A fundamental question in cognitive science is whether animals can represent numerosity (a property of a stimulus that is defined by the number of discriminable elements it contains) and use numerical representations computationally. Here, it was shown that rhesus monkeys represent the numerosity of visual stimuli and detect their ordinal disparity. Two monkeys were first trained to respond to exemplars of the numerosities 1 to 4 in an ascending numerical order (1 --> 2 --> 3 --> 4). As a control for non-numerical cues, exemplars were varied with respect to size, shape, and color. The monkeys were later tested, without reward, on their ability to order stimulus pairs composed of the novel numerosities 5 to 9. Both monkeys responded in an ascending order to the novel numerosities. These results show that rhesus monkeys represent the numerosities 1 to 9 on an ordinal scale.

Responses to S_ ("errors") are not a necessary condition for the formation of an operant discrimination of color. Errors do not occur if discrimination training begins early in conditioning and if S+ and S_ initially differ with respect to brightness, (luration and wavelength. After training starts, S-'s duration and brightness is progressively increased until S+ and Sdiffer only with respect to wavelength. Errors do occur if training starts after much conditioning in the presence of S+ has occurred or if S+ and S_ differ only with respect to wavelength throughout training. Performance following discrimination learning without errors lacks three characteristics that are found following learning with errors. Only those birds that learned the discrimination with errors showed (1) "emotional" responses in the presence of S-, (2) an increase in the rate (or a decrease in the latency) of its response to S+, and (3) occasional bursts of responses to S-.

Metacognition is knowledge that can be expressed as confidence judgments about what one knows (monitoring) and by strategies for learning what one does not know (control). Although there is a substantial literature on cognitive processes in animals, little is known about their metacognitive abilities. Here we show that rhesus macaques, trained previously to make retrospective confidence judgments about their performance on perceptual tasks, transferred that ability immediately to a new perceptual task and to a working memory task. We also show that monkeys can learn to request "hints" when they are given problems that they would otherwise have to solve by trial and error. This study demonstrates, for the first time, that nonhuman primates share with humans the ability to monitor and transfer their metacognitive ability both within and between different cognitive tasks, and to seek new knowledge on a need-to-know basis.

Three rhesus monkeys (Macaca mulatto) were trained to respond to exemplars of 1,2,3, and 4 in an ascending, descending, or a nonmonotonic numerical order (1-»2->3->4,4->3-*2-'1, 3-•!-»4->2). The monkeys were then tested on their ability to order pairs of the novel numerosities 5-9. In Experiment 1, all 3 monkeys ordered novel exemplars of the numerosities 1-4 in ascending or descending order. The attempt to train a nonmonotonic order (3-"1->4-*2) failed. In Experiment 2A, the 2 monkeys who learned the ascending numerical rule ordered pairs of the novel numerosities 5-9 on unreintorced trials. The monkey who learned the descending numerical rule failed to extrapolate the descending rule to new numerosities. In Experiment 2B all 3 monkeys ordered novel exemplars of pairs of the numerosities 5-9. Accuracy and latency of responding revealed distance and magnitude effects analogous to previous findings with human participants (R. S. Moyer & T. K. Landaeur. 1967). Collectively these studies show that monkeys represent the numerosities 1-9 on at least an ordinal scale.

Acquisition and maintenance of autoshaped key pecking by pigeons were studied as a function of the duration of trial and intertrial intervals'. In Experiment 1, trial durations were fixed and intertrial durations were variable. Twenty-five groups of birds were studied at trial durations ranging from 1 to 64 sec and mean intertrial interval durations ranging from 6 to 768 sec. Values were chosen so as to obtain several groups with the same ratio of intertrial interval to trial duration. Ratios ranged from 2:1 to 96:1. Over most durations studied, constant values of the ratio produced an approximately constant number of trials to acquisition. High ratios resulted in faster acquisition than did low ratios. Following acquisition, response rate varied inversely with absolute trial duration. In Experiment 2, intertrial interval as well as trial durations were fixed. Intertrial intervals ranged from 48 to 384 sec, and trial durations ranged from 8 to 32 sec. Trials to acquisition again varied inversely with the ratio of intertrial interval to trial duration. In both experiments, this relationship was well approximated by a single power function. These results are shown to strain several current accounts of the classical conditioning process.