Conserved fMRI and LFP signals during new associative learning in the human and macaque monkey medial temporal lobe

We measured local field potential (LFP) and blood-oxygen-level-dependent (BOLD) functional magnetic resonance imaging (fMRI) in the medial temporal lobes of monkeys and humans, respectively, as they performed the same conditional motor associative learning task. Parallel analyses were used to examine both data sets. Despite significantly faster learning in humans relative to monkeys, we found equivalent neural signals differentiating new versus highly familiar stimuli, first stimulus presentation, trial outcome, and learning strength in the entorhinal cortex and hippocampus of both species. Thus, the use of parallel behavioral tasks and analyses in monkeys and humans revealed conserved patterns of neural activity across the medial temporal lobe during an associative learning task.     

A model of associative learning in the mushroom body

The mushroom body is a prominent invertebrate neuropil strongly associated with learning and memory. We built a high-level computational model of this structure using simplified but realistic models of neurons and synapses, and developed a learning rule based on activity dependent pre-synaptic facilitation. We show that our model, which is consistent with mushroom body Drosophila data and incorporates Aplysia learning, is able to both acquire and later recall CS-US associations. We demonstrate that a highly divergent input connectivity to the mushroom body and strong periodic inhibition both serve to improve overall learning performance. We also examine the problem of how synaptic conductance, driven by successive training events, obtains a value appropriate for the stimulus being learnt. We employ two feedback mechanisms: one stabilises strength at an initial level appropriate for an association; another prevents strength increase for established associations.     

A model of chemotaxis and associative learning in C. elegans

The nematode C. elegans has attracted a great deal of interest from the neuroscience community due to the simplicity of its nervous system, which in the hermaphrodite is composed of just 302 neurons. C. elegans is known to engage in a number of sophisticated behaviours such as chemo- and thermotaxis. Experimental work has shown that these behaviours can be modified by experience and that C. elegans is capable of associative learning. In this paper, we focus on the chemotactic response of C. elegans to sodium chloride mediated by the ASE sensory neurons. We construct a biophysical model of the ASEL and ASER neurons that captures the time course of the ASE responses in response to up- and down-steps in NaCl concentration. We use this model to show that the time course of the ASE responses provide sufficient temporal resolution to successfully drive chemotaxis in C. elegans via steering, pirouettes and control of final turn angle. We show that these different locomotion strategies are individually capable of driving chemotaxis and that by working together they produce the best chemotactic response. We find that there is a separation into upward and downward drives mediated by the left and right ASE neurons. We show that the connectivity from ASEL and ASER must be of opposite polarity and that ASER, and the concomitant ability to sense when the worm is moving down the gradient, is more important for chemotaxis than ASEL, findings that are consistent with existing modelling studies in the literature. Finally, we examine associative learning in the network and show that experimental data can be explained by changes that occur at either the synaptic or sensory neuron level, the choice of which has distinct consequences for network function.     

Neural circuits underlying imitation learning of hand actions: an event-related fMRI study

The neural bases of imitation learning are virtually unknown. In the present study, we addressed this issue using an event-related fMRI paradigm. Musically naive participants were scanned during four events: (1) observation of guitar chords played by a guitarist, (2) a pause following model observation, (3) execution of the observed chords, and (4) rest. The results showed that the basic circuit underlying imitation learning consists of the inferior parietal lobule and the posterior part of the inferior frontal gyrus plus the adjacent premotor cortex (mirror neuron circuit). This circuit, known to be involved in action understanding, starts to be active during the observation of the guitar chords. During pause, the middle frontal gyrus (area 46) plus structures involved in motor preparation (dorsal premotor cortex, superior parietal lobule, rostral mesial areas) also become active. Given the functional properties of area 46, a model of imitation learning is proposed based on interactions between this area and the mirror neuron system.     

Mental retardation in heterozygotes for the fragile-X mutation: evidence in favor of an X inactivation-dependent effect.

The still debated question of whether the expression of mental retardation in heterozygous carriers of the Martin-Bell syndrome is influenced by X inactivation has been investigated in a group of phase-known double heterozygotes for the FRA-X mutant and the G6PD Mediterranean variant. In these individuals, the number of somatic cells (fibroblasts or red cells) with an active FRA-X chromosome could be assessed through the G6PD phenotype at the single-cell level. The data reported indicate a significant inverse correlation between the IQ level (as measured by the Wechsler-Bellevue test) and the percentage of fibroblast cells with an FRA-X active chromosome. In contrast, no significant correlation was found when the IQ level and red cell data were compared, thus suggesting the occurrence of somatic selection against hematopoietic stem cells with an active FRA-X chromosome.     

