LiCl-induced aversions to audiovisual cues as a function of response measure and CS—US interval

In four experiments using 312 adult albino rats, locomotor avoidance and suppression of drinking were used to monitor learned associations between spatial cues and injections of poison or saline. Rats injected with hypertonic LiCl or hypertonic NaCl immediately upon removal from one side of a two-compartment shuttlebox free of ingestible substances later showed locomotor avoidance of that side. Other animals that were identically trained and later offered novel fluids in either the compartment paired with injection or the compartment paired with noninjection did not differentially suppress drinking. When injections were isotonic, the NaCl-injected animals displayed neither ingestive nor locomotor aversions while the LiCl-injected animals showed a place-specific reluctance to drink before any locomotor avoidance appeared. When injections were administered 4 min after removal from the treatment-paired compartment, LiCl-injected rats developed discriminated suppression of drinking more rapidly than locomotor position preferences whereas NaCl-injected rats displayed no acquisition. Animals injected with LiCl or NaCl 4 min before being placed into the treatment-paired compartment showed no significant acquisition by either ingestive or locomotor measure. These interactions between type of test, nature and concentration of injected agent, and time of injection, not explicable in terms of different response thresholds for a single association, are interpreted as the consequence of hypertonically induced somatic pain associations formed at short CS—US intervals being prepotent to affect locomotor activity, and toxin-induced visceral pain associations formed at short or long CS—US intervals being prepotent to affect ingestive behavior. When both were potentially present with short interstimulus intervals, acute somatic pain apparently overshadowed visceral pain. Some implications of prepotent response systems for the manifestation of information about specific USs are considered in the light of recent failures of the stimulus equivalence hypothesis.