Several techniques previously used to describe behavioral correlates of hippocampal unit and slow-wave activity are combined in a single odor-discrimination paradigm. Rats repetitively performed a sequence of behaviors during each trial: approach to a stimulus-sampling port, investigatory sniffing of the odor cue, orientation and approach toward a separate reward location, and water reward consumption. In a series of post hoc analyses, spike activity was timelocked to variations of each task event to uncover behavioral and physiological parameters that best synchronized unit firing. Three major categories of cells were identified:(1) "Cue-sampling" cells fired after onset of odor-cue sampling. Response magnitude was related to cue valence on both the current and past trials. (2) "Goal-approach" cells fired prior to arrival at either the odor-sampling port or reward cup. A number of sampling and approach cells also had place correlates. However, detailed analyses indicated that specific behaviors associated with increased firing reliably occurred at the same place. Unit activity was at least as well described by behavioral as spatial parameters. (3) "Theta" cells fired at high rates in strict relation to the ongoing limbic theta rhythm.This categorization suggests a functional organization of the hippocampus in which different cell types play complementary roles. Cue-sampling cells activated by discriminative stimuli during attentive fixations may be involved in comparing relative cue valence. Goal-approach cells may be involved in orientation movements for successive cuesampling periods. Theta cells may provide synchronization of sensory acquisition during sampling, as well as in orientation movements during approach.Despite extensive investigations of the hippocampus, we have only a vague understanding of its global functional role and little insight into its fundamental organization for information coding. The hippocampus is many synapses away from both receptor and effector neurons, so it is perhaps not surprising that progress in clarifying the functional correlates of hippocampal neurons has lagged behind that of sensory and motor neurons,

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In this study we examine how the introduction of a reference lottery with nonrandom outcomes alters the way in which choices among pairs of lotteries are made, even if it does not alter the choices. We use different domains (some of the lotteries produce gains, other losses) and different contexts (one member of the pair, the reference lottery, may be either risky or certain). In our experiment, the change from gain to loss domain affects choices: subjects are risk averse in the gain domain, but not in the loss domain. On the contrary, the context effect of the certain lottery does not affect choices. However, the introduction of the certainty reference lottery affects two behavioral variables, response time and brain activation, in a dramatic way. This result suggests that the certainty lottery promotes a different process through which preferences are revealed, even if the differences among lotteries may not be large enough to induce different choices. W e study behavioral and neuronal responses associated with economic choices. We manipulated the nature of one of the two lotteries (called the reference lottery) in a choice task between pairs of lotteries. In one condition the reference lottery was risky; in the other condition it was certain: that is, the outcome won or lost was fixed. We demonstrate that although choice behavior is relatively insensitive to the type of reference lottery, the case is dramatically different for response times and neuronal activation: these two strongly depend on it. Thus, very different neurophysiologic processes appear to govern similar choice behavior. This study unequivocally indicates that, in humans, choice behavior alone does not reveal completely how choices are made. Our findings also show how the process of choice is distributed throughout the brain and that the activation of different functional units is sensitive to minute differences in environmental conditions, in particular the structure of comparison lotteries.Two lines of inquiry are related to and inspired our study: the research on the effect of certainty on choice (as found in the Allais paradox, ref. 1), and on a context effect on ordering (2). Like most studies of choice, both studies rely strictly on subjects' announced preferences to hypothesize indirectly that choice processes themselves will change as a function of the alternatives that are being compared; however, no evidence is gathered to indicate directly how the choice processes are different. By gathering evidence on reaction time as well as brain activation, we directly assess how decision processes differ.