Investigation of the modulation of spatial frequency preferences with attentional load within human visual cortex

Performance in visual tasks improves with attention, and this improvement has been shown to stem, in part, from changes in sensory processing. However, the mechanism by which attention affects perception remains unclear. Considering that neurons within the visual areas are selective for basic image statistics, such as orientation or spatial frequency (SF), it is plausible that attention modulates these sensory preferences by altering their so-called ‘tuning curves’. The goal of this project is to investigate this possibility by measuring and comparing the SF tuning curves across a range of attentional states in humans. In Experiment 1, a model-driven approach to fMRI analysis was introduced that allows for fast and efficient estimation of population spatial frequency tuning (pSFT) for individual voxels within human visual cortices. Using this method, I estimated pSFTs within early visual cortices of 8 healthy, young adults. Consistent with previous studies, the estimated SF optima showed a decline with retinotopic eccentricity. Moreover, my results suggested that the bandwidth of pSFT depends on eccentricity, and that populations with lower SF peaks possess broader bandwidths. In Experiment 2, I proposed a new visual task, coined the Numerosity Judgement Paradigm (NJP), for fine-grained parametric manipulation of attentional load. Eight healthy, young adults performed this task in an MRI scanner, and the analysis of the BOLD signal indicated that the activity within the putative dorsal attention network was precisely modulated as a function of the attentional load of the task. In Experiment 3, I used the NJP to modulate attentional load, and exploited the model-based approach to estimate pSFTs under different attentional states. fMRI results of 9 healthy, young adults did not reveal any changes in either peak or the bandwidth of the pSFTs with attentional load. This study yields a full visuocortical map of spatial frequency sensitivity and introduces a new paradigm for modulating attentional load. Although under this paradigm I did not find any changes in SF preferences within human visual areas with attentional load, I cannot preclude the possibility that changes emerge under different attentional manipulations

Novel Paradigms For Visual Field Mapping With Functional Magnetic Resonance Imaging

The overall goal of this study is to evaluate the existing, and develop new visual field mapping paradigms, which consist of visual stimulation scheme, post-processing and displaying tools using fMRI for both research and clinical applications. We first directly compared phase mapping and random multifocal mapping paradigms with respect to clinically relevant factors. Multifocal mapping was superior in immunity to noise and was able to accurately decompose the response of single voxels to multiple stimulus locations. In contrast, phase mapping activated more extrastriate visual areas and was more efficient per run in achieving a statistically efficient response in a minimum time but required separate runs to map eccentricity and angle. Multifocal mapping became less efficient as the number of simultaneous stimulus locations increased from 13 to 25 to 49 and when duty cycle increased from 25% to 50%. In sum, each paradigm offers advantages that may be optimal for different applications. Given the respective advantages and weaknesses of phase-encoded design and random multifocal design, we further developed a novel paradigm by combining the phase-encoded stimuli and one or two isolated random segments. The addition of the random stimuli was shown to have insignificant effect on the retinotopic mapping by the phase-encoded stimuli. Three applications were demonstrated for this combined paradigm: Simultaneous mapping the retinotopy and selected ROIs, automated calibration of the temporal phases, and delineation of the hemodynamic response function for selected voxels. At present the representation of the visual field by the visual cortex is displayed as a diagram of a subject\u27s visual field with circular symbols placed at locations to which voxels have shown a response. The diagram provides an intuitive way of interpreting the fMRI cortical maps in terms of visual function. However, it provides little information about the relative probability of obtaining a brain response from different locations within the field of view. Therefore, we derived a quantitative form of such a diagram, on which a probability distribution could be drawn. The quantitative diagrams from five subjects showed highly variable patterns of coverage, which made it questionable whether any meaningful probabilistic distribution can be obtained

Investigation of Spatio-Temporal Effects of fMRI Visual Field Mapping Techniques on V1

Blood oxygenation level dependent functional magnetic resonance imaging has been used extensively for mapping the representation of the visual field within the human brain. Visual field mapping using fMRI has been used clinically to assess patients with cortical pathology and to plan surgical treatment impacting the visual system. The accuracy of fMRI-based visual field mapping methods needs to be better understood for clinical use. This accuracy can be important for presurgical mapping of brain function near a tumor resection site since inaccurate rendition of the underlying neural function could lead to inappropriate resection of viable brain tissue. The most widely used method for visual field mapping is temporal phase mapping. This dissertation investigates the accuracy of temporal phase mapping, specifically focused on the detection of polar angle visual field locations in primary visual cortex. Early studies show that polar angle positions are not uniformly distributed as suggested by animal studies. These non-uniformities are seen as relatively under-represented areas in the visual field maps used to display the fMRI data. This dissertation shows that temporal phase mapping is susceptible to hemodynamic distortions that lead to missassignment of visual field locations. Further analysis of the non-uniformity in the frequency distribution of voxels representing different angular position within the visual field shows an under-representation of locations near the vertical meridia in V1. These results led to the development of a new retinotopic mapping technique, code-based mapping. The main reason for developing a new retinotopic mapping technique was to reduce the under-representations of vertical meridia posed by using temporal phase mapping when assigning a stimulus location to a voxel. This dissertation shows that code-based mapping is a viable method for mapping visual field locations and produces a uniform distribution of voxels representing different angular positions within the visual field. Furthermore, the code-based mapping method is less susceptible to the hemodynamic biases than temporal phase mapping. With respect to clinical utility of fMRI mapping techniques, the code-based mapping shows a greater potential to accurately map a patient\u27s visual field in the presence of a tumor or other malformations that can induce large noise effects in the fMRI voxel responses

fMRI Analysis-by-Synthesis Reveals a Dorsal Hierarchy That Extracts Surface Slant.