Reconciling the influence of predictiveness and uncertainty on stimulus salience: a model of attention in associative learning

Theories of selective attention in associative learning posit that the salience of a cue will be high if the cue is the best available predictor of reinforcement (high predictiveness). In contrast, a different class of attentional theory stipulates that the salience of a cue will be high if the cue is an inaccurate predictor of reinforcement (high uncertainty). Evidence in support of these seemingly contradictory propositions has led to: (i) the development of hybrid attentional models that assume the coexistence of separate, predictiveness-driven and uncertainty-driven mechanisms of changes in cue salience; and (ii) a surge of interest in identifying the neural circuits underpinning these mechanisms. Here, we put forward a formal attentional model of learning that reconciles the roles of predictiveness and uncertainty in salience modification. The issues discussed are relevant to psychologists, behavioural neuroscientists and neuroeconomists investigating the roles of predictiveness and uncertainty in behaviour.     

Response inhibition during cue reactivity in problem gamblers: an fMRI study

Disinhibition over drug use, enhanced salience of drug use and decreased salience of natural reinforcers are thought to play an important role substance dependence. Whether this is also true for pathological gambling is unclear. To understand the effects of affective stimuli on response inhibition in problem gamblers (PRGs), we designed an affective Go/Nogo to examine the interaction between response inhibition and salience attribution in 16 PRGs and 15 healthy controls (HCs).Four affective blocks were presented with Go trials containing neutral, gamble, positive or negative affective pictures. The No-Go trials in these blocks contained neutral pictures. Outcomes of interest included percentage of impulsive errors and mean reaction times in the different blocks. Brain activity related to No-Go trials was assessed to measure response inhibition in the various affective conditions and brain activity related to Go trials was assessed to measure salience attribution.PRGs made fewer errors during gamble and positive trials than HCs, but were slower during all trials types. Compared to HCs, PRGs activated the dorsolateral prefrontal cortex, anterior cingulate and ventral striatum to a greater extent while viewing gamble pictures. The dorsal lateral and inferior frontal cortex were more activated in PRGs than in HCs while viewing positive and negative pictures. During neutral inhibition, PRGs were slower but similar in accuracy to HCs, and showed more dorsolateral prefrontal and anterior cingulate cortex activity. In contrast, during gamble and positive pictures PRGs performed better than HCs, and showed lower activation of the dorsolateral and anterior cingulate cortex.This study shows that gambling-related stimuli are more salient for PRGs than for HCs. PRGs seem to rely on compensatory brain activity to achieve similar performance during neutral response inhibition. A gambling-related or positive context appears to facilitate response inhibition as indicated by lower brain activity and fewer behavioural errors in PRGs.     

The internet as a research tool in the study of associative learning: an example from overshadowing

The present study aimed to replicate an associative learning effect, overshadowing, both in the traditional laboratory conditions and over the internet. The experimental task required participants to predict an outcome based on the presence of several cues. When a cue that was always trained together with a second cue was presented on isolation at test, the expectancy of the outcome was impaired, which revealed overshadowing. This experimental task was performed by undergraduate students (N=106) in the laboratory and by a different set of anonymous participants over the internet (N=91). Similar levels of overshadowing were obtained in both locations. These similarities show that web-delivered experiments can be used as a complement of traditional experiments.     

The evolution of associative learning: A factor in the Cambrian explosion

The Cambrian explosion is probably the most spectacular diversification in evolutionary history, and understanding it has been a challenge for biologists since the time of Darwin. We propose that one of the key factors that drove this great diversification was associative learning. Although the evolutionary emergence of associative learning required only small modifications in already existing memory mechanisms and may have occurred in parallel in several groups, once this type of learning appeared on the evolutionary scene, it led to extreme diversifying selection at the ecological level: it enabled animals to exploit new niches, promoted new types of relations and arms races, and led to adaptive responses that became fixed through genetic accommodation processes. This learning-based diversification was accompanied by neurohormonal stress, which led to an ongoing destabilization and re-patterning of the epigenome, which, in turn, enabled further morphological, physiological, and behavioral diversification. Our hypothesis combines several previous ideas about the dynamics of the Cambrian explosