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BOLD responses to stimuli with four different contrasts. On left (A and C) is the raw signal and on the right (B and D) are the comparisons. Note that in (D), the relationship between the electrophysiologic measures and BOLD is linear, yet, clearly, to reach the zero point, the relationship needs to change, suggesting a high level of nonlinearity at low levels of neuronal activity. Reproduced with permission from Figure 5 of Logothetis and colleagues (2001).
