Behavioral and neuronal attributes of short- and long-term habituation in the crab Chasmagnathus

Investigations using invertebrate species have led to a considerable progress in our understanding of the mechanisms underlying learning and memory. In this review we describe the main behavioral and neuronal findings obtained by studying the habituation of the escape response to a visual danger stimulus in the crab Chasmagnathus granulatus. Massed training with brief intertrial intervals lead to a rapid reduction of the escape response that recovers after a short term. Conversely, few trials of spaced training renders a slower escape reduction that endures for many days. As predicted by Wagner’s associative theory of habituation, long-term habituation in the crab proved to be determined by an association between the contextual environment of the training and the unconditioned stimulus. By performing intracellular recordings in the brain of the intact animal at the same time it was learning, we identified a group of neurons that remarkably reflects the short- and long-term behavioral changes. Thus, the visual memory abilities of crabs, their relatively simple and accessible nervous system, and the recording stability that can be achieved with their neurons provide an opportunity for uncovering neurophysiological and molecular events that occur in identifiable neurons during learning.     

Influence of long-term sensitization on long-term habituation of the acoustic startle response in rats: central gray lesions, preexposure, and extinction

The relation between long-term decrements of the acoustic startle response in rats and the development of freezing behavior during habituation training was examined. Freezing behavior developed over the initial trials of habituation training, and the rate of long-term response decrements was found to be inversely related to the development of freezing. Manipulations (neurological or behavioral) that either reduced the level of freezing or retarded its development promoted startle response decrements. In Experiment 1, rats receiving electrolytic lesions of the ventrolateral periaqueductal gray demonstrated both accelerated long-term startle response decrements and retarded development of freezing behavior. In Experiment 2, preexposure to the startle apparatus (i.e., latent inhibition) accelerated long-term startle decrements and inhibited development of freezing. In Experiment 3, exposure to the startle apparatus following initial habituation training (i.e., extinction) reduced both freezing behavior and startle response amplitudes. The results are discussed in terms of the influence of Pavlovian fear conditioning on long-term habituation of the acoustic startle response.     

A new group-training procedure for habituation demonstrates that presynaptic glutamate release contributes to long-term memory in Caenorhabditis elegans

In the experiments reported here we have developed a new group-training protocol for assessing long-term memory for habituation in Caenorhabditis elegans. We have replicated all of the major findings of the original single-worm protocol using the new protocol: (1) distributed training produced long-term retention of training, massed training did not; (2) distributed training at long interstimulus intervals (ISIs) produced long-term retention, short ISIs did not; and (3) long-term memory for distributed training is protein synthesis-dependent as it could be blocked by heat shock during the inter-block interval. In addition, we have shown that long-term memory for habituation is graded, depending on the number of blocks of stimuli in training. The inter-block interval must be >40 min for long-term retention of training to occur. Finally, we have tested long-term memory for habituation training in a strain of worms with a mutation in a vesicular glutamate transporter in the sensory neurons that transduce tap (eat-4). The results from these eat-4 worms indicate that glutamate release from the sensory neurons has an important role in the formation of long-term memory for habituation.     

Latent memory for sensitization in Aplysia

In the analysis of memory it is commonly observed that, even after a memory is apparently forgotten, its latent presence can still be revealed in a subsequent learning task. Although well established on a behavioral level, the mechanisms underlying latent memory are not well understood. To begin to explore these mechanisms, we have used Aplysia, a model system that permits the simultaneous study of memory at the behavioral, cellular, and molecular levels. We first demonstrate that robust latent memory is induced by long-term sensitization training of the tail-elicited siphon withdrawal reflex. It is revealed by its ability to facilitate the subsequent induction of three mechanistically distinct temporal domains of sensitization memory: short-term, intermediate-term, and long-term memory. Under our training conditions, the latent memory persists for at least 2 d following the decay of original memory expression but appears to be gone by 4 d. Interestingly, we also find that latent memory is induced even in the absence of overt memory for the original training. These findings now permit the analysis of the cellular and molecular architecture of a common feature of learning and memory.     

