Showing papers by "Brian A. Wandell published in 2006"

No functional magnetic resonance imaging evidence for brightness and color filling-in in early human visual cortex.

Abstract: The brightness and color of a surface depends on its contrast with nearby surfaces. For example, a gray surface can appear very light when surrounded by a black surface or dark when surrounded by a white surface. Some theories suggest that perceived surface brightness and color is represented explicitly by neural signals in cortical visual field maps; these neural signals are not initiated by the stimulus itself but rather by the contrast signals at the borders. Here, we use functional magnetic resonance imaging (fMRI) to search for such neural "filling-in" signals. Although we find the usual strong relationship between local contrast and fMRI response, when perceived brightness or color changes are induced by modulating a surrounding field, rather than the surface itself, we find there is no corresponding local modulation in primary visual cortex or other nearby retinotopic maps. Moreover, when we model the obtained fMRI responses, we find strong evidence for contributions of both local and long-range edge responses. We argue that such extended edge responses may be caused by neurons previously identified in neurophysiological studies as being brightness responsive, a characterization that may therefore need to be revised. We conclude that the visual field maps of human V1 and V2 do not contain filled-in, topographical representations of surface brightness and color.

Assessment of stimulus-induced changes in human V1 visual field maps.

Abstract: Visual cortex contains a set of field maps in which nearby scene points are represented in the responses of nearby neurons. We tested a recent hypothesis that the visual field map in primary visual cortex (V1) is dynamic, changing in response to stimulus motion direction. The original experimental report replicates, but further experimental and analytical investigations do not support, the interpretation of the results. The V1 map remains invariant when measured using stimuli moving in different directions. The measurements can be explained by small and systematic response amplitude differences that arise when probing with stimuli moving in different directions.

Computational Neuroimaging : Color Signals in the Visual Pathways

Abstract: This review describes measurements of visual field maps and color signals in human visual cortex. One of the most exciting advances in recent years has been the ability to measure more than a dozen visual field maps in human visual cortex. These maps are grouped into small clusters that share a common eccentricity representation; we speculate that maps within a cluster have shared visual functions. The distribution of color (cone photoreceptor) signals differs strikingly across these clusters. The cluster of maps in ventral occipital cortex responds to all three color dimensions powerfully, and lesions within these maps disturb color appearance. The ventral occipital maps appear to be essential for normal interpretation of colors. Dorsal regions receive input from all three cone classes, but dorsal color coding but the signals are quite unlike human visual perception. Quantitative measurements of the responses in these dorsal and ventral regions promise to clarify the cortical circuitry that is essential for color appearance and related color phenomena. (Neuro-ophthalmol Jpn 23: 000～000, 2006)

Computational neuroimaging: maps and tracts in the human brain

Abstract: During the last decade, a number of remarkable magnetic resonance imaging (MRI) techniques have been developed for measuring human brain activity and structure. These MRI techniques have been accompanied by the development of signal processing, statistical and visualization methodologies. We review several examples of these methods, drawn mainly from work on the human visual pathways. We provide examples of how two methods- functional MRI (fMRI) and diffusion tensor imaging (DTI) - are used. First, we explain how fMRI enables us to identify and measure several distinct visual field maps and measure how these maps reorganize following disease or injury. Second we explain how DTI enables us to visualize neural structures within the brain's wires (white matter) and measure the patterns of connectivity in individual brains. Throughout, we identify signal processing, statistical, and visualization topics in need of further methodological development.

Optical interaction of space and wavelength in high-resolution digital imagers

Abstract: Precise simulation of digital camera architectures requires an accurate description of how the radiance image is transformed by optics and sampled by the image sensor array. Both for diffraction-limited imaging and for all practical lenses, the width of the optical-point-spread function differs at each wavelength. These differences are relatively small compared to coarse pixel sizes (6μm-8μm). But as pixel size decreases, to say 1.5μm-3μm, wavelength-dependent point-spread functions have a significant impact on the sensor response. We provide a theoretical treatment of how the interaction of spatial and wavelength properties influences the response of high-resolution color imagers. We then describe a model of these factors and an experimental evaluation of the model's computational accuracy.