Showing papers by "Brian A. Wandell published in 2015"

Stimulus Dependence of Gamma Oscillations in Human Visual Cortex

Abstract: A striking feature of some field potential recordings in visual cortex is a rhythmic oscillation within the gamma band (30–80 Hz). These oscillations have been proposed to underlie computations in perception, attention, and information transmission. Recent studies of cortical field potentials, including human electrocorticography (ECoG), have emphasized another signal within the gamma band, a nonoscillatory, broadband signal, spanning 80–200 Hz. It remains unclear under what conditions gamma oscillations are elicited in visual cortex, whether they are necessary and ubiquitous in visual encoding, and what relationship they have to nonoscillatory, broadband field potentials. We demonstrate that ECoG responses in human visual cortex (V1/V2/V3) can include robust narrowband gamma oscillations, and that these oscillations are reliably elicited by some spatial contrast patterns (luminance gratings) but not by others (noise patterns and many natural images). The gamma oscillations can be conspicuous and robust, but because they are absent for many stimuli, which observers can see and recognize, the oscillations are not necessary for seeing. In contrast, all visual stimuli induced broadband spectral changes in ECoG responses. Asynchronous neural signals in visual cortex, reflected in the broadband ECoG response, can support transmission of information for perception and recognition in the absence of pronounced gamma oscillations.

Evaluating the Accuracy of Diffusion MRI Models in White Matter

Abstract: Models of diffusion MRI within a voxel are useful for making inferences about the properties of the tissue and inferring fiber orientation distribution used by tractography algorithms. A useful model must fit the data accurately. However, evaluations of model-accuracy of commonly used models have not been published before. Here, we evaluate model-accuracy of the two main classes of diffusion MRI models. The diffusion tensor model (DTM) summarizes diffusion as a 3-dimensional Gaussian distribution. Sparse fascicle models (SFM) summarize the signal as a sum of signals originating from a collection of fascicles oriented in different directions. We use cross-validation to assess model-accuracy at different gradient amplitudes (b-values) throughout the white matter. Specifically, we fit each model to all the white matter voxels in one data set and then use the model to predict a second, independent data set. This is the first evaluation of model-accuracy of these models. In most of the white matter the DTM predicts the data more accurately than test-retest reliability; SFM model-accuracy is higher than test-retest reliability and also higher than the DTM model-accuracy, particularly for measurements with (a) a b-value above 1000 in locations containing fiber crossings, and (b) in the regions of the brain surrounding the optic radiations. The SFM also has better parameter-validity: it more accurately estimates the fiber orientation distribution function (fODF) in each voxel, which is useful for fiber tracking.

Sex differences in the corpus callosum in preschool-aged children with autism spectrum disorder

Abstract: Background: Abnormalities in the corpus callosum have been reported in individuals with autism spectrum disorder (ASD), but few studies have evaluated young children. Sex differences in callosal organization and diffusion characteristics have also not been evaluated fully in ASD. Methods: Structural and diffusion-weighted images were acquired in 139 preschool-aged children with ASD (112 males/27 females) and 82 typically developing (TD) controls (53 males/29 females). Longitudinal scanning at two additional annual time points was carried out in a subset of these participants. Callosal organization was evaluated using two approaches: 1) diffusion tensor imaging (DTI) tractography to define subregions based on cortical projection zones and 2) as a comparison to previous studies, midsagittal area analysis using Witelson subdivisions. Diffusion measures of callosal fibers were also evaluated. Results: Analyses of cortical projection zone subregions revealed sex differences in the patterns of altered callosal organization. Relative to their sex-specific TD counterparts, both males and females with ASD had smaller regions dedicated to fibers projecting to superior frontal cortex, but patterns differed in callosal subregions projecting to other parts of frontal cortex. While males with ASD had a smaller callosal region dedicated to the orbitofrontal cortex, females with ASD had a smaller callosal region dedicated to the anterior frontal cortex. There were also sex differences in diffusion properties of callosal fibers. While no alterations were observed in males with ASD relative to TD males, mean diffusivity (MD), axial diffusivity (AD), and radial diffusivity (RD) were all increased in females with ASD relative to TD females. Analyses of Witelson subdivisions revealed a decrease in midsagittal area of the corpus callosum in both males and females with ASD but no regional differences in specific subdivisions. Longitudinal analyses revealed no diagnostic or sex differences in the growth rate or change in diffusion measures of the corpus callosum from 3 to 5 years of age. Conclusions: There are sex differences in the pattern of altered corpus callosum neuroanatomy in preschool-aged children with ASD.