Associative learning versus fear habituation as predictors of long-term extinction retention

Violation of unconditioned stimulus (US) expectancy during extinction training may enhance associative learning and result in improved long-term extinction retention compared to within-session habituation. This experiment examines variation in US expectancy (i.e., expectancy violation) as a predictor of long-term extinction retention. It also examines within-session habituation of fear-potentiated startle (electromyography, EMG) and fear of conditioned stimuli (CS) throughout extinction training as predictors of extinction retention. Participants (n?=?63) underwent fear conditioning, extinction and retention and provided continuous ratings of US expectancy and EMG, as well as CS fear ratings before and after each phase. Variation in US expectancy throughout extinction and habituation of EMG and fear was entered into a regression as predictors of retention and reinstatement of levels of expectancy and fear. Greater variation in US expectancy throughout extinction training was significantly predictive of enhanced extinction performance measured at retention test, although not after reinstatement test. Slope of EMG and CS fear during extinction did not predict retention of extinction. Within-session habituation of EMG and self-reported fear is not sufficient for long-term retention of extinction learning, and models emphasizing expectation violation may result in enhanced outcomes.     

Behavioral and genetic characterization of habituation using Caenorhabditis elegans

This review surveys the literature that investigates the behavioral characterization and cellular and molecular mechanisms of habituation using the model organism Caenorhabditis elegans. In 1990, C. elegans was first observed to show habituation to a non-localized mechanical tap. The parameters that govern this behavioral plasticity in C. elegans were subsequently characterized, which lead to the important hypothesis that habituation is mediated by multiple mechanisms. Many tools are available to C. elegans researchers that allow for relatively easy genetic manipulation. This has lead to a number of recent genetic studies that have begun to identify key genes and molecules that play a role in the mechanisms of habituation. Some of these genes include a vesicular glutamate transporter, a glutamate receptor subunit, a dopamine receptor and downstream intracellular signaling molecules, such as G proteins and kinases. Some of these genes only affect certain parameters of habituation, but not others supporting the hypothesis that multiple mechanisms mediate habituation. The field of research has also led to the dissection of different phases of memory (short-term vs. long-term memory for habituation), which are triggered by different training paradigms. The differences in mechanism between these various forms of memory are also beginning to be revealed.     

Generalization of Habituation and Intrinsic Sensitization in the Leech

Using the shortening reflex of the medicinal leech Hirudo medicinalis we examined stimulus generalization of habituation learning. Preparations received mechanosensory stimulus at two positions on the leech body wall, one site used to carry out habituation training and a second novel site to test for generalization of habituation. After training, the specific mechanosensory neurons activated by each stimulus were assessed using intracellular recordings. As expected, the closer the two sites were to each other, the greater the degree of generalization of habituation at the novel site and the more sensory cells were shared. However, a form of behavioral facilitation was observed at the trained site that resembled behavioral sensitization, but differed from the standard sensitization process in several respects. (1) Facilitation was induced by stimulation of the novel site before habituation training at the trained site, although the stimulus intensity at the novel site was equivalent to the training stimuli and was not the strong, noxious stimuli that normally induce sensitization. (2) The magnitude of the facilitating effect was proportional to the proximity of the novel and trained stimulation sites. (3) Although behavior at the trained site was facilitated, behavior at the novel site was habituated, indicating that the induced behavioral facilitation did not generalize throughout the animal, as normally occurs during sensitization, but was limited to a single stimulus–response pathway.     

Feeding behavior of Aplysia: a model system for comparing cellular mechanisms of classical and operant conditioning

Feeding behavior of Aplysia provides an excellent model system for analyzing and comparing mechanisms underlying appetitive classical conditioning and reward operant conditioning. Behavioral protocols have been developed for both forms of associative learning, both of which increase the occurrence of biting following training. Because the neural circuitry that mediates the behavior is well characterized and amenable to detailed cellular analyses, substantial progress has been made toward a comparative analysis of the cellular mechanisms underlying these two forms of associative learning. Both forms of associative learning use the same reinforcement pathway (the esophageal nerve, En) and the same reinforcement transmitter (dopamine, DA). In addition, at least one cellular locus of plasticity (cell B51) is modified by both forms of associative learning. However, the two forms of associative learning have opposite effects on B51. Classical conditioning decreases the excitability of B51, whereas operant conditioning increases the excitability of B51. Thus, the approach of using two forms of associative learning to modify a single behavior, which is mediated by an analytically tractable neural circuit, is revealing similarities and differences in the mechanisms that underlie classical and operant conditioning.     

