Effects of the stimulus and chamber size on unlearned fear across development

Predator odors have been found to induce unconditioned fear in adult animals and provide the opportunity to study the mechanisms underlying unlearned and learned fear. Predator threats change across an animal's lifetime, as do abilities that enable the animal to learn or engage in different defensive behaviors. Thus, the objective of this study was to determine the combination of factors that successfully induce unlearned fear to predator odor across development. Infant, juvenile, adolescent, and adult rats were exposed to one of the three odor stimuli (control odor, cat urine, or cat fur) in either a small or large chamber. Though all ages displayed fear-related behavior to cat odors, differences were reflected only in freezing behavior and not, as expected, risk-assessment. Infant and juvenile animals also increased freezing to cat urine compared to the control odor, possibly because these age groups possess limited defensive options to cope with threat and so may respond with freezing to all predator stimuli. Unexpectedly, chamber size had no effect on either freezing or risk-assessment in this study. Once the parameters of unconditioned fear are understood, they can be exploited to develop a learning paradigm to predator odors that could be used in early life.     

Learning to learn: theta oscillations predict new learning, which enhances related learning and neurogenesis

Animals in the natural world continuously encounter learning experiences of varying degrees of novelty. New neurons in the hippocampus are especially responsive to learning associations between novel events and more cells survive if a novel and challenging task is learned. One might wonder whether new neurons would be rescued from death upon each new learning experience or whether there is an internal control system that limits the number of cells that are retained as a function of learning. In this experiment, it was hypothesized that learning a task that was similar in content to one already learned previously would not increase cell survival. We further hypothesized that in situations in which the cells are rescued hippocampal theta oscillations (3-12 Hz) would be involved and perhaps necessary for increasing cell survival. Both hypotheses were disproved. Adult male Sprague-Dawley rats were trained on two similar hippocampus-dependent tasks, trace and very-long delay eyeblink conditioning, while recording hippocampal local-field potentials. Cells that were generated after training on the first task were labeled with bromodeoxyuridine and quantified after training on both tasks had ceased. Spontaneous theta activity predicted performance on the first task and the conditioned stimulus induced a theta-band response early in learning the first task. As expected, performance on the first task correlated with performance on the second task. However, theta activity did not increase during training on the second task, even though more cells were present in animals that had learned. Therefore, as long as learning occurs, relatively small changes in the environment are sufficient to increase the number of surviving neurons in the adult hippocampus and they can do so in the absence of an increase in theta activity. In conclusion, these data argue against an upper limit on the number of neurons that can be rescued from death by learning.     

Learning characteristics and long-term memory in rhesus macaques

The use of entropy of choices distribution in studying the processes of learning and long-term memory in rhesus monkeys during differentiation of periodic and non-periodic visual stimuli, showed that the transition from the unlearned state to the learned one proceeds not gradually but in two stages; the first stage--the accumulation of information and externally random behaviour, the second stage--rapid transition to the learned behaviour. For "filters" selective to spatial frequency, the change of stimuli orientation leads to an increase of the behaviour entropy. For "filters" selective to the bar width, the entropy practically does not change. During prolonged storage of acquired information, in the long-term memory, its partial loss takes place leading to an increase of the behaviour entropy. The difference between the entropy determined directly after the learning and that determined again after a prolonged storage of the memorized habit characterizes the quantity of information lost during the storage period.     

A simple spatial alternation task for assessing memory function in zebrafish

A series of studies was initiated to examine learning and memory function in the zebrafish (Danio rerio) by using a simple spatial alternation paradigm for a food reward. Fish were fed on alternating sides of a divided fish tank, with a red card displayed on one side serving as a visual means of orientation. Although responses were recorded at cue (light tap on the tank), 5 s after cue (as food was delivered), and 5 s after food delivery, the learning test was choice of a correct side of the tank to receive food. Therefore, an accurate level of an animal's achievement of the spatial task was represented by responses at food delivery. Data collected from 11 separate experiments indicated that zebrafish learned to alternate for a food reward. Further, statistical analysis showed that the zebrafish learned the task in the first half of the experiment as exhibited by a calculated t1/2 of 13.9 trials. Zebrafish could recall the task after a short period of 10 days with no testing. The alternating behavior was extinguished by withholding the food reward. Thus, the spatial alternation task can be learned easily by zebrafish, and may be useful in addressing learning and memory functions in vertebrate animals using zebrafish as a model organism.     