The brain's skill in estimating the 3-D orientation of viewed surfaces supports a range of behaviors, from placing an object on a nearby table, to planning the best route when hill walking. This ability relies on integrating depth signals across extensive regions of space that exceed the receptive fields of early sensory neurons. Although hierarchical selection and pooling is central to understanding of the ventral visual pathway, the successive operations in the dorsal stream are poorly understood. Here we use computational modeling of human fMRI signals to probe the computations that extract 3-D surface orientation from binocular disparity. To understand how representations evolve across the hierarchy, we developed an inference approach using a series of generative models to explain the empirical fMRI data in different cortical areas. Specifically, we simulated the responses of candidate visual processing algorithms and tested how well they explained fMRI responses. Thereby we demonstrate a hierarchical refinement of visual representations moving from the representation of edges and figure-ground segmentation (V1, V2) to spatially extensive disparity gradients in V3A. We show that responses in V3A are little affected by low-level image covariates, and have a partial tolerance to the overall depth position. Finally, we show that responses in V3A parallel perceptual judgments of slant. This reveals a relatively short computational hierarchy that captures key information about the 3-D structure of nearby surfaces, and more generally demonstrates an analysis approach that may be of merit in a diverse range of brain imaging domains.This project was supported by the Wellcome Trust (095183/Z/ 10/Z) and the Japan Society for the Promotion of Science (H22.290 and KAKENHI 26870911).This is the final published version. It first appeared at http://www.jneurosci.org/content/35/27/9823

The multisensory attentional consequences of tool use : a functional magnetic resonance imaging study

Background: Tool use in humans requires that multisensory information is integrated across different locations, from objects seen to be distant from the hand, but felt indirectly at the hand via the tool. We tested the hypothesis that using a simple tool to perceive vibrotactile stimuli results in the enhanced processing of visual stimuli presented at the distal, functional part of the tool. Such a finding would be consistent with a shift of spatial attention to the location where the tool is used. Methodology/Principal Findings: We tested this hypothesis by scanning healthy human participants’ brains using functional magnetic resonance imaging, while they used a simple tool to discriminate between target vibrations, accompanied by congruent or incongruent visual distractors, on the same or opposite side to the tool. The attentional hypothesis was supported: BOLD response in occipital cortex, particularly in the right hemisphere lingual gyrus, varied significantly as a function of tool position, increasing contralaterally, and decreasing ipsilaterally to the tool. Furthermore, these modulations occurred despite the fact that participants were repeatedly instructed to ignore the visual stimuli, to respond only to the vibrotactile stimuli, and to maintain visual fixation centrally. In addition, the magnitude of multisensory (visual-vibrotactile) interactions in participants’ behavioural responses significantly predicted the BOLD response in occipital cortical areas that were also modulated as a function of both visual stimulus position and tool position. Conclusions/Significance: These results show that using a simple tool to locate and to perceive vibrotactile stimuli is accompanied by a shift of spatial attention to the location where the functional part of the tool is used, resulting in enhanced processing of visual stimuli at that location, and decreased processing at other locations. This was most clearly observed in the right hemisphere lingual gyrus. Such modulations of visual processing may reflect the functional importance of visuospatial information during human tool use

Spatial Updating in Human Cortex

Single neurons in several cortical areas in monkeys update visual information in conjunction with eye movements. This remapping of stimulus representations is thought to contribute to spatial constancy. The central hypothesis here is that spatial updating also occurs in humans and that it can be visualized with functional MRI.In Chapter 2, we describe experiments in which we tested the role of human parietal cortex in spatial updating. We scanned subjects during a task that involved remapping of visual signals across hemifields. This task is directly analogous to the single-step saccade task used to test spatial updating in monkeys. We observed an initial response in the hemisphere contralateral to the visual stimulus, followed by a remapped response in the hemisphere ipsilateral to the stimulus. Our results demonstrate that updating of visual information occurs in human parietal cortex and can be visualized with fMRI.The experiments in Chapter 2 show that updated visual responses have a characteristic latency and response shape. Chapter 3 describes a statistical model for estimating these parameters. The method is based on a nonlinear, fully Bayesian, hierarchical model that decomposes the fMRI time series data into baseline, smooth drift, activation signal, and noise. This chapter shows that this model performs well relative to commonly-used general linear models. In Chapter 4, we use the statistical method described in Chapter 3 to test for the presence of spatial updating activity in human extrastriate visual cortex. We identified the borders of several retinotopically defined visual areas in the occipital lobe. We then tested for spatial updating using the single step saccade task. We found a roughly monotonic relationship between the strength of updating activity and position in the visual area hierarchy. We observed the strongest responses in area V4, and the weakest response in V1. We conclude that updating is not restricted to brain regions involved primarily in attention and the generation of eye movements, but rather, is present in occipital lobe visual areas as well