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By demonstrating that fMRI responses in human MT+ increase linearly with motion coherence and comparing these responses with slopes of single-neuron firing rates in monkey MT, a new paper provides the best evidence so far that fMRI responses are proportional to firing rates. to infer something about the changes in underlying neuronal activity. The vascular source of the fMRI signal places important limits on the usefulness of the technique. At best, the fMRI signal (at each image position and temporal frame) would be proportional to the local firing rate, averaged over a small region of cortex and a short period of time. If that were true, then it would allow us to make direct inferences about firing rates from fMRI data. There is some indirect empirical support for a proportional relationship between average firing rates of neurons and the fMRI news and views