Two alternative models for orienting response habituation to complex sequences of stimuli were formulated. Model 1 assumed complete generalization of habituation, while Model 2 assumed independent habituation processes for the different stimuli making up the complex sequence. Two experiments were conducted in order to compare the models. Experiment 1 was conducted to obtain a simple habituation function to tones. Experiment 2 utilized complex sequences, each comprising two tones. The degree of similarity between these two tones was manipulated by systematically varying the frequency of one of these tones, using a betweensubjects design. The predictions of the two models were correlated with the SCRs of each subject. The results showed that the validities of the models depend on the similarity between the stimuli making up the complex sequence. For a sequence comprising very similar stimuli, Model 1 is a better description of the habituation process. Model 2, however, yields a closer fit to the habituation process to sequences comprising clearly discriminable stimuli.The nature of orienting response (OR) habituation to complex sequences of stimuli is of theoretical importance for the understanding of neuronal models. Several studies have tried to clarify the habituation pattern in such sequences (e.g., Furedy, 1968;Zimny, Pawlick, & Saur, 1969). Frith (1978a, 1978b) have raised the possibility that different laws may govern the habituation process in complex and simple sequences of stimuli. These authors have suggested that the neuronal model operates within a range of specific limits. It follows that if the stimuli making up the complex sequence vary within these limits, the habituation pattern should not differ from that of habituation to a simple sequence. Only when the stimuli set includes a stimulus that falls outside these limits should we expect habituation pattern to the complex sequence that is different from that of habituation to a simple sequence.Ben-Shakhar (1980) has also dealt with habituation processes to complex sequences, and has tried to formulate, by means of simple mathematical models, the relationship between habituation in complex and simple sequences. Two main models have been formulated: Modell, which assumes complete generalization of habituation, states that the response in a given trial of a complex sequence is a function of all the previous stimuli in that sequence. In more formal terms, Modell states: g(i/j) =f(i + j), where f denotesThe present study was supported by a grant from the Eshkol Institute, Faculty of Social Sciences, The Hebrew University of Jerusalem. We wish to thank Ms. Judy Lustig and Ms. Aya Poznanski for their assistance.the habituation function in a simple sequence, g denotes the habituation function in a complex sequence of stimuli, i and j denote the trial numbers of two stimuli, and g(ilj) denotes the response to a stimulus in its [th presentation, given that the other stimuli have appeared j times before it. Model 2 assumes that the response to a given stim...