Operant Conditioning Task Research Articles

A novel behavioral apparatus for spontaneous exploration and operant conditioning of social information under spatial conditions in rats.

Evidence from the fields of evolutionary biology and neuroscience supports the theory that spatial cognition and social cognition share neural mechanisms. Rodent models are widely used to study either spatial or social cognition, but few studies have explored the interactions between the spatial and social cognitive domains due to the lack of appropriate paradigms. Our study introduces the Vertical Maze (VM), a novel behavioral apparatus designed to measure multiple aspects of spatial and social cognition. The VM features a standard 3-chamber maze positioned above multilevel columns allowing for the presentation of conspecifics at varying spatial distances and familiarity levels. This arrangement enables conspecifics to serve as discriminative stimuli for both social and spatial spontaneous and goal-oriented assessments. The three-dimensional design of the VM allows rats to use multisensory cues to judge distance, direction, and social identity of conspecifics. In the present study, we found that rats can 1) discriminate the spatial distance of conspecifics located below them in an operant conditioning task, and 2) discriminate social novelty when conspecifics are presented below at the near, middle, and far distances in a spontaneous exploration task. Critically, it was necessary for rats to explore all levels of the maze to perform these discriminations. This new method advances the field by permitting the presentation of social information (conspecifics) at different spatial distances. The use of conspecifics to serve as stimuli for both social and spatial discriminations allows more direct comparison of behavioral measures across these information domains. Importantly, the presentation of conspecifics as stimuli below the 3-chamber level of the maze engages auditory, visual, and olfactory systems, encouraging a robust multisensory representation of conspecifics presented at a distance. Our results confirm that the VM is an effective tool for studying both spatial and social cognition, facilitating the development of novel automated tasks in these areas. This new method opens new avenues for investigating the neural and cognitive foundations of spatial and social behavior, as well as for exploring the possibility of shared mechanisms across these cognitive domains.

Developmental trajectory of social reward motivation from early adolescence into adulthood in female and male Long-Evans rats.

Most studies of adolescent and adult behavior involved one age group of each, whereas the dynamic changes in brain development suggest that there may be behavioral flux in adolescence. In two studies, we investigated developmental changes in social reward motivation in female and male Long-Evans rats from prepuberty to early adulthood in a social operant conditioning task. Given the earlier onset of puberty in females than in males, we predicted the course of social reward development would differ between the sexes. Overall, the pattern of results from both studies suggests that the trajectory of social motivation across adolescence is characterized by upward and downward shifts that do not depend on the sex of the rats. During training, in both studies, the mean number of social gate openings and percentage of social gate openings was higher at P30 (prepubertal, early adolescence) and P50 (late adolescence) than at P40 (mid adolescence) and P70 (adulthood) irrespective of sex. Nevertheless, the specific age comparisons that were significant depended on the study. In both studies, P30 rats had greater levels of social motivation than did adults in accessing a social reward when increased effort was required (progressive ratio tests). In an extinction test, only P30 and P50 rats continued to show more nose-pokes at the previously social gate than at the nonsocial gate, suggesting resistance to extinction. The results highlight the importance of characterizing behavior at several timepoints in adolescence to understand the neural mechanisms, many of which show similar discontinuities as they develop across adolescence.

Social complexity affects cognitive abilities but not brain structure in a Poeciliid fish.

Some cognitive abilities are suggested to be the result of a complex social life, allowing individuals to achieve higher fitness through advanced strategies. However, most evidence is correlative. Here, we provide an experimental investigation of how group size and composition affect brain and cognitive development in the guppy (Poecilia reticulata). For 6 months, we reared sexually mature females in one of 3 social treatments: a small conspecific group of 3 guppies, a large heterospecific group of 3 guppies and 3 splash tetras (Copella arnoldi)-a species that co-occurs with the guppy in the wild, and a large conspecific group of 6 guppies. We then tested the guppies' performance in self-control (inhibitory control), operant conditioning (associative learning), and cognitive flexibility (reversal learning) tasks. Using X-ray imaging, we measured their brain size and major brain regions. Larger groups of 6 individuals, both conspecific and heterospecific groups, showed better cognitive flexibility than smaller groups but no difference in self-control and operant conditioning tests. Interestingly, while social manipulation had no significant effect on brain morphology, relatively larger telencephalons were associated with better cognitive flexibility. This suggests alternative mechanisms beyond brain region size enabled greater cognitive flexibility in individuals from larger groups. Although there is no clear evidence for the impact on brain morphology, our research shows that living in larger social groups can enhance cognitive flexibility. This indicates that the social environment plays a role in the cognitive development of guppies.

