The independent variable in the early origin of higher taxa.

Taxa of high rank tend to originate at earlier times in the fossil record than do taxa of lower rank. Statistical analysis of published stratigraphic ranges shows that the early origin of higher taxa is a secondary consequence of taxonomic structure. For instance, the probability that the origination of an order also begins a new class is a decreasing function of the number of orders that have already originated in the same phylum. Similar decreasing functions also apply to families and orders within classes, and to genera and families within orders, indicating that the pattern of origination is self-similar across at least three taxonomic ranks. These functions are sufficient to predict the extent to which higher taxa originate earlier, but the converse is not true. These results are more consistent with genetic constraints or taxonomic hindsight than with ecological saturation as explanations for the early origin of higher taxa.

Rewarding and aversive effects of stimulant drugs in infant rats.

Four experiments investigated the reinforcing effects of cocaine, amphetamine, and phenylpropanolamine using odor as the conditioned stimulus and infant rats as the subjects. At the age of 2 days the rats were injected with saline or with one of the three drugs and placed for 30 min in a plastic bag with almond-scented shavings. At the age of 18 days the rats were deprived of water for 12 hr and then given two tests that required different responses. In the consumption test they were given 30 min access to two water spouts, one surrounded by almond-scented cotton and the other surrounded by plain cotton. In the place test, they spent 5 min in a shuttle box with almond-scented shavings under one side and plain shavings under the other side. Compared to saline, all three drugs decreased water intake from the almond-scented spout and increased time spent over the almond-scented shavings.

Aversion to the reinforcer differentially affects conditioned reinforcement and instrumental responding.

Rats were trained to press a bar for sucrose solution delivered by a dipper; they then received pairings of sucrose and lithium chloride (LiCl) in the home cage and were tested for bar pressing. In the first and second experiments, the conditioned aversion to sucrose had no effect on unreinforced bar pressing in the test, but the aversion reduced bar pressing reinforced either by sucrose or by the operation of the empty dipper. In the third and fourth experiments, presses during training were reinforced by sucrose only in the presence of an audiovisual discriminative stimulus; in the test, the aversion to sucrose reduced pressing reinforced by the discriminative stimulus, but the aversion had no effect on unreinforced pressing in either the presence or the absence of the discriminative stimulus. Thus, pairings of sucrose and LiCl reduced the reinforcing value of sucrose and also of a stimulus previously paired with sucrose, but they had no effect of an instrumental response previously reinforced by sucrose or on the discriminative properties of a stimulus paired with sucrose.

Rewarding and aversive effects of morphine: temporal and pharmacological properties.

To assess morphine-induced location preferences and flavor aversions, rats were administered morphine sulfate (10 mg/kg, IP) either immediately before (Experiment 1) or immediately after (Experiment 2) confinement for 20 min in one side of a shuttlebox with access to a flavored solution. On control trails the rats were administered saline and confined for 20 min on the opposite side with a differently flavored solution. In subsequent choice tests, it was found that morphine injections before confinement produced a preference for the side associated with morphine and indifference to the flavors, whereas morphine injections after confinement produced an aversion to the flavor paired with morphine and indifference to the sides. Experiments 3 and 4, using a procedure similar to that of Experiment 1, showed that naloxone (1 mg/kg, IP) blocked the morphine-induced side preference, although given alone it was without effect in this test.

Temporal properties of the rewarding and aversive effects of amphetamine in rats.

To associate amphetamine with a location and a flavor, rats were given amphetamine injections and then confined for 20 min in one side of a shuttlebox with access to a flavored solution; on control trials with saline injections, they were confined for 20 min on the opposite side with a different flavor. Three groups of rats were placed in the shuttlebox either 5 min, 120 min, or 240 min after the injections. In subsequent choice tests, the 5 min and 120 min groups preferred the side and avoided the flavor associated with amphetamine; the 240 min group was indifferent between the sides and the flavors.

Irrelevant-incentive learning with flavors in rats.

In four experiments, rats were trained to associate one cue with one incentive and another cue with a different incentive. Experiment 1 used food and water as the incentives, and Experiments 2-4 used arbitrarily flavored sugar solutions as the incentives; Experiments 1-3 used flavor cues, and Experiment 4 used exteroceptive cues. Preference between the cues was tested under deprivation conditions different from training. With flavor cues, the rats preferred the cue for the incentive with which they were not satiated in the test (Experiments 1 and 2), but they showed no tendency to prefer the cue for the incentive with which they had not been satiated during training (Experiments 2 and 3). With exteroceptive cues, on the other hand, preference in the test depended upon satiation during training but not upon satiation in the test (Experiment 4). These results indicate latent learning but no learning of a habit with flavor cues, and learning of a habit but no latent learning with exteroceptive cues.