Color responses of the human lateral geniculate nucleus: selective amplification of S-cone signals between the lateral geniculate nucleno and primary visual cortex measured with high-field fMRI

The lateral geniculate nucleus (LGN) is the primary thalamic nucleus that relays visual information from the retina to the primary visual cortex (V1) and has been extensively studied in non-human primates. A key feature of the LGN is the segregation of retinal inputs into different cellular layers characterized by their differential responses to red-green (RG) color (L/M opponent), blue-yellow (BY) color (S-cone opponent) and achromatic (Ach) contrast. In this study we use high-field functional magnetic resonance imaging (4 tesla, 3.6 × 3.6 × 3 mm3) to record simultaneously the responses of the human LGN and V1 to chromatic and Ach contrast to investigate the LGN responses to color, and how these are modified as information transfers between LGN and cortex. We find that the LGN has a robust response to RG color contrast, equal to or greater than the Ach response, but a significantly poorer sensitivity to BY contrast. In V1 at low temporal rates (2 Hz), however, the sensitivity of the BY color pathway is selectively enhanced, rising in relation to the RG and Ach responses. We find that this effect generalizes across different stimulus contrasts and spatial stimuli (1-d and 2-d patterns), but is selective for temporal frequency, as it is not found for stimuli at 8 Hz. While the mechanism of this cortical enhancement of BY color vision and its dynamic component is unknown, its role may be to compensate for a weak BY signal originating from the sparse distribution of neurons in the retina and LGN

Texture variations suppress suprathreshold brightness and colour variations

Discriminating material changes from illumination changes is a key function of early vision. Luminance cues are ambiguous in this regard, but can be disambiguated by co-incident changes in colour and texture. Thus, colour and texture are likely to be given greater prominence than luminance for object segmentation, and better segmentation should in turn produce stronger grouping. We sought to measure the relative strengths of combined luminance, colour and texture contrast using a suprathreshhold, psychophysical grouping task. Stimuli comprised diagonal grids of circular patches bordered by a thin black line and contained combinations of luminance decrements with either violet, red, or texture increments. There were two tasks. In the Separate task the different cues were presented separately in a two-interval design, and participants indicated which interval contained the stronger orientation structure. In the Combined task the cues were combined to produce competing orientation structure in a single image. Participants had to indicate which orientation, and therefore which cue was dominant. Thus we established the relative grouping strength of each cue pair presented separately, and compared this to their relative grouping strength when combined. In this way we observed suprathreshold interactions between cues and were able to assess cue dominance at ecologically relevant signal levels. Participants required significantly more luminance and colour compared to texture contrast in the Combined compared to Separate conditions (contrast ratios differed by about 0.1 log units), showing that suprathreshold texture dominates colour and luminance when the different cues are presented in combination

Combining S-cone and luminance signals adversely affects discrimination of objects within backgrounds

The visual system processes objects embedded in complex scenes that vary in both luminance and colour. In such scenes, colour contributes to the segmentation of objects from backgrounds, but does it also affect perceptual organisation of object contours which are already defined by luminance signals, or are these processes unaffected by colour’s presence? We investigated if luminance and chromatic signals comparably sustain processing of objects embedded in backgrounds, by varying contrast along the luminance dimension and along the two cone-opponent colour directions. In the first experiment thresholds for object/non-object discrimination of Gaborised shapes were obtained in the presence and absence of background clutter. Contrast of the component Gabors was modulated along single colour/luminance dimensions or co-modulated along multiple dimensions simultaneously. Background clutter elevated discrimination thresholds only for combined S-(L + M) and L + M signals. The second experiment replicated and extended this finding by demonstrating that the effect was dependent on the presence of relatively high S-(L + M) contrast. These results indicate that S-(L + M) signals impair spatial vision when combined with luminance. Since S-(L + M) signals are characterised by relatively large receptive fields, this is likely to be due to an increase in the size of the integration field over which contour-defining information is summed

Ensemble coding of color and luminance contrast

Ensemble coding has been demonstrated for many attributes including color, but the metrics on which this coding is based remain uncertain. We examined ensemble percepts for stimulus sets that varied in chromatic contrast between complementary hues, or that varied in luminance contrast between increments and decrements, in both cases focusing on the ensemble percepts for the neutral gray stimulus defining the category boundary. Each ensemble was composed of 16 circles with four contrast levels. Observers saw the display for 0.5 s and then judged whether a target contrast was a member of the set. False alarms were high for intermediate contrasts (within the range of the ensemble) and fell for higher or lower values. However, for ensembles with complementary hues, gray was less likely to be reported as a member, even when it represented the mean chromaticity of the set. When the settings were repeated for luminance contrast, false alarms for gray were higher and fell off more gradually for out-of-range contrasts. This difference implies that opposite luminance polarities represent a more continuous perceptual dimension than opponent-color variations, and that “gray” is a stronger category boundary for chromatic than luminance contrasts. For color, our results suggest that ensemble percepts reflect pooling within rather than between large hue differences, perhaps because the visual system represents hue differences more like qualitatively different categories than like quantitative differences within an underlying color “space.” The differences for luminance and color suggest more generally that ensemble coding for different visual attributes might depend on different processes that in turn depend on the format of the visual representation

El espiritu africano y el espacio trinitario. Percepciones de la obra de Earl Lovelace. (Traducción: Jhilda Méndez).

