The effects of signal intensity upon conditioned suppression: Effects upon responding during signals and intersignal intervals

Abstract

Two rat experiments were run to study the effects of a wide range of signal (CS) intensities on the suppression of licking. In Experiment I, a light CS was varied over five levels, including zero intensity. Conditioned suppression was found to vary directly with CS intensity, but basal lick rates were not different among groups. In Experiment 2, an attempt was made to disturb the basal rate of licking, while a tone CS was varied over four levels, including zero intensity. Here the suppression of the CS rates was found to be directly related and basal rates inversely related to CS intensity. The results as a whole were consistent with the Perkins-Logan hypothesis regarding the effects of CS intensity upon conditioning, but not with the Rescorla-Wagner model of classical conditioning.The intensity of the conditioned stimulus (CS) has been regarded as one of the important variables in both classical and instrumental conditioning experiments. Although the results of these studies are somewhat equivocal (Gray, 1965), they generally support the view that the relation between CS intensity and conditioned response magnitude is positive, a phenomenon known by the Hullian name of "stimulus intensity dynamism" (Hull, 1949).Stimulus intensity dynamism has also been reported in conditioned suppression (Kamin, 1965;Kamin & Brimer, 1963;Kamin & Schaub, 1963). The effects of CS intensity in an aversive conditioning situation, however, can be treated in a completely different manner, as suggested by recent developments in studies of aversive conditioning (lmada & Soga, 1971; Nageishi & lmada, 1974;Odling-Smee, 1975;Seligman, 1968Seligman, , 1969Weiss, 1970). Seligman (1968), for example, had demonstrated that unsignaled shocks produce greater suppression of basal rate of leverpressing than do signaled shocks, a result he explains with his safety signal hypothesis, which assumes that rats under a signaled shock condition learn to discriminate the signal (CS), or more correctly, the signal plus background cues (BC), from the BC alone, and the former comes to serve as a good predictor of danger and the latter of safety. When the shocks are given unsignaled or signaled by a cue of subliminal or zero intensity, no such discrimination should be possible, and hence animals in such a situation would be persistently fearful. Viewed in this way, it becomes immediately obvious that the problem of CS intensity in an on-the-baseline condiThe present research was supported by a grant in aid of research awarded to Hiroshi Imada by the Murao Foundation, Kobe, Japan. tioned suppression experiment can be discussed in terms of discriminability of the CS + BC from the BC alone. Decreasing CS intensity, therefore, should make this discrimination difficult, and thus should lead to the suppression of baseline responding. Unfortunately, in no previous studies of conditioned suppression dealing with CS intensity, has reference been made to the basal rate of responding. Also, in previous studies that dealt with the effect of predictability of shock upon...

The concept of uncertainty in animal experiments using aversive stimulation.

“…All of the results of the experiments conducted in the first author's laboratory using the same licking device have confirmed the signaled-shock effects (Imada, 1972;Imada & Okamura, 1975;Imada & Soga, 1971;Imada, Sugioka, Ohki, Ninohira, & Yamazaki, 1978;Imada, Yamazaki, & Morishita, 1981;Miyashita, 1971; Table 1 Summary of the Experimental Procedures of Nageishi and Imada (1974, Experiment I) Imada, 1974). Imada, 1975).…”

“…Even when shocks are all predicted by a fixed-intensity signal, if the signal intensity is so weak that it is just barely perceptible, the condition becomes virtually an unsignaled shock condition. Imada, Yamazaki et al (1981) studied the effects of signal intensity (or better, salience, in this context) in a licking-suppression experiment from the point of view of predictability/unpredictability of shock. They found in Experiment 2 that the suppression of the basal rate of licking became pronounced when the signal was of just supraliminal intensity.…”

The nature of the activity deficit produced by inescapable shock

“…Our data imply only that these sorts of changes are not necessary. However, the fact that sizable differences occurred here in a situation in which a shock-mitigating effect of overt postural adjustments was minimized suggests that this sort of behavioral mechanism might typically play only a minor role (Fanselow, 1979;Imada, Yamazaki, & Morishita, 1981).…”

Inhibition and facilitation by B over C after A+, AB–, and ABC+ training with multimodality stimulus combinations.

“…More than 120 experiments have been conducted in our laboratory with the conditioned suppression preparation used here (e.g., Fujii, Asada, Takata, Yamano, & Imada, 1989; Imada & Okamura, 1975; Imada, Yamazaki, & Morishita, 1981; Kawai & Imada, 1996; Kitaguchi & Imada, 1995; Nageishi & Imada, 1974; Yoshida, Kai, & Imada, 1969), and an inhibitory property of B has been successfully demonstrated in retardation assays after A+ and AB− training (e.g., Fujii & Imada, 1981). However, no experiment has been explored subtractive summation of B and a target excitor after the A+ and AB− training.…”