A Lack of Experience-Dependent Plasticity After More Than a Decade of Recovered Sight

Abstract: In 2000, monocular vision was restored to M. M., who had been blind between the ages of 3 and 46 years. Tests carried out over 2 years following the surgery revealed impairments of 3-D form, object, and face processing and an absence of object- and face-selective blood-oxygen-level-dependent responses in ventral visual cortex. In the present research, we reexamined M. M. to test for experience-dependent recovery of visual function. Behaviorally, M. M. remains impaired in 3-D form, object, and face processing. Accordingly, we found little to no evidence of the category-selective organization within ventral visual cortex typically associated with face, body, scene, or object processing. We did observe remarkably normal object selectivity within lateral occipital cortex, consistent with M. M.’s previously reported shape-discrimination performance. Together, these findings provide little evidence for recovery of high-level visual function after more than a decade of visual experience in adulthood.

Computational Modeling of Responses in Human Visual Cortex

Abstract: A new generation of models and experimental designs are clarifying the computational principles in human visual cortex Over the first two decades of functional magnetic resonance imaging, steady progress in measuring visual cortex led to the identification of more than twenty retinotopically mapped cortical areas New models are being developed to predict responses in these maps and thus clarify their functional roles Many of these models are based on a stimulus-referred approach, so that the computational principles can be tested using multiple types of measurements – spanning functional MRI, intracranial recordings, and single-unit measurements The stimulus-referred approach promises to build an integrated view of neural computations measured across temporal and spatial scales

Data management to support reproducible research

Abstract: We describe the current state and future plans for a set of tools for scientific data management (SDM) designed to support scientific transparency and reproducible research. SDM has been in active use at our MRI Center for more than two years. We designed the system to be used from the beginning of a research project, which contrasts with conventional end-state databases that accept data as a project concludes. A number of benefits accrue from using scientific data management tools early and throughout the project, including data integrity as well as reuse of the data and of computational methods.

Disrupted fornix integrity in children with chromosome 22q11.2 deletion syndrome.

Abstract: The fornix is the primary subcortical output fiber system of the hippocampal formation. In children with 22q11.2 deletion syndrome (22q11.2DS), hippocampal volume reduction has been commonly reported, but few studies as yet have evaluated the integrity of the fornix. Therefore, we investigated the fornix of 45 school-aged children with 22q11.2DS and 38 matched typically developing (TD) children. Probabilistic diffusion tensor imaging (DTI) tractography was used to reconstruct the body of the fornix in each child׳s brain native space. Compared with children, significantly lower fractional anisotropy (FA) and higher radial diffusivity (RD) was observed bilaterally in the body of the fornix in children with 22q11.2DS. Irregularities were especially prominent in the posterior aspect of the fornix where it emerges from the hippocampus. Smaller volumes of the hippocampal formations were also found in the 22q11.2DS group. The reduced hippocampal volumes were correlated with lower fornix FA and higher fornix RD in the right hemisphere. Our findings provide neuroanatomical evidence of disrupted hippocampal connectivity in children with 22q11.2DS, which may help to further understand the biological basis of spatial impairments, affective regulation, and other factors related to the ultra-high risk for schizophrenia in this population.

Image Systems Simulation

Abstract: Image systems simulation plays an important role in the design and evaluation of a range of devices, from large-scale remote sensing to the smallest biomedical imaging instruments. We review principles of image systems simulation that are applicable to the design and evaluation of consumer imaging devices. In this application the main goal is to help engineering team members measure and understand how each of the many system components influences image quality. We show how to combine models for scene radiance, optics, sensors, image processing and displays into a unified simulation. We then illustrate how to use simulations to evaluate design tradeoffs, invent new imaging systems, and optimize image-processing algorithms. Keywords: color constancy; end-to-end simulation; plenoptic function; light field; L3

ISETBIO: Computational tools for modeling early human vision

Abstract: Vision begins with the formation of the retinal image, sampling by photoreceptors, and retinal processing of photoreceptor excitations. We describe software tools for modeling how these fundamental processes limit human performance on visual tasks. Article not available.

Correction: Evaluating the Accuracy of Diffusion MRI Models in White Matter.

Abstract: There is an error in Equation 3. Please see the correct Equation 3 here. S(θ,b)= βoe−bD+∑i=1Fβie−bθtQiθ

Automatically designing an image processing pipeline for a five-band camera prototype using the local, linear, learned (L3) method

Abstract: The development of an image processing pipeline for each new camera design can be time-consuming. To speed camera development, we developed a method named L3 (Local, Linear, Learned) that automatically creates an image processing pipeline for any design. In this paper, we describe how we used the L3 method to design and implement an image processing pipeline for a prototype camera with five color channels. The process includes calibrating and simulating the prototype, learning local linear transforms and accelerating the pipeline using graphics processing units (GPUs).

