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Brian A Wandell

Publications and source records attributed to Brian A Wandell.

At least 19 recordsLinked to original sources
PubMed2006

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

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.

Algorithms↗
PubMed2006

Differential sensitivity to words and shapes in ventral occipito-temporal cortex.

Efficient extraction of shape information is essential for proficient reading but the role of cortical mechanisms of shape analysis in word reading is not well understood. We studied cortical responses to written words while parametrically varying the amount of visual noise applied to the word stimuli. In only a few regions along the ventral surface, cortical responses increased with word visibility. We found consistently increasing responses in bilateral posterior occipito-temporal sulcus (pOTS), at an anatomical location that closely matches the "visual word form area". In other cortical regions, such as V1, responses remained constant regardless of the noise level. We performed 3 additional tests to assess the functional specialization of pOTS responses for written word processing. We asked whether pOTS responses are 1) left lateralized, 2) more sensitive to words than to line drawings or false fonts, and 3) invariant for visual hemifield of words but not other stimuli. We found that left and right pOTS response functions both had highest sensitivity for words, intermediate for line drawings, and lowest for false fonts. Visual hemifield invariance was similar for words and line drawings. These results suggest that left and right pOTS are both involved in shape processing, with enhanced efficiency for processing visual word forms.

Adult↗
PubMed2006

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

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.

Action Potentials↗
PubMed2005

Cone signal interactions in direction-selective neurons in the middle temporal visual area (MT).

Many experimental measurements support the hypothesis that the middle temporal visual area (MT) of the rhesus monkey has a central role in processing visual motion. Most of these studies were performed using luminance stimuli, leaving open the question of how color information is used during motion processing. We investigated the specific question of how S-cone signals, an important source of color information, interact with L,M-cone signals, the dominant source of luminance information. In MT, S-cone-initiated signals combine synergistically with L,M-cone (luminance) signals over most of the stimulus range, regardless of whether the stimuli are added or subtracted. A quantitative analysis of the responses to the combination of S- and L,M-cone signals shows that for a significant minority of cells, these S-cone signals are carried to MT by a color-opponent ("blue-yellow") pathway, such that in certain limited contrast ranges, a small amount of S- and L,M-cone cancellation is observed. Both S- and L,M-cone responses are direction-selective, suggesting that MT processes a wide range of motion signals, including those carried by luminance and color. To investigate this possibility further, we measured MT responses while monkeys discriminated the direction of motion of luminance and S-cone-initiated gratings. The sensitivity of single MT neurons and the correlation between trial-to-trial variations in single neuron firing and perception are similar for S- and L,M-cone stimuli, further supporting a role for MT in processing chromatic motion.

Animals↗
PubMed2005

Visual field maps and stimulus selectivity in human ventral occipital cortex.

Human visual cortex is organized into distinct visual field maps whose locations and properties provide important information about visual computations. There are two conflicting models of the organization and computational role of ventral occipital visual field maps. We report new functional MRI measurements that test these models. We also present the first coordinated measurements of visual field maps and stimulus responsivity to color, objects and faces in ventral occipital cortex. These measurements support a model that includes a hemifield map, hV4, adjacent to the central field representation of ventral V3. In addition, the measurements demonstrate a cluster of visual field maps in ventral occipital cortex (VO cluster) anterior to hV4. We describe the organization and stimulus responsivity of two new hemifield maps, VO-1 and VO-2, within this cluster. The maps and stimulus responsivity support a general organization of visual cortex based on clusters of maps that serve distinct computational functions.

Humans↗
PubMed2005

Lack of long-term cortical reorganization after macaque retinal lesions.

Several aspects of cortical organization are thought to remain plastic into adulthood, allowing cortical sensorimotor maps to be modified continuously by experience. This dynamic nature of cortical circuitry is important for learning, as well as for repair after injury to the nervous system. Electrophysiology studies suggest that adult macaque primary visual cortex (V1) undergoes large-scale reorganization within a few months after retinal lesioning, but this issue has not been conclusively settled. Here we applied the technique of functional magnetic resonance imaging (fMRI) to detect changes in the cortical topography of macaque area V1 after binocular retinal lesions. fMRI allows non-invasive, in vivo, long-term monitoring of cortical activity with a wide field of view, sampling signals from multiple neurons per unit cortical area. We show that, in contrast with previous studies, adult macaque V1 does not approach normal responsivity during 7.5 months of follow-up after retinal lesions, and its topography does not change. Electrophysiology experiments corroborated the fMRI results. This indicates that adult macaque V1 has limited potential for reorganization in the months following retinal injury.

Animals↗
PubMed2005

Functional organization of human occipital-callosal fiber tracts.

