Spatial tuning studies of the pattern evoked electroretinogram

The locus of origin of the pattern evoked electroretinogram, (PERG), has been the subject of considerable discussion. A novel approach was adopted in this study to further elaborate the nature of the PERG evoked by pattern onset/offset presentation. The PERG was found to be linearly related to stimulus contrast and in particular was linearly related to the temporal contrast of the retinal image, when elicited by patterns of low spatial frequency. At high spatial frequencies the retinal image contrast is significantly reduced because of optical degradation. This is described by the eye's modulation transfer function (MTF). The retinal contrast of square wave grating and chequerboard patterns of increasing spatial frequency were found by filtering their Fourier transforms by the MTF. The filtered pattern harmonics were then resynthesised to constitute a profile of retinal image illuminance from which the temporal and spatial contrast of the image could be calculated. If the PERG is a pure illuminance response it should be spatially insensitive and dependent upon the temporal contrast of stimulation. The calculated loss of temporal contrast for finer patterns was expressed as a space-averaged temporal contrast attentuation factor. This factor, applied to PERGs evoked by low spatial frequency patterns, was used to predict the retinal illuminance response elicited by a finer pattern. The predicted response was subtracted from the recorded signal and residual waveform was proposed to represent specific activity. An additional correction for the attenuation of spatial contrast was applied to the extracted pattern specific response. Pattern specific responses computed for different spatial frequency patterns in this way are the predicted result of iso-contrast pattern stimulation. The pattern specific responses demonstrate a striking bandpass spatial selectivity which peaks at higher spatial frequencies in the more central retina. The variation of spatial sensitivity with eccentricity corresponds closely with estimated ganglion receptive field centre separation and psychophysical data. The variation of retinal structure with eccentricity, in the form of the volumes of the nuclear layers, was compared with the amplitudes of the computed retinal illuminance and pattern specific responses. The retinal illuminance response corresponds more closely to the outer and inner nuclear layers whilst the pattern specific response appears more closely related to the ganglion cell layer. In general the negative response transients correspond to the more proximal retinal layers. This thesis therefore supports the proposed contribution of proximal retinal cell activity to the PERG and describes techniques which may be further elaborated for more detailed studies of retinal receptive field dimensions.

Depth profile of uncompensated spins in an exchange bias system

We have used the unique spatial sensitivity of polarized neutron and soft x-ray beams in reflection geometry to measure the depth dependence of magnetization across the interface between a ferromagnet and an antiferromagnet. The net uncompensated magnetization near the interface responds to applied field, while uncompensated spins in the antiferromagnet bulk are pinned, thus providing a means to establish exchange bias.

Pulsiones del espacio : Oteiza y la aproximación a la arquitectura como vacío activo

