High conditioning to B

Blocking, Overshadowing and Related Concepts. Fig. 1.

Blocking in human participants shown in normal participants as an increased number of trials (to the learning criterion) required in the blocking condition relative to the overshadowing controls. This blocking effect was abolished or even reversed in schizophrenic participants (Schiz) and their relatives (Rels), irrespective of levels of schizotypy (st) as measured by questionnaire. (Adapted from Jones et al. (1997) Behav Brain Res 88:103-114, see text for full details.)

Animal Studies of Blocking and Overshadowing

In animals, psychopharmacological studies have shown that treatment with indirect ► dopamine agonists given systemically (typically ► amphetamine at around 1.5 mg/kg in rats) abolishes blocking and overshadowing. In every case, abolition of the normal effect results in overconditioning, with the consequence that the CS that would normally be treated as uninformative, whether because of redundancy in the case of blocking or relatively low intensity in overshadowing, accrues more associative strength than in non-drug-treated groups (Cassaday and Moran 2010).

In part for practical reasons, because of the need to test for dose-related effects, psychopharmacological studies are not always run with full behavioral controls. In the case of blocking, further reduction in learning produced by the Stage 1 pre-training can be a relatively small effect when compared to that seen in an overshadowing group. In some studies, apparent amphetamine-induced abolition of blocking may be entirely attributable to impaired overshadowing under amphetamine (Fig. 2).

However, patients with schizophrenia have been reported to show impaired blocking, over and above any change in overshadowing (Fig. 1) and in animal studies blocking and overshadowing may be dissociable in terms of their differential sensitivity to dopamine D2-like (Fig. 2) vs. dopamine D1-like (Fig. 3) receptor agents.

Blocking, Overshadowing and Related Concepts. Fig. 2.

Overshadowing shown as reduced learning relative to the conditioning control group in a standard fear conditioning (lick suppression) procedure. The dependent variable is the log transform of the time taken to complete 10 licks in the presence of the CS. Overshadowing was shown in the saline- (SAL) and haloperidol- (HAL) injected groups. Overshadowing was abolished by treatment with amphetamine (AMP) and the amphetamine-induced abolition of overshadowing was not reversed by treatment with haloperidol, suggesting that dopamine D2-like receptors do not mediate the amphetamine-induced abolition of overshadowing. (Adapted from O'Tuathaigh and Moran (2002) Psychopharmacology 162:225-231, see text for full details.)

With respect to the neural mediation of the systemic effects of amphetamine, dopamine activity in part of the ventral striatum, specifically ► nucleus accumbens, has been reported to modulate blocking in the absence of any effect on overshadowing in the control group (Fig. 4). Similarly, the evidence for a pivotal role of nucleus accumbens and related structures is overwhelming in the case of latent inhibition (Gray et al. 1991; Weiner 2003). However, it is important to note that the nucleus accumbens is a heterogeneous structure and opposing effects of drug treatments and lesions can be seen in relation to very small differences in the laterality of the cannula placement. Moreover, the evidence with respect to the neural substrates of other selective learning phenomena is patchy, in part because of the poor selectivity of the lesion methods used to date. Techniques in neuro-psycho-pharmacology such as lesions selective to dopa-mine and precisely targeted microinjection studies are required to delineate the underlying substrates of

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