The intensity of RNA synthesis in rat hippocampus during learning]

Intensity of hippocampal RNA synthesis was studied in albino rats on days when elementary links of the chain motor conditioned reflex with food reinforcement were formed. A gradual increase in the nuclear RNA synthesis intensity was found in both learned and unlearned rats from the 1st towards the 5th day. This increase was, however, higher in the learned rats. A decrease was recorded in the difference in the intensity of the hippocampal nuclear RNA synthesis between the learned and unlearned rats from the 1st to the 5th day; this is possibly connected with the formation of certain mechanisms of memory consolidation, since the learned and unlearned rats differed in this property only.     

Bilateral imprinting in the chicks of precocial birds in response to monocular presentation of an object

The paper deals with lateralization of memory of imprinted object in chickens of precocial birds after monocular imprinting. Significant preference is shown of the imprinted stimulus in situation of choice from "learned" and "unlearned" eye in 24 hours after imprinting. Lesion of the ventral hyperstriatum of "learned" hemisphere in 28-30 hours after learning retained the preference of the imprinted stimulus. Analogous lesions made directly after learning, eliminated this preference. Conclusion is made about bilateral imprinting at monocular stimulus presentation.     

Evidence for hippocampal role in place avoidance other than merely memory storage

Spatial navigation is used as a popular animal model of higher cognitive functions in people. The data suggest that the hippocampus is important for both storing spatial memories and for performing spatial computations necessary for navigation. Animals use multiple behavioral strategies to solve spatial tasks often using multiple memory systems. We investigated how inactivation of the rat hippocampus affects performance in a place avoidance task to determine if the role of the hippocampus in this task could be attributed to memory storage/retrieval or to the computations needed for navigation. Injecting tetrodotoxin (TTX) into both hippocampi impaired conditioned place avoidance, but after injecting only one hippocampus, the rats learned the place avoidance as well as without any injections. Retention of the place avoidance learned with one hippocampus was not impaired when the injection was switched to the hippocampus that had not been injected during learning. The result suggests that during learning, the hippocampus did not store the place avoidance memory.     

Implicit and Explicit Contributions to Object Recognition: Evidence from Rapid Perceptual Learning

The present study investigated implicit and explicit recognition processes of rapidly perceptually learned objects by means of steady-state visual evoked potentials (SSVEP). Participants were initially exposed to object pictures within an incidental learning task (living/non-living categorization). Subsequently, degraded versions of some of these learned pictures were presented together with degraded versions of unlearned pictures and participants had to judge, whether they recognized an object or not. During this test phase, stimuli were presented at 15 Hz eliciting an SSVEP at the same frequency. Source localizations of SSVEP effects revealed for implicit and explicit processes overlapping activations in orbito-frontal and temporal regions. Correlates of explicit object recognition were additionally found in the superior parietal lobe. These findings are discussed to reflect facilitation of object-specific processing areas within the temporal lobe by an orbito-frontal top-down signal as proposed by bi-directional accounts of object recognition.     

Neural aromatization accelerates the acquisition of spatial memory via an influence on the songbird hippocampus

Circulating estrogens affect the neural circuits that underlie learning and memory in several vertebrates via an influence on the hippocampus. In the songbird hippocampus local estrogen synthesis due to the abundant expression of aromatase may modulate hippocampal function including spatial memory performance. Here, we examined the effect of estradiol, testosterone, and dihydrotestosterone on the structure and function of the songbird hippocampus. Adult male zebra finches were castrated, implanted with one of these steroids or a blank implant, and trained on a spatial memory task. The rate of acquisition and overall performance on this task was recorded by direct observation. The size and density of cells in the hippocampus and its volume were measured. Estradiol-treated birds learned the task more rapidly than any other group. Although testosterone- and blank-implanted birds did learn the task, we found no evidence of learning in dihydrotestosterone-implanted subjects. Cells in the rostral hippocampus were larger in estradiol- and testosterone-treated birds relative to other groups. A corresponding decrease in the density of cells was apparent in estradiol-implanted subjects relative to all other groups. These data suggest that estradiol may accelerate the acquisition of a spatial memory task and increase the size of neurons in the rostral hippocampus. Since testosterone-mediated changes in acquisition and cell size were similar to those of estradiol, but not dihydrotestosterone, we conclude that neural aromatization of testosterone to estrogen is responsible for effects on the structure and function of the songbird hippocampus.     

