FIG. 1.

Facilitatory and inhibitory combination-sensitive neurons may arise from different combinations of excitatory and inhibitory inputs. Top: delay curves show the spike discharge as a function of the delay of the high-frequency sounds (HI) after the low-frequency sound (LO). Bottom: representation of spiking responses and the sum of excitatory and inhibitory postsynaptic potentials (EPSPs and IPSPs, respectively) in response to high- and low-frequency sweeps alone and in response to combination stimuli at 3 different delays. A: our operating hypothesis is that facilitation arises in inferior colliculus (IC) through a postinhibitory rebound mechanism. Thus IPSPs should be observed in response to both HI and LO signals, and facilitation should occur when the 2 rebounds are aligned (5-ms delay). B: inhibitory combination sensitivity seems to originate mostly within the NLL but may include low frequency inhibitory inputs to IC. Thus EPSPs should be observed in response to the HI sound, whereas IPSPs may (shown in B) or may not (not shown) be observed in response to the LO sound. Filled hexagons and response curves indicate HI stimulus and HI PSPs, respectively, whereas unfilled hexagons and response curves indicate LO stimulus and PSPs, respectively. Depolarizing and hyperpolarizing responses are shown as areas above and below the horizontal lines, respectively.