At What Stage of Neural Processing Does Cocaine Act to Boost Pursuit of Rewards?
Figure 12
The sensitivity of brain reward circuitry determines the stimulation strength required to produce a reward of a given intensity whereas the gain of the circuit determines the maximum intensity attainable. a) Simulated curves showing how a cocaine-induced increase in sensitivity shifts the intensity-growth function leftwards along the pulse-frequency axis. At the intersection of the black vertical line with the solid dark-red curve representing the saline condition, reward intensity is twice that at the intersection of this curve with the gray vertical line. In contrast, reward intensities are almost identical at the intersections of the two vertical lines with the dashed pink curve representing the cocaine condition. Thus, changes in sensitivity alter the relative values of rewards. b) Simulated data from panel a re-plotted in double logarithmic coordinates. c) The contour line halfway between the minimum and maximum time allocation. The cocaine-induced increase in sensitivity shifts the contour line downwards along the pulse-frequency axis. Note, that the form of the contour line mirrors the shape of the intensity-growth function in panel b. d) Simulated curves showing how a cocaine-induced increase in the gain of the BSR substrate produces equal proportional changes in all reward-intensity values. The intensity-growth function has been rescaled by the drug. Note that the ratios of reward intensities are equivalent at the intersections of the vertical lines with the solid dark-red curve for the saline condition (10∶5) and the dashed pink curve for the cocaine condition (50∶25). Thus, changes in gain alter absolute, but not relative, reward intensities. e) Simulated data from panel d re-plotted in double logarithmic coordinates. f) Effect of a gain increase on the position of the contour line halfway between the minimum and maximum time allocation. The contour line is shifted rightwards along the price axis.