Este trabajo se centra en el estudio de las investigaciones de Jorge Oteiza en torno a la funcionalidad estética del espacio, en especial, en la actividad artística que desarrolló en el año 1958, un año decisivo en la vida del escultor en el que dio por finalizado su proceso de experimentación sobre la naturaleza espacial de la estatua. En este desenlace tuvo un papel fundamental la relación funcional que planteó, a la hora de retomar su trabajo después de su triunfo en la IV Bienal de São Paulo de 1957, entre la escultura y la arquitectura. La primera, entendida como organismo puramente espacial, debía de responder a las condiciones de su mundo circundante, el espacio arquitectónico. Su función: acondicionarlo estéticamente para satisfacer las necesidades espirituales del habitante. Siguiendo el canon estético que para la escultura acababa de anunciar en Brasil, la desocupación espacial (la liberación de la energía espacial de la estatua, el rompimiento de la neutralidad del espacio libre) no se trataba de embellecer superficialmente la arquitectura sino de activar su vacío interior. Oteiza, que siempre estuvo muy interesado por la arquitectura y que había colaborado con anterioridad en numerosas ocasiones con los mejores arquitectos del país, fue durante este año cuando profundizó de manera más sistemática (teórica y prácticamente) sobre la relación arte-arquitectura. De hecho, él mismo nombraba como el último trabajo de su línea de experimentación en escultura a su propuesta para el concurso del Monumento a José Batlle en Montevideo, que junto al arquitecto Roberto Puig acabaron a finales de año. En el proyecto se planteaba a escala urbana, y como ejemplo concreto, el modelo teórico de integración arquitectura + (arte=0) que había elaborado los meses anteriores, la integración vacía. En el texto explicativo que acompañaba al proyecto (un texto que desbordaba los límites de una memoria al uso) demandaba la necesidad de la toma de conciencia estética del espacio, como acto de libertad individual, y declaraba el fin del rol de espectador del hombre frente a la obra de arte, reclamando su participación activa en la misma. Para él, la noción del espacio estético no era una condición innata en el hombre, se descubría, se aprendía, evolucionaba y se olvidaba (una vez convertido en hábito). Frente a la ceguera de la sensibilidad espacial del hombre, proponía la educación de la percepción espacial, condicionar emocionalmente la reflexión espontánea ante el juego espacial de las formas en la naturaleza y el espectáculo natural de la ciudad. Aprender a leer el lenguaje emocional del espacio, a pensar visualmente. La obra de arte era así un catalizador espiritual del contorno del mundo, modificador de la vida espacial circundante que corregía hábitos visuales y condicionaba estímulos y reflejos. Desde una resonancia afectiva con la definición psicológica del término (como energía psíquica profunda que invita o incita a pasar a la acción), a diferencia del instinto, la pulsión (siendo la fuente de toda conducta espontánea) es susceptible de ser modificada por la experiencia, por la educación, por la cultura, por el deseo. Es desde esta aproximación en términos de energía desde la que se propone la noción pulsiones del espacio como fórmula (reversible) entre la energía espacial liberada en el proceso de desocupación definido por Oteiza y caracterizadora de la obra como vacío activo (en escultura, en arquitectura), y la energía psíquica profunda que invita o incita a la toma de posesión del espacio (la voluntad espacial absoluta con la que Oteiza definía su modelo de arte=0, cero como expresión formal). Si el hombre modifica su entorno al mismo tiempo que es condicionado por él, es indispensable una conciencia estética del espacio que le enseñe, de entre todas las posibilidades que este le ofrece, qué es lo que necesita (qué es lo que le falta), para tomar posesión de él, para un efectivo ser o existir en el espacio. Es desde esta caracterización como energía por lo que las pulsiones del espacio se sitúan entre el hombre y su entorno (construido) y permiten la transformación entre energía espacial y energía psíquica; entre su hábitat y sus hábitos. Por estas mismas fechas, Oteiza definía una casa como un conjunto articulado de vacíos activos, como una obra de plástica pura que no es arte sino en función del habitante. Es este habitante, educado en la toma de conciencia estética del espacio, el que participando activamente en la interpretación de los espacios previstos por el arquitecto, sintiendo y movido por las pulsiones del espacio, hará uso adecuado de la arquitectura; pasando de un arte como objeto a un arte como comportamiento, transformará su habitar en un arte, el arte de habitar. ABSTRACT This work focuses on the study of Jorge Oteiza’s investigations on the aesthetic functionality of space, especially on his artistic activity developed in 1958, a decisive year in the life of the sculptor, in which he gave end to his process of experimentation on the spatial nature of the statue. In this outcome it was fundamental the functional relationship that he propounded, at the time of returning to work after his triumph in the IV Bienal de São Paulo in 1957, between sculpture and architecture. The first, understood as a purely spatial organism, should respond to the conditions of its environment (umwelt), the architectonic space. Its function: set it up aesthetically to meet the spiritual needs of the inhabitant. Following the aesthetic canon that he had just announced in Brazil for sculpture, the spatial disoccupation (the liberation of the spatial energy of the statue, the breaking of the neutrality of the free space) the aim was not to superficially beautify architecture but to activate its inner void. Oteiza, who had always been very interested in architecture and who had previously collaborated on numerous occasions with the best architects in the country, was in this year when he deepened in a more systematic way (theoretically and practically) about the art-architecture relationship. In fact, he named as the last work of his line of experimentation in sculpture to his proposal for the competition of the Monument to José Batlle in Montevideo, which, developed together with the architect Roberto Puig, was ended at the end of the year. The project proposed on an urban scale, and as a concrete example, the theoretical model of integration architecture + (art = 0) which he had elaborated the previous months, the empty integration. In the explanatory text accompanying the project (a text that exceeded the normal extents of a competition statement) he demanded the need of the aesthetic awareness of space, as an act of individual freedom, and it declared the end of the role of man as passive spectator in front of the work of art, claiming his actively participation in it. For him, the notion of the aesthetic space was not an inborn condition in man; first it was discovered, then learned, evolved and finally forgotten (once converted into a habit). To counteract blindness of the spatial sensitivity of man, he proposed the education of spatial perception, to emotionally influence the spontaneous reflection in front of the spatial game of forms in nature and the natural spectacle of the city. Learn to read the emotional language of space, to think visually. The work of art was thus a spiritual catalyst of the world’s contour, a modifier of the surrounding spatial life that corrected visual habits and conditioned stimuli and reflexes. From an emotional resonance with the psychological definition of the term (such as deep psychic power that invites or urges action), as opposed to instinct, drive (being the source of all spontaneous behavior) is likely to be modified by experience, by education, by culture, by desire. It is from this approach in terms of energy from which the notion drives of space is proposed, as a (reversible) formula between the spatial energy released in the process of disoccupation defined by Oteiza and characterizing of the work as a charged void (in sculpture, in architecture), and the deep psychic energy that invites or encourages the taking possession of the space (the absolute spatial will with which Oteiza defined its model of Art = 0, zero as a formal expression). If man changes his environment at the same time that is conditioned by it, it is essential an aesthetic awareness of space that shows him, among all the possibilities that it offers, what he needs (what is what he lacks), in order to take possession of it, for an effective being or existing in space. It is this characterization as energy by what drives of space lie between man and his (built) environment and allow the transformation between spatial and psychological energy; between his habitat and his habits. Around this same time, Oteiza defined a House as an articulated set of charged voids, as a work of pure plastic that is not art but according to the inhabitant. It is this inhabitant, educated in aesthetic awareness of space, who actively participating in the interpretation of the spaces provided by the architect, feeling and moved by the drives of the space, will make proper use of the architecture; from an art as object to an art as behavior, he will transform his inhabitation into an art, the art of inhabitation.

