Classical conditioning data show that a conditioned stimulus (CS) can act either as a simple CS--eliciting conditioned responses (CRs) by signaling the occurrence of an unconditioned stimulus (US)--or as an occasion setter--controlling the responses generated by another CS. In this article, the authors apply a simple extension of a network model of conditioning, originally presented by N. A. Schmajuk and J. J. DiCarlo (S-D; 1992), to the description of these 2 different CS functions. In the model, CS inputs are connected to the CR output both directly and indirectly through a hidden unit layer that codes configural stimuli. In this framework, a CS acts as (a) a simple stimulus through its direct connections with the output units and as (b) an occasion setter through its indirect configural connections via the hidden units. Computer simulations demonstrate that the network accounts for a large part of the data on occasion setting.

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One experiment assessed predictions from the attentional theory of context processing (ATCP, J. M. Rosas, J. E. Callejas-Aguilera, M. M. Ramos-Álvarez, & M. J. F. Abad, 2006, Revision of retrieval theory of forgetting: What does make information context-specific? International Journal of Psychology & Psychological Therapy, Vol. 6, pp. 147-166) that extinction arouses attention to contextual stimuli. In a video-game method, participants learned a biconditional discrimination (RG+/BG-/RY-/BY+) either after extinction of another stimulus had occurred, or not. When contextual stimuli were relevant to solving the discrimination (i.e., all RG+/BG- trials occurred in one context and all RY-/BY+ in another), prior extinction of another stimulus facilitated the discrimination, as if extinction enhanced attention to the contexts. Results are discussed briefly in terms of ATCP and the model of N. A. Schmajuk, Y. W. Lam, & J. A. Gray (1996, Latent inhibition: A neural network approach, Journal of Experimental Psychology: Animal Behavior Processes, Vol. 22, pp. 321-349).

The effects of prenatal choline availability on contextual processing in a 30-s peak-interval (PI) procedure with gaps (1, 5, 10, and 15 s) were assessed in adult male rats. Neither supplementation nor deprivation of prenatal choline affected baseline timing performance in the PI procedure. However, prenatal choline availability significantly altered the contextual processing of gaps inserted into the to-be-timed signal (light on). Choline-supplemented rats displayed a high degree of context sensitivity as indicated by clock resetting when presented with a gap in the signal (light off). In contrast, choline-deficient rats showed no such effect and stopped their clocks during the gap. Control rats exhibited an intermediate level of contextual processing in between stop and full reset. When switched to a reversed gap condition in which rats timed the absence of the light and the presence of the light served as a gap, all groups reset their clocks following a gap. Furthermore, when filling the intertrial interval (ITI) with a distinctive stimulus (e.g., sound), both choline-supplemented and control rats rightward shifted their PI functions less on trials with gaps than choline-deficient rats, indicating greater contextual sensitivity and reduced clock resetting under these conditions. Overall, these data support the view that prenatal choline availability affects the sensitivity to the context in which gaps are inserted in the to-be-timed signal, thereby influencing whether rats run, stop, or reset their clocks.

The effects of prenatal choline availability on Pavlovian conditioning were assessed in adult male rats (3-4 mo). Neither supplementation nor deprivation of prenatal choline affected the acquisition and extinction of simple Pavlovian conditioned excitation, or the acquisition and retardation of conditioned inhibition. However, prenatal choline availability significantly altered the contextual control of these learned behaviors. Both control and choline-deprived rats exhibited context specificity of conditioned excitation as exhibited by a loss in responding when tested in an alternate context after conditioning; in contrast, choline-supplemented rats showed no such effect. When switched to a different context following extinction, however, both choline-supplemented and control rats showed substantial contextual control of responding, whereas choline-deficient rats did not. These data support the view that configural associations that rely on hippocampal function are selectively sensitive to prenatal manipulations of dietary choline during prenatal development.There is increasing evidence that variations in maternal dietary choline intake during the second half of pregnancy cause structural, biochemical, and physiological changes in basal forebrain neurons and their projections to the hippocampal complex as well as long-term cognitive changes in the offspring (e.g., Meck and Williams 2003;McCann et al. 2006;Meck et al. 2008). We know, for instance, that the adult offspring of pregnant rats supplemented with 4.5 times the amount of choline in the standard laboratory diet display improved memory capacity and precision on the radial-arm maze (e.g., Meck et al.