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Indifferent sensory stimuli, such as light-onset, can be reinforcers (for reviews, see Berlyne, 1969; Eisenberger, 1972; Glanzer, 1958; Kish, 1966; Lockard, 1963; Tapp, 1969). In comparison to important biological reinforcers such as food and water, the reinforcing effectiveness of sensory reinforcers is weak and transient. Responding for sensory reinforcers has often been characterized as investigatory behavior. Experiments with light reinforcement were first reported by Girdner (1953), who considered the operant responding for novel light-onset analogous to investigatory loco-motor responses that expose organisms to novel environmental change (as described by Kish, 1966, p. 111). For example, exploratory activity is high when a rat is first placed into a novel locomotor chamber and then decreases within the same session. Similarly, operant responding for novel light-onset is initially high at the start of a test session and then shows a within-session decline (Gancarz, Ashrafioun, et al., 2012; Gancarz, Robble, Kausch, Lloyd, & Richards, 2012). Within-session declines in locomotor activity and responding for novel light-onset are often characterized as indicators of habituation involving memory processes. A rat, when placed in the novel chamber, forms an internal representation (memory) of the chamber and the objects within and around it (Antunes & Biala, 2012; Leussis & Bolivar, 2006). Locomotor activity decreases as a consistent internal representation of the test chamber is formed and perceived stimuli are recognized. This interpretation is consistent with memory-based explanations of habituation (Konorski, 1963; Sokolov, 1963; Wagner, 1979). According to memory-based explanations of habituation, perceived stimuli are compared with existing memory. If the perceived sensory stimuli do not match memory, they are novel and cause dishabituation. On the other hand, if the perceived stimuli match memory, then they are not novel or surprising and do not cause dishabituation. McSweeney and Murphy (2009) have argued that within-session declines in operant responding are a function of habituation to the sensory properties of repeatedly presented reinforcers. In agreement with their work, we have provided evidence that within-session declines in the effectiveness of sensory reinforcers is attributable to habituation (Lloyd, Gancarz, Ashrafioun, Kausch, & Richards, 2012). Within-session decreases in light-reinforced responding can be reasonably attributed to habituation of reinforcer effectiveness because there is a programmed contingency between responding and light-onset. In contrast, locomotor activity in a novel locomotor chamber is more likely to be described as elicited or evoked by exposure to novel environmental stimuli, suggesting that locomotor activity in a novel environment is a Pavlovian process. However, as has been previously pointed out (Berlyne, 1960), exploratory behavior can be characterized as a mixture of Pavlovian and operant processes. Novel sensory stimuli such as light-onset activate orienting or targeting reflexes that bring sensory receptors into contact with the sensory stimulus (Sokolov, 1963). Some of the sensory stimuli (e.g., localized changes in relative illumination) evoke targeting reflexes and are also reinforcers that increase the probability of the responses that produced them. It is likely that naturally occurring contingencies between responding and sensory stimulation that occur while an animal is exploring a novel environment may similarly increase locomotor activity in a novel environment. Thus, even though there are no programmed operant contingencies in a novel locomotor chamber, the habituation of the reinforcing effectiveness of inherent operant contingencies may contribute to declines in exploratory activity. This is supported by evidence from McSweeney and Swindell (1999) who showed that decreases in exploratory activity follow the same mathematical relationship as habituation in other paradigms. As was originally suggested by Girdner (1953), operant responding to produce light-onset may be an effective way to precisely measure exploratory behavior. Habituation to the reinforcing effectiveness of sensory stimuli may be mediated by decreases in dopamine (DA) neurotransmission evoked by novel or surprising sensory stimuli. Novel sensory stimuli increase phasic firing of DA neurons. This effect rapidly habituates when sensory stimuli are repeatedly presented (Ljungberg, Apicella, & Schultz, 1992). Redgrave and Gurney (2006) have hypothesized that phasic firing of DA neurons elicited by novel sensory stimuli activate the animal to repeat responses that produced the novel sensory stimulation. According to Redgrave and Gurney, this DA-mediated mechanism allows organisms to “discover” contingencies between investigatory behaviors and novel sensory events. Following initial “discovery,” phasic firing of DA neurons is cancelled by timed inhibitory input to DA as the sensory stimulus becomes predictable (familiar) through repetition. Redgrave and Gurney (2006) have described the phasic firing of DA neurons evoked by novel sensory stimulation as a sensory prediction error signal. The concept of phasic firing of DA neurons as a function of the difference between perceived and remembered stimuli fits well with memory-based explanations of habituation, where comparison of perceived and remembered stimuli leads to the identification of novel stimuli that produce dishabituation (arousal). However, instead of arousal, Redgrave and Gurney emphasize that an increase in DA neurotransmission evoked by novel stimuli determines the reinforcing effectiveness (or effects) of indifferent sensory stimuli. More specifically, they hypothesize that novelty-induced increases in DA cause the animal to repeat actions that precede novel stimuli, leading to the discovery of the action that produced it. The hypothesis that the reinforcing effects of sensory stimuli rapidly habituate, in combination with the concept of phasic DA as a sensory prediction error signal that mediates the behavioral expression of habituation, makes predictions that can be tested with both behavioral and drug manipulations. To test these predictions we have developed a method for quantitatively characterizing the rate of habituation for operant responding that takes into account relative differences in response rate so that the habituation rate (HR) measure is independent of absolute response rate measures. In Phase 1 of the current experiment, we use this method to measure the HR of the operant level of snout poking in a novel environment. Memory-based theories of habituation predict that the rate of habituation will increase with repeated exposure to the novel environment as sensory consequences of investigatory behaviors become familiar. The DA as a sensory error signal hypothesis predicts that phasic firing of DA neurons will decrease with repeated exposure, resulting in decreased activation. Both theories therefore predict that repeated exposure will lead to a decrease in measured operant level responding (snout-poking). In Phase 2, the effects of the stimulant drugs nicotine (NIC) and methamphetamine (METH) on HR are measured. The DA as a sensory error signal hypothesis predicts that stimulant drugs such as NIC and METH which artificially increase DA neurotransmission will disrupt (decrease) expression of normally occurring habituation. In Phase 3, the effects on HR of making a novel sensory stimulus contingent upon snout poking are tested. Memory-based theories of habituation predict that introduction of a novel response-contingent sensory stimulus will decrease the rate of habituation. In addition, the DA as a sensory error signal hypothesis predicts that introduction of a novel stimulus will increase DA neurotransmission and increase activation. The apparatus and procedures used in this experiment have previously been found to reliably produce light reinforced responding (Gancarz, Ashrafioun, et al., 2012; Gancarz, Robble, Kausch, Lloyd, & Richards, 2012, 2013; Lloyd, Gancarz, et al., 2012; Lloyd, Kausch, Gancarz, Beyley, & Richards, 2012). We selected 0.75 mg/kg METH for this experiment because it was within the range of known effective doses (Gancarz, Ashrafioun, et al., 2012), and we selected the NIC dose of 0.4 mg/kg because this dose was found to effectively increase light reinforced responding (Palmatier et al., 2007).