Diffusion tensor imaging (DTI) and fiber tracking (FT) were used to measure the occipital lobe fiber tracts connecting the two hemispheres in individual human subjects. These tracts are important for normal vision. Also, damage to portions of these tracts is associated with alexia. To assess the reliability of the DTI-FT measurements, occipital-callosal projections were estimated from each subject's left and right hemispheres independently. The left and right estimates converged onto the same positions within the splenium. We further characterized the properties of the estimated occipital-callosal fiber tracts by combining them with functional MRI. We used functional MRI to identify visual field maps in cortex and labeled fibers by the cortical functional response at the fiber endpoint. This labeling reveals a regular organization of the fibers within the splenium. The dorsal visual maps (dorsal V3, V3A, V3B, V7) send projections through a large band in the middle of the splenium, whereas ventral visual maps (ventral V3, V4) send projections through the inferior-anterior corner of the splenium. The agreement between the independent left/right estimates, further supported by previous descriptions of homologous tracts in macaque, validates the DTI-FT methods. However, a principal limitation of these methods is low sensitivity: a large number of fiber tracts that connect homotopic regions of ventral and lateral visual cortex were undetected. We conclude that most of the estimated tracts are real and can be localized with a precision of 1-2 mm, but many tracts are missed because of data and algorithm limitations.

Adult↗
PubMed2005

Visual field map clusters in human cortex.

We describe the location and general properties of nine human visual field maps. The cortical location of each map, as well as many examples of the eccentricity and angular representations within these maps, are shown in a series of images that summarize a large set of functional MRI data. The organization and properties of these maps are compared and contrasted with descriptions by other investigators. We hypothesize that the human visual field maps are arranged in several clusters, each comprising a group of maps that share a common foveal representation and semicircular eccentricity map. The spatial organization of these clusters suggests that the perceptual processing within each cluster serves related functions.

Brain Mapping↗
PubMed2005

Specializations for chromatic and temporal signals in human visual cortex.

Neurological case studies and qualitative measurements suggest that regions within human extrastriate cortex are specialized for different perceptual functions, including color. However, there are few quantitative measurements of human extrastriate color specializations. We studied the chromatic and temporal responses in several different clusters of human visual field maps using functional magnetic resonance imaging. Contrast response functions were measured for luminance [(L + M)-cone], red-green [(L - M)-cone] and blue-yellow (S-cone) modulations at various temporal frequencies. In primary visual cortex (V1), temporal responsivities to luminance and red-green modulations are approximately constant up to 10 Hz, but responsivities to blue-yellow modulations decrease significantly. In ventral occipital cortex (VO), all colors elicit strong responses, and, for each color, low temporal frequency modulations are more effective than high temporal frequency modulations. Hence, VO represents the full range of color information but does not respond well to rapid modulations. Conversely, in human motion-selective cortex (MT+) and V3A, blue-yellow modulations elicit very weak responses, whereas luminance and red-green high temporal frequency modulations are equally or more effective than low temporal frequency modulations. Hence, these dorsal occipital regions respond well to rapid modulations, but not all color information is represented. Similar to human motion perception, MT+ and V3A respond powerfully to all temporal frequencies but only to some colors. Similar to human color perception, VO responds powerfully to all colors but only to relatively low temporal frequencies.

Brain Mapping↗
PubMed2005

Occipital-callosal pathways in children: Validation and atlas development.

Diffusion tensor imaging and fiber tracking were used to measure fiber bundles connecting the two occipital lobes in 53 children of 7-12 years of age. Independent fiber bundle estimates originating from the two hemispheres converge onto the lower half of the splenium. This observation validates the basic methodology and suggests that most occipital-callosal fibers connect the two occipital lobes. Within the splenium, fiber bundles are organized in a regular pattern with respect to their cortical projection zones. Visual cortex dorsal to calcarine projects through a large band that fills much of the inferior half of the splenium, while cortex ventral to calcarine sends projections through a band at the anterior inferior edge of the splenium. Pathways projecting to the occipital pole and lateral-occipital regions overlap the dorsal and ventral groups slightly anterior to the center of the splenium. To visualize these pathways in a typical brain, we combined the data into an atlas. The estimated occipital-callosal fiber paths from the atlas form the walls of the occipital horn of the lateral ventricle, with dorsal paths forming the medial wall and the ventral paths bifurcating into a medial tract to form the inferior-medial wall and a superior tract that joins the lateral-occipital paths to form the superior wall of the ventricle. The properties of these fiber bundles match those of the hypothetical pathways described in the neurological literature on alexia.

Anatomy, Artistic↗
PubMed2004

Predominantly extra-retinotopic cortical response to pattern symmetry.