Effects of post-weaning environment on learning in the rat

Rats were reared in three different kinds of post-weaning environment: (1) with litter-mates and a variety of playthings (objects/social), (2) without either litter-mates or playthings (no-objects/isolates), (3) with litter-mates, but without playthings (no-objects/social). The animals were subsequently tested on a variety of learning tasks. In one task, the rats had to remove an obstacle from an alley in order to enter a food compartment; subsequently they were required to remove the obstacle in a different way from the one they had learned. Another task was to open a door leading to a food compartment; when the rats and learned this, the floor of the apparatus was lowered so they had to reach the door by climbing a ladder. Object-deprived/isolates were slower than the objects/social group in the transfer phase of the above tasks, though not in original acquisition. Object-deprived/social animals were not inferior to an objects/social group. Isolates had a higher free feeding weight than social animals, were more active in an open field, and ran faster for food reward when deprived.     

Spatial learning induces predominant downregulation of cytosolic proteins in the rat hippocampus

Spatial learning is known to depend on protein synthesis in the hippocampus. Whereas the role of the hippocampus in spatial memory is established, the biochemical and molecular mechanisms underlying this process are poorly understood. To comprehend the complex pattern of protein expression induced by spatial learning, we analyzed alterations in the rat hippocampus proteome after 7 days of spatial learning in the Morris water maze. Forty Wistar rats were randomized into two groups. Animals of group A learned to localize a hidden platform in the water maze. Animals of group B served as controls and spent exactly the same time in the water maze as animals of group A. However, no platform was used in this test and the rats could not learn to localize the target. After the last trial, hydrophilic proteins from the hippocampus were isolated. A proteome-wide study was performed, based on two-dimensional gel electrophoresis and mass spectrometry. Compared with non-learning animals, 53 (70%) proteins were downregulated and 23 (30%) proteins were upregulated after 7 days in rats with spatial learning. The overall changes in protein expression, as quantified by the induction factor, ranged from -1.62 (downregulation to 62%) to 2.10 (upregulation by 110%) compared with controls (100%). Most identified proteins exhibit known functions in vesicle transport, cytoskeletal architecture, and metabolism as well as neurogenesis. These findings indicate that learning in the Morris water maze has a morphological correlate on the proteome level in the hippocampus.     

Zebra Finch Mates Use Their Forebrain Song System in Unlearned Call Communication

Unlearned calls are produced by all birds whereas learned songs are only found in three avian taxa, most notably in songbirds. The neural basis for song learning and production is formed by interconnected song nuclei: the song control system. In addition to song, zebra finches produce large numbers of soft, unlearned calls, among which “stack” calls are uttered frequently. To determine unequivocally the calls produced by each member of a group, we mounted miniature wireless microphones on each zebra finch. We find that group living paired males and females communicate using bilateral stack calling. To investigate the role of the song control system in call-based male female communication, we recorded the electrical activity in a premotor nucleus of the song control system in freely behaving male birds. The unique combination of acoustic monitoring together with wireless brain recording of individual zebra finches in groups shows that the neuronal activity of the song system correlates with the production of unlearned stack calls. The results suggest that the song system evolved from a brain circuit controlling simple unlearned calls to a system capable of producing acoustically rich, learned vocalizations.     

