Choose-long errors at delays shorter than the training delay persist when enhanced between-trial summation of duration memories are precluded in pigeons

Pigeons were trained to match 2- and 10-s durations of houselight to red and green comparisons. Following acquisition with a 0-s baseline delay, they were tested with delays of 0, 10 and 20 s. There was a strong tendency to choose the short-associated comparison stimulus at both the 10- and 20-s delays (i.e., a choose-short effect) and no bias at the 0-s delay. This test was repeated after baseline training with a constant 10-s delay. As in the first delay test, a choose-short effect was obtained at the 20-s delay. In contrast to the first test, no bias was obtained at the 10-s delay and a strong tendency to choose the long-associated comparison (i.e., a choose-long effect) was obtained at the 0-s delay. Importantly, the choose-long effect was obtained under conditions which insured that the temporal spacing between a 0-s delay trial and the preceding trial was equal to that in training. These results are inconsistent with the temporal summation account of the choose-long effect and are most readily interpreted within a perspective emphasizing the subjective shortening of temporal memories over time.     

Effects of delays on human performance on a temporal discrimination procedure: evidence of a choose-short effect

Studies of temporal discrimination in non-human subjects have reliably shown a choose-short effect: higher matching accuracy on short-duration-sample trials than on long-duration-sample trials. This effect occurs as a function of increasing the delay between the onset of sample and comparison stimuli in a delayed matching-to-sample procedure. The present experiment investigated whether the choose-short effect could be demonstrated in human subjects under conditions which paralleled those used with non-human subjects. Subjects responded under a discrete-trial procedure in which they were required to push one of two buttons depending on the duration of a sample stimulus (a blue square on a computer monitor). Delays (0, 8, 16, and 32s) separated sample and comparison stimuli (two white boxes) and were tested both within and across several sessions. Intermediate durations (probe stimuli between 2 and 4s) were also presented. The addition of a delay between the sample and comparison stimuli produced a bias to judge intervals as short when the 8 and 32-s delays were tested across sessions and when the 0, 16, and 32-s delays were tested within the same session. Thus, the choose-short effect was produced in human subjects using the interval bisection procedure regardless of delay length.     

Training delays reduce the choose-short effect with keylight duration samples in pigeons

Pigeons were trained to matching 2- and 8-s keylight samples. The delay on training trials was either 0s (group 0sF), 2s (group 2sF), or varied between 1 and 3s (M=2s, group 2sV). Testing at delays of 10 and 20s revealed a choose-short tendency in all three groups. The magnitude of this tendency was largest in group 0sF and was highly similar in magnitude in groups 2sF and 2sV. In Experiment 2, the training delay remained at 0s in group 0sF and was increased to 5s in group 2sF (now group 5sF). Group 2sV (now group 5sV) received variable training delays ranging from 2 to 8s (M=5s). Testing at a 20-s delay and at a delay that exceeded the training delay by 15s for each group revealed a robust choose-short effect only in group 0sF. Groups 5sF and 5sV both demonstrated a weak and statistically nonsignificant choose-short tendency. When different durations of keylight are employed as samples, training with a nonzero delay, whether fixed or variable, reduces the magnitude of the choose-short effect, with longer training delays producing a greater reduction in the choose-short effect.     

Training delays reduce the choose-short effect with keylight, but not with food, duration samples in pigeons

Pigeons were trained to match 2- and 8-s food samples. The delay on training trials was either 0s (group 0sF), 5s (group 5sF), or varied between 2 and 8s (M=5s, group 5sV). Testing at a delay that exceeded the training delay by 15s in each group revealed a robust choose-short effect in each group. The same pigeons then reacquired a previously trained matching task involving 2- and 8-s keylight samples. Different comparison stimuli were used on food-sample and keylight-sample trials. The delay on training trials was the same on both food- and keylight-sample trials. Extended-delay testing revealed a robust choose-short effect in all three groups when the durations were conveyed by food presentations, but only group 0sF revealed a choose-short effect when the durations were conveyed by keylight presentations. Hence, training with a nonzero delay, whether fixed or variable, reduces the choose-short effect with keylight durations but not with food durations. It was concluded that at least some of the psychological processes mediating performance differ as a function of the event that conveys the duration.     

