Contextual effects in the perception of quinine HCl/NaCl mixtures

Stimulus distribution, stimulus spacing and stimulus range affectmean intensity ratings of solutions of unmixed tastants. Thepresent study compares contextual effects for mixture ratingswith those for unmixed stimuli: if ratings for mixed and unmixedstimuli are differentially affected by context, the degree ofmixture suppression inferred from the responses is context-dependent.Bitterness intensity ratings for unmixed quinine and quinineHCl/NaCl mixtures were not differentially affected by shiftsin stimulus frequency distribution. The subjects' tendency tobe consistent in their responses to identical stimuli, resultsin a transfer of stimulus context of previous sessions to thenext.     

Context-dependent encoding of fear and extinction memories in a large-scale network model of the basal amygdala

The basal nucleus of the amygdala (BA) is involved in the formation of context-dependent conditioned fear and extinction memories. To understand the underlying neural mechanisms we developed a large-scale neuron network model of the BA, composed of excitatory and inhibitory leaky-integrate-and-fire neurons. Excitatory BA neurons received conditioned stimulus (CS)-related input from the adjacent lateral nucleus (LA) and contextual input from the hippocampus or medial prefrontal cortex (mPFC). We implemented a plasticity mechanism according to which CS and contextual synapses were potentiated if CS and contextual inputs temporally coincided on the afferents of the excitatory neurons. Our simulations revealed a differential recruitment of two distinct subpopulations of BA neurons during conditioning and extinction, mimicking the activation of experimentally observed cell populations. We propose that these two subgroups encode contextual specificity of fear and extinction memories, respectively. Mutual competition between them, mediated by feedback inhibition and driven by contextual inputs, regulates the activity in the central amygdala (CEA) thereby controlling amygdala output and fear behavior. The model makes multiple testable predictions that may advance our understanding of fear and extinction memories.     

Dopamine neurons can represent context-dependent prediction error

Midbrain dopamine (DA) neurons are thought to encode reward prediction error. Reward prediction can be improved if any relevant context is taken into account. We found that monkey DA neurons can encode a context-dependent prediction error. In the first noncontextual task, a light stimulus was randomly followed by reward, with a fixed equal probability. The response of DA neurons was positively correlated with the number of preceding unrewarded trials and could be simulated by a conventional temporal difference (TD) model. In the second contextual task, a reward-indicating light stimulus was presented with the probability that, while fixed overall, was incremented as a function of the number of preceding unrewarded trials. The DA neuronal response then was negatively correlated with this number. This history effect corresponded to the prediction error based on the conditional probability of reward and could be simulated only by implementing the relevant context into the TD model.     

Regularities of context-dependent codon bias in eukaryotic genes

Nucleotides surrounding a codon influence the choice of this particular codon from among the group of possible synonymous codons. The strongest influence on codon usage arises from the nucleotide immediately following the codon and is known as the N₁ context. We studied the relative abundance of codons with N₁ contexts in genes from four eukaryotes for which the entire genomes have been sequenced: Homo sapiens, Drosophila melanogaster, Caenorhabditis elegans and Arabidopsis thaliana. For all the studied organisms it was found that 90% of the codons have a statistically significant N₁ context-dependent codon bias. The relative abundance of each codon with an N₁ context was compared with the relative abundance of the same 4mer oligonucleotide in the whole genome. This comparison showed that in about half of all cases the context-dependent codon bias could not be explained by the sequence composition of the genome. Ranking statistics were applied to compare context-dependent codon biases for codons from different synonymous groups. We found regularities in N₁ context-dependent codon bias with respect to the codon nucleotide composition. Codons with the same nucleotides in the second and third positions and the same N₁ context have a statistically significant correlation of their relative abundances.     

Context-dependent memory: colour versus odour

An olfactory stimulus and a visual stimulus were employed in a context- dependent memory study using a prose passage as the to-be-remembered item. Ninety-five university students (aged 17-35 years) learned the passage of prose in the presence of one of the stimuli and were then asked to recall the passage with the original context either reinstated or not reinstated. The results revealed a significant context-dependent memory effect for the olfactory cue but not for the visual cue. They demonstrate support for the effectiveness of odours as context cues and it is suggested that context-dependent memory processes may underlie the formation and retrieval of odour-evoked autobiographical memories.      