Crack cocaine use is more associated with impulsivity and a propensity to take risks than heroin use, yet no studies have examined this relationship in the absence of acute drug effects. The current study examined impulsivity (using the Delay Discounting Task) and risk-taking propensity (using the Balloon Analogue Risk Task) across independent groups of primary crack cocaine users with minimal heroin use (n = 16) and primary heroin users with minimal crack cocaine use (n = 11) in residential treatment, with all participants drug abstinent during participation. Crack cocaine users evidenced greater levels of impulsivity and risk-taking propensity, with only the difference in impulsivity persisting after controlling for age and gender. These data hold potential theoretical importance in understanding differences between crack cocaine and heroin users, as the findings cannot be attributed solely to acute pharmacological drug effects.

There are reports that diazepam can increase, decrease, or have no effect on measures of impulsive behavior, which may be related, in part, to differences among the tasks used to measure impulsivity. This study examined the effects of a relatively high dose of diazepam (20 mg) on 5 measures of impulsive behavior in healthy adult men and women. Volunteers (N = 18) participated in a 2-session double-blind randomized design in which they received 20 mg diazepam or placebo. One hour after ingesting the capsule, participants completed mood questionnaires and several impulsivity tasks to measure subtypes of impulsive behavior, including behavioral inhibition, delay and probability discounting, and risk taking. Diazepam impaired behavioral inhibition but had no effect on measures of discounting or risk taking. These results are discussed in the context of other recent findings suggesting that different behavioral indices of impulsivity are dissociable and governed by separate underlying mechanisms.

Agreement between computer and questionnaire measures of delay discounting in smokers was compared. Correlations between measures for small, medium, or large rewards were significant. Log k values decreased as the reward delay increased, with values lower for the computer task than the questionnaire, with significant differences for small rewards. The 2 measures were related to smoking rate but not to age, gender, or obesity. The Bland-Altman test of agreement indicated large within-subject differences in k values between the 2 measures. The size of the difference between the log k values and magnitude of the log k values were positively related. Results suggest k values from the 2 measures are related but may not be used interchangeably.

In trying to better understand why individuals begin and continue to smoke despite the obvious health consequences, researchers have become interested in identifying relevant personality variables, such as risk taking. In this study, the authors compared the ability of 2 behavioral measures of risk taking, the Bechara Gambling Task (BGT) and the Balloon Analogue Risk Task (BART), to differentiate smokers and nonsmokers. Self-report measures of impulsivity and sensation seeking were taken for comparison with the 2 behavioral risk-taking tasks. Results indicate that behavior on the BART, and not the BGT, was related to smoking status. Further, when considered in a logistic regression analysis, only the Sensation Seeking total score and the BART score contributed uniquely to the differentiation of smokers and nonsmokers.