Daniel Coulombe

The curve-shift paradigm in self-stimulation

Eleven rats were trained to press a lever in an operant chamber in order to earn rewarding trains of cathodal rectangular pulses of fixed intensity and variable frequency. The rate-frequency functions were examined under administration of two neuroleptics (pimozide and chlorpromazine) and three manipulations that interfered with bar pressing (muscular relaxation with methocarbamol, increased lever weight, and limitation of maximum response rates by an F1 reinforcement schedule). Chlorpromazine, and pimozide at low dosages produced a near parallel shift of the rate-frequency functions on the logarithmic axis of pulses, suggesting that these drugs decreased the reinforcing efficacy of the stimulation. The three conditions that interfered with bar-pressing decreased the asymptotic rates and produced small or moderate lateral shifts. Changes in the reinforcing efficacy of the stimulation following the above manipulations were inferred from the shift in the number of pulses required at zero and half-maximal performance (theta 0 and M50 indices, respectively). In the cases of the manipulations that interfered with bar-pressing, M50 indicated a larger artifactual change in the efficacy of the stimulation, compared to theta 0. This phenomenon was mainly due to the fact that the asymptote of the altered functions was shifted towards higher pulse numbers.

Fitting intracranial self-stimulation data with growth models.

Until now, the problem of fitting self-stimulation rate-frequency functions has been dealt with by using linear models applied to the linear portion of the empirical curve. In this article, an alternative procedure is presented, together with three sigmoid growth models that seem to accurately fit rate-frequency data. From any of these models, it is possible to compute the two indices of stimulation efficacy in use in the parametric study of brain stimulation reward (M50 and theta 0), in addition to the inflection point of the curve, which can be used as an alternative to M50. Important relations allowing initial estimation of each parameter are provided, allowing use of computer programs derived from the Gauss-Newton algorithm for nonlinear regression. The considerations relevant to the choice of a nonlinear model are discussed in terms of each efficacy index.

Investigation of the robust rank-order test for non-normal populations with unequal variances : The case of reaction time

Abstract This article examines the performance of the robust rank-order (Fligner-Policello) test of treatment effects for populations with unequal variances. Both symmetric (normal) and skewed (ex-Gaussian) distributions are examined. The ex-Gaussian distribution is particularly relevant because it is representative of skewed distributions found in experimental psychology, most notably reaction time data. The results indicate that when testing the hypothesis of equal medians for the symmetric distribution, the Fligner-Policello test is conservative, whereas it performs inconsistently for the skewed population. In light of the findings, we discuss the various options presently available to researchers confronted with unequal variances and skewed data, including a discussion of whether testing for treatment effects for populations with unequal variances is of practical relevance. Experimental psychologists are often interested in testing hypotheses about treatment effects in their experimental designs. The commonly used significance tests using t or F statistics involve the assumptions that the populations are normally distributed and the population variances are equal. The violation of the latter assumption is often referred to as the Behrens-Fisher problem (Scheffe, 1970) in honour of two pioneering statisticians who drew attention to the violation of this assumption and provided an early solution. In this article, we consider the simultaneous violation of both normality and equal variances, and we evaluate a nonparametric statistical method that is designed for use when neither of these assumptions is met. Consider a common scenario in experimental research: A researcher is interested in the reaction times (recorded in milliseconds) in a perceptual judgement task. The design consists of two independent groups, wherein group A is a control group and group B is an experimental group. As expected with reaction time data, the distribution of scores in each group is skewed (mean minus median equals 92 milliseconds); however, the variance in the experimental group is 2.25 times larger than that in the control group. The researcher is interested in testing the equality of the two sample means or, alternatively, medians. Given that the samples are skewed, it is generally recommended that a nonparametric inference procedure, usually the Wilcoxon-Mann-Whitney test, be used (Harwell & Serlin, 1989). However, the Wilcoxon-Mann-Whitney test is inappropriate for cases with unequal variances (see, e.g., Harwell, Rubinstein, Hayes, & Olds, 1992; Zimmerman & Zumbo, 1993a; 1993b). In fact, it is noteworthy that the Wilcoxon-Mann-Whitney test behaves very much like the Student t test: When the larger variance is associated with the larger sample size, there is a depression of the Type I error rate; when the larger variance is associated with the smaller sample size, there is an elevation of the Type I error rate (Zimmerman & Zumbo, 1993a; 1993b). Siegel and Castellan (1988), however, recommend that in the case of non-normality and unequal variances, an alternative nonparametric inference procedure is the Fligner-Policello (1981) test, also known as the robust rank-order test. It should be noted that the Behrens-Fisher problem was originally formulated only for the case of normal population distributions (Scheffe, 1970). The scenario described in our example, non-normality and unequal variances, is therefore referred to in the statistical literature as the generalized Behrens-Fisher problem. Although solutions to the Behrens-Fisher problem are discussed in some textbooks on experimental design (e.g., Winer, Brown, & Michels, 1991), to our knowledge, the only such discussion of solutions to the generalized Behrens-Fisher problem is Siegel and Castellans (1988; pp. 137-144) presentation of the robust rank-order test (Fligner & Policello, 1981). Therefore, the purposes of the present article are: (a) to present the Fligner-Policello test in more detail, noting some important points overlooked by Siegel and Castellan in their presentation, and (b) to examine the performance of the Fligner-Policello test when applied to a non-normal distribution representative of those found in experimental psychology, especially in reaction time data. …

Factorial structure and psychometric properties of the reminiscence functions scale.