Optical imaging of retina in response to grating stimuli in cats

We examined the intrinsic signals in response to grating stimuli in order to determine whether the light-evoked intrinsic signals of the retina are due to changes in the photoreceptor activities induced by the image projected on to the retina or are due to neural activities of the inner retina. The retinas of the left eye of 12 cats under general anesthesia were examined by a functional imaging fundus camera. Near infrared light was used to monitor the reflectance changes (RCs) of the retina. Vertical grating were used to stimulate the retina at 4 Hz. The spatial frequencies of the gratings were 0.05, 0.11, 0.22, 0.43, 0.86, 1.73, and 3.46 cycles/degree (cpd). Ten images were averaged and used to analyze the RCs to obtain the peak value (PV) of a two dimensional fast Fourier transfer of the RCs. The wavefront aberrations (WA) were measured with a compact wavefront aberrometer and the spatial modulation transfer function (MTF) of the eye was calculated. The retinal reflectance image had a grating pattern. The PV of the spatial sensitivity curve was highest at low spatial frequencies (0.05 and 0.11 cpd), and the sensitivity decreased steeply with an increase in the spatial frequency. RCs were not detectable at 3.46 cpd. The MTF decreased gradually with increases in the spatial frequencies and was 0.68 at 3.46 cpd. The reflectance pattern of the retinal intrinsic signal elicited by grating stimuli of different spatial frequencies was different from that of the MTF. This suggests that the intrinsic signal represents not only the response of the photoreceptors but also other neuronal or vascular changes in the retina.

Coding Strategies and Implementations of Compressive Sensing

This dissertation studies the coding strategies of computational imaging to overcome the limitation of conventional sensing techniques. The information capacity of conventional sensing is limited by the physical properties of optics, such as aperture size, detector pixels, quantum efficiency, and sampling rate. These parameters determine the spatial, depth, spectral, temporal, and polarization sensitivity of each imager. To increase sensitivity in any dimension can significantly compromise the others.