Learning and Neural Activity: Beyond Localization

Learning is classified into two types: "classical conditioning," which modifies simple reflexes, and "operant conditioning," which modifies complex voluntary behaviors. The neural circuits underlying these two types differ significantly. During the learning process of operant conditioning tasks, various changes in firing rate and firing synchrony of neurons can be observed across multiple brain regions. Additionally, neuronal firing rate and synchrony in several brain regions can be voluntarily controlled through operant conditioning. Consequently, it is evident that neurons in widespread brain regions have the potential for plastic changes to facilitate learning. It may be suggested that the learning of complex voluntary behaviors is underlined by widespread dynamic changes in neural activity and is not restricted to only a few brain regions.

The role of the GABAergic cells of the median raphe region in reinforcement-based learning

Learning and memory are important in everyday life as well as in pathological conditions. The median raphe region (MRR) contributes to memory formation; however, its precise role and the neurotransmitters involved have yet to be elucidated. To address this issue, we stimulated the MRR neurons of mice by chemogenetic technique and studied them in the operant conditioning and active avoidance tests. The virus carrier infected a variety of neuron types including both GABAergic and glutamatergic ones. Behavior was not influenced by stimulation. We hypothesize that the lack of effect was due to opposing effects exerted via GABAergic and glutamatergic neurons. Therefore, next we used VGAT-Cre mice that allowed the specific manipulation of MRR-GABAergic neurons. The stimulation did not affect behavior in the learning phase of the operant conditioning task, but increased reward preference and total responses when operant contingencies were reversed. The enhanced responsiveness might be a proclivity to impulsive behavior. Stimulation facilitated learning in the active avoidance test but did not affect reversal learning in this paradigm. Our findings suggest that MRR-GABAergic neurons are involved in both learning and reversal learning, but the type of learning that is affected depends on the task.

A comprehensive assay of social motivation reveals sex-specific roles of autism-associated genes and oxytocin

Social motivation is critical to the development of typical social functioning. Social motivation, specifically one or more of its components (e.g., social reward seeking or social orienting), could be relevant for understanding phenotypes related to autism. We developed a social operant conditioning task to quantify effort to access a social partner and concurrent social orienting in mice. We established that mice will work for access to a social partner, identified sex differences, and observed high test-retest reliability. We then benchmarked the method with two test-case manipulations. Shank3B mutants exhibited reduced social orienting and failed to show social reward seeking. Oxytocin receptor antagonism decreased social motivation, consistent with its role in social reward circuitry. Overall, we believe that this method provides a valuable addition to the assessment of social phenotypes in rodent models of autism and the mapping of potentially sex-specific social motivation neural circuits.

A Pilot Study on Behavioural and Physiological Indicators of Emotions in Donkeys.

Recognizing animal emotions is critical to their welfare and can lead to a better relationship with humans and the environment, especially in a widespread species like the donkey, which is often prone to welfare issues. This study aims to assess the emotional response of donkeys through an operant conditioning task with two presumed different emotional contents. Specifically, a within-subject design including positive and negative conditions was conducted, collecting behavioural and physiological (heart rate variability and HRV) parameters. Facial expressions, postures, and movements were analysed by principal component analysis and behavioural diversity indexes (frequencies, activity budgets, richness, Shannon and Gini-Simpson). During the positive condition, both ears were held high and sideways (left: r = -0.793, p < 0.0001; right: r = -0.585, p = 0.011), while the ears were frontally erected (left: r = 0.924, p < 0.0001; right: r = 0.946, p < 0.0001) during the negative one. The latter was also associated with an increased tendency to walk (r = 0.709, p = 0.001), walk away (r = 0.578, p = 0.012), more frequent changes in the body position (VBody position = 0, p = 0.022), and greater behavioural complexity (VGini-Simpson Index = 4, p = 0.027). As for HRV analysis, the root mean square of successive beat-to-beat differences (rMSSD) was significantly lower after the negative condition. These non-invasive parameters could be considered as possible indicators of donkeys' emotional state.

Noise exposure in early adulthood causes age-dependent and brain region-specific impairments in cognitive function.