SUMARIO 1.- Artículos Interpretación de un pasaje ambiguo del Quijote: el juicio del cura sobre Tirante el Blanco durante el escrutinio de los libros del ingenioso Hidalgo. (Primera parte, Capítulo VI). Morales Ardaya, Francisco Poéticas del viaje en la narrativa de la alta modernidad latinoamericana: Los Pasos Perdidos, de Alejo Carpentier. Palmero González, Elena 2.- Dossier Presentación. Mansoor, Ramón Solidaridad de clase y política sexual de las relaciones entre indios/africanos en la literatura trinitaria (Traducción: Fabiola Reyes). Rampersad, Sheila Reflexiones sobre una nación: un acercamiento a la cuestión de la identidad en la poesía trinitaria de Jennifer Rahim. Roberts, Nicole V. S. Naipul y the enigma of arrival (Traducción: José Francisco Velásquez). Singh Datoo, Arlene V. S. Naipaul, la escritura que dilucida el imaginario del mundo colonial. Valero, Arnaldo E. Haciendo palabra: la poesía de Jennifer Rahim. (Traducción: Andrés Seijas). Antoine-Dunne, Jean El espiritu africano y el espacio trinitario. Percepciones de la obra de Earl Lovelace. (Traducción: Jhilda Méndez). Sankeralli, Burton El carnaval canibalizado o el caníbal carnavalizado. Descontextualizando el chiste del caníbal en el calipso y la literatura. Rohlehr, Gordon La contribución de la música hispánica al calipso. (Traducción: Andrés Seijas). Regis, Louis 3.- Entrevista Entrevista con Brother Resistance (Hermano Resistencia). Mansoor, Ramón Creo que escribo desde la vida misma. Entrevista a Earl Lovelace. Pérez Sisto, Edith 4.- Reseñas La refiguración del viaje de Victoria de Stefano. Alba, Alexandra Escrituras híbridas. Juego intertextual y ficción en García Márquez y Jean Rhys de Mireya Fernández Merino. Navarro, Bernardo Relatar el tiempo: Alejo Carpentier de Elena Palmero. Pacheco O., Bettina 5.- Índice acumulado Índice acumulado. 6.- Portadas de secciones Puja (1993). Boudlov, Isaiah Dancing the limbo (1961). Boudlov, Isaiah Arrival (2002). Singh, Parmana Krishna playing the flute (1996). Boudlov, [email protected] analíticoanua

A quantitative analysis of normal post-receptoral chromatic mechanisms and its applications to visual dysfunction

Measurements of acquired colour deficits can aid in the detection of certain visual ailments (King-Smith (1991)). Such ailments cause selective deficits among the different neural colour and luminance coding mechanisms at the post-receptoral level. These mechanisms consist of varying combinations of inputs from the three cone types (L, M, S) responsible for phototransduction in the human visual system. An efficient colour test can measure the sensitivities of these mechanisms independently. In order to achieve this, precise estimates of the cone inputs (or cone weightings) to these mechanisms need to be obtained.We obtained these estimates through measurement of contrast detection thresholds for sinusoidal gratings in 69 different axes in colour space in three normal human subjects. The colour space used (cone contrast space) permitted direct estimation of the cone weightings from the threshold information. We sensitized each mechanism in turn by varying the spatiotemporal properties of the grating. At each spatiotemporal condition, planar and three-dimensional "super-elliptical" fits were made to the threshold data. The cone weightings of the sensitized mechanism were obtained directly from the parameters of these fits. (Abstract shortened by UMI.

Investigation of the chromatic postreceptoral detection mechanisms of human colour vision using noise masking in cone contrast space

Human colour vision is subserved by three photoreceptor, or cone, types. To optimise the coding of natural colour scenes, the neural projections from these cones are recombined in the retina to form the postreceptoral mechanism. Several psychophysical studies (e.g., Krauskopf et al., 1982; Cole et al., 1993) have consistently revealed three principal postreceptoral mechanisms, termed red-green, blue-yellow and luminance (or achromatic). However, it is still unknown whether these are the only three mechanisms responsible for the detection of coloured stimuli. In fact, recent studies (e.g., Gegenfurtner & Kiper, 1992) propose the existence of additional postreceptoral detection mechanisms. These studies measured contrast sensitivity in the presence of noise masking that effectively desensitised the principal mechanisms so that other mechanisms might be observed. In this thesis, we have, for the first time, applied to this technique a colour representation (cone contrast space) that provides a more suitable framework with which to study the postreceptoral mechanisms and their cone inputs.We used psychophysical techniques to address three specific questions: (1) the number of postreceptoral mechanisms, (2) the functional composition of each mechanism, and (3) the manner in which the mechanism responses are combined to yield the detectability of a given stimulus. Our investigation produced three main results: (1) there are only three postreceptoral mechanisms, (2) each postreceptoral mechanism consists of an opponent pair of submechanisms, and (3) the responses of the three postreceptoral mechanisms at detection threshold are combined according to a model of probability summation. The third result suggests that the mechanism responses can be treated as probabilities of seeing a given stimulus, and that these three responses are statistically independent. A final result shows that suprathreshold discrimination depends on the ratio of postreceptoral mechanism responses, indicative of a simple mechanism for hue discrimination