Figure 7.

Ventral striatal units with a firing rate ramp up to the first reward site were preferentially theta modulated and phase precessing. To quantify the degree of “rampiness,” the skewness of firing rate over space was used; negative numbers indicate an upward-sloping ramp as animals approach the reward site, and positive skewness indicates a downward-sloping ramp. To identify ramp cell candidates, only data points for which the spatial tuning curve was well fit by linear regression (p < 0.05) are shown; note that this criterion does not contain a bias in the upward- or downward-sloping direction. Nevertheless, the population of ventral striatal units was biased in the negative skewness/upward-ramping direction (A, B). Upward ramping was associated with higher theta modulation and stronger phase precession.