The effect of predictability on subjective duration

Events can sometimes appear longer or shorter in duration than other events of equal length. For example, in a repeated presentation of auditory or visual stimuli, an unexpected object of equivalent duration appears to last longer. Illusions of duration distortion beg an important question of time representation: when durations dilate or contract, does time in general slow down or speed up during that moment? In other words, what entailments do duration distortions have with respect to other timing judgments? We here show that when a sound or visual flicker is presented in conjunction with an unexpected visual stimulus, neither the pitch of the sound nor the frequency of the flicker is affected by the apparent duration dilation. This demonstrates that subjective time in general is not slowed; instead, duration judgments can be manipulated with no concurrent impact on other temporal judgments. Like spatial vision, time perception appears to be underpinned by a collaboration of separate neural mechanisms that usually work in concert but are separable. We further show that the duration dilation of an unexpected stimulus is not enhanced by increasing its saliency, suggesting that the effect is more closely related to prediction violation than enhanced attention. Finally, duration distortions induced by violations of progressive number sequences implicate the involvement of high-level predictability, suggesting the involvement of areas higher than primary visual cortex. We suggest that duration distortions can be understood in terms of repetition suppression, in which neural responses to repeated stimuli are diminished.     

The effect of context change upon long-term memory of CS duration

An experiment with rats investigated forgetting of inhibition of delay in the conditioned suppression paradigm. The combined effects of contextual change and retention interval were tested. After a reliable temporal discrimination was reached, half of the rats received a test in the training context after a retention interval of 3 or 20 days. The other half received it in a different but equally familiar context at either retention interval. The longest retention interval flattened the temporal discrimination gradient and increased suppression to the CS. A similar but weaker pattern was found with the change of context; this effect was independent of the retention interval. The implications for retrieval and interference theory [Bouton, M.E., 1993. Psychol. Bull., 114: 80–99] and hypotheses concerning the forgetting of specific features of stimuli over time [Riccio, D.C., Richardson, R. and Ebner, D.L., 1984. Psychol. Bull., 96: 152–165] are discussed.     

The choose-short effect in rat memory for event duration: the subjective-shortening model

Two predictions derived from the subjective-shortening model were tested in rats. The predictions concerned the temporary occurrence of the choose-short effect with extended training at a given retention interval (RI) and the occurrence of a temporary choose-long effect, when RIs shorter than those used during training were applied. In a first experiment, using a stepwise delay procedure with training 0-s RI sessions interpolated between each series of increasing RIs, results showed: (1) a choose-short effect during the stepwise increase in the delay procedure, (2) a temporary occurrence of the choose-short effect during testing at a given RI and (3) a choose-long effect in half of the animals, when a RI shorter than that used previously was applied. These contrasting results suggest that the disappearance of the choose-short effect could be, as proposed by the model, either the consequence of the foreshortening of the reference memory (for rats choosing-long) or the consequence of an adaptation of the working memory (for rats which did not choose long). Results were discussed in relation with the procedure which could have contributed, by the interposition of 0-s RI sessions, to maintain a stable reference memory. In order to test this interpretation, a second experiment, using the classical stepwise delay procedure without training sessions interpolated, was carried out. In these conditions, rats did never present a significant choose-long effect when the RI was shortened. These results suggest that rats maintained a stable reference memory and could improve their performances during retention testing sessions either by an adaptation of their working memory or by the adoption of an alternative strategy which consisted in learning to maintain an orientation towards the location of the correct lever.     

Sex differences in the long-term effect of preweanling isolation stress on memory retention

Social experiences during development can powerfully modulate later neuroendocrine and behavioral system. In the present study, male and female rat pups experienced daily bouts of social isolation for 6 h per day or control conditions during the third postnatal week. Performance on a 12-arm radial maze with 8 arms consistently baited with food reward was examined in adulthood. During the social isolation, both male and female pups exhibited a significant increase in plasma corticosterone levels. When tested on the radial arm maze as adults, the performance of female rats that had experienced social isolation during development was not affected; however, male rats in the isolation condition initially exhibited impairments in working memory but not reference memory. Despite achieving comparable asymptotic levels of performance on the maze, male rats that experienced social isolation during the third week demonstrated disruption in working memory retention when radial arm maze trials were interrupted after the fourth arm choice. Thus, while male rats that experience social isolation during the third week of life eventually perform comparably to controls on the standard radial arm maze task, their ability to retain information over a delay remains impaired. These findings highlight an important sex difference in the long-term effects of stress during this period of late preweanling development.     

The relationships between working memory and long-term memory

Recent studies have led to the proposal that working memory operates not as a gateway between sensory input and long-term memory but as a workspace. The core of argument is that access to acquired knowledge and prior learning occurs before information becomes available to working memory. This proposition is a way to accomodate Baddeley's multiple component working memory model and the view that considers that working memory is nothing other than temporary activations of representations and procedures in long-term memory. However, this ‘workspace’ conception of working memory raises the question of the relationships between the central executive system and long-term memory.     

The effect of duration of sprinkling on total moisture losses from pigs

Total moisture losses (TML) from 2 Large White X Landrace pigs sprinkled (800 ml/min) for 0, 30, 60, 90 or 120 seconds at 35C ambient temperature were measured in a hygrometric tent. TML increased to peak values of 5–6 g/min after sprinkling for 60–120 seconds. Irrespective of duration, TML declined to control levels within 60–70 minutes. To obtain most benefit in terms of feed intake and growth in a hot climate it is suggested that pigs should be sprinkled with approximately 800 ml water every 30 minutes.     

