Short-term item memory in successive same-different discriminations

Pigeons were tested in a successive same-different (S/D) discrimination procedure to examine the short-term memory for individual items in sequences of different or identical pictures. Item-by-item analyses of pecking behavior within single trials revealed this S/D discrimination emerged at the earliest possible point in the sequence--the presentation of the second item. Further, by comparing peck rates at points where different types of sequences diverged (e.g. ABA versus ABC), we determined that the pigeons remembered the first item for at least 4-8s and across one to two intervening items. These results indicate that this S/D discrimination was controlled by relational comparisons of pictorial content across memories of specific items, rather than the detection of low-level perceptual "transients" between items. A second experiment supported this conclusion by showing increased discrimination with longer first item viewing times, consistent with encoding of details about individual pictures. These findings further support a qualitative similarity among birds and primates in possessing a general capacity to judge certain types of stimulus relations, such as stimulus identity and difference. Implications for the temporal continuity of experience in animals are also considered.     

Response acquisition with immediate and delayed conditioned reinforcement

Groups comprising eight rats initially were exposed to response-independent water deliveries, then to conditions under which a lever-press response raised an empty dipper immediately or after a resetting delay of 15, 30, or 45 s. When their performance was compared to that of control animals using a 90% confidence level, six rats in the immediate-reinforcement group met the primary criterion for response acquisition during a single 6-h session; 4, 4, and 3 did so in the 15, 30, and 45 s delay groups, respectively. Similar evidence of acquisition was obtained when a 95% confidence level was used. With a 99% confidence level, however, evidence of acquisition was not compelling. Although these data appear to provide the first demonstration of response acquisition in the absence of handshaping or autoshaping under conditions where the putative reinforcer is both conditioned and delayed, they also demonstrate that whether response acquisition occurs depends, in part, on how it is defined.     

Response acquisition with delayed reinforcement in Lewis and Fischer 344 rats

Previous work has shown neurochemical and behavioral differences between Lewis rats and Fischer 344 rats. Some of this work suggests that there might be differential sensitivity to delayed reinforcement between the two strains. To further explore this possibility, Lewis (n=8) and Fischer 344 (n=8) rats were exposed to a response-acquisition task with a non-resetting 20s delay to reinforcement. A tandem fixed-ratio 1, fixed-time 20s schedule of reinforcement was programmed for one of two levers; presses on the alternate lever had no programmed consequences. A greater number of Lewis rats (5/8) acquired lever pressing compared to the Fischer 344 rats (2/8). Future work with these strains may lead to a better understanding of the genetic and/or neurochemical factors involved in temporal control of behavior.     

Effects of reinforcer magnitude on response acquisition with unsignaled delayed reinforcement

Sixteen rats received eight 1-h sessions of a tandem fixed-ratio 1 differential-reinforcement-of-other-behavior 30-s schedule with reinforcer magnitude at one or six food pellet(s) across groups of eight rats. The larger reinforcer magnitude established the lever press more effectively. First, mean schedule completions differed across groups, in terms of both their overall difference and in their increase across sessions. Second, whereas the larger reinforcer magnitude established reliable response acquisition in all eight rats by the end of the experiment, the smaller reinforcer magnitude only established reliable response acquisition in four of eight rats. The present results systematically replicate earlier findings obtaining more reliable response acquisition with unsignaled delayed reinforcement with greater food deprivation. Taken together, this work demonstrates the influence of motivational variables on response acquisition with unsignaled delayed reinforcement.     

Effects of prenatal stress on lever-press acquisition with delayed reinforcement in male and female rats

The prenatally stressed (PS) rat shows enhanced conditioned fear and increased behavioral inhibition in response to footshock compared to control (CON) rats. It is unclear whether this facilitated learning will occur only with aversive stimulation, or if it will also be observed in the context of positive reinforcement. There are limited and inconsistent data regarding sex differences and the impact of prenatal stress on learning. The present study was designed to examine lever-press acquisition with a 10-s delay to food reinforcement in male and female PS and CON rats. Overall, twice as many PS male rats acquired the lever-press response than the PS female rats, CON male rats, and CON female rats. PS male rats also earned significantly more reinforcers and responded on the operative lever at a significantly greater rate than the other three rat groups. These findings suggest that PS rats exhibit altered learning with a task involving positive reinforcement, and this effect of PS is sex specific for male rats.     

