisions for reversible letters was slower when letters were transformed in orientation (mirrored or rotated) relative to control and identical primes. For nonreversible letters, orientation contrasts facilitated target recognition, leading to faster word decisions relative to controls, but slower than identical primes in Experiment 1. Given the similar effects for rotated and mirrored primes, a mechanism of mirror invariance cannot fully explain the present results. We propose that, when visual features are compatible with multiple letter representations, those representations will be activated, leading to competition effects between them (through mutual inhibition). The same principle of recognition-by-components, originally proposed in visual object recognition, seems to apply to letter identification.De forma a discriminar letras reversíveis (e.g., b e d) eficazmente, durante a aprendizagem da leitura é necessário ultrapassar a invariância ao espelho (uma propriedade original do sistema visual que trata reflexos laterais como sendo perceptos equivalentes). Estudos anteriores sugerem que a invariância ao espelho pode ainda ocorrer durante a identificação de letras, e que em contraste com letras não-reversíveis (que diferem de outras letras do alfabeto na forma, sendo a sua orientação uma propriedade irrelevante: f, R), a discriminação de letras reversíveis (para as quais a orientação é uma propriedade de diagnóstico: d, p, b; N, Z) depende de um mecanismo específico de supressão de imagens em espelho. Neste estudo, exploramos qual a influência de contrastes de orientação na discriminação de letras reversíveis (b; d; p) e não-reversíveis (f; r; t) durante o reconhecimento de palavras. Na Experiência 1, adotamos uma tarefa de decisão lexical com o paradigma priming sandwich, para reduzir influências lexicais. Os primes em minúsculas diferem dos alvos em maiúsculas (e.g., IDEIA) na letra critica apenas: prime identical (versão do alvo em minúsculas: ideia); prime mirrored (imagem em espelho da letra crítica: ibeia); prime rotated (letra crítica rodada no plano 180º: ipeia); condição control (letra crítica substituída por uma máscara: ). Na Experiência 2, adotamos uma tarefa same-different com priming mascarado, utilizando as mesmas condições prime-alvo. O padrão de resultados foi semelhante nas duas experiências. A transformação da orientação (imagem-espelho ou rotação) em letras reversíveis resultou em respostas mais lentas relativamente a condição de controlo. Para letras não-reversíveis, os contrastes de orientação facilitaram o reconhecimento do alvo, levando a respostas mais rápidas relativamente à condição de controlo, mas mais lentas do que a condição identical na Experiência 1. Dada a semelhança nos efeitos obtidos para ambos os contrastes de orientação, estes resultados não podem ser totalmente explicados segundo um mecanismo de invariância ao espelho. Propomos que, quando os traços visuais são compatíveis com múltiplas representações de letras, estas representações são ativadas, levando a efeitos de competição entre elas (através de mútua inibição). O mesmo princípio de reconhecimento-por-componentes, proposto originalmente no reconhecimento visual de objetos, parece aplicar-se à identificação de letras

Determining cosmic microwave background structure from its peak distribution

We present a new method for time-efficient and accurate extraction of the power spectrum from future cosmic microwave background (CMB) maps based on properties of peaks and troughs of the Gaussian CMB sky. We construct a statistic describing their angular clustering - analogously to galaxies, the 2-point angular correlation function, $\xi_\nu(\theta)$. We show that for increasing peak threshold, $\nu$, the $\xi_\nu(\theta)$ is strongly amplified and becomes measurable for $\nu\geq$1 on angular scales $\leq 10^\circ$. Its amplitude at every scale depends uniquely on the CMB temperature correlation function, $C(\theta)$, and thus the measured $\xi_\nu$ can be uniquely inverted to obtain $C(\theta)$ and its Legendre transform, the power spectrum of the CMB field. Because in this method the CMB power spectrum is deduced from high peaks/troughs of the CMB field, the procedure takes only $[f(\nu)]^2N^2$ operations where $f(\nu)$ is the fraction of pixels with $|\delta T|\geq\nu$ standard deviations in the map of $N$ pixels and is e.g. 0.045 and 0.01 for $\nu$=2 and 2.5 respectively. We develop theoretical formalism for the method and show with detailed simulations, using MAP mission parameters, that this method allows to determine very accurately the CMB power spectrum from the upcoming CMB maps in only $\sim(10^{-4}-10^{-3})\times N^2$ operations.Comment: To be published in Ap.J. Letters. Minor changes to match the journal versio