The effect of protein synthesis inhibition in the central nervous system on long-term memory formation in some behavioral tasks

The effect of the maximum protein synthesis inhibition in brain and spinal cord on long-term memory formation in extreme situations was studied in various new behavioral tasks in rats. Cycloheximide injected bilaterally into the lateral ventricles three hours before learning suppressed protein synthesis in the central nervous system by 96% during one hour after learning. Forty-four hours after learning in a standard Morris water maze, the information about the platform position was not retained, whereas no memory disorder was observed in case of learning in a simplified Morris maze or a new test learned jump-out-of-water task. A more prolonged suppression of protein synthesis (76%, ten hours after learning) elicited amnesia in five out of eight rats learned in a simplified Morris maze but not disturbed information storage after 48 h and 14 days in the learned jump-out-of-water task. It was concluded that protein synthesis inhibitors are not a universal tool for disrupting formation of long-term memory. It was assumed that under extreme conditions, sometimes procedural long-term (to two weeks) memory is formed without de novo protein synthesis.     

The effect of sample duration on the quantification of stream drift

1. We performed computer simulations and a field experiment to determine the effect that sample duration and, thus, sample volume had on estimates of drift density and sample variance. 2. In computer simulations, when the spatial arrangement of individuals in the water column approximated a random and a contagious-random distribution, estimated mean drift density was not significantly affected by sample duration, but sample variance decreased curvilinearly as sample duration increased. 3. Similar results were obtained in field experiments in habitats of high and low water velocity. 4. Our findings from an Albertan stream indicate that the relationship between sample variance (i.e. coefficient of variation) and duration of drift samples is curvilinear. This relationship affected the number of samples required to achieve a specific level of precision (i.e. a standard error within 10% of the mean). For estimates in low and high current velocities, sample variation was halved by increasing the duration of sample collections from 10 to 20 min. The increased precision obtained with samples of 20 min duration reduced the amount of drift material that needed to be processed by approximately 50% compared with an equivalent 10% level of precision for samples of 10 min duration. This reduction in the number of samples required to obtain a given level of precision has important consequences to the cost of processing drift samples. 5. Thus to optimize studies of stream invertebrate drift, both in terms of sample precision and processing effort, researchers must consider the effect that sample volume has on the variance of drift density estimates. Because researchers generally use drift nets with similar-sized apertures (>300cm²), the problem for specific field applications becomes one of optimizing sample duration relative to variance estimates for drift density.     

Mechanisms of long-term memory

Long-term potentiation (LTP) of the hippocampus provides an excellent model on which to build hypotheses for the laying down of memories in the cerebral cortex. After repetitive activation, the primary happening seems to be the increase in transmitter sensitivity brought about by the increased Ca2+ in the recipient neurons. There may be secondary presynaptic changes. The extreme duration of the LTP may require structural changes of the synaptic spines. The hippocampus plays an essential role in the laying down of cognitive memories, the pathway to the frontal lobe being via the MD thalamus. The thalamo-cortical fibres activate stellate cells whose axons make climbing fibre-like ramifications up the apical dendrites of the pyramidal cells. On the Marr hypothesis repetitive conjunction of synaptic activation by these climbing fibres with synaptic activation by horizontal fibres on the apical dendrites produces prolonged potentiation of the horizontal fibre synapses, which is the neural basis of memory. Presumably this is a consequence of the raised Ca2+ of the apical dendrites, acting as it does on the hippocampal LTP. It will be considered how this elemental unit for cerebral memory can be developed into the varieties of cerebral memories that have been located by study of the regional cerebral blood flow during their retrieval. These sites for memory are in the frontal lobe, usually in the superior prefrontal area.     

The effect of a moving foveal target on the subjective sensation of motion

In this paper, we report on two experiments concerning the effect of the visual field of fovea on the subjective estimation of angular velocity. Experiment 1 investigates the effect of a slow moving target on the perception of self motion. The result of this experiment can be summarized as follows: a slow moving target seen in the visual field of fovea by a stationary person generates in this person a sensation of self rotation in the same direction as the motion of the target. This phenomenon will be called foveal induced ego motion. Experiment 2 investigates the latency for the detection of a self angular acceleration when the person focusses his fovea on a slowly moving target. From the results of this experiment we conclude that the latency for detection of a small self angular acceleration is shorter if the person sees a small foveal target moving with respect to the person in the direction of self rotation than if that small foveal target is moving (with respect to the person) in the opposite direction. The results of these experiments help us in refining existing models of visual-vestibular interaction, by providing a model which accounts for the phenomenon of oculogyral illusion.This research was conducted while serving as a Visiting Professor at the Man Vehicle Laboratory, Department of Aeronautics and Astronautics, Massachusetts Institute of Technology, Cambridge, MA 02139, USA     

Cell adhesion molecules and the transition from short- to long-term memory

Training chicks on a one-trial passive avoidance task results in a cascade of molecular and cellular processes in two forebrain regions, culminating within 60–90 min in post-translational glycosylation of synaptic membrane proteins and expression of immediate early genes c-fos and c-jun. We have now found a second window of vulnerability of memory to the protein synthesis inhibitor anisomycin, 4 h downstream of training. By 5.5 h post-training this window closes, to be replaced by a window of sensitivity to blockade of glycoprotein synthesis, presumably representing post-translational modification of the newly synthesised proteins. Amongst the pre- and post-synaptic membrane glycoproteins involved at both first and second time windows are the cell adhesion molecules, L1 (at both times) and NCAM (at the later). Molecular dissection of the external membrane domains of L1 distinguishes between a requirement for the IgG domain at the early time, the fibronectin-like domain at the later. The second time window only occurs if the animal is trained on a stimulus strong enough to be remembered for a long period. Weak memories do not persist beyond 6–8 h and the second wave of glycoprotein synthesis does not occur. Thus the second wave may represent the molecular processes required for the alterations in synaptic configuration, by way of the adhesion molecules amongst others, required for the morphological changes in neuronal connectivity hypothesised to encode memory.