Hearing loss is a chronic health condition that affects millions of people worldwide. In addition to age-related hearing impairment, excessive noise exposure is a leading cause of hearing loss. Beyond the devastating effects of hearing impairment itself, epidemiological studies have identified hearing loss as a major risk factor for age-related cognitive decline, including dementia. At present, we currently lack a full understanding of the brain regions and underlying molecular changes that are responsible for mediating the link between hearing loss and cognitive impairment across aging. In the present study, we exposed 6-month-old rats to an occupational-like noise (100 dB SPL, 4 h/day × 30 days) or sham exposure and investigated both hippocampal-dependent (i.e., spatial learning and memory, assessed using the Morris water maze) and striatal-dependent (i.e., visuomotor associative learning, assessed using an operant-conditioning task) cognitive function across aging at 7, 10, and 13 months of age. We also investigated brain region-specific changes in microglial expression following noise/sham exposure in order to assess the potential contribution of this cell type to noise-induced cognitive impairments. Consistent with human studies, the occupational-like noise exposure resulted in high-frequency hearing loss, evidenced by a significant increase in hearing thresholds at 20 kHz. Ultimately, our results suggest that not all higher-level cognitive tasks or their associated brain regions appear to be equally susceptible to noise-induced deficits during aging, as the occupational-like noise exposure caused an age-dependent deficit in spatial but not visuomotor associative learning, as well as altered microglial expression in the hippocampus but not the striatum. Interestingly, we found no significant relationships between spatial learning ability and the level of hearing loss or altered microglial density in the hippocampus following noise exposure, suggesting that other changes in the brain likely contribute to hippocampal-dependent cognitive dysfunction following noise exposure. Lastly, we found that a subset of younger animals also showed noise-induced deficits in spatial learning; findings which suggest that noise exposure may represent an increased risk for cognitive impairment in vulnerable subjects. Overall, our findings highlight that even a mild occupational-like noise exposure earlier in adulthood can have long lasting implications for cognitive function later in life.

Investigation of donkeys learning capabilities through an operant conditioning

Despite donkeys being involved in various activities with humans, their cognitive and learning abilities are still little known. A deeper understanding of their perceptive, cognitive and learning processes is, thus, necessary to preserve their well-being and establish a good human-animal bond. An operant conditioning task was applied to explore donkeys’ learning abilities. Nine out of 14 adult and non-working donkeys of both sexes fully completed a three-phases training procedure. In the first phase, animals could approach the manipulandum, a specifically designed cabin with a button on the front side. Thereafter, donkeys had to learn how to interact with the button to obtain a feed reward. To evaluate donkeys’ learning capabilities, two linear models were built: i) a fixed-effects model, exploring how the average between-pressures time (BPT) was affected by individual characteristics (i.e. age, sex, and donkeys’ height at the withers) and the training session; and ii) a mixed-effects model to evaluate the difference in the average between-pressures time among consecutive sessions (BPTcs) in the function of the animals’ characteristics, including the sessions-lags as the random effect. In the first model, all the explanatory variables resulted significantly associated with the BPT observed variability. Male donkeys presented a BPT significantly higher, increased by 23.14 s (S.E.=9.71, p = 0.003) compared to females of the same height and age. Age was significant with a positive coefficient (Est.=1.21, S.E.=0.55, p = 0.032). The ‘high’ height class estimate was significant (Est.=13.06, S.E.=6.26, p = 0.032), while no significant effects were identified between the ‘medium’ and ‘short’ and the ‘medium’ and ‘high’ height classes. Lastly, the variable ‘session’ was significant with a negative coefficient (Est.=−10.64, S.E.=1.56, p < 0.0001), indicating an increase in the average speed to perform the desired behaviour for each additional training session. In the second model, the variable ‘sex’ was the only predictor significantly associated with the BPTcs, indicating that the male group progressively improved performance time faster than females (Est.=−8.71, S.E.=4.19. p = 0.045). This pilot study: i) provides insights into donkeys learning abilities by applying an effective methodology for operant conditioning; ii) it highlights how intrinsic animal characteristics might affect asses training performances, although further points need to be explored in future research; iii) it confirms that feed represents an effective positive reinforcement in operant conditioning with donkeys. The development of appropriate handling and training methods, respectful of animals’ subjective experiences and based on positive practices, can improve donkeys’ welfare and their relationship with humans.

