The Functional Dissection of Motion Processing Pathways in the Human Visual Cortex: An fMRI-guided TMS Study

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The development of modern brain imaging techniques, such as functional magnetic resonance imaging (fMRI), has given neuroscientists unparalleled access to the inner workings of the living human brain. Visual processing in particular has proven to be particularly amenable to study with fMRI. Studies using this technique have revealed the existence of different networks in the brain that are activated by different kinds of visual stimuli, such as motion, colour, faces, objects and so forth. In this research project we are interested in how the brain analyses information about moving stimuli. The analysis of motion within our visual environment is vitally important to our interaction with the outside world. It provides us with a rich source of information that helps us orientate ourselves within our surroundings, aiding in the avoidance as well as in the recognition of objects as we move around in this environment. The importance of motion perception is reflected by the fact brain imaging experiments reveal that when we look at moving stimuli, this generates neural activity across a large number of visual areas within the brain. Many of these brain areas contain their own individual representation of the outside world or visual field. But it seems unlikely, not to say inefficient, that each of these brain areas performs exactly the same kind of analysis and contributes in exactly the same way to our perception of motion. Yet precisely what roles these different areas do play in our perception of motion, is far from clear. Whilst fMRI provides us with an excellent means by which we can localise and map different areas across the visual brain, it is less well suited to providing information as to whether neural activity within a particular brain area is crucial for perception or behaviour. However, these kinds of direct or causal links can be made when fMRI is combined with transcranial magnetic stimulation (TMS). TMS is a non-invasive and non-harmful technique which can bring about transient disruption of neural function in small areas of the brain. If this neural activity is important for perception, then its disruption can induce impairments in the ability of human observers to performance specific visual tasks, such a determining the direction motion of stimulus, for example. Experimental evidence from the monkey brain has shown that different motion areas are responsive to different kinds of moving stimuli such as left-right (translational) motion, optic flow (a kind of radial motion produced on the retina as we physically move through our environment) or boundaries defined by moving stimuli. This work has provided a theoretical framework within which the organisation of motion processing can be studied in the human brain. Using our fMRI-guided TMS approach we aim to establish direct relationships between neural activity within particular brain areas and the ability to perceive different kinds of moving stimuli. In so doing we aim to provide a more complete description as to how each of these brain areas that are responsive to motion contribute to its perception.