This research implements various coding strategies subject to optical multidimensional imaging and acoustic sensing in order to extend their sensing abilities. The proposed coding strategies combine hardware modification and signal processing to exploiting bandwidth and sensitivity from conventional sensors. We discuss the hardware architecture, compression strategies, sensing process modeling, and reconstruction algorithm of each sensing system.

Optical multidimensional imaging measures three or more dimensional information of the optical signal. Traditional multidimensional imagers acquire extra dimensional information at the cost of degrading temporal or spatial resolution. Compressive multidimensional imaging multiplexes the transverse spatial, spectral, temporal, and polarization information on a two-dimensional (2D) detector. The corresponding spectral, temporal and polarization coding strategies adapt optics, electronic devices, and designed modulation techniques for multiplex measurement. This computational imaging technique provides multispectral, temporal super-resolution, and polarization imaging abilities with minimal loss in spatial resolution and noise level while maintaining or gaining higher temporal resolution. The experimental results prove that the appropriate coding strategies may improve hundreds times more sensing capacity.

Human auditory system has the astonishing ability in localizing, tracking, and filtering the selected sound sources or information from a noisy environment. Using engineering efforts to accomplish the same task usually requires multiple detectors, advanced computational algorithms, or artificial intelligence systems. Compressive acoustic sensing incorporates acoustic metamaterials in compressive sensing theory to emulate the abilities of sound localization and selective attention. This research investigates and optimizes the sensing capacity and the spatial sensitivity of the acoustic sensor. The well-modeled acoustic sensor allows localizing multiple speakers in both stationary and dynamic auditory scene; and distinguishing mixed conversations from independent sources with high audio recognition rate.

Psychophysical measurements of luminance and chromatic spatial and temporal contrast sensitivity in Duchenne muscular dystrophy

In children with Duchenne muscular dystrophy, color vision losses have been related to dystrophin deletions downstream of exon 30, which affect a dystrophin isoform, Dp260, present in the retina. To further evaluate visual function in DMD children, we measured spatial, temporal, and chromatic red-green and blue-yellow contrast sensitivity in two groups of DMD children with gene deletion downstream and upstream of exon 30. Psychophysical spatial contrast sensitivity was measured for low, middle, and high spatial frequencies with achromatic gratings and for low and middle frequencies with red-green and blue-yellow chromatic gratings. Temporal contrast sensitivity was also measured with achromatic stimuli. A reduction in sensitivity at all spatial luminance contrasts was found for the DMD patients with deletion downstream of exon 30. Similar results were found for temporal luminance contrast sensitivity. Red-green chromatic contrast sensitivity was reduced in DMD children with deletion downstream of exon 30, whereas blue-yellow chromatic contrast sensitivity showed no significant differences. We conclude that visual function is impaired in DMD children. Furthermore, we report a genotype-phenotype relationship because the visual impairment occurred in children with deletion downstream but not upstream of exon 30, affecting the retinal isoform of dystrophin Dp260.

Sensitivity of Spring Phenology to Warming Across Temporal and Spatial Climate Gradients in Two Independent Databases

Disparate ecological datasets are often organized into databases post hoc and then analyzed and interpreted in ways that may diverge from the purposes of the original data collections. Few studies, however, have attempted to quantify how biases inherent in these data (for example, species richness, replication, climate) affect their suitability for addressing broad scientific questions, especially in under-represented systems (for example, deserts, tropical forests) and wild communities. Here, we quantitatively compare the sensitivity of species first flowering and leafing dates to spring warmth in two phenological databases from the Northern Hemisphere. One-PEP725-has high replication within and across sites, but has low species diversity and spans a limited climate gradient. The other-NECTAR-includes many more species and a wider range of climates, but has fewer sites and low replication of species across sites. PEP725, despite low species diversity and relatively low seasonality, accurately captures the magnitude and seasonality of warming responses at climatically similar NECTAR sites, with most species showing earlier phenological events in response to warming. In NECTAR, the prevalence of temperature responders significantly declines with increasing mean annual temperature, a pattern that cannot be detected across the limited climate gradient spanned by the PEP725 flowering and leafing data. Our results showcase broad areas of agreement between the two databases, despite significant differences in species richness and geographic coverage, while also noting areas where including data across broader climate gradients may provide added value. Such comparisons help to identify gaps in our observations and knowledge base that can be addressed by ongoing monitoring and research efforts. Resolving these issues will be critical for improving predictions in understudied and under-sampled systems outside of the temperature seasonal mid-latitudes.

