Figure 1.

Representative effects of LHb stimulation on the spontaneous activity of midbrain DA neurons. A, C, Peristimulus time histograms comprising 100 consecutive sweeps. Stimuli consisted of rectangular current pulses (0.5 mA, 100 μs) delivered 100 ms after the start of each sweep (vertical dashed line). B, D, CUMSUM plots of the data illustrated in A and C, respectively. Graphs were generated by adding the contents of consecutive bins in the corresponding peristimulus time histogram to create a cumulative total of the number of spikes recorded across trials. Decimal values denote the slope of a line fit to each component of the response by least-squares regression (solid line, control; long dash, inhibition; dotted line, delayed excitation; short dash, recovery). A, B, Biphasic response. In these cells, typical of 66% of the DA neurons recorded, LHb stimulation resulted in a cessation in spontaneous firing, followed by a delayed excitation. C, D, Inhibitory response. In ∼33% of the cells studied, stimulation suppressed DA cell firing without inducing a delayed excitation.