Habituation in children within a behavior suppression paradigm

Using an instrumental response suppression paradigm, Experiment 1 demonstrated habituation and long-term retention of habituation to a tone stimulus in third-grade children, but specificity of habituation was not obtained when a new tone stimulus was presented. In Experiment 2 first graders showed specificity of habituation to auditory stimuli on the response suppression measure whereas fifth graders did not. However, both age groups evidenced specificity when a skin conductance measure was employed. In addition, specificity of habituation was not affected by amount of training at either age level. Experiment 3 demonstrated specificity of habituation in fifth graders on the response suppression measure when a cross-modality stimulus change was introduced. Implications of these data for analyses of habituation and traditional learning are discussed.     

NMDA receptors and voltage-dependent calcium channels mediate different aspects of acquisition and retention of a spatial memory task

Activity dependent calcium entry into neurons can initiate a form of synaptic plasticity called long-term potentiation (LTP). This phenomenon is considered by many to be one possible cellular mechanism underlying learning and memory. The calcium entry that induces this phenomenon can occur when N-methyl-D-aspartate receptors (NMDARs) and/or voltage-dependent calcium channels (VDCCs) are activated. While much is known about synaptic plasticity and the mechanisms that are triggered by activation of these two Ca(2+) channels, it is unclear what roles they play in learning. To better understand the role activation of these channels may play in learning we systemically administered pharmacological antagonists to block NMDARs, VDCCs, or both during training trials and retention tests in a radial arm maze task. Wistar rats injected with the NMDAR antagonist MK-801 (0.1mg/kg) were impaired in the acquisition of this task. In contrast, rats injected with verapamil (10mg/kg), an antagonist to VDCCs, acquired the task at the same rate as control animals, but were impaired on a 10-day retention test. A group of animals injected with both antagonists were unable to learn the task. The results suggest that each of the calcium channels and the processes they trigger are involved in a different stage of memory formation or expression.     

The genetic versus pharmacological invalidation of the cannabinoid CB(1) receptor results in differential effects on 'non-associative' memory and forebrain monoamine concentrations in mice

The endocannabinoid CB(1) receptor has been implicated in the inhibitory control of learning and memory. In the present experiment, we compared the behavioral response of CB(1) receptor knockout mice (CB(1)R(-/-)) with animals administered CB(1) receptor antagonist/inverse agonist SR141716A (rimonabant; 3 mg/kg IP, 30 min pre-trial) in terms of acquisition and retention of a habituation task and changes in cerebral monoamines. The results can be summarized as follows: (i) the acute and chronic invalidation of the CB(1) receptor resulted in an increase of behavioral habituation during the first exposure to an open field, indicative of enhanced acquisition of the task; (ii) CB(1)R(-/-) mice, but not rimonabant-treated animals, showed enhanced retention of the habituation task when re-tested 48 h and 1 week subsequent to the first exposure to the open field, respectively; (iii) the facilitation of retention of the habituation task in CB(1)R(-/-) mice was accompanied by a selective and site-specific increase in serotonin activity in hippocampus; and (iv) rimonabant-treated animals displayed 'antidepressant-like' neurochemical alterations of cerebral monoamines, that is, most parameters of monoaminergic activity were increased especially in dorsal striatum and hippocampus. Taken together, the present findings demonstrate that the genetic disruption of the CB(1) receptor gene can cause an improvement of behavioral habituation, which is considered to represent a form of 'non-associative' learning. Furthermore, our data support the assumption of a rimonabant-sensitive cannabinoid receptive site that is different from the 'classical' CB(1) receptor and which, under physiological conditions, might be involved in the inhibitory control of the acquisition but not retention of non-associative learning tasks.     

Interaction between consecutive learnings: inhibitory avoidance and habituation

Rats were submitted to step-down inhibitory avoidance training and to habituation of a rearing response to a tone with a 2-h interval between the two tasks, and were tested for retention of both tasks on the next day. When animals were trained first in inhibitory avoidance and then in habituation, retention of the avoidance behavior was impaired. When the animals were trained first in the habituation task and then in the avoidance task, retention of the two tasks was normal. The same results were obtained regardless of the order in which the two tasks were presented on the day of testing. This asymmetrical influence of habituation training on inhibitory avoidance retention could be due either to cognitive or, more likely, to task-specific neurochemical interactions.     