Neuronal Encoding of Emotional Valence and Intensity in the Monkey Amygdala.

Neuropsychological and neuroimaging studies have suggested that the primate amygdala plays an essential role in processing the emotional valence and intensity of visual stimuli, which is necessary for determining whether to approach or avoid a stimulus. However, the neuronal mechanisms underlying the evaluation of emotional value remain unknown. In the present study, we trained male macaque monkeys to perform an operant conditioning task in which fractal visual patterns were associated with three different amounts of air puff delivered to the cheek (negative) or liquid reward (positive). After confirming that the monkeys successfully differentiated the emotional valence and intensity of the visual stimuli, we analyzed neuronal responses to the stimuli in the amygdala. Most amygdala neurons conveyed information concerning the emotional valence and/or intensity of the visual stimuli, and the majority of those conveying information about emotional valence responded optimally to negative stimuli. Further, some amygdala neurons conveyed information related to both emotional valence and intensity, whereas a small portion conveyed information related to emotional intensity alone. These results indicate that the primate amygdala encodes both emotional valence and intensity, highlighting its important role in the avoidance of dangerous stimuli and animal survival.SIGNIFICANCE STATEMENT Evaluating the emotional value of visual stimuli is essential for animal survival, especially in primates. Emotional value is estimated from the emotional valence and intensity of stimuli, and evidence indicates that the amygdala is likely to play a major role in processing these types of information. To our knowledge, the current study is the first to examine the responses of neurons in the monkey amygdala to visual stimuli that differ in emotional valence and intensity simultaneously. Our data suggest that the amygdala plays an important role in the evaluation of emotional stimuli and in the decision to escape negative and harmful stimuli.

Motivational disturbances in rodent models of neuropsychiatric disorders.

Motivated behavior is integral to the survival of individuals, continuously directing actions toward rewards or away from punishments. The orchestration of motivated behavior depends on interactions among different brain circuits, primarily within the dopaminergic system, that subserve the analysis of factors such as the effort necessary for obtaining the reward and the desirability of the reward. Impairments in motivated behavior accompany a wide range of neuropsychiatric disorders, decreasing the patients’ quality of life. Despite its importance, motivation is often overlooked as a parameter in neuropsychiatric disorders. Here, we review motivational impairments in rodent models of schizophrenia, depression, and Parkinson’s disease, focusing on studies investigating effort-related behavior in operant conditioning tasks and on pharmacological interventions targeting the dopaminergic system. Similar motivational disturbances accompany these conditions, suggesting that treatments aimed at ameliorating motivation levels may be beneficial for various neuropsychiatric disorders.

Characterizing the transcriptionally‐activated ensembles recruited by cocaine in the nucleus accumbens

While significant effort has been made to understand the neural basis of addiction, it remains unclear exactly how the brain controls motivated behaviors and how drugs of abuse alter these neural systems to control drug taking and seeking. The goal of this study was to understand how cocaine and associated stimuli are encoded in the brain and how neuronal activation in response to cocaine acts to drive drug seeking. In a given brain region, only a small percentage of cells are activated to any stimulus ‐ termed an “ensemble”. Here we aimed to determine if ensembles transcriptionally activated by cocaine in the nucleus accumbens (NAc) – a brain region central to reward encoding ‐ were capable of driving motivated behaviors in isolation. Using transgenic animals that allowed for the temporally specific tagging of ensembles with optical tools, we were able to record from, identify, and manipulate the neural ensembles that are selectively activated by cocaine experience. Using conditional viruses in combination with these transgenic mice, we expressed opsins in cocaine‐ or saline‐ (for control) activated neuronal ensembles within the NAc. A fiberoptic was placed above the region and animals performed operant conditioning tasks where a nosepoke triggered a laser pulse that would reactivate or inhibit cocaine‐activated ensembles to determine their role in motivated behaviors. Mice were highly motivated to optically reactivate the ensembles that were activated by the first cocaine experience, highlighted by their high rates of responding for reactivation. Repeated cocaine injections in these animals did not change the reinforcing efficacy of ensemble activation, suggesting that the increases in motivated behavior that occur following repeated cocaine exposure may be due to the recruitment of a novel ensemble. Indeed, via immunohistochemistry, we find minimal overlap between the neurons activated by the first cocaine experience and the tenth. Using a similar approach as described above, we conditionally deleted the cocaine‐ or saline‐activated ensembles using a diphtheria toxin and had the mice perform cocaine self‐administration to assess changes in acquisition, performance, or intake of cocaine. We observed no changes in cocaine self‐administration behavior, suggesting that, while the cocaine ensemble is sufficient for reinforcing motivated behavior, it is not necessary for reinforcement learning about cocaine or cocaine‐associated cues. We also show that repeated cocaine exposure recruits a new population of neurons to guide behavior. Overall, we hypothesize that the neural ensemble initially recruited by cocaine may be important for reinforcement learning, but that other ensembles of neurons in the nucleus accumbens may be recruited in order to mediate the transition to addiction.

