Functional models of fMRI bold signal in the visual cortex.

摘要

Functional magnetic resonance imaging (fMRI) is a functional imaging procedure that measures the neural activity by detecting the associate changes in the blood flow. It has become one of the most widely used technologies for in-vivo human brain imaging.;Since the first sets of fMRI images were acquired to compare the response in visual areas with visual stimuli either on or off, fMRI has been prominent in the study of the visual system. Not only fMRI extends our knowledge about the human visual system with different stimuli and tasks, but also the visual system provides a venue for understanding the mechanism of the fMRI signal and for testing new analysis methods. My three studies follow this tradition of fruitful interactions between fMRI methods and findings about the visual system, and these three projects are fully elaborated from chapter 2 to chapter 4.;In chapter 2, we identify and explain the correlational structures of the spontaneous BOLD fMRI in the low-level visual areas. We measured the BOLD response while subjects rested with their eyes closed, without stimuli or any explicit tasks. We studied the measured spontaneous activity in the visual cortex (V1-V3) at a fine scale. We found that the strongest correlations in spontaneous activity are between points on the cortex with functional receptive fields at the same distance from the point of gaze in the visual space, irrespective of whether the receptive fields are in the same quadrant or hemi-field. No iso-eccentricity connections have ever been reported in the visual area using anatomical methods. We used a simulation to determine if temporally varying and spatially diffused feedback from at least two higher cortical areas with different amounts of coverage of the peripheral visual field can provide a quantitative account of these findings.;The purpose of the second study, described in chapter 3, is to build a signal strength response model which accounts for category specificity and attention effect across different visual areas. Using fMRI, we conducted three complementary experiments in which observers had to compare either the face or scene component of a pair of briefly presented images, each being an amalgamation of face, scene and random noise pattern. We found that the BOLD response to each non-noise image component (face or scene) is linear in the "signal proportion" of the component, defined as the ratio of the contrast energy of the component to the contrast energy of the entire image. For a cortical area along the ventral visual pathway, the slope of this linear function depends on whether the component is attended to and if it is preferred by the cortical area. The net BOLD response of a cortical area is a simple sum of the responses to all non-noise components. We validated this linear sum-of-components model by showing that a model fitted to the data from any two of the three experiments can accurately predict the empirical results of the third.;The aim of my third study, described in chapter 4, is to understand the relationship between the neural response and BOLD response with an achiasmatic subject. Achiasma is a rare congenital condition that prevents the normal crossing of the optic nerves from developing. Because the optic chiasm is absent, the left and right halves of the visual field project to the same cerebral hemisphere, ipsilateral to the eye of origin. In such an achiasmatic individual, subject S, we found that each fMRI voxel in the retinotopic areas V1-V3 responds equally to two positions in the visual field. These two population receptive fields (pRFs) of a single voxel are located mirror-symmetrically about the vertical meridian. Contrast masking and fMRI adaptation experiments revealed that these pRFs are served by two non-interacting groups of neurons of comparable number and excitability. By presenting identical stimuli to both of these receptive fields instead of just one, we can double the local neural activity, regardless of the definition of "neural activity." Using this in-vivo model, we found that BOLD response amplitude is proportional to approximately the square root of the underlying neural activity.