Fig. 2. The stimulus intensity–response functions (SIRFs). The SIRF shown in A  represents a proportional increase in blood oxygenation level–dependent response with respect to stimulus intensity. Brain regions with this type of response are thought to encode stimulus intensity. The SIRF shown in B  does not show a response to P1 (i.e ., does not differentiate between rest [R] and P1), but shows a proportional increase over the painful trials (P2, P3). Brain regions with this type of response are thought to encode pain intensity. The SIRF shown in C  is a step function that does not differentiate between R, P1, and P2, but shows blood oxygen level–dependent (BOLD) response only to P3. This SIRF models brain regions that respond only to high pain (high-pain–specific responses).