Academic journal article The Psychological Record

A Matching Law Analysis of the Effect of Amphetamine on Responding Reinforced by the Opportunity to Run

Academic journal article The Psychological Record

A Matching Law Analysis of the Effect of Amphetamine on Responding Reinforced by the Opportunity to Run

Article excerpt

"Runner's high" is a phenomenon that is widely known but poorly understood. The pharmacological basis of this phenomenon has been attributed to a release of endogenous opiates that occurs with intense exercise (Thoren, Floras, Hoffmann, & Seals, 1990); however, evidence in support of this opiate hypothesis is less than substantial (Steinberg & Sykes, 1985). An alternative, lesser known hypothesis is that the pharmacological basis of the rewarding effect of running is dopaminergic (Lambert, 1992). According to this hypothesis, running produces an increase in the release of dopamine in the mesolimbic pathway that is associated with the biological basis of reward (Lambert, 1992; Watson, Trujillo, Herman, & Akil, 1989). The purpose of the present study was to investigate this dopamine hypothesis by observing the effects of a dopamine agonist on indices of motor performance and reinforcement efficacy derived from a matching law analysis of the relationship between response and reinforcement rates in a context where responding was reinforced by the opportunity to run.

Lambert (1992) proposed that the pharmacological basis of the rewarding effects of running is a function of a dopamine rather than an opiate mechanism. Specifically, "after moderate running, increased mesolimbic dopaminergic activity provides physiological reinforcement, which sustains maladaptive running in the face of low food consumption" (Lambert, 1992, p. 27). Evidence cited in support of this hypothesis comes from studies of the effects of dopamine agonists and antagonists on running. Dopamine agonists such as amphetamine and cocaine increase running (Evans & Vaccarino, 1986; Glavin, Pare, Vincent, & Tsuda, 1981; Jakubczak & Gomer, 1973; Tainter, 1943) whereas dopamine antagonists such as pimozide and chlorpromazine decrease running (Beninger & Freedman, 1982; Routtenberg, 1968; Routtenberg & Kuznesof, 1967). However, this evidence must be viewed with a degree of skepticism because it fails to distinguish between motor and motivational effects of the drugs. That is, a decrease in running under the influence of a drug may be the result of either a decrease in motivation to run or an impairment of motor behavior.

One procedure that offers the potential to discriminate between motor and motivational changes in reinforced responding due to the effect of a drug is based on Herrnstein's (1970, 1974) matching law (Hamilton, Stellar, & Hart, 1985; Heyman, 1983, 1992; Heyman & Monaghan, 1987, 1990; Heyman, Kinzie, & Seiden, 1986; Heyman & Seiden, 1985). Herrnstein (1970) formulated an elementary matching law equation for the case where there is only a single measured source of reinforcement and a single measured response rate. The relationship between response and reinforcement rates in this case generally takes the form of a negatively accelerated monotonic function that is described by the following equation:

B1 = k R1 / R1 + Re. (1)

In Equation 1, B1 is response rate, R1 is reinforcement rate, and k and Re are estimated parameters. Specifically, k refers to the asymptotic rate of responding (i.e., the maximal response rate) and Re is the rate of reinforcement associated with one half the asymptotic rate of responding. Re describes how quickly response rate rises toward asymptote as reinforcement rate increases and at one half the asymptotic response rate (k/2), the value of Re can be estimated because R1 and Re are equivalent.

The k parameter in Equation 1 has been interpreted as an index of the motor aspects of a reinforced response (Hamilton et al., 1985; Herrnstein, 1974; Heyman, 1983). This interpretation derives from empirical observations that showed that when k changed, but Re remained stable, the experimenter changed some aspect of the response requirement (Heyman & Monaghan, 1987). For example, the value of k changed independent of Re with a change in response topography when the response manipulandum for pigeons was changed from a pecking key to a treadle (McSweeney, 1978) and with a change in response force on the same manipulandum when the force required to make a response was increased (Belke & Heyman, 1994; Heyman & Monaghan, 1987). …

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