Effect of training delays and start and stop markers on the choose-short effect in pigeons

Pigeons were trained in a matching-to-duration task using short and long filled intervals. Group 2/8 was trained with 2- and 8-s intervals, Group 4/10 with 4- and 10-s intervals, and group marker with 2- and 8-s intervals presented between 1-s start and stop markers. Extended-delay testing showed no significant choose-short effect (CSE) in any group. It was hypothesized that the lack of a CSE may have resulted from use of a variable delay (range 1-3s) during training. The same subjects were employed in Experiment 2 and were trained with a new set of comparisons and one of the alternate types of samples employed in Experiment 1. All training trials involved a 0-s delay. Extended-delay testing revealed a significant CSE in Groups 2/8 and 4/10 but only a weak, and statistically nonsignificant, CSE in group marker. It was concluded that use of a variable delay during training reduced the CSE. The notion that subjective shortening underlies the CSE provided an adequate account of these findings.     

Rats' memory for event duration in delayed matching-to-sample with nonspatial comparison response alternatives

Previous research has suggested that using stationary and moving levers as nonspatial response alternatives can significantly enhance the speed of acquiring a temporal discrimination in rats. In Experiment 1, rats were trained to discriminate 2 and 8s of magazine light illumination by responding to either a stationary lever or a moving lever with a cue light illuminated above it. Rats learned to discriminate event durations at a high level of accuracy after 25 sessions of training. During subsequent delay tests, rats exhibited a strong choose-long bias, indicating that they were timing from the onset of the magazine light until the entry of levers into the chamber. This occurred regardless of whether intertrial intervals and delay intervals were dark or illuminated. On test trials in which the sample was omitted, rats responded as if the short sample had been presented. In Experiment 2, the rats received extensive training with dark and illuminated variable delay intervals (1-4 s). However, they continued to exhibit a tendency to time from the onset of the magazine light until entry of the levers into the chamber. Although the use of stationary/uncued and moving/cued levers as response alternatives enhanced the speed of acquisition of the event duration discrimination in rats, additional procedural modifications will be necessary to prevent rats from timing during the delay interval.     

The effect of intruded events on peak time: the role of reinforcement history during the intruded event

Pigeons were studied in an extension of a study by Aum et al. [Aum, S., Brown, B.L., Hemmes, N.S. 2004. The effects of concurrent task and gap events on peak time in the peak procedure. Behav. Process. 65, 43-56] on timing behavior under a discrete-trial fixed-interval (FI) procedure during which 6-s intruded events were superimposed on peak-interval (PI) test trials. In Aum et al., one event consisted in termination of the timing cue (gap trial); the other was a stimulus in the presence of which subjects had been trained to respond under an independent random-interval (RI) schedule of reinforcement (concurrent task trial). Aum et al. found a disruption of timing on concurrent task trials that was greater than that on gap trials. The present study investigated history of reinforcement associated with intruded events as a possible explanation of this earlier finding. After training to peck a side key on a 30-s PI procedure, discrimination training was conducted on the center key in separate sessions; red or green 6-s stimuli were associated with RI 24s or EXT (extinction) schedules. During testing under the PI procedure, three types of intruded events were presented during probe trials--the stimulus associated with the RI (S+) or EXT (S-) schedule during discrimination training, or a gap (termination of the side-keylight). Intruded events occurred 3, 9, or 15s after PI trial onset. Effects of reinforcement history were revealed as substantial disruption of timing during the S+ event and relatively little disruption during the S- event. Intermediate effects were found for the gap event. Results indicate that postcue effects are at least partially responsible for the disruptive effects of the S+ event.     

