Strength-duration characteristics of lateral hypothalamic and periaqueductal gray reward-path neurons

Behaviorally measured strength-duration curves were plotted for reward related neurons stimulated by electrodes in the lateral hypothalamus (LH) and ventral periaqueductal gray (PG). The average curves for the two groups were only marginally different, suggesting that similar neural populations mediate self-stimulation at the two sites. Analysis of the data for both groups indicates that stimulating pulses shorter than 0.1 msec stimulate mainly a short time-constant substrate with an average time-constant of about 0.1 msec. This is the same time-constant that has been reported in the literature for myelinated axons having a wide range of conduction velocities. It is suggested that the marked departure of behaviorally determined strength-duration curves from the approximately exponential form usually exhibited by single elements is due to the stimulation of unmyelinated elements by long (but not by short) stimulation pulses.