Person: Sierra Vázquez, Vicente
Universidad Complutense de Madrid
Faculty / Institute
Now showing 1 - 9 of 9
- PublicationLow spatial frequency filtering modulates early brain processing of affective complex pictures(Elsevier, 2007-11-05) Alorda, Catalina; Serrano Pedraza, Ignacio; Campos Bueno, José Javier; Sierra Vázquez, Vicente; Montoya, PedroRecent research on affective processing has suggested that low spatial frequency information of fearful faces provide rapid emotional cues to the amygdala, whereas high spatial frequencies convey fine-grained information to the fusiform gyrus, regardless of emotional expression. In the present experiment, we examined the effects of low (LSF, <15 cycles/image width) and high spatial frequency filtering (HSF, >25 cycles/image width) on brain processing of complex pictures depicting pleasant, unpleasant, and neutral scenes. Event-related potentials (ERP), percentage of recognized stimuli and response times were recorded in 19 healthy volunteers. Behavioral results indicated faster reaction times in response to unpleasant LSF than to unpleasant HSF pictures. Unpleasant LSF pictures and pleasant unfiltered pictures also elicited significant enhancements of P1 amplitudes at occipital electrodes as compared to neutral LSF and unfiltered pictures, respectively; whereas no significant effects of affective modulation were found for HSF pictures. Moreover, mean ERP amplitudes in the time between 200 and 500ms post-stimulus were significantly greater for affective (pleasant and unpleasant) than for neutral unfiltered pictures; whereas no significant affective modulation was found for HSF or LSF pictures at those latencies. The fact that affective LSF pictures elicited an enhancement of brain responses at early, but not at later latencies, suggests the existence of a rapid and preattentive neural mechanism for the processing of motivationally relevant stimuli, which could be driven by LSF cues. Our findings confirm thus previous results showing differences on brain processing of affective LSF and HSF faces, and extend these results to more complex and social affective pictures.
- PublicationPower spectrum model of visual masking: simulations and empirical data(Optical Society of America, 2013-06) Serrano Pedraza, Ignacio; Sierra Vázquez, Vicente; Derrington, Andrew MIn the study of the spatial characteristics of the visual channels, the power spectrum model of visual masking is one of the most widely used. When the task is to detect a signal masked by visual noise, this classical model assumes that the signal and the noise are previously processed by a bank of linear channels and that the power of the signal at threshold is proportional to the power of the noise passing through the visual channel that mediates detection. The model also assumes that this visual channel will have the highest ratio of signal power to noise power at its output. According to this, there are masking conditions where the highest signal-to-noise ratio (SNR) occurs in a channel centered in a spatial frequency different from the spatial frequency of the signal (off-frequency looking). Under these conditions the channel mediating detection could vary with the type of noise used in the masking experiment and this could affect the estimation of the shape and the bandwidth of the visual channels. It is generally believed that notched noise, white noise and double bandpass noise prevent off-frequency looking, and high-pass, low-pass and bandpass noises can promote it independently of the channel's shape. In this study, by means of a procedure that finds the channel that maximizes the SNR at its output, we performed numerical simulations using the power spectrum model to study the characteristics of masking caused by six types of one-dimensional noise (white, high-pass, low-pass, bandpass, notched, and double bandpass) for two types of channel's shape (symmetric and asymmetric). Our simulations confirm that (1) high-pass, low-pass, and bandpass noises do not prevent the off-frequency looking, (2) white noise satisfactorily prevents the off-frequency looking independently of the shape and bandwidth of the visual channel, and interestingly we proved for the first time that (3) notched and double bandpass noises prevent off-frequency looking only when the noise cutoffs around the spatial frequency of the signal match the shape of the visual channel (symmetric or asymmetric) involved in the detection. In order to test the explanatory power of the model with empirical data, we performed six visual masking experiments. We show that this model, with only two free parameters, fits the empirical masking data with high precision. Finally, we provide equations of the power spectrum model for six masking noises used in the simulations and in the experiments.
- PublicationEstudio de algunos procesos perceptivos en el sistema visual del gato(Universidad Complutense de Madrid, 2015) Sierra Vázquez, Vicente
- PublicationThe effect of white-noise mask level on sinewave contrast detection thresholds and the critical-band-masking model(2006-11) Serrano Pedraza, Ignacio; Sierra Vázquez, VicenteIt is known that visual noise added to sinusoidal gratings changes the typical U-shaped threshold curve which becomes flat in log-log scale for frequencies below 10c/deg when gratings are masked with white noise of high power spectral density level. These results have been explained using the critical-band-masking (CBM) model by supposing a visual filter-bank of constant relative bandwidth. However, some psychophysical and biological data support the idea of variable octave bandwidth. The CBM model has been used here to explain the progressive change of threshold curves with the noise mask level and to estimate the bandwidth of visual filters. Bayesian staircases were used in a 2IFC paradigm to measure contrast thresholds of horizontal sinusoidal gratings (0.25-8 c/deg) within a fixed Gaussian window and masked with one-dimensional, static, broadband white noise with each of five power density levels. Raw data showed that the contrast threshold curve progressively shifts upward and flattens out as the mask noise level increases. Theoretical thresholds from the CBM model were fitted simultaneously to the data at all five noise levels using visual filters with log-Gaussian gain functions. If we assume a fixed-channel detection model, the best fit was obtained when the octave bandwidth of visual filters decreases as a function of peak spatial frequency.