The rodent medial prefrontal cortex and associated circuits in orchestrating adaptive behavior under variable demands

Emerging evidence implicates rodent medial prefrontal cortex (mPFC) in tasks requiring adaptation of behavior to changing information from external and internal sources. However, the computations within mPFC and subsequent outputs that determine behavior are incompletely understood. We review the involvement of mPFC subregions, and their projections to the striatum and amygdala in two broad types of tasks in rodents: 1) appetitive and aversive Pavlovian and operant conditioning tasks that engage mPFC-striatum and mPFC-amygdala circuits, and 2) foraging-based tasks that require decision making to optimize reward. We find support for region-specific function of the mPFC, with dorsal mPFC and its projections to the dorsomedial striatum supporting action control with higher cognitive demands, and ventral mPFC engagement in translating affective signals into behavior via discrete projections to the ventral striatum and amygdala. However, we also propose that defined mPFC subdivisions operate as a functional continuum rather than segregated functional units, with crosstalk that allows distinct subregion-specific inputs (e.g., internal, affective) to influence adaptive behavior supported by other subregions.

Effects of CSF1R inhibitor PLX3397 on reinforcement learning ability

PLX3397 is an orally administrable CSF1 receptor inhibitor that is expected to be a therapeutic agent for cell tumors such as tendon synovial giant cell tumors. Administration of high doses of PLX3397 is known to eliminate microglia selectively and may affect neuronal functions via the loss of microglia. The elimination of microglia has been known to affect exploratory tasks involving aversive stimuli. However, the effect of the microglial elimination on the ability of reward-based reinforcement learning has been unknown. In this study, C57BL6 mice were administrated with PLX3397 for three weeks and were tested with a 5-armed bandit task (5-ABT). 5-ABT is an exploratory operant conditioning task in which each of the five choices has a different reward probability. The task contains three subtasks: one in which all choices had a reward probability of 30% (ALL), a subtask in which only one choice had a reward probability of 50% and the rest had a reward probability of 0% (BIT), and a subtask in which the correct choice in BIT was reversed and four options had a reward probability of 30% (REV). As a result, there was no significant difference in the entropy of choice (exploration pattern) in the subtask ALL between the PLX3397-treated group and the control group. There was no significant difference between the groups in the number of steps required for the BIT and REV subtasks to exceed 50% correct. We also estimated the learning rates of the mice by fitting the behavioral sequences to a Q-learning model and found no significant differences between the groups. These results suggest that microglial elimination has no significant impact on reinforcement learning ability.

Return of the Righting Reflex Does Not Portend Recovery of Cognitive Function in Anesthetized Rats.

As the number of individuals undergoing general anesthesia rises globally, it becomes increasingly important to understand how consciousness and cognition are restored after anesthesia. In rodents, levels of consciousness are traditionally captured by physiological responses such as the return of righting reflex (RORR). However, tracking the recovery of cognitive function is comparatively difficult. Here we use an operant conditioning task, the 5-choice serial reaction time task (5-CSRTT), to measure sustained attention, working memory, and inhibitory control in male and female rats as they recover from the effects of several different clinical anesthetics. In the 5-CSRTT, rats learn to attend to a five-windowed touchscreen for the presentation of a stimulus. Rats are rewarded with food pellets for selecting the correct window within the time limit. During each session we tracked both the proportion of correct (accuracy) and missed (omissions) responses over time. Cognitive recovery trajectories were assessed after isoflurane (2% for 1 h), sevoflurane (3% for 20 min), propofol (10 mg/kg I.V. bolus), ketamine (50 mg/kg I.V. infusion over 10 min), and dexmedetomidine (20 and 35 μg/kg I.V. infusions over 10 min) for up to 3 h following RORR. Rats were classified as having recovered accuracy performance when four of their last five responses were correct, and as having recovered low omission performance when they missed one or fewer of their last five trials. Following isoflurane, sevoflurane, and propofol anesthesia, the majority (63–88%) of rats recovered both accuracy and low omission performance within an hour of RORR. Following ketamine, accuracy performance recovers within 2 h in most (63%) rats, but low omission performance recovers in only a minority (32%) of rats within 3 h. Finally, following either high or low doses of dexmedetomidine, few rats (25–32%) recover accuracy performance, and even fewer (0–13%) recover low omission performance within 3 h. Regardless of the anesthetic, RORR latency is not correlated with 5-CSRTT performance, which suggests that recovery of neurocognitive function cannot be inferred from changes in levels of consciousness. These results demonstrate how operant conditioning tasks can be used to assess real-time recovery of neurocognitive function following different anesthetic regimens.