Enhancement of performances in a learning task in suckler calves after weaning and relocation: Motivational versus cognitive control?: A pilot study

Weaning in suckler calves influences performance in a learning task. The aim of the present study was to investigate whether the improved performance after weaning, including relocation, is due to differences in motivation for the reward or in learning abilities. Forty Aubrac calves were used; half of them were weaned from their dams at around eight months, the other half were weaned one month later. After weaning, calves were housed in groups of four in a new setting. From the day after weaning of the last group of calves, the animals were subjected to two tests: (1) an arena test, (2) a T-maze test where one arm led to either a social or a food reward. The T-maze test consisted of three sessions: in Session 1, trials were conducted until the animal acquired the task (i.e. did not take the unrewarded arm on three consecutive trials); in Session 2, the motivation for the reward was assessed via the walking time of the animal to reach the reward; in Session 3, the place of reward was reversed and the animals were trained until they acquired the new task.

Calves weaned for one day explored more (P < 0.05) and had lower heart rates during the arena test (P < 0.05) compared to the ones weaned for one month. During the T-maze test, calves weaned for one month versus one day did not differ in their capacities to learn the initial route (Session 1) or in their motivation for either the social or food reward (Session 2). Calves weaned for one day learned significantly faster (P < 0.05) the reversed route (Session 3) than calves weaned for one month. Hence, the better performances at reversal in the T-maze by calves that have just been weaned cannot be accounted for by a higher motivation for the reward. A better cognitive control of their behaviour due to a lower stress state is suggested by our results.     

Cross-Limb Interference during Motor Learning

It is well known that following skill learning, improvements in motor performance may transfer to the untrained contralateral limb. It is also well known that retention of a newly learned task A can be degraded when learning a competing task B that takes place directly after learning A. Here we investigate if this interference effect can also be observed in the limb contralateral to the trained one. Therefore, five different groups practiced a ballistic finger flexion task followed by an interfering visuomotor accuracy task with the same limb. Performance in the ballistic task was tested before the training, after the training and in an immediate retention test after the practice of the interference task for both the trained and the untrained hand. After training, subjects showed not only significant learning and interference effects for the trained limb but also for the contralateral untrained limb. Importantly, the interference effect in the untrained limb was dependent on the level of skill acquisition in the interfering motor task. These behavioural results of the untrained limb were accompanied by training specific changes in corticospinal excitability, which increased for the hemisphere ipsilateral to the trained hand following ballistic training and decreased during accuracy training of the ipsilateral hand. The results demonstrate that contralateral interference effects may occur, and that interference depends on the level of skill acquisition in the interfering motor task. This finding might be particularly relevant for rehabilitation.     

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.     

Estimation of mental effort in learning visual search by measuring pupil response

Perceptual learning refers to the improvement of perceptual sensitivity and performance with training. In this study, we examined whether learning is accompanied by a release from mental effort on the task, leading to automatization of the learned task. For this purpose, we had subjects conduct a visual search for a target, defined by a combination of orientation and spatial frequency, while we monitored their pupil size. It is well known that pupil size reflects the strength of mental effort invested in a task. We found that pupil size increased rapidly as the learning proceeded in the early phase of training and decreased at the later phase to a level half of its maximum value. This result does not support the simple automatization hypothesis. Instead, it suggests that the mental effort and behavioral performance reflect different aspects of perceptual learning. Further, mental effort would be continued to be invested to maintain good performance at a later stage of training.     

Ability of predatory native Australian fishes to learn to avoid toxic larvae of the introduced toad Bufo marinus

Of two native Australian fishes naïve to the introduced toad Bufo marinus most barramundi Lates calcarifer rapidly learned to avoid B. marinus tadpoles, while sooty grunter Hephaestus fuliginosus exhibited considerable intraspecific variation in their learning ability. Some sooty grunter learned to avoid tadpoles after only a few attacks, while other individuals continued to attack and reject tadpoles throughout the entire laboratory trials. Individuals of both species recognized and avoided tadpoles 1 day after their previous encounter. None of the fishes died during the trials. The observed variation in behavioural responses of fishes to B. marinus may be due to differences in (1) learning ability, (2) fish hunger levels, and or (3) tadpole palatability and toxicity. The results demonstrate that most barramundi and sooty grunter learn to avoid B. marinus tadpoles with minimal trauma. Consequently, it is anticipated that the toads are unlikely to have a significant negative impact on wild populations of these fishes through direct toxic effects.     