Occasion setting in Pavlovian ambiguous target discriminations

Rats were trained in a Pavlovian serial ambiguous target discrimination, in which a target cue was reinforced if it was preceded by one stimulus (P-->T+) but was not reinforced if it was preceded by another stimulus (N-->T-). Test performance indicated that stimulus control by these features was weaker than that acquired by features trained within separate serial feature positive (P-->T+, T-) and serial feature negative (N-->W-, W+) discriminations. The form of conditioned responding and the patterns of transfer observed suggested that the serial ambiguous target discrimination was solved by occasion setting. The data are discussed in terms of the use of retrospective coding strategies when solving Pavlovian serial conditional discriminations, and the acquisition of special properties by both feature and target stimuli.     

Testing meter, rhythm, and tempo discriminations in pigeons

Rhythmic grouping and discrimination is fundamental to music. When compared to the perception of pitch, rhythmic abilities in animals have received scant attention until recently. In this experiment, four pigeons were tested with three types of auditory rhythmic discriminations to investigate their processing of this aspect of sound and music. Two experiments examined a meter discrimination in which successively presented idiophonic sounds were repeated in meters of different lengths in a go/no-go discrimination task. With difficulty, the birds eventually learned to discriminate between 8/4 and 3/4 meters constructed from cymbal and tom drum sounds at 180 beats per minute. This discrimination subsequently transferred to faster tempos, but not to different drum sounds or their combination. Experiment 3 tested rhythmic and arrhythmic patterns of sounds. After 40 sessions of training, these same pigeons showed no discrimination. Experiment 4 tested repetitions of a piano sound at fast and slow tempos. This discrimination was readily learned and showed transfer to novel tempos. The pattern of results suggests that pigeons can time periodic auditory events, but their capacity to understand generalized rhythmic groupings appears limited.     

Effects of successive carbon monoxide exposures on delayed neuronal death in mice under the maintenance of normal body temperature.

The 3 time carbon monoxide (CO) exposures potentiated the delayed neuronal death (DND) in comparison with that induced by single CO exposure. Deterioration of DND induced by CO exposures was observed when normal body temperature was maintained during the exposures, since CO exposure fell the body temperature to about 34 degrees C. Pretreatment with noncompetitive NMDA receptor antagonist, MK-801 (30 nmol/mouse), ameliorated DND induced by successive CO exposures under the maintenance of normal body temperature. These results suggest that the mice exposed successively to CO under the maintenance of normal body temperature is a useful hypoxic model.     

Undermatching in humans to amount of reinforcement

Human subjects were exposed to a concurrent-chain procedure in which amount of reinforcement in the terminal links was varied. The experimental procedure was designed to resemble as closely as possible animal operant procedures: verbal instructions were eliminated, the key-press operant response was shaped, and a “consummatory” response was required to receive reward. In addition to varying amount of reward, three different pairs of initial-link values in the concurrent chain were studied. The human subjects showed undermatching to amount of reinforcement (as do animal subjects). Moreover, the degree of undermatching tended to increase as the values of the initial links increased, consistent with Fantino's delay reduction hypothesis (1977) that choice for a larger reward decreases as the length of the initial link increases.     

Sensitivity and Resistance to Spectinomycin in Escherichia coli

Inhibition of growth and division of Escherichia coli by spectinomycin is reversible, and the kinetics of its interference with deoxyribonucleic and ribonucleic acid synthesis may be interpreted as secondary effects of inhibition of protein synthesis on the ribosome. Spontaneous mutations to spectinomycin resistance occur in E. coli K-12 at a rate of about 2 x 10(-10). Resistance is transducible with a discrete lag in phenotypic expression, and the kinetics of its development is about the same as that for streptomycin resistance. All spectinomycin-resistant mutants tested contain resistant ribosomes, and all map in a locus (spc) counterclockwise to and 70% cotransducible with the classical str locus. Differences in the residual drug sensitivity of various spectinomycin-resistant mutants, and of their ribosomes, indicate the existence of more than one phenotypic class of resistance.     

Using network models of absolute pitch to compare frequency-range discriminations across avian species

The spectral frequency ranges of song notes are important for recognition in avian species tested in the field. Frequency-range discriminations in both the field and laboratory require absolute pitch (AP). AP is the ability to perceive pitches without an external referent. The authors provided a network model designed to account for differences in AP among avian species and evaluated it against discriminative performance in eight-frequency-range laboratory tests of AP for five species of songbirds and two species of nonsongbirds. The model's sensory component describes the neural substrate of avian auditory perception, and its associative component handles learning of the discrimination. Using only two free parameters to describe the selectivity and the sensitivity of each species’ auditory sensory filters, the model provided highly accurate predictions of frequency-range discrimination in songbirds and in a parrot species, but performance and its prediction were less accurate in pigeons: the only species tested that does not learn its vocalizations. Here for the first time, the authors present a model that predicted individual species’ performance in frequency-range discriminations and predicted differences in discrimination among avian species with high accuracy.