My main interest is to study how the brain processes visual information, what is the connectivity between different visual areas and the rest of the brain and how we perceive this information. My PhD research was mainly focused on the ability of the visual cortex to reorganize following injury of the visual pathways.
Visual cortical plasticity
The visual cortex is the part of our brain that processes information that receives from the retina and helps us to perceive the visual world. The primary visual cortex (V1) is the main source of feed-forward visual information to higher order visual cortical areas. If V1 or its inputs are damaged as a result of stroke or other brain lesions this leads to a loss of conscious vision in the affected region of the contralateral visual hemifield which is called scotoma. Cortical blindness affects many activities on a patient's daily life such as driving, reading, and navigating complex visual environments while there are currently no widely accepted treatment options available. Understanding brain repair processes is an important step in the effort to design treatments aimed at enhancing the ability of the nervous system to recover after injury.
I study visual cortical plasticity in human subjects with lesions of the visual pathway. The aim is to map visual cortex organization after injury, gathering information about the role that specific networks of brain areas play in cortical reorganization and recovery. This is done by: 1) Mapping changes in human visual cortex organization and comparing with control subjects without lesion, and 2) Determining whether rehabilitative training increases the degree of cortical reorganization.
Nonlinear Population Receptive Field Changes in hV5/MT+ of Healthy Subjects with Simulated Visual Field Scotomas
An important question is whether the adult visual cortex is able to reorganize in subjects with visual field defects (scotomas) as a result of retinal or cortical lesions. Functional magnetic resonance imaging (fMRI) methods provide a useful tool to study the population receptive field (pRF) properties and assess the capacity of the human visual cortex to reorganize following injury. However, these methods are prone to biases near the boundaries of the scotoma. Distinguishing pRF changes that occur as the result of true reorganization versus different test-stimulus presentation conditions is an important task that needs to be undertaken when studying the organization of visual cortex in patients with visual field deficits. The purpose of this work was to point out some of the issues involved. We measured responses in human area V5/MT+ in five healthy subjects after masking the stimulus in the left upper quadrant of the visual field (“artificial scotoma” or AS). This simulates a homonymous quadrantanopic scotoma that occurs often as result of partial V1 or optic radiation lesions. We compared responses obtained under the AS condition with simulations obtained from a linear AS model (or LAS model). We found pRF changes in hV5/MT+ under the AS condition that are significantly different than those obtained with the LAS model suggesting that the pRFs are nonlinearly affected by the truncated stimulus presented. This was signified by a shift of the pRF centers towards the border of the AS, a decrease in pRF size and an increase in pRF amplitude near the AS border. In addition, we found erroneous pRF estimates inside the area corresponding to the AS, when we used the full bar stimulus model for predicting the pRF topography when the actual stimulus presented included the AS. These biases are not the result of a trivial methodological artifact but appear to originate from asymmetric BOLD responses occurring when the stimulus moves from seeing to non-seeing locations of the visual field. We argue that these responses are not simply neural anticipatory responses but likely contain a significant hemodynamic component.
V1 Population Receptive Field Analysis Complements Perimetry in Patients with Homonymous Visual Field Defects
Partial damage of the primary visual cortex (V1), or damage to the white matter inputs to V1 (optic radiation), cause blindness in specific regions of the visual field. In this study, we used fMRI and quantitative pRF analysis to measure responses in patients with chronic V1 injury that resulted in blindness in a quarter of the visual field. The fMRI responses of patients and controls were generally similar, but in some patients differences from controls could be measured suggesting that there is a (limited) degree of reorganization following V1 injury. Importantly, we demonstrate that responses in spared early visual cortex are not always congruent with visual perception. Two different patterns of mismatch between responses in early visual areas and visual perception as measured by perimetry mapping were identified. In some patients, spared V1 pRF maps overlapped significantly with dense regions of the perimetric scotoma. Visual stimuli presented inside the scotoma could modulate neural activity in these voxels even though they generate no visual percept, suggesting that pRF analysis may help identify visual field locations amenable to rehabilitation. Conversely, in the remaining patients, spared V1 pRF maps failed to cover sighted locations in the perimetric map, indicating the existence of V1 bypassing pathways able to mediate useful vision. Identifying these patterns of mismatch and understanding the capacity of early visual areas to reorganize after injury is an important step which will allow us in the course of time to adopt more rational strategies for rehabilitation.
Population Receptive Field Topography
The visual cortex is retinotopically organized. That means that adjacent locations in the visual space map on to nearby locations in the visual cortex. Thanks to new analysis techniques we are able to estimate the population receptive fields (pRF) voxel by voxel in the visual cortex. Our team has developed a method to estimate a linear pRF topography model. The pRF structure is modeled as a set of weights that can be estimated by solving a linear model that predicts the Blood Oxygen Level-Dependent (BOLD) signal using the stimulus protocol and the canonical hemodynamic response function. The resulting linear equations can be solved for the pRF weight vector using the ridge regression, giving us the pRF topography. This method does not make a priori assumptions about the specific pRF shape and is therefore a useful tool for uncovering the underlying pRF structure at different spatial locations in an unbiased way. In addition, it is particularly attractive for monitoring pRF properties in the visual areas of subjects with lesions of the visual pathways, where it is difficult to anticipate what shape the reorganized pRF might take.