Fig. 5. Functional interaction between sustaining fiber (SF) O38 and a head-down motoneuron (HDMN). (A) Simultaneous recording of responses of SF O38 and an HDMN to a step increase in illumination delivered to the dorsoposterior quadrant of the visual field. (B) Cross-correlation histogram of SF O38 and the HDMN responses to a 5-minute exposure to continuous illumination. The ordinate is the conditional probability of a motoneuron impulse in a 1.0ms time bin following a sustaining fiber impulse. The abscissa is the time lag from the sustaining fiber impulse. The dashed line indicates the expected conditional probability of a motoneuron impulse on the basis of its mean firing rate (9impulsess^-1) and assuming that motoneuron impulses occur at random times after a sustaining fiber impulse. The peak of the correlogram is at +5ms and the total conduction time to and from the inferred synapse in the brain is 3–4ms (modified from Glantz et al., 1984).