High resolution, high field strength MRI studies of the mechanisms and architecture underlying human stereoscopic vision

Functional magnetic resonance imaging (fMRI) can be used to visualise activity in the brains of human subjects. When a region of the brain becomes active the cells begin to fire action potentials to communicate information. An increased flow of oxygen is required in that local region to fuel this activity. Since oxygenated (fresh) and de-oxygenated blood have different magnetic properties, it is possible to use the ratio of these two substances to measure changes in brain activity. The techniques used for looking at the human brain, both its function and its connections, are constantly evolving, allowing us to gain more and more information.
This project is designed to determine how we use our 2 eyes to see in depth. There are many pieces of information that can tell us where objects are in relation to each other, but if a slight difference is introduced between the images of the two eyes artificially, such as in ?Magic Eye? pictures, an object may ?jump? out without any other information available. Using stimuli that are similar to Magic Eye pictures we will investigate 3 main issues:
1) We will look in the normal population to see whether different areas of the brain are specialised for processing different stages of depth perception. We will also investigate whether the responses of the brain are related to how good people are at seeing depth.
2) Deficits in depth perception are common, as any misalignment of the two eyes during early childhood due to ?squint? or ?lazy eye? can prevent the development of binocular neurons; those sensitive to input from both eyes. Although these conditions can often be corrected surgically, not all children regain their binocular vision. We will investigate the neural responses in such subjects lacking binocular vision to examine how their brains process information differently from people with normal binocular vision.
3) The amount of detail that can be seen with fMRI depends on various factors, but an important one is the strength of the magnet used for the experiments. We are very fortunate in Oxford as we will have access to a very strong magnet that we will use to investigate the very fine organisation believed to underlie the perception of depth.

We propose to use high resolution, high field-strength functional magnetic resonance imaging (fMRI) to determine how the human visual cortex processes binocular depth. Using 3-Tesla or 7-Tesla, state-of-the-art, scanning methods, we will image, at a much higher resolution than before, the organisation for binocular disparity in sub-compartments of human visual cortical areas.

Our brains can compute information about the depth of objects by exploiting the slightly different images on the retinas of the two eyes. However, the development of binocular vision can be disrupted in childhood, often leading to permanent visual deficits, typically amblyopia. Subjects with normal binocular vision show a link between the amount of neural activity and psychophysical performance in many visual cortical areas. Interestingly, however, our recent work has shown that the fMRI responses of lateral occipital areas LO-1 and LO-2 appear to be driven primarily by the disparity of the stimulus, regardless of the performance of the subject. Thus, these areas may have a pure stimulus-related signal, rather than a signal mingled with other factors, such as attention or task difficulty.

To investigate this hypothesis further, Part 1 aims (i) to use multivariate analysis to determine whether these areas can resolve different disparities within the stimulus and (ii) to correlate the levels of neural activation to disparity-defined stimuli with the ability of the subject to detect binocular depth. Comparisons across several of the multiple, independently-defined visual areas will indicate whether the neural activation of LO-1 & LO-2 is indeed tightly linked to the disparity stimulus alone.

One of our longer-term, strategic goals is to develop MRI into a tool for evaluating the efficacy of treatments for amblyopia. Part 2 of this project will measure the neural activity in two simple binocular experiments in patients with dysfunctional binocular vision. The patients will be classified according to the severity of their condition, allowing a correlation between neural activity and binocular performance.

Recent optical imaging studies have confirmed long-standing claims of a functional organisation for disparity within the identifiable compartments of cortical area V2 in non-human primates, suggesting the importance of depth processing in the visual system. This architecture has never been mapped functionally in humans, but should be within the reach of the new 7-Tesla scanner at FMRIB. Combined with the imaging of ocular dominance columns in V1, it will be possible to image compartments of cortical areas that may be altered in binocular dysfunction.