Effects of retention interval on performance in a numerical reproduction task

The retention interval (RI) between the sample and production phase in a numerical reproduction task was varied to determine whether a "produce-small" effect would be obtained with increased delays. Four pigeons were trained with a retention interval of 2s, and then tested with intervals of 0.5s and 8s. Results showed a number-dependent, "produce-large" effect-response number increased when RI was increased-analyses of average response number and accuracy suggested RI affected responding most on the 2-flash trials with an 8-s RI. Additionally, discrimination between trial types decreased as RI increased. Existing explanations for the "choose-short/small" effect appear unable to account for these results; however the "produce-large" effect may be attributed to a disruption in stimulus control over responding.     

Dopamine agonists and antagonists can produce an attenuation of response bias in a temporal discrimination task depending on discriminability of target duration

The current study examined the effects of the D2 agonist (quinpirole) and D2 antagonist (eticlopride) on temporal discrimination performance in a conditional discrimination task (Experiment I) and a delayed conditional discrimination task (Experiment II). In both experiments rats discriminated between a scheduled stimulus duration of 3 s versus 9 s. Consistent with previous reports, overall discrimination performance decreased in a dose-dependent manner with both drugs. Changes in response bias (the tendency to choose-short or choose-long irrespective of actual stimulus duration), however, were best characterized in terms of both drugs tending to attenuate the bias effects normally observed during baseline drug-free performance. Specifically, the 'choose-short' bias observed in Experiment I and at a relatively short, 0.1 s, delay in Experiment II became less extreme with increasing doses. In addition, the 'choose-long' bias observed at a relatively long, 6 s, delay in Experiment II also became less extreme with increasing doses. Thus, whether there was an apparent shift from a short response bias to long, or vice versa, was the product of the delay interval between stimulus presentation and choice rather than whether the drug in question was a D2 agonist or antagonist. Such an attenuation of bias may have arisen because of subjects confounding the delay interval with the actual discriminative stimulus duration.     

Reinforcer concentration effects on a fixed-interval schedule

Four rats received training on a mixed FI 30-s FI 150-s schedule, where the different FI values were associated with different levers. During baseline, the reinforcer was a 30% concentration of condensed milk. During subsequent testing sessions, the reinforcer concentration was varied within sessions over values of 10, 30, 50, and 70%. Measures of behaviour were taken from the FI 30-s lever during trials where the reinforcer was delivered for responses on the other lever. Increasing the reinforcer concentration which began the interval (a) increased the time to start responding in the interval, and (b) increased the location of the response peak on the FI 30-s lever (often to values well above 30s). Response rate at the peak, and spread of the response rate versus time function, changed much less with reinforcer concentration. The data are discussed relative to predictions derived from Scalar Expectancy Theory, the Behavioural Theory of Timing, and the Tuned-trace model.     

Temporal discrimination learning by pigeons

Memory for time by animals appears to undergo a systematic shortening. This so-called choose-short effect can be seen in a conditional temporal discrimination when a delay is inserted between the sample and comparison stimuli. We have proposed that this temporal shortening may result from a procedural artifact in which the delay appears similar to the intertrial interval and thus, produces an inadvertent ambiguity or 'instructional failure'. When this ambiguity is avoided by distinguishing the intertrial interval from the delay, as well as the samples from the delay, the temporal shortening effect and other asymmetries often disappear. By avoiding artifacts that can lead to a misinterpretation of results, we may understand better how animals represent time. An alternative procedure for studying temporal discriminations is with the psychophysical bisection procedure in which following conditional discrimination training, intermediate durations are presented and the point of subjective equality is determined. Research using the bisection procedure has shown that pigeons represent temporal durations not only as their absolute value but also relative to durations from which they must be discriminated. Using this procedure, we have also found that time passes subjectively slower when animals are required to respond to the to-be-timed stimulus.     