Symmetry along one or more axes is a key property of objects and biological organisms. We report on a bilateral visual region of occipital cortex that responds strongly to the presence of multiple symmetries in the viewed image. The stimuli consisted of random dots organized in fourfold and onefold mirror-symmetric patterns, against random control stimuli. The contrast between symmetric and random patterns produced negligible or inconsistent activation of the primary visual projection area V1 or of other medial occipital projection areas. However, there was strong symmetry-specific activation in extra-retinotopic lateral occipital cortex. The high level of activation in this region of cortex may represent part of a general class of computations that require integration of information across a large span of the visual field.

Brain Mapping↗
PubMed2004

Interpreting the BOLD signal.

The development of functional magnetic resonance imaging (fMRI) has brought together a broad community of scientists interested in measuring the neural basis of the human mind. Because fMRI signals are an indirect measure of neural activity, interpreting these signals to make deductions about the nervous system requires some understanding of the signaling mechanisms. We describe our current understanding of the causal relationships between neural activity and the blood-oxygen-level-dependent (BOLD) signal, and we review how these analyses have challenged some basic assumptions that have guided neuroscience. We conclude with a discussion of how to use the BOLD signal to make inferences about the neural signal.

Animals↗
PubMed2003

Visual field representations and locations of visual areas V1/2/3 in human visual cortex.

The position, surface area and visual field representation of human visual areas V1, V2 and V3 were measured using fMRI in 7 subjects (14 hemispheres). Cortical visual field maps of the central 12 deg were measured using rotating wedge and expanding ring stimuli. The boundaries between areas were identified using an automated procedure to fit an atlas of the expected visual field map to the data. All position and surface area measurements were made along the boundary between white matter and gray matter. The representation of the central 2 deg of visual field in areas V1, V2, V3 and hV4 spans about 2100 mm2 and is centered on the lateral-ventral aspect of the occipital lobes at Talairach coordinates -29, -78, -11 and 25, -80, -9. The mean area between the 2-deg and 12-deg eccentricities for the primary visual areas was: V1: 1470 mm2; V2: 1115 mm2; and V3: 819 mm2. The sizes of areas V1, V2 and V3 varied by about a factor of 2.5 across individuals; the sizes of V1 and V2 are significantly correlated within individuals, but there is a very low correlation between V1 and V3. These in vivo measurements of normal human retinotopic visual areas can be used as a reference for comparison to unusual cases involving developmental plasticity, recovery from injury, identifying homology with animal models, or analyzing the computational resources available within the visual pathways.

Brain Mapping↗
PubMed2003

Functional imaging of the visual pathways.

Functional neuroimaging has provided a new view of activity in human visual cortex. There have been a series of interesting developments in understanding the relationship between the functional signals, particularly functional MRI, and basic measurements of action potentials and local field potentials. The new human neuro-imaging measurements have clarified some of the similarities and differences between the general organization of visual areas in human and macaque visual cortex, and there have been some interesting new results concerning cortical visual plasticity and dysfunction. The new fMRI focus on measurements of the human brain will drive new relationships between neurology and visual neuroscience that should help us learn much more about the neural basis of perception.

Electroencephalography↗
PubMed2003

Spectral estimation theory: beyond linear but before Bayesian.

Most color-acquisition devices capture spectral signals by acquiring only three samples, critically undersampling the spectral information. We analyze the problem of estimating high-dimensional spectral signals from low-dimensional device responses. We begin with the theory and geometry of linear estimation methods. These methods use linear models to characterize the likely input signals and reduce the number of estimation parameters. Next, we introduce two submanifold estimation methods. These methods are based on the observation that for many data sets the deviation between the signal and the linear estimate is systematic; the methods incorporate knowledge of these systematic deviations to improve upon linear estimation methods. We describe the geometric intuition of these methods and evaluate the submanifold method on hyperspectral image data.

Journal Article↗
PubMed2003

Integrated color pixels in 0.18-microm complementary metal oxide semiconductor technology.

Following the trend of increased integration in complementary metal oxide semiconductor (CMOS) image sensors, we have explored the potential of implementing light filters by using patterned metal layers placed on top of each pixel's photodetector. To demonstrate wavelength selectivity, we designed and prototyped integrated color pixels in a standard 0.18-microm CMOS technology. Transmittance of several one-dimensional (1D) and two-dimensional (2D) patterned metal layers was measured under various illumination conditions and found to exhibit wavelength selectivity in the visible range. We performed (a) wave optics simulations to predict the spectral responsivity of an uncovered reference pixel and (b) numerical electromagnetic simulations with a 2D finite-difference time-domain method to predict transmittances through 1D patterned metal layers. We found good agreement in both cases. Finally, we used simulations to predict the transmittance for more elaborate designs.

Journal Article↗