Methodological approaches to planar and volumetric scintigraphic imaging of small volume targets with high spatial resolution and sensitivity

Single-photon emission computed tomography (SPECT) is a non-invasive imaging technique, which provides information reporting the functional states of tissues. SPECT imaging has been used as a diagnostic tool in several human disorders and can be used in animal models of diseases for physiopathological, genomic and drug discovery studies. However, most of the experimental models used in research involve rodents, which are at least one order of magnitude smaller in linear dimensions than man. Consequently, images of targets obtained with conventional gamma-cameras and collimators have poor spatial resolution and statistical quality. We review the methodological approaches developed in recent years in order to obtain images of small targets with good spatial resolution and sensitivity. Multipinhole, coded mask- and slit-based collimators are presented as alternative approaches to improve image quality. In combination with appropriate decoding algorithms, these collimators permit a significant reduction of the time needed to register the projections used to make 3-D representations of the volumetric distribution of target’s radiotracers. Simultaneously, they can be used to minimize artifacts and blurring arising when single pinhole collimators are used. Representation images are presented, which illustrate the use of these collimators. We also comment on the use of coded masks to attain tomographic resolution with a single projection, as discussed by some investigators since their introduction to obtain near-field images. We conclude this review by showing that the use of appropriate hardware and software tools adapted to conventional gamma-cameras can be of great help in obtaining relevant functional information in experiments using small animals.

Chromatic spatial contrast sensitivity estimated by visual evoked cortical potential and psychophysics

The purpose of the present study was to measure contrast sensitivity to equiluminant gratings using steady-state visual evoked cortical potential (ssVECP) and psychophysics. Six healthy volunteers were evaluated with ssVECPs and psychophysics. The visual stimuli were red-green or blue-yellow horizontal sinusoidal gratings, 5° × 5°, 34.3 cd/m2 mean luminance, presented at 6 Hz. Eight spatial frequencies from 0.2 to 8 cpd were used, each presented at 8 contrast levels. Contrast threshold was obtained by extrapolating second harmonic amplitude values to zero. Psychophysical contrast thresholds were measured using stimuli at 6 Hz and static presentation. Contrast sensitivity was calculated as the inverse function of the pooled cone contrast threshold. ssVECP and both psychophysical contrast sensitivity functions (CSFs) were low-pass functions for red-green gratings. For electrophysiology, the highest contrast sensitivity values were found at 0.4 cpd (1.95 ± 0.15). ssVECP CSF was similar to dynamic psychophysical CSF, while static CSF had higher values ranging from 0.4 to 6 cpd (P < 0.05, ANOVA). Blue-yellow chromatic functions showed no specific tuning shape; however, at high spatial frequencies the evoked potentials showed higher contrast sensitivity than the psychophysical methods (P < 0.05, ANOVA). Evoked potentials can be used reliably to evaluate chromatic red-green CSFs in agreement with psychophysical thresholds, mainly if the same temporal properties are applied to the stimulus. For blue-yellow CSF, correlation between electrophysiology and psychophysics was poor at high spatial frequency, possibly due to a greater effect of chromatic aberration on this kind of stimulus.