Visualisation of plasmid replication intermediates containing reversed forks

Blockage of replication forks can have deleterious consequences for the cell as it may prompt premature termination of DNA replication. Moreover, the blocked replication intermediate (RI) could be particularly sensitive to recombination processes. We analysed the different populations of RIs generated in vivo in the bacterial plasmid pPI21 after pausing of replication forks at the inversely oriented ColE1 origin. To achieve this goal, a new method was developed based on two-dimensional agarose gel electrophoresis. This method allows the isolation of specific RIs, even when they were rather scarce, from the total DNA. Here we describe the occurrence of RI restriction fragments containing reversed forks. These Holliday-like structures have been postulated but never observed before.     

Effects of free-food deliveries and delays on choice under concurrent-chains schedules

Pigeons responded on a concurrent-chains schedule with two equal variable-interval (VI) schedules as initial links and delays to food of 3 and 12s as the two terminal links. In even-numbered sessions, no other reinforcement schedule was present, and all pigeons showed a strong preference for the response key that had the shorter, 3-s terminal-link delay. In odd-numbered sessions, the initial links were interrupted at random times by one of three different types of events. When the interruptions were immediate food deliveries, the response percentages increased on the key that had the 3-s delay. When the interruptions were 30-s delays followed by food, the response percentages remained approximately unchanged. When the interruptions were 30-s delays with no food, the response percentages decreased. The results were used to compare the predictions of different mathematical models of concurrent-chains performance. The results favored models that assume that preference is determined by the relative amount of advantage that is gained when a terminal link is entered.     

The effect of between-set rest intervals on the oxygen uptake during and after resistance exercise sessions performed with large- and small-muscle mass

Between-set rest intervals (RIs) may influence accumulated fatigue, work volume, and therefore oxygen uptake (VO2) and energy expenditure (EE) during resistance training. The study investigated the effects of different RIs on VO2 and EE in resistance exercises performed with multiple sets and recruiting large and small-muscle mass. Ten healthy men performed 4 randomized protocols (5 sets of 10 repetitions with 15 repetition maximum workloads in either horizontal leg press [LP] or chest fly [CF] with an RI of 1 and 3 minutes). The VO2 was measured at rest, within sets, and during 90-minute postexercise recovery (excess postexercise oxygen consumption [EPOC]). The EE was estimated from VO2net (total VO2 - rest VO2). The VO2 increased in all protocols, being higher within the exercises and during EPOC in the LP than in the CF regardless of the RI. The 1-minute RI induced higher accumulated VO2 during LP (p < 0.05) but not during CF. The EPOC lasted approximately 40 minutes after LP1, LP3, and CF1, being longer than after CF3 (20 minutes, p < 0.05). Total EE was mainly influenced by muscle mass (p < 0.001) (LP3 = 91.1 ± 13.5 kcal ～ LP1 = 88.7 ± 18.4 kcal > CF1 = 50.3 ± 14.4 kcal ～ CF3 = 54.1 ± 12.0 kcal). In conclusion, total VO2 was always higher in LP than in CF. Shortening RI enhanced the accumulated fatigue throughout sets only in LP and increased VO2 in the initial few minutes of EPOC, whereas it did not influence total VO2 and EE in both exercises. Therefore, (a) the role of RI in preventing early fatigue seems to be more important when large-muscle groups are recruited; (b) resistance exercises recruiting large-muscle mass induce higher EE because of a greater EPOC magnitude.     

Both PKMζ and KIBRA are closely related to reference memory but not working memory in a T-maze task in rats

Protein kinase M zeta (PKMζ) and the kidney and brain protein (KIBRA) play important roles in various forms of memories. However, whether they are involved in performing the T-maze task is still unknown. In this study, the delayed nonmatch-to-sample (DNMS) task in a T-maze was given to rats. The percentage of correct choices denoting the performance accuracy was calculated and the protein levels of PKMζ and KIBRA in rat’s prefrontal cortex were measured. The results showed significantly increased performance accuracy after the training phase, which was maintained on the next day in groups with a delay of 10 s but not 30 s, indicating that 30 s is too long for rats to maintain working memory. As for the expressions of PKMζ and KIBRA, significant increases were observed 1 day after the training phase, indicating that the formation of reference memory accompanies an increase in PKMζ and KIBRA. No significant difference was found among groups with various delay intervals, indicating that the expressions of PKMζ and KIBRA exert no effects on the performance of working memory. These results provide the first evidence that KIBRA as well as PKMζ is closely related to reference memory but not working memory in rats.     