An iterative algorithm for spectral estimation with spatial smoothing

Abstract: Many multispectral imaging systems are computational in nature and require processing of raw data in order to obtain radiance spectra. In this paper, we derive a fast and scalable spectral estimation algorithm based on the Alternating Direction Method of Multipliers (ADMM). Using this approach we solve for the unknown surface spectral reflectance simultaneously for all pixels in the image. This global formulation allows us to incorporate spatial as well as spectral regularizers, such as total variation penalty or non-negativity. We show that the estimates derived with our solver are more accurate and more robust in the presence of noise.

17.3: D-CIELAB: A Color Metric For Dichromatic Observers

Abstract: A color metric for dichromatic subjects is proposed and evaluated empirically. The color metric extends the widely used CIELAB color difference metric, an international standard for trichromatic observers. The dichromatic extension (D-CIELAB) is implemented by specifying a linear transformation that maps the two types of cone absorptions in dichromats to the three types of cone absorptions in trichromats and then applies the standard CIELAB color metric to these values. The linear mapping is derived from between-eye color-matches of observers who are dichromatic in one eye and trichromatic in the other eye. We evaluate the accuracy of the D-CIELAB metric by predicting and measuring color discrimination thresholds for two dichromatic observers.

Erratum: Sex differences in the corpus callosum in preschool-aged children with autism spectrum disorder (Molecular Autism (2015) 6 (26))

Abstract: [This corrects the article DOI: 10.1186/s13229-015-0005-4.].

Efficient illuminant correction in the local, linear, learned (L3) method

Abstract: To speed the development of novel camera architectures we proposed a method, L 3 (Local, Linear and Learned),that automatically creates an optimized image processing pipeline. The L 3 method assigns each sensor pixel into one of 400 classes, and applies class-dependent local linear transforms that map the sensor data from a pixel and its neighbors into the target output (e.g., CIE XYZ rendered under a D65 illuminant). The transforms are precomputed from training data and stored in a table used for image rendering. The training data are generated by camera simulation, consisting of sensor responses and rendered CIE XYZ outputs. The sensor and rendering illuminant can be equal (same-illuminant table) or different (cross-illuminant table). In the original implementation, illuminant correction is achieved with cross-illuminant tables, and one table is required for each illuminant. We find, however, that a single same-illuminant table (D65) effectively converts sensor data for many different same-illuminant conditions. Hence, we propose to render the data by applying the same-illuminant D65 table to the sensor data, followed by a linear illuminant correction transform. The mean color reproduction error using the same-illuminant table is on the order of 4u E units, which is only slightly larger than the cross-illuminant table error. This approach reduces table storage requirements significantly without substantially degrading color reproduction accuracy.

A spectral estimation method for predicting between-eye color matches in unilateral dichromats

Abstract: INTRODUCTION There are several reports describing color vision in subjects who are dichromatic in one eye and trichromatic in the other. Between-eye color matches in these unilateral dichromats have been used to model color appearance for dichromats (Brettel et al, 1997 ). METHODS We describe a theoretical principle that makes specific predictions about the mapping from two cone class absorptions in the dichromat eye to three cone class absorptions in the trichromatic eye. Specifically, we propose that the brain estimates a spectral power distribution consistent with the two measured cone absorption rates; we use this estimate to predict the equivalent absorption rate for the missing cone type. We examined the implications of different spectral estimation methods, which is a severely under constrained estimation problem. These include (a) a smoothness constraint, (b) non-negativity constraint, and (c) natural scene priors. We implemented these calculations in open-source software. RESULTS Under some assumptions (smoothness only), a single linear transformation converts the dichromatic cone absorptions to the estimate for the missing cone class. This transformation matches some, but not all, of the color matches in unilateral dichromats. Adding additional assumptions (non-negativity) results in a nonlinear relationship between the two measured cone class absorptions and the estimated absorptions for the missing cone class. This observation predicts which wavelengths of light will appear the same to the dichromatic and trichromatic eyes (isochromes). The non-negativity constraint improves the agreement between predictions and measurements in unilateral dichromats (Alpern et al, 1983). CONCLUSION Establishing a quantitative map from the two cone classes in a dichromat to a missing cone class has practical value for estimating color appearance matches between dichromats and trichromats (Brettel, et al.; Vischeck). In addition, we explain how the method can be useful for implementing a color difference metric for dichromatic observers. Meeting abstract presented at VSS 2015.

P2SF: Physically-based Point Spread Function for digital image processing

Abstract: P2SF is a tool aimed at estimating the spatially variant point spread function based on the ray-tracing computation of the pupil function. Only 6 parameters are required to describe a monochromatic PSF.

I2.2: Invited Paper: Image Systems Simulation

Abstract: We describe a suite of software tools for image system simulation. The tools model the three-dimensional scene radiance, image formation by the optics, sensor transduction, image processing and display rendering.