Objectives: This study reports on the psychometric properties and the factorial structure of the Reminiscence Functions Scale (RFS), a 43-item self-report instrument used to assess the frequencies of reminiscence for distinct functions. Method: The factorial validity (exploratory factor analysis, n = 453; confirmatory factor analysis, n = 456), the invariance of factorial structure across gender (males = 228; females = 240), and psychometric properties were examined. Results: They support an eight-factor structure similar to the original one, yet question the value of a few of the items. Cronbachs alphas for the various subscales ranged from 0.76 to 0.87. Test–retest reliability ranged from r = 0.48–0.63. Conclusion: The RFS is confirmed as a psychometrically sound instrument for use in research on the functions of reminiscence with samples of older adults.

The effects of pimozide on the reinforcing efficacy of central grey stimulation in the rat

The present report describes the effects of the neuroleptic pimozide on the reinforcing efficacy of central grey stimulation. Six adult rats were implanted with a monopolar moveable stimulating electrode in the pontine central grey. Each bar-press in an operant chamber delivered a 0.3-s train of cathodal rectangular pulses of constant duration (0.1 ms) and intensity (from 180 to 440 microA depending on the subject) and of variable frequency. The function relating the rate of bar-pressing to the logarithm of the number of pulses per train (rate-frequency function) was recorded following administration of vehicle and increasing doses of pimozide (from 0.15 to 0.5 mg/kg). Pimozide produced a dose-dependent parallel shift of the rising segment of the rate-frequency function towards higher pulse numbers in 4 subjects, indicating that the drug reduced the reinforcing efficacy of the stimulation. In two subjects, the shift was accompanied by a decrease in slope of the rising segment. Depending on the subject, the greatest shift observed varied from 0.087 to 0.489 log units. Further attempt to increase the magnitude of shift with higher doses resulted in complete abolition of self-stimulation. The fact that pimozide reduced the value of reward in an area containing only marginal amounts of dopaminergic cells adds support to the hypothesis that dopamine modulates reward indirectly. The fact that the shift could not be increased with higher doses was interpreted as an indication that dopaminergic neurons are involved in a gate-like synaptic arrangement in which a limited decrease in dopaminergic activity is sufficient to obliterate the transmission in the reward pathway or the conversion of the signal into a reinforcing effect.

Amygdaloid self-stimulation : a movable electrode mapping study

The distribution of electrical self-stimulation foci within the amygdala (AMY) was mapped using movable electrodes in rats. Each barpress delivered a 0.4-s train of cathodal rectangular pulses of fixed intensity and duration and variable frequency. The rate-frequency function was recorded for successive dorsoventral sites. Self-stimulation was found throughout the AMY, except in the lateral nucleus. Depending on the site, maximum rates varied from 3 to 37 barpresses/min, whereas threshold frequencies varied from 9.2 to 40 pulses/train; no correlation between these two aspects of self-stimulation was found. Most threshold frequencies lay within the range of 10 to 20 pulses/train, which suggests a relatively homogeneous distribution of rewarding efficacy within the positive areas. The lowest threshold estimates are comparable to those usually obtained for pontine and medial forebrain bundle areas, which suggests that the AMY is an important focus for self-stimulation.

Interactions between amygdaloid and hypothalamic self-stimulation: a re-examination

The function relating bar-pressing rate to the frequency of cathodal pulses was obtained in rats self-stimulating with amygdaloid (AMY) and lateral hypothalamic (LH) electrodes. The maximum self-stimulation (SS) rates in the AMY was found to be very low, compared to the LH. Concurrent stimulation with pairs of AMY-LH pulses did not shift the rate-frequency functional laterally, indicating the absence of summation of the two rewarding effects. In a second experiment, concurrent AMY-LH stimulation (using sub-threshold intensity LH pulses) facilitated bar-pressing for AMY stimulation (it increased the slope of the AMY rate-frequency function) without shifting this function laterally. In a third experiment, subjects were given a choice between a pulse frequency yielding maximal AMY rate and a series of higher pulse frequencies. Subjects consistently preferred the higher frequency values, attesting that the maximum AMY rates were not constrained by a saturating reinforcing effect. In a fourth experiment, subjects were given a choice between AMY stimulation and concurrent AMY-LH stimulation, using low intensity LH pulses. Subjects showed no preference for either stimulation condition, although rates were higher for the latter condition. These findings suggest that the maximum rate for AMY stimulation was constrained by factors interfering with bar-pressing and that the effect of these factors was attenuated by co-activation of the LH. In a fifth experiment, pre-treatment with phenobarbital mimicked the rate-enhancing effect of concurrent AMY-LH stimulation for 2 of the 4 subjects tested. This finding suggests that the LH pulses contributed to attenuate seizure activity accompanying AMY SS. In a final experiment, AMY SS rates were also increased by co-activation of rewarding sites in the rostral MFB but not the dorsal raphe, suggesting an anatomical specificity of this effect.