Traveling Around the Ouija Board

Supposedly guided by spirits form another world, the planchette glides from letter to letter on a Ouija board, spelling out words. One can characterize the difficulty of a word by the distance that planchette must travel to spell the words (visiting the letters in the order of their appearance in the word). The table below gives the shortest-travel and the longest-travel words of various lengths, in pixels using the Ouija board

Skin detection by dual maximization of detectors agreement for video monitoring

This is the author’s version of a work that was accepted for publication in Journal Pattern Recognition Letters. Changes resulting from the publishing process, such as peer review, editing, corrections, structural formatting, and other quality control mechanisms may not be reflected in this document. Changes may have been made to this work since it was submitted for publication. A definitive version was subsequently published in Journal Pattern Recognition Letters, 34, 16 (2013) DOI: 10.1016/j.patrec.2013.07.016This paper presents an approach for skin detection which is able to adapt its parameters to image data captured from video monitoring tasks with a medium field of view. It is composed of two detectors designed to get high and low probable skin pixels (respectively, regions and isolated pixels). Each one is based on thresholding two color channels, which are dynamically selected. Adaptation is based on the agreement maximization framework, whose aim is to find the configuration with the highest similarity between the channel results. Moreover, we improve such framework by learning how detector parameters are related and proposing an agreement function to consider expected skin properties. Finally, both detectors are combined by morphological reconstruction filtering to keep the skin regions whilst removing wrongly detected regions. The proposed approach is evaluated on heterogeneous human activity recognition datasets outperforming the most relevant state-of-the-art approaches.This work has been partially supported by the Spanish Government (TEC2011-25995 EventVideo)

Reanalysis of the spectrum of the z=10 galaxy

In a recent paper Pello et al. reported observations of a faint galaxy, gravitationally lensed by the galaxy cluster Abell 1835. Deep J-band spectroscopy revealed a weak emission line near 1.34 microns, detected in two spectra with different central wavelengths. The line was interpreted as Lyman-alpha at redshift z=10.0. This interpretation is supported by the broad-band photometric spectral energy distribution, and by the location of the galaxy close to the lens critical line for this redshift. We have reanalysed the two spectra, just released from the data archive. Our analysis includes allowance for wavelength shifts due to transverse drift of the object in the slit. We do not detect a significant emission line at the reported location, or nearby, at either grating setting, nor in the combined spectrum. We provide a possible explanation for the reported detection as due to spurious positive flux introduced in the sky-subtraction stage as a result of variable hot pixels. We provide our final reduced 2D frame, and corresponding error array.Comment: 4 pages, 1 figure. To appear in A&A Letters. Added possible explanation for reported emission line as due to variable hot pixel

Pushing the limits: K2 observations of the trans-Neptunian objects 2002 GV31 and (278361) 2007 JJ43

We present the first photometric observations of trans-Neptunian objects (TNOs) taken with the Kepler space telescope, obtained in the course of the K2 ecliptic survey. Two faint objects have been monitored in specifically designed pixel masks that were centered on the stationary points of the objects, when their daily motion was the slowest. In the design of the experiment, only the apparent path of these objects were retrieved from the detectors, i.e. the costs in terms of Kepler pixels were minimized. Because of the faintness of the targets we employ specific reduction techniques and co-added images. We measure rotational periods and amplitudes in the unfiltered Kepler band as follows: for (278361) 2007 JJ43 and 2002 GV31 we get P_rot=12.097 h and P_rot=29.2 h while 0.10 and 0.35 mag for the total amplitudes, respectively. Future space missions, like TESS and PLATO are not well suited to this kind of observations. Therefore, we encourage to include the brightest TNOs around their stationary points in each observing campaign to exploit this unique capability of the K2 Mission -- and therefore to provide unbiased rotational, shape and albedo characteristics of many objects.Comment: Accepted for publication in ApJ Letters, 5.2 pages in emulateapj style, misspelled 2007 JJ43 designation correcte

Nano-optical observation of cascade switching in a parallel superconducting nanowire single photon detector

The device physics of parallel-wire superconducting nanowire single photon detectors is based on a cascade process. Using nano-optical techniques and a parallel wire device with spatially-separate pixels we explicitly demonstrate the single- and multi-photon triggering regimes. We develop a model for describing efficiency of a detector operating in the arm-trigger regime. We investigate the timing response of the detector when illuminating a single pixel and two pixels. We see a change in the active area of the detector between the two regimes and find the two-pixel trigger regime to have a faster timing response than the one-pixel regime.Comment: 11 pages, 2 figure