No abstract

Pigeons were exposed to a multiple schedule which provided equally frequent reinforcement in the presence of two stimuli but which produced markedly different rates of key-pecking. Generalization gradients were displaced away from the stimulus associated with the lower rate of key-pecking. Another group of pigeons had similar training, except that a low rate of key-pecking was established in a stimulus with a much higher frequency of food reinforcement. In this case, the generalization gradients were not affected by the training on the schedule producing a low response rate.

were presented a dim light and then tested for recognition (generalization) with test stimuli including the original and several brighter values. Seven groups (re = 20 in each) differed in the range (and number) of test values employed (from 3 to 9). On each trial, 5 judged the test stimulus as same as or different from the original and also rated it on a 9-point brightness scale. This provided both generalization and adaptation level (AL) measures. Both AL and maximal generalized responding shifted to higher stimulus values with longer test series, i.e., with greater test asymmetry. In Experiment II, four groups of 5s (n = 20 in each) were tested with a long or short range of stimuli with few or many stimuli within each range. Only range affected both AL and the location of maximal responding. These results strongly support an AL interpretation of the "central tendency effect" in stimulus generalization.

An experiment was performed to test the hypothesis that in a voluntary generalization task subjects encode the training stimulus (TS) in relation to the adaptation-level (AL) prevailing at the time of TS presentation. If so, then during a subsequent generalization test the point of maximal generalized responding should bear this same relationship to the AL of the generalization test series. To test this hypothesis, three major groups of subjects (n = 80) were given preexposures with dim lights (low preexperimental AL group), bright lights (high preexperimental AL group), or no preexposure (control group). Next, half of each group was exposed to a TS and a subsequent generalization (recognition) test including this brightness and four others. The other half of each group was exposed to the same TS and subsequent generalization stimuli, but their task was to rate the stimuli on a 7-point bright-dim scale. This provided AL measures which could be correlated with generalization performance in the comparable generalization groups. The preexposure condition was successful in producing the expected low or high preexperimental ALs, as reflected in different ratings of the TS when it was presented. Furthermore, these effects persisted throughout testing, as evidenced in ALs (i.e., rating scale midpoints) which reflected the preexposure condition. Finally, the difference' between the point of maximal generalized responding and the AL of the generalization test series was highly and significantly correlated with the difference between the original rating of the TS and scale midpoint. This supports the major experimental hypothesis of the study. Thomas and Jones (1962), in a study of voluntary stimulus generalization, found that the location of the training stimulus within the test series had a marked effect on the symmetry of the resulting generalization gradients. Their subjects were given a 6O-sec exposure to a 525-nm training stimulus (TS) with instructions to remember the stimulus so that they could identify it during a subsequent wavelength generalization test. The location of the TS in the test series was systematically varied, with the TS being in the center position of the test series for one group and asymmetrical (i.e., displaced from the center) for four other groups. Only with the symmetrical group did a symmetrical generalization gradient (with a peak at the TS) result. For the other groups, the peak was shifted from the TS toward the center of the test series. This finding, which Thomas and J ones called a "central tendency effect," has been replicated many times with different stimulus

Removal of the epicardium of the dog's heart with 95 per cent phenol permits anastomoses between pericardial vessels and the coronary arteries that are large enough to carry the Schlesinger mass (40 micra or larger). In 18 patients suffering from intractable angina pectoris an operation consisting of phenolization, instillation of talc and pneumonopexy has resulted in consistent relief of pain. The simplicity of the procedure exposes the patient to minimum risk of fresh thrombosis; its effectiveness justifies further trial.