c-Fos after incentive shifts: expectancy, incredulity, and recovery.
Rats were used in a successive negative contrast procedure to determine which brain structures were activated by sucrose concentration downshifts, and on what day this occurred. Subjects were given preshift solutions for 12 days before being shifted to their postshift concentrations. Groups included 2 unshifted controls (32%-32% and 4%-4%) and 1 shifted group (32%-4%). Half of each group was killed 1 hr after the first exposure to the shifted solution (Shift 1), and half after the second exposure (Shift 2). Brains were processed for c-Fos-like immunoreactivity (FLI). Two major patterns emerged. Terminal drinking of 32% sucrose activated minimal brainstem regions involved in palatable taste, visceral feedback, and fluid homeostasis. In contrast, shifted subjects showed extensive cortical activation with selective activation in cerebral nuclei and brainstem. Robust FLI on Shift 1 was absent on Shift 2, consistent with evidence for rapid (1-trial) changes in all major motor outputs mediated by expectancy.