From acquisition to consolidation: on the role of brain-derived neurotrophic factor signaling in hippocampal-dependent learning

One of the most rigorously investigated problems in modern neuroscience is to decipher the mechanisms by which experience-induced changes in the central nervous system are translated into behavioral acquisition, consolidation, retention, and subsequent recall of information. Brain-derived neurotrophic factor (BDNF) has recently emerged as one of the most potent molecular mediators of not only central synaptic plasticity, but also behavioral interactions between an organism and its environment. Recent experimental evidence indicates that BDNF modulates synaptic transmission and plasticity by acting across different spatial and temporal domains. BDNF signaling evokes both short- and long-term periods of enhanced synaptic physiology in both pre- and postsynaptic compartments of central synapses. Specifically, BDNF/TrkB signaling converges on the MAP kinase pathway to enhance excitatory synaptic transmission in vivo, as well as hippocampal-dependent learning in behaving animals. Emerging concepts of the intracellular signaling cascades involved in synaptic plasticity induced through environmental interactions resulting in behavioral learning further support the contention that BDNF/TrkB signaling plays a fundamental role in mediating enduring changes in central synaptic structure and function. Here we review recent literature showing the involvement of BDNF/TrkB signaling in hippocampal-dependent learning paradigms, as well as in the types of cellular plasticity proposed to underlie learning and memory.     

Place learning in the Morris water task: making the memory stick

Although the Morris water task has been used in hundreds of studies of place learning, there have been no systematic studies of retention of the place memory. We report that retention, as measured by selective search behavior on a probe trial, is excellent when the retention interval is short (5-10 min). However, performance rapidly deteriorates, so that by approximately 4 h the search is no longer selective. Additional experiments revealed that selective search at longer retention intervals was improved by inserting gaps between blocks of training trials, but this effect was a non-monotonic function of the interval separating trial blocks. Our experiments also revealed that the location of the first block of trials (Room A or Room B) was irrelevant to long-term retention. A memory modulation theoretical framework may provide a useful way to understand these findings.     

An analysis of behavioral plasticity in male Caenorhabditis elegans.

Caenorhabditis elegans is a simple soil-dwelling nematode which has two sexes, hermaphrodite and male. The male C. elegans is differentiated from the hermaphrodite by the presence of 14 sensory structures in the tail. In this study, we compared the behavioral responses of males and hermaphrodites to head-touch and to tap. We hypothesized that the anatomical difference in sensory structures might result in behavioral differences in the reversal response to vibratory stimulation (a tap to the side of the holding dish). In the response to increasing intensities of tap, both sexes showed an increase in response magnitude, with the males showing larger responses than hermaphrodites. In addition, the male was shown to be capable of simple nonassociative learning: it demonstrated habituation and recovery from habituation in a similar manner as the hermaphrodite. Tail-touch-induced inhibition of the reversal response appeared to be similar in males and hermaphrodites. The evidence suggests that the touch withdrawal circuit in hermaphrodites is also present in the male C. elegans, and that the subtle differences in response to tap seen in males may result from the additional sensory receptors of the copulatory bursa of the tail. It seems clear from these studies that these structures do not play a key role in the male worm's response to tap.     

Infant visual habituation

The use of visual habituation in the study of infant cognition and learning is reviewed. This article traces the history of the technique, underlying theory, and procedural variation in its measurement. In addition, we review empirical findings with respect to the cognitive processes that presumably contribute to habituation, studies of developmental course and long-term prediction, as well as recent attempts to address or explain the phenomenon of visual habituation through the use of mathematical or quantitative models. The review ends with an appeal for a return to the study of habituation per se as a valid measure of infant learning, rather than relegating the phenomenon to its use as a technique for familiarizing infants in procedures testing for discrimination or recognition.     

