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Assessing two control procedures for latent inhibition.

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A conditioned freezing preparation with four groups of rats (C, L, T, N) was used to assess two control procedures for latent inhibition. C was preexposed to the context alone, L to a light, and T to a tone, all followed by light-shock pairings. N
  Assessing Two Control Procedures for Latent Inhibition  Cristina Dos Santos and José E. BurgosUniversidad de Guadalajara (CEIC)A conditioned freezing preparation with four groups of rats (C, L, T, N) wasused to assess two control procedures for latent inhibition. C was preexposed to the context alone, L to a light, and T to a tone, all followed bylight-shock pairings. N received light-shock pairings without any preexposure treatment. C showed significantly more conditioned freezingthan L and N. This result confirms the context facilitation effect, whichquestions preexposure to the context as a control procedure. T showedsignificantly more conditioned freezing than the other groups. This resultconfirms a prediction by a neural-network model and questions preexposureto a different stimulus as a control procedure. INTRODUCTION Latent Inhibition (LI) is a significant retardation in the acquisition of a conditionedresponse after continued preexposure to a conditioned stimulus (CS) (Lubow & Moore,1959).Traditionally, LI experiments consider at least one of two different control procedures toassess the effects of CS preexposure on conditioning. One of these procedures is the socalled context control  . It consists in placing the subject in the conditioning context, with noother programmed consequence. The other control procedure typically used in LI researchconsists in preexposing the control subjects to a non-target CS. Both procedures haveshown few disadvantages, and some authors have pointed out the inconveniences of usingeither one of them alone to demonstrate the CS-preexposure effect.Boughner, Thomas, & Papini (2004) argue that comparisons between the experimentalgroup and the context control group would tend to overestimate the size of LI, because preexposure to the context alone has shown to facilitate acquisition under certaincircumstances. The authors suggest the use of what they call the srcinal acquisition group(one that receives no preexposure to the CS or context) together with either the controlcontext group or a non-target preexposure group.Previous simulations with neural-networks have suggested that preexposure to a non-targetCS facilitates subsequent acquisition of the target CS (Burgos, 2003). Although, this prediction has not been tested yet.  The present experiment was designed to evaluate the possibility that preexposure to a non-target CS could facilitate acquisition of the target CS; which could question the use of that procedure alone in LI experiments as a control procedure. METHODSubjects. Thirty-two experimentally naïve, female Wistar rats (rattus norvegicus). Theyweighed between 200-273 g and were about 120 days old. Rats were randomly assigned toone of 4 groups (n=8). Procedure  Preexposure. There were 10 sessions of 6 trials, with a variable interval of 5 min betweeneach presentation (47 min in total). Each trial consisted in a 2 min presentation of therespective CS. Group C reamined in the chamber for the entire duration of the session withno planned event. Group N remained in their home cage during this phase.   Conditioning. All rats were given a single conditioning session. This consisted of placingrats into the context and delivering a shock (0.6 mA, 1-s duration) 5 min later. Shock offsetco-occurred with the off-set of a 30-s presentation of the light. Test. Consisted of four presentations of the light, each of which lasted for 30-s with aninterval of 6 min between each presentation. The test was videotaped using the Video-Freeze Software © (Med Associates, Albany, VT). During the test, each rat’s behavior was scored in terms of the percentage of freezing during the CS. RESULTS Across the 30-s period prior to the initial CS presentation, there were no significantdifferences between the groups  X  2 (3, N = 32) = 4.44,  p = .22. The median levels of freezingin each group across the four test presentations are shown in Table 1.---------- Table 1----------Kruskal-Wallis non-parametric analysis of variance suggested significative differences between groups  X  2 (3, N = 32) = 9.67,  p = .022. Conover post hoc tests were conducted inorder to evaluate differences between groups. Table 2 resumes the results.-------Table 2 -----------  This experiment confirms that rats freeze as a conditional reaction to a CS paired withshock and that this reaction is reduced by preexposure to that CS.The higher levels of freezing in group C than in group N suggest that preexposure to thecontext alone facilitated the acquisition of conditioned freezing to the light, thus,confirming the context effect.Statistically significant differences between group T and N confirm the facilitatory effect of  preexposure to a tone (non-target CS) on acquisition of the light.The use of either one of these control procedures alone seems not to be sufficient in order to demonstrate LI. Both, the context control procedure and the non-target preexposure procedure, tend to inflate the acquisition base-rate of the target stimulus, overestimating thesize of the effect of the CS-preexposure treatment. REFERENCES Boughner, R. L., Thomas, B. L. & Papini, M.R. (2004). Effect of nonreinforced preexposure to the context on autoshaping in rats: methodological implications for demonstrations of latent inhibition.  Int. J. Comp. Psych, 17, 168.Burgos, J.E. (2003). Theoretical note: simulating latent inhibition with selection neuralnetworks.  Behav. Proc., 62, 183.Lubow, R. & Moore, A. (1959). Latent inhibition: the effect of non-reinforced preexposureto the conditional stimulus.  J. Comp. Phys. Psych., 52 , 415.  Table 1.  Experimental design Table 2.  Post-hoc analysis Factor n Averange rank Different (  p <0.05) from factor C 8 21.44 (L) (N)L 8 9.94 (C) (T) N 8 13.00 (C) (T)T 8 21.62 (L) (N)  Figure 1. Median percentage of freezing to the light at test. 0510152025303540Group   C L N T Group Preexposure Conditioning Test L Context + LightLight  Shock Light T Context + Tone C Context alone N  None
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