Sensitivity of the neural circuitry to the rewarding properties of stimulation is assessed by analyzing left- or right-shifts in the M50, or the frequency at which 50% of the maximum number of responses was recorded. Reaching 50% of the MCR at a lower frequency is characteristic of a left-shift in the frequency-rate curve and sensitization of the reward circuitry to stimulation. An increase in the M50 indicates that a greater stimulation frequency was required to reach 50% of the MCR, and the reward circuitry has been desensitized by the experimental manipulation. Another way of analyzing the frequency-rate curve between control and experimental conditions is to do a linear regression through the ascending data points in a plot of raw data (which has not been normalized to the MCR). The point where y=0, or the x-intercept, is called the threshold frequency or theta zero (θ0). This is the frequency at which ICSS response rates are equal to 0 (and any frequency above this will theoretically elicit ICSS responding).