Axonal tracing requires local injection of a tracer molecule followed by death and histological analysis, all of which is ethically infeasible in humans. Studying brain function in humans alone is also limiting in that very little is known about the structure of the human brain-at least when compared to what we know about the macaque brain, for example. 2009) in the monkey do not exist for human imaging, and direct microstimulation or temporary deactivation by cortical infusion in a normal human subject is ethically unthinkable. 2010) and local electrical microstimulation ( Tolias, Sultan et al. While some coarse techniques exist to manipulate human brain function, such as transcranial magnetic stimulation, non-invasive methods comparable to localized chemical deactivations ( Liu, Yttri et al. In order to draw causal links between brain function and behavioural performance, it is necessary to directly alter the brain either prior to or during fMRI. While human fMRI can go a long way to answering fundamental questions about organization of function in the brain, much of the information we obtain from fMRI is correlative-for example, while activity in an area may correlate with a task, it is impossible to assume that the activity causes the observed behaviour. The combination of non-invasiveness and the broad availability of equipment and tools to perform fMRI have made this technique popular and broadly adopted. Well-developed commercial MRI systems with built-in support for functional imaging are widely available, as are a plethora of tools, both free and commercial, for the analysis of human functional brain images. The technique allows for noninvasive whole-volume measurements of brain function in humans and repeatable non-invasive measurement of activity over the whole brain, providing a correlate of neural activity without use of tracers or electrodes. Functional magnetic resonance imaging (fMRI) is perhaps the most important development in the measurement of human brain function in recent neuroscience history.
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