Effects of lateral hypothalamic stimulation on acquisition,reversal and extinction of a visual discriminative learning task

The present experiment was carried out to investigate the effects of lateral hypothalamic (LH) stimulation on a negatively reinforced complex learning task including acquisition, reversal and extinction of a visual discrimination in an Y-maze. Male swiss mice were stimulated 45 sec after each training session during 60 sec (group ST). The stimulation intensity administrated in post-session was that which produced a rate of 50 responses by min during intracranial self-stimulation (ICSS) testing carried out prior to the learning experiment. Three control groups were constituted by animals either submitted to ICSS testing but not stimulated after training session (group NST). The post-session stimulated (group I) or non-implanted (group NI). The post-session stimulation improved the learning performance of the animals, but this effect was significant only during reversal learning of the task. ICSS testing carried out before learning, as well as electrode implantation, had no effect by themselves on the acquisition of the visual discrimination task. Moreover, no sign of extinction was observed in any group tested. These results suggest that facilitating effects induced by LH stimulation may depend on the complexity of the task and that cues of the learning situation have to reach a minimum of salience in order for LH stimulation to be effective.     

Plasticity in the Rat Prefrontal Cortex: Linking Gene Expression and an Operant Learning with a Computational Theory

The plasticity in the medial Prefrontal Cortex (mPFC) of rodents or lateral prefrontal cortex in non human primates (lPFC), plays a key role neural circuits involved in learning and memory. Several genes, like brain-derived neurotrophic factor (BDNF), cAMP response element binding (CREB), Synapsin I, Calcium/calmodulin-dependent protein kinase II (CamKII), activity-regulated cytoskeleton-associated protein (Arc), c-jun and c-fos have been related to plasticity processes. We analysed differential expression of related plasticity genes and immediate early genes in the mPFC of rats during learning an operant conditioning task. Incompletely and completely trained animals were studied because of the distinct events predicted by our computational model at different learning stages. During learning an operant conditioning task, we measured changes in the mRNA levels by Real-Time RT-PCR during learning; expression of these markers associated to plasticity was incremented while learning and such increments began to decline when the task was learned. The plasticity changes in the lPFC during learning predicted by the model matched up with those of the representative gene BDNF. Herein, we showed for the first time that plasticity in the mPFC in rats during learning of an operant conditioning is higher while learning than when the task is learned, using an integrative approach of a computational model and gene expression.     

Cognition Enhancing Activity of Sulforaphane Against Scopolamine Induced Cognitive Impairment in Zebra Fish (<Emphasis Type="Italic">Danio rerio</Emphasis>)

Several epidemiological studies have shown that consumption of large quantities of vegetables especially cruciferous vegetables (Broccoli and Brussels sprouts) can protect against chronic diseases. Sulforaphane, an isothiocynate found in cruciferous vegetables has been demonstrated to have neuroprotective effects in several experimental paradigms. This study was undertaken to examine the effect of sulforaphane on cognitive impairment in zebra fish model using a novel method of fear conditioning. Initially, the normal behaviour of zebra fishes was studied in light-dark tank for 10 min daily for 10 days. Fishes were then divided into seven groups of twelve in each. Group I served as normal, group II served as fear conditioned control, group III and group IV were sulforaphane (25 µM/L) and piracetam (200 mg/L) treated respectively. Group V served as scopolamine (400 µM/L) induced memory impairment fishes. Group VI and VII were sulforaphane (25 µM/L) and piracetam (200 mg/L) treated scopolamine induced memory impairment groups respectively. In normal behavioural analysis, fishes preferred to stay in dark compartment. The average number of entries into the dark and time spent in dark were significantly more. Fishes in group II to VII were individually subjected to fear conditioning passive avoidance task and evaluated for learned task memory. It was observed that the average number of entries into dark and time spent in dark were significantly decreased. After exposure to respective treatment fishes in group III to VII were subjected to cognitive evaluation. There was no significant difference in cognition of group III and IV fishes exposed to sulforaphane and piracetam alone respectively. Fishes exposed to scopolamine showed a significant cognitive impairment. Sulforaphane exposure prior to scopolamine significantly retained the memory of learned task. These findings suggest that sulforaphane might be a promising therapeutic agent for cognitive enhancement in Alzheimer’s disease.