- PublicationSingle-band amplitude demodulation of Müller-Lyer illusion images(Complutense University of Madrid, 2007-05) Sierra Vázquez, Vicente; Serrano Pedraza, IgnacioThe perception of the Müller-Lyer illusion has previously been explained as a result of visual low band-pass spatial filtering, although, in fact, the illusion persists in band-pass and high-pass filtered images without visible low-spatial frequencies. A new theoretical framework suggests that our perceptual experience about the global spatial structure of an image corresponds to the amplitude modulation (AM) component (or its magnitude, also called envelope) of its AM-FM (alternatively, AM-PM) decomposition. Because demodulation is an ill-posed problem with a non-unique solution, two different AM-FM demodulation algorithms were applied here to estimate the envelope of images of Müller-Lyer illusion: the global and exact Daugman and Downing (1995) AMPM algorithm and the local and quasi-invertible Maragos and Bovik (1995) DESA. The images used in our analysis include the classic configuration of illusion in a variety of spatial and spatial frequency content conditions. In all cases, including those of images for which visual low-pass spatial filtering would be ineffective, the envelope estimated by single-band amplitude demodulation has physical distortions in the direction of perceived illusion. It is not plausible that either algorithm could be implemented by the human visual system. It is shown that the proposed second order visual model of pre-attentive segregation of textures (or "back-pocket" model) could recover the image envelope and, thus, explain the perception of this illusion even in Müller-Lyer images lacking low spatial frequencies.
- PublicationComparing the effect of the interaction between fine and coarse scales and surround suppression on motion discrimination.(Scholar One, 2013) Serrano Pedraza, Ignacio; Gamonoso Cruz, María J; Sierra Vázquez, Vicente; Derrington, Andrew MOur ability to discriminate motion direction in a Gabor patch diminishes with increasing size and contrast, indicating surround suppression. Discrimination is also impaired by a static low-spatial-frequency patch added to the moving stimulus, suggesting an antagonism between sensors tuned to fine and coarse features. Using Bayesian staircases, we measured duration thresholds in motion-direction discrimination tasks using vertically oriented Gabor patches moving at 2°/s. In two experiments, we tested two contrasts (2.8% and 46%), five window sizes (from 0.7° to 5°), and two spatial frequencies (1 c/deg and 3 c/deg), either presented alone or added to a static pattern. When the moving pattern was presented alone, duration thresholds increased with size at high contrast and decreased with size at low contrast. At low contrast, when a static pattern of 3 c/deg was added to a moving pattern of 1 c/deg, duration thresholds were similar to the case when the moving pattern was presented alone; however, at high contrast, duration thresholds were facilitated, eliminating the effect of surround suppression. When a static pattern of 1 c/deg was added to a moving pattern of 3 c/deg, duration thresholds increased about 4 times for high contrast and 2 times for low contrast. These results show that the antagonism between sensors tuned to fine and coarse scales is more complex than surround suppression, suggesting that it reflects the operation of a different mechanism.
- PublicationObtención de datos experimentales sobre imágenes con contenido emocional en el campus virtual UCM(Editorial Complutense, 2005) Campos Bueno, José Javier; Serrano, Ignacio; Alorda, Tina; Sierra Vázquez, Vicente; Montoya, Pedro; Fernández-Valmayor Crespo, Alfredo; Fernández-Pampillón Cesteros, Ana María; Merino Granizo, JorgeLa realización de experimentos psicológicos on line es una práctica cada vez más frecuente en muchos laboratorios por las indudables ventajas que proporciona. El presente trabajo presenta un procedimiento que permite obtener datos experimentales sobre la evaluación de imágenes filtradas del International Affective Picture System en las dimensiones de valencia y afecto. Se ha utilizado la herramienta que proporciona WebCT (versión 4.1.) para construir exámenes y cuestionarios, describiendo los problemas y las estrategias seguidas.
- PublicationDesarrollo de software en entorno Matlab para la generación y presentación de sonidos e imágenes 2D y 3D en el aula(2017) Serrano Pedraza, Ignacio; Sierra Vázquez, Vicente; López Bascuas, Luis Enrique; Campos Bueno, José Javier; Luna del Valle, Raúl; Arranz Paraíso, SandraEl resultado de este Proyecto de Innovación Docente consta de doce programas en Matlab con más de cien funcionalidades. Cinco programas están dedicados a demonstraciones para el estudio del sistema auditivo humano, seis están dedicados al estudio del sistema visual humano y un programa está dedicado al análisis y explicación de resultados experimentales analizados de acuerdo con la Teoría de Detección de Señales. Estos programas ayudarán al profesor a explicar el análisis de Fourier aplicado a imágenes y sonidos, el concepto de filtro, la técnica de enmascaramiento, el concepto de canal visual, generar estímulos 3D utilizando estereogramas de puntos aleatorios, y a presentar de modo controlado sonidos e imágenes (2D y 3D) en el aula.
- PublicationLibrerías Matlab para la presentación y procesamiento de sonidos e imágenes 2D y 3D en el aula(2020-04-30) Serrano Pedraza, Ignacio; Sierra Vázquez, Vicente; Arranz Paraíso, Sandra; Luna del Valle, Raul; López Bascuas, Luis Enrique; Campos Bueno, José JavierEl resultado del Proyecto consta de 15 programas en Matlab con más de cien funcionalidades. Ocho están dedicados a demostraciones para el estudio del Sistema Auditivo humano (e.g. diferentes tipos de espectrogramas, Transformadas Wavelet, el filtro gammatono, el filtro gammachirp, etc), cinco están dedicados a al estudio del Sistema Visual (e.g. determinación de la estereoagudeza, de la sensibilidad al contraste, de la sensibilidad a la disparidad, etc.) y dos están dedicados a la Colorimetría de luces y objetos. En el Anexo de este documento se presentan capturas de los quince programas con una breve descripción de las características, objetivos y funcionalidades de cada programa. Todos los programas están sujetos a mejoras que se irán añadiendo en sucesivas versiones. Las actualizaciones de estos programas se harán accesibles en esta página web: https://www.ucm.es/serranopedrazalab/.