The evolutionary significance of habituation and sensitization across phylogeny: A behavioral homeostasis model

The phenomenon of habituation may be interpreted as a process that has evolved for filtering out iterative stimuli of little present relevance. That habituation is seen in aneural as well as neural organisms throughout phylogeny with remarkably similar characteristics suggests that its role is an important one in animal survival. If habituation is to be viewed as a process to filter out iterative stimuli that have no significant consequences, then how is sensitization to be viewed? One way of viewing these two behavioral changes, i.e. habituation and sensitization, is that they are homeostatic processes which optimize an organism’s likelihood of detecting and assessing the significance of a stimulus in a new iterative series or a change in it. If one views the level of initial responsiveness to a new stimulus as a function of an organism’s threshold just prior to stimulus occurrence, then “high responders” (i.e. those who initially react more strongly) are assumed to have a lower threshold for detecting and assessing the significance of this stimulus than are the “low responders” (i.e. those who initially react more weakly). Thus, high-responders would initially receive more sensory input and progressively decrease their responsiveness to a non-threatening stimulus (habituation). Likewise, initial low-responders would receive less sensory input followed by a decreased threshold and an increased response to the next stimulus occurrence (sensitization). The level of responsiveness achieved in both habituaters and sensitizers, as an asymptote is approached, is a balance between being too sensitive to an unimportant stimulus (and possibly missing other significant stimuli) and being too insensitive, and missing a change in the relevance of the present stimulus. These response changes can be taken as indices of the organism’s mechanisms for achieving an appropriate threshold level to an iterative stimulus in order to accurately assess its present significance and then eventually to asymptote at an optimal stable response level. This approach toward an asymptote is a behavioral homeostatic process that reflects the accumulated significance of the iterative stimulus at each occurrence. The purpose of adding “behavioral” to the term “homeostasis” is to extend the usual meaning of the concept from primarily internal processes to also include (a) iterative external stimulation, (b) the organism’s initial threshold to the initial stimulus as well as (c) the behavior which results from it. Since we are discussing organisms that range from intact, single-celled protozoa to intact mammals, as well as surgically simplified preparations, the termsstimulus, response andbehavior will be used broadly. While other investigators have focused on specific cellular mechanisms underlying habituation and sensitization in a given organism, this paper focuses on the adaptive significance of these two behavioral processes viewed across phylogeny.     

Habituation to repeated stress: get used to it

Habituation, as described in the landmark paper by Thompson et al. [Thompson, R. F., & Spencer, W. A. (1966). Habituation: A model phenomenon for the study of neuronal substrates of behavior. Psychological Review, 73(1), 16–43], is a form of simple, nonassociative learning in which the magnitude of the response to a specific stimulus decreases with repeated exposure to that stimulus. A variety of neuronal and behavioral responses have been shown to be subject to habituation based on the criteria presented in that paper. It has been known for several decades that the magnitude of hypothalamic–pituitary–adrenal (HPA) activation occurring in response to a stressor declines with repeated exposure to that same stressor. For some time this decline has been referred to as “habituation” in the stress neurobiology literature. However, how this usage compares to the definition proposed by Thompson and Spencer has not been systematically addressed. For this special issue, we review the stress neurobiology literature and examine the support available for considering declines in HPA response to repeated stress to be response habituation in the sense defined by Thompson and Spencer. We conclude that habituation of HPA activity meets many, but not all, important criteria for response habituation, supporting the use of this term within the context of repeated stress. However, we also propose that response habituation can, at best, only partially explain the phenomenon of HPA habituation, which also involves well-known negative feedback mechanisms, activation of broad stress-related neural circuitry and potentially more complex associative learning mechanisms.     

A one-trial inhibitory avoidance task to zebrafish: Rapid acquisition of an NMDA-dependent long-term memory

The behavioral tasks aiming to evaluate learning and memory mechanisms currently available to zebrafish (Danio rerio) involve long training sessions frequently along multiple days and are based on shuttle box or active-avoidance protocols, preventing a detailed analysis of cellular and molecular time-dependent processes involved in memory acquisition and consolidation. In order to explore zebrafish’s potential contribution to the characterization of the molecular machinery underlying learning and memory rapidly acquired and reliable paradigms are necessary. In this study we present a rapid and effective learning protocol in a single-trial inhibitory avoidance in zebrafish. In a simple apparatus, adult animals learned to refrain from swimming from a white into a dark compartment in order to avoid an electric shock during a single-trial training session that lasted less than 2 min. The resulting memory is robust, long-lasting and sensitive to NMDA-receptor antagonist MK-801 given in the tank water immediately after training. Experiments aiming to further characterize the events underlying memory formation, retrieval or extinction or those looking for cognitive profiling of mutants, neurotoxicological studies and disease models may benefit from this task, and together with complementary strategies available for zebrafish may significantly improve our current knowledge on learning and memory mechanisms.