Deprivation of direct adult contact during development affects social representation in a songbird.

Social cognition involves a wide array of skills that are built largely through interactions with conspecifics and therefore depend upon early social experience. Motivation for social stimuli is a key feature of social behavior and an operant conditioning task showed that isolated wild-caught adult starlings (Sturnus vulgaris) are highly motivated to access pictures of other starlings. Here, we show that hand-raised adult starlings maintained in groups of peers throughout development but without any contact with adult models were not or only poorly motivated to access pictures of conspecifics. Moreover, they did not prefer pictures of starlings to pictures of landscapes, unlike birds wild-caught as adults. These results raise questions about the role of social experience during development, particularly with adult models, in the development of social motivation and of social representation in general.

Auditory hypersensitivity and processing deficits in a rat model of fragile X syndrome

Fragile X (FX) syndrome is one of the leading inherited causes of autism spectrum disorder (ASD). A majority of FX and ASD patients exhibit sensory hypersensitivity, including auditory hypersensitivity or hyperacusis, a condition in which everyday sounds are perceived as much louder than normal. Auditory processing deficits in FX and ASD also afford the opportunity to develop objective and quantifiable outcome measures that are likely to translate between humans and animal models due to the well-conserved nature of the auditory system and well-developed behavioral read-outs of sound perception. Therefore, in this study we characterized auditory hypersensitivity in a Fmr1 knockout (KO) transgenic rat model of FX using an operant conditioning task to assess sound detection thresholds and suprathreshold auditory reaction time-intensity (RT-I) functions, a reliable psychoacoustic measure of loudness growth, at a variety of stimulus frequencies, bandwidths, and durations. Male Fmr1 KO and littermate WT rats both learned the task at the same rate and exhibited normal hearing thresholds. However, Fmr1 KO rats had faster auditory RTs over a broad range of intensities and steeper RT-I slopes than WT controls, perceptual evidence of excessive loudness growth in Fmr1 KO rats. Furthermore, we found that Fmr1 KO animals exhibited abnormal perceptual integration of sound duration and bandwidth, with diminished temporal but enhanced spectral integration of sound intensity. Because temporal and spectral integration of sound stimuli were altered in opposite directions in Fmr1 KO rats, this suggests that abnormal RTs in these animals are evidence of aberrant auditory processing rather than generalized hyperactivity or altered motor responses. Together, these results are indicative of fundamental changes to low-level auditory processing in Fmr1 KO animals. Finally, we demonstrated that antagonism of metabotropic glutamate receptor 5 (mGlu5) selectively and dose-dependently restored normal loudness growth in Fmr1 KO rats, suggesting a pharmacologic approach for alleviating sensory hypersensitivity associated with FX. This study leverages the tractable nature of the auditory system and the unique behavioral advantages of rats to provide important insights into the nature of a centrally important yet understudied aspect of FX and ASD.

Sleep and conditioning of the siphon withdrawal reflex in Aplysia.

