Posttraining lesion of the superior colliculus interferes with feature-negative discrimination of fear-potentiated startle.

Though much is known about the neural circuits involved in the elicitation of fear, little is known about the neural circuits responsible for the reduction of fear. The present experiments investigated the contribution of the superior colliculus (SC) and the dorsal periacquaductal gray (dPAG) in the reduction of conditioned fear produced by an auditory feature trained in a feature-negative discrimination procedure. In this procedure, light plus foot shock training trials are interspersed with trials in which the light is preceded by a noise and this noise and light compound is not followed by foot shock. At the end of this feature-negative discrimination training, rats were given excitotoxic lesions of the SC or dPAG. Feature-negative discrimination of fear was assessed with the fear-potentiated startle effect in which conditioned fear is operationally defined as potentiated startle amplitude in the presence versus the absence of the light. Feature-negative discrimination of fear is evidenced by a reduction in fear-potentiated startle to the light when the noise feature accompanies the light. Lesions of the SC, but not the dPAG, interfered with feature-negative discrimination of fear-potentiated startle suggesting that the SC plays a role in feature-negative discrimination of fear. Both SC and dPAG lesions facilitated startle amplitude in the absence of the light suggesting that these structures may exert a tonic inhibition on the acoustic startle reflex. The SC receives polymodal sensory information and is known to project forebrain areas involved in the production of conditioned fear. Thus, the SC may be an important component of the feature-negative discrimination circuit.