Differential effects of aging on spatial contrast sensitivity to linear and polar sine-wave gratings

Changes in visual function beyond high-contrast acuity are known to take place during normal aging. We determined whether sensitivity to linear sine-wave gratings and to an elementary stimulus preferentially processed in extrastriate areas could be distinctively affected by aging. We measured spatial contrast sensitivity twice for concentric polar (Bessel) and vertical linear gratings of 0.6, 2.5, 5, and 20 cycles per degree (cpd) in two age groups (20-30 and 60-70 years). All participants were free of identifiable ocular disease and had normal or corrected-to-normal visual acuity. Participants were more sensitive to Cartesian than to polar gratings in all frequencies tested, and the younger adult group was more sensitive to all stimuli tested. Significant differences between sensitivities of the two groups were found for linear (only 20 cpd; P<0.01) and polar gratings (all frequencies tested; P<0.01). The young adult group was significantly more sensitive to linear than to circular gratings in the 20 cpd frequency. The older adult group was significantly more sensitive to linear than to circular gratings in all spatial frequencies, except in the 20 cpd frequency. The results suggest that sensitivity to the two kinds of stimuli is affected differently by aging. We suggest that neural changes in the aging brain are important determinants of this difference and discuss the results according to current models of human aging.

Spatial Aggregation of Forest Songbird Territories and Possible Implications for Area Sensitivity

Habitat area requirements of forest songbirds vary greatly among species, but the causes of this variation are not well understood. Large area requirements could result from advantages for certain species when settling their territories near those of conspecifics. This phenomenon would result in spatial aggregations much larger than single territories. Species that aggregate their territories could show reduced population viability in highly fragmented forests, since remnant patches may remain unoccupied if they are too small to accommodate several territories. The objectives of this study were twofold: (1) to seek evidence of territory clusters of forest birds at various spatial scales, lags of 250-550 m, before and after controlling for habitat spatial patterns; and (2) to measure the relationship between spatial autocorrelation and apparent landscape sensitivity for these species. In analyses that ignored spatial variation of vegetation within remnant forest patches, nine of the 17 species studied significantly aggregated their territories within patches. After controlling for forest vegetation, the locations of eight out of 17 species remained significantly clustered. The aggregative pattern that we observed may, thus, be indicative of a widespread phenomenon in songbird populations. Furthermore, there was a tendency for species associated with higher forest cover to be more spatially aggregated [ERRATUM].

Chromatic spatial contrast sensitivity estimated by visual evoked cortical potential and psychophysics

The purpose of the present study was to measure contrast sensitivity to equiluminant gratings using steady-state visual evoked cortical potential (ssVECP) and psychophysics. Six healthy volunteers were evaluated with ssVECPs and psychophysics. The visual stimuli were red-green or blue-yellow horizontal sinusoidal gratings, 5^° × 5^°, 34.3 cd/m2 mean luminance, presented at 6 Hz. Eight spatial frequencies from 0.2 to 8 cpd were used, each presented at 8 contrast levels. Contrast threshold was obtained by extrapolating second harmonic amplitude values to zero. Psychophysical contrast thresholds were measured using stimuli at 6 Hz and static presentation. Contrast sensitivity was calculated as the inverse function of the pooled cone contrast threshold. ssVECP and both psychophysical contrast sensitivity functions (CSFs) were low-pass functions for red-green gratings. For electrophysiology, the highest contrast sensitivity values were found at 0.4 cpd (1.95 ± 0.15). ssVECP CSF was similar to dynamic psychophysical CSF, while static CSF had higher values ranging from 0.4 to 6 cpd (P < 0.05, ANOVA). Blue-yellow chromatic functions showed no specific tuning shape; however, at high spatial frequencies the evoked potentials showed higher contrast sensitivity than the psychophysical methods (P < 0.05, ANOVA). Evoked potentials can be used reliably to evaluate chromatic red-green CSFs in agreement with psychophysical thresholds, mainly if the same temporal properties are applied to the stimulus. For blue-yellow CSF, correlation between electrophysiology and psychophysics was poor at high spatial frequency, possibly due to a greater effect of chromatic aberration on this kind of stimulus.

Comparative neurophysiology of spatial luminance contrast sensitivity

The luminance contrast sensitivity function has been investigated using behavioral and electrophysiological methods in many vertebrate species. Some features are conserved across species as a shape of the function, but other features, such as the contrast sensitivity peak value, spatial frequency contrast sensitivity peak, and visual acuity have changed. Here, we review contrast sensitivity across different classes of vertebrates, with an emphasis on the frequency contrast sensitivity peak and visual acuity. We also correlate the data obtained from the literature to test the power of the association between visual acuity and the spatial frequency of the contrast sensitivity function peak.