A formal conditioning model is proposed that adds a dynamic attention rule and a novel response mapping rule to the Rescorla-Wagner associative axiom. This model retains the virtues of its predecessor and, in addition, accurately simulates many conditioning phenomena that are not encompassed by the original Rescorla-Wagner model: (a) The acquisition function is S-shaped, and abrupt shifts in responsiveness occur during acquisition and extinction; (b) reacquisition is characteristically more rapid than the initial acquisition; (c) the behavioral effect of a set of conditioning operations is not necessarily independent of the subject's prior conditioning history. In addition, the new model copes better than the Rescorla-Wagner model with latent inhibition and with the ineffectiveness of nonreinforcing a conditioned inhibitor.The Rescorla-Wagner conditioning model differs from the traditional linear operator model proposed by Estes and Burke (19S3) and by Bush and Hosteller (19SS) in assuming that all stimuli share a common pool of associative strength so that their aggregate values, rather than their individual values, approach an overall asymptote. When one stimulus gains associative strength, the unconditional stimulus (US) becomes less effective in supporting conditioning to other stimuli that are presented in conjunction with the original stimulus. This novel assumption permits a successful extension of the linear model to account for overshadow-

A series of experiments were conducted to determine what factors control responding to the first element of a two-element serial compound in rabbit eyelid conditioning. An examination of response topography indicated that the eye-blink CR is rigidly timed to occur when the US is expected. This response-system characteristic prevents the occurrence of a CR during the first element of a serial compound or during the second-order CS in second-order conditioning. The comparison of a serial gap procedure with conventional serial and trace conditioning procedures suggested that the associative strength of the first element of a serial CS is not strongly influenced by either a second-order conditioning process or by the variable-reinforcement principle.

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