Evolutionary Lability of Context-Dependent Codon Bias in Bacteria

In bacteria, synonymous codon usage can be considerably affected by base composition at neighboring sites. Such context-dependent biases may be caused by either selection against specific nucleotide motifs or context-dependent mutation biases. Here we consider the evolutionary conservation of context-dependent codon bias across 11 completely sequenced bacterial genomes. In particular, we focus on two contextual biases previously identified in Escherichia coli; the avoidance of out-of-frame stop codons and AGG motifs. By identifying homologues of E. coli genes, we also investigate the effect of gene expression level in Haemophilus influenzae and Mycoplasma genitalium. We find that while context-dependent codon biases are widespread in bacteria, few are conserved across all species considered. Avoidance of out-of-frame stop codons does not apply to all stop codons or amino acids in E. coli, does not hold for different species, does not increase with gene expression level, and is not relaxed in Mycoplasma spp., in which the canonical stop codon, TGA, is recognized as tryptophan. Avoidance of AGG motifs shows some evolutionary conservation and increases with gene expression level in E. coli, suggestive of the action of selection, but the cause of the bias differs between species. These results demonstrate that strong context-dependent forces, both selective and mutational, operate on synonymous codon usage but that these differ considerably between genomes. Received: 6 May 1999 / Accepted: 29 October 1999     

A neurocomputational model for the processing of conflicting information in context-dependent decision tasks

Context-dependent computation is a relevant characteristic of neural systems, endowing them with the capacity of adaptively modifying behavioral responses and flexibly discriminating between relevant and irrelevant information in a stimulus. This ability is particularly highlighted in solving conflicting tasks. A long-standing problem in computational neuroscience, flexible routing of information, is also closely linked with the ability to perform context-dependent associations. Here we present an extension of a context-dependent associative memory model to achieve context-dependent decision-making in the presence of conflicting and noisy multi-attribute stimuli. In these models, the input vectors are multiplied by context vectors via the Kronecker tensor product. To outfit the model with a noisy dynamic, we embedded the context-dependent associative memory in a leaky competing accumulator model, and, finally, we proved the power of the model in the reproduction of a behavioral experiment with monkeys in a context-dependent conflicting decision-making task. At the end, we discuss the neural feasibility of the tensor product and made the suggestive observation that the capacities of tensor context models are surprisingly in alignment with the more recent experimental findings about functional flexibility at different levels of brain organization.

     

Contextual modulation of V1 receptive fields depends on their spatial symmetry

The apparent receptive field characteristics of sensory neurons depend on the statistics of the stimulus ensemble—a nonlinear phenomenon often called contextual modulation. Since visual cortical receptive fields determined from simple stimuli typically do not predict responses to complex stimuli, understanding contextual modulation is crucial to understanding responses to natural scenes. To analyze contextual modulation, we examined how apparent receptive fields differ for two stimulus ensembles that are matched in first- and second-order statistics, but differ in their feature content: one ensemble is enriched in elongated contours. To identify systematic trends across the neural population, we used a multidimensional scaling method, the Procrustes transformation. We found that contextual modulation of receptive field components increases with their spatial extent. More surprisingly, we also found that odd-symmetric components change systematically, but even-symmetric components do not. This symmetry dependence suggests that contextual modulation is driven by oriented On/Off dyads, i.e., modulation of the strength of intracortically-generated signals. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

A Feedback Model of Attention and Context Dependence in Visual Cortical Networks

We have modeled biologically realistic neural networks that may be involved in contextual modulation of stimulus responses, as reported in the neurophysiological experiments of Motter (1994a, 1994b) (Journal of Neuroscience, 14:2179–2189 and 2190–2199). The networks of our model are structured hierarchically with feedforward, feedback, and lateral connections, totaling several thousand cells and about 300,000 synapses. The contextual modulation, arising from attention cues, is explicitly modeled as a feedback signal coming from the highest-order cortical network. The feedback signal arises from mutually inhibitory neurons with different stimulus preferences. Although our model is probably the simplest one consistent with available anatomical and physiological evidence and ignores the complexities that may exist in high-level cortical networks such as the prefrontal cortex, it reproduces the experimental results quite well and offers some guidance for future experiments. We also report the unexpected observation of 40 Hz oscillations in the model.     