Sleep is essential for memory consolidation after learning as shown in mammals and invertebrates such as bees and flies. Aplysia californica displays sleep, and sleep in this mollusk was also found to support memory for an operant conditioning task. Here, we investigated whether sleep in Aplysia is also required for memory consolidation in a simpler type of learning, i.e. the conditioning of the siphon withdrawal reflex. Two groups of animals (Wake, Sleep, each n=11) were conditioned on the siphon withdrawal reflex, with the training following a classical conditioning procedure where an electrical tail shock served as the unconditioned stimulus (US) and a tactile stimulus to the siphon as the conditioned stimulus (CS). Responses to the CS were tested before (pre-test), and 24 and 48 h after training. While Wake animals remained awake for 6 h after training, Sleep animals had undisturbed sleep. The 24 h test in both groups was combined with extinction training, i.e. the extended presentation of the CS alone over two blocks. At the 24 h test, siphon withdrawal duration in response to the CS was distinctly enhanced in both Sleep and Wake groups with no significant difference between groups, consistent with the view that consolidation of a simple conditioned reflex response does not require post-training sleep. Surprisingly, extinction training did not reverse the enhancement of responses to the CS. On the contrary, at the 48 h test, withdrawal duration in response to the CS was even further enhanced across both groups. This suggests that processes of sensitization, an even simpler non-associative type of learning, contributed to the withdrawal responses. Our study provides evidence for the hypothesis that sleep preferentially benefits consolidation of more complex learning paradigms than conditioning of simple reflexes.

Information capacity and robustness of encoding in the medial prefrontal cortex are modulated by the bioavailability of serotonin and the time elapsed from the cue during a reward-driven task

Serotonin (5-HT) is a key neuromodulator of medial prefrontal cortex (mPFC) functions. Pharmacological manipulation of systemic 5-HT bioavailability alters the electrical activity of mPFC neurons. However, 5-HT modulation at the population level is not well characterized. In the present study, we made single neuron extracellular recordings in the mPFC of rats performing an operant conditioning task, and analyzed the effect of systemic administration of fluoxetine (a selective serotonin reuptake inhibitor) on the information encoded in the firing activity of the neural population. Chronic (longer than 15 days), but not acute (less than 15 days), fluoxetine administration reduced the firing rate of mPFC neurons. Moreover, fluoxetine treatment enhanced pairwise entropy but diminished noise correlation and redundancy in the information encoded, thus showing how mPFC differentially encodes information as a function of 5-HT bioavailability. Information about the occurrence of the reward-predictive stimulus was maximized during reward consumption, around 3 to 4 s after the presentation of the cue, and it was higher under chronic fluoxetine treatment. However, the encoded information was less robust to noise corruption when compared to control conditions.

Accumbal D1 and D2 medium spiny neurons control distinct learning parameters in complex behavior

Value-based decision-making is at the core of nearly all motivated behaviors and requires the ability to associate outcomes with specific actions and make adaptive decisions about future behavioral action. At the core of value-based decision-making and reinforcement is the nucleus accumbens (NAc), which is integrally involved in learning, selecting, and executing goal-oriented behaviors. The NAc is a heterogeneous population primarily composed of D1 and D2 medium spiny projection (MSN) neurons that are thought to have opposing roles in behavior, in which D1 MSNs promote reward and D2 MSNs promote aversion. By expressing channelrhodopsin selectively in D1- and D2- populations (using D1-Cre and A2A-Cre mice) in the NAc core, we show that mice will nose poke for optical self-stimulation of both cell types, suggesting D2-MSN activity is not inherently aversive. While optogenetic approaches give some information about how cellular activation can modulate behavior, they eliminate the temporally specific neural activity patterns that encode information in behaving animals. To understand how real-time activity in these populations is linked to behavioral execution, we expressed the genetically encoded calcium indicator (GCaMP6f) within D1 and D2 MSNs coupled with in vivo fiber photometry to record from these cell populations in awake and behaving animals during pavlovian and operant conditioning tasks. Utilizing complex reinforcement schedules that allow dissociation of stimulus value, outcome, cue learning, and action, we show that D1 MSNs respond to the presence and intensity of unconditioned stimuli – regardless of value. Conversely, D2 MSNs respond to a mismatch between what is expected and what is received and thus encode errors in prediction in a value-independent fashion. To establish a causal link between the information encoded within these neuronal populations and behavioral output, we next expressed an inhibitory opsin (halorhodopsin) selectively in either D1- or D2-MSNs to inhibit these cells at specific time points during a fear conditioning task. We show that inhibition at select time points, either by inhibiting D2-MSN activity during the predictive cue or D1-MSN activity during the shock outcome, disrupts learning of the association. We thus provide foundational evidence for the discrete aspects of information that are encoded within these cellular populations.