Polyoma virus minichromosomes: characterization of the products of in vitro DNA synthesis.

A two-dimensional (neutral-alkali) agarose gel electrophoretic system was used to separate three families of replicative intermediate (RI) polyoma virus DNA molecules (form I, form II, and form III RIs). Two of these families, form II and III RIs, are the result of artifactual nicking of one of the parental strands of supercoiled RIs (form I RIs) during in vitro replication of soluble minichromosomes. Kinetic studies in vitro showed that the nicked RIs serve as templates for limited DNA synthesis. The nicked species are not converted into normal products, however. The nicking reaction, which appears to be specific for the parental strands, is dependent on magnesium ions and occurs concurrently with the in vitro synthesis of DNA.     

Formation, fixation and reproduction of temporary connections during adaptation to physical loading

It is shown that long-term adaptation to physical loads of various intensity and duration has an influence on formation, fixation and reproduction of temporary connections in rats. Periodicity of conditioned activity has been found in the process of protracted training. After twenty days of adaptation to submaximal physical loads, the processes of fixation and reproduction of temporary connections improve whereas with elongation of adaptation the conditioned activity somewhat deteriorates. In conditions of long-term training by maximal physical loads, both elaboration and preservation of conditional reflexes in all adaptation terms is affected.     

Blood pressure variations real-time reflect the conditioned fear learning and memory

The conditioned fear learning and memory occurs when a neutral conditioned stimulus (CS) is paired with an aversive unconditioned stimulus (US). This process is critically dependent on the amygdala and inevitably involves blood pressure (BP) alterations. We hypothesized that BP variations could instantaneously reveal individual steps during conditioned fear learning and memory. An implanted telemetric probe was used to monitor the BP real-time in rats during training and testing sessions of the fear-potentiated startle. Our results showed that (i) the conditioned fear learning during the training sessions was reflected by light (CS)-induced rapid BP elevations and by electric shock (US)-evoked sympathetic tone elevations; (ii) these two BP-related parameters were not only negatively correlated with each other but also coupled to each other in the training session trials; (iii) both parameters closely predicted the performance of fear-potentiated startle on the next day; and (iv) although local blocking of one of the two fear-conditioned pathways in the training session partially inhibited fear learning, the fear memory retrieval still used both pathways. Altogether, real-time blood pressure variations faithfully revealed the critical steps involved in conditioned fear learning and memory, and our results supported a coupling between the cued learning and the post-shock calmness.     

Hippocampal Sharp Wave/Ripples during Sleep for Consolidation of Associative Memory

The beneficial effect of sleep on memory has been well-established by extensive research on humans, but the neurophysiological mechanisms remain a matter of speculation. This study addresses the hypothesis that the fast oscillations known as ripples recorded in the CA1 region of the hippocampus during slow wave sleep (SWS) may provide a physiological substrate for long term memory consolidation. We trained rats in a spatial discrimination task to retrieve palatable reward in three fixed locations. Hippocampal local field potentials and cortical EEG were recorded for 2 h after each daily training session. There was an increase in ripple density during SWS after early training sessions, in both trained rats and in rats randomly rewarded for exploring the maze. In rats learning the place -reward association, there was a striking further significant increase in ripple density correlated with subsequent improvements in behavioral performance as the rat learned the spatial discrimination aspect of the task. The results corroborate others showing an experience-dependent increase in ripple activity and associated ensemble replay after exploratory activity, but in addition, for the first time, reveal a clear further increase in ripple activity related to associative learning based on spatial discrimination.