State-dependency in C. elegans

Memory and the expression of learned behaviors by an organism are often triggered by contextual cues that resemble those that were present when the initial learning occurred. In state-dependent learning, the cue eliciting a learned behavior is a neuroactive drug; behaviors initially learned during exposure to centrally acting compounds such as ethanol are subsequently recalled better if the drug stimulus is again present during testing. Although state-dependent learning is well documented in many vertebrate systems, the molecular mechanisms underlying state-dependent learning and other forms of contextual learning are not understood. Here we demonstrate and present a genetic analysis of state- dependent adaptation in Caenorhabditis elegans. C. elegans normally exhibits adaptation, or reduced behavioral response, to an olfactory stimulus after prior exposure to the stimulus. If the adaptation to the olfactory stimulus is acquired during ethanol administration, the adaptation is subsequently displayed only if the ethanol stimulus is again present. cat-1 and cat-2 mutant animals are defective in dopaminergic neuron signaling and are impaired in state dependency, indicating that dopamine functions in state-dependent adaptation in C. elegans.     

Startle responding and context conditioning. Naloxone pretreatment and stimulus intensity

The possibility that acoustic startle stimuli could support a conditional response (freezing) to contextual stimuli was investigated. Rats were exposed to three acoustic startle stimuli on the first day, and one on the second day. On day 1, 20 rats received naloxone pretreatment and another 20 received saline (placebo) pretreatment. Half of each group received a high-intensity acoustic stimulus, the other half a low-intensity acoustic stimulus. Both the higher stimulus intensity and the naloxone pretreatment led to greater freezing behavior during the 3-minute test period before the single startle stimulus on day 2. These findings support the notion that increased actual or perceived intensity of the acoustic startle stimulus increases conditioning to contextual stimuli as indexed by freezing behavior.     

Contextual cues are not unique for motor learning: Task-dependant switching of feedback controllers

The separation of distinct motor memories by contextual cues is a well known and well studied phenomenon of feedforward human motor control. However, there is no clear evidence of such context-induced separation in feedback control. Here we test both experimentally and computationally if context-dependent switching of feedback controllers is possible in the human motor system. Specifically, we probe visuomotor feedback responses of our human participants in two different tasks—stop and hit—and under two different schedules. The first, blocked schedule, is used to measure the behaviour of stop and hit controllers in isolation, showing that it can only be described by two independent controllers with two different sets of control gains. The second, mixed schedule, is then used to compare how such behaviour evolves when participants regularly switch from one task to the other. Our results support our hypothesis that there is contextual switching of feedback controllers, further extending the accumulating evidence of shared features between feedforward and feedback control.     

Context dependence, ancestral misidentification, and spurious signatures of natural selection

Population genetic analyses often use polymorphism data from one species, and orthologous genomic sequences from closely related outgroup species. These outgroup sequences are frequently used to identify ancestral alleles at segregating sites and to compare the patterns of polymorphism and divergence. Inherent in such studies is the assumption of parsimony, which posits that the ancestral state of each single nucleotide polymorphism (SNP) is the allele that matches the orthologous site in the outgroup sequence, and that all nucleotide substitutions between species have been observed. This study tests the effect of violating the parsimony assumption when mutation rates vary across sites and over time. Using a context-dependent mutation model that accounts for elevated mutation rates at CpG dinucleotides, increased propensity for transitional versus transversional mutations, as well as other directional and contextual mutation biases estimated along the human lineage, we show (using both simulations and a theoretical model) that enough unobserved substitutions could have occurred since the divergence of human and chimpanzee to cause many statistical tests to spuriously reject neutrality. Moreover, using both the chimpanzee and rhesus macaque genomes to parsimoniously identify ancestral states causes a large fraction of the data to be removed while not completely alleviating problem. By constructing a novel model of the context-dependent mutation process, we can correct polymorphism data for the effect of ancestral misidentification using a single outgroup.     

Situational Specificity in Alpine-marmot Alarm Communication

We studied the degree to which alpine marmot (Marmota marmota L.) alarm calls function as communication about specific external stimuli. Alpine marmots emit variable alarm calls when they encounter humans, dogs, and several species of aerial predators. The first part of the study involved observations and manipulations designed to document contextual variation in alarm calls. Alarm calls varied along several acoustic parameters, but only along one that we examined, the number of notes per call, was significantly correlated with the type of external stimulus. Marmots were more likely to emit single-note alarm calls as their first or only call in response to an aerial stimulus, and multiple-note alarm calls when first calling to a terrestrial stimulus. This relationship was not without exceptions; there was considerable variation in the number of notes they emitted to both aerial and terrestrial stimuli, and a single stimulus type — humans — elicited a wide range of acoustic responses. The second part of the study involved playing back three types of alarm calls to marmots and observing their responses. Marmots did not have overtly different responses to the three types of played-back alarm calls. Our results are consistent with the hypotheses that: 1. Alarm calls do not refer to specific external stimuli; 2. Alarm calls function to communicate the degree of risk a caller experiences; and 3. Alarm calls require additional contextual cues to be properly interpreted by conspecifics.     

Contextual modulation of visuomotor associations in bumble-bees (Bombus terrestris)

We performed the following experiment to investigate whether contextual cues can prevent interference during the acquisition of potentially competing visuomotor associations. In the bumble-bee (Bombus terrestris). Bees discriminated between horizontally and vertically orientated gratings of black and white stripes in order to reach a feeder and between different diagonally orientated gratings to gain access to their nest. Once bees were well trained on these two tasks, the discrimination task at the nest was changed so that bees had to distinguish between horizontal and vertical gratings at both sites. Whereas they still approached the horizontal grating to reach food, they now had to approach a vertical grating in order to return to their nest. The new task was learnt rapidly. Errors at the feeder did not increase during or after the acquisition of this potentially competing behaviour at the nest entrance. For a brief period during acquisition, bees showed some hesitation at the feeder and hovered for slightly longer before choosing between horizontals and verticals. After acquisition was complete, bees showed a slight increase in their preference for verticals over a more ambiguous stimulus of an array of dots. These findings are consistent with the hypothesis that different contextual signals are associated with approaching the nest or approaching the feeder, and that these contextual signals facilitate learnt associations between orientation detectors and motor commands.     

Contextual control of rats' foraging behaviour in a radial maze

This study demonstrated that with experience rats were able to discriminate the background contextual cues in order to foraging correctly in an eight-arm radial maze if the background contexts signaled to the rats which of two bait patterns was in effect in that context. Four of the eight arms were context-dependent: two arms were baited in the "night" context, while the other two were baited in the "daytime" context. The remaining four arms were context-independent: three of them were never baited, while the remaining one was baited in both contexts. The rats gradually began to avoid the arms that were uniquely baited in the other context, suggesting a contextual or conditional control of arm selection. Interestingly, these rats also showed better performance in avoiding the never-baited arms, compared with control rats. Namely, disambiguating the incentive values of the four arms by contextual cues collaterally facilitated the context-independent arm performance. Another interesting finding of the present study is that the rats did not visit the always-baited arm earlier than the arms uniquely baited in that context.     

Effect of Isolation-Rearing on Conditioned Dopamine Release In Vivo in the Nucleus Accumbens of the Rat

Abstract: Intracerebral microdialysis in conjunction with HPLC coupled to electrochemical detection was used to investigate the effect of isolation-rearing in the rat on extracellular dopamine (DA) and its metabolites in vivo, in the shell region of the nucleus accumbens, in response to footshock and in relation to a conditioned emotional response. Male Lister hooded rats were reared from weaning for 6–8 weeks in either social isolation or groups of five. In the training phase, rats were exposed to a novel environment for 10 min where they experienced mild footshock. Footshock caused an immediate increase in basal extracellular DA levels in both rearing groups relative to control rats. However, the increase in extracellular DA was prolonged in the case of the isolation-reared rats and significantly greater than in group-reared rats. Exposure to the novel environment without shock (control groups) did not significantly alter basal extracellular DA in the nucleus accumbens shell; 140 min later rats were returned to the testing box (contextual stimulus) without receiving footshock. The contextual stimulus increased basal extracellular DA in the nucleus accumbens of both groups of rats with respect to controls; however, this increase was significantly greater and more prolonged in isolates. Extracellular levels of the metabolites 3,4-dihydroxyphenylacetic acid and homovanillic acid did not differ between isolation- and group-reared rats, and they were not significantly affected by either footshock or the contextual stimulus. These results suggest that exposure to footshock and a contextual stimulus are associated with increases in basal extracellular DA levels in the nucleus accumbens shell. The results also support evidence in favour of an isolation-induced enhancement in dopaminergic activity in the nucleus accumbens, which probably underlies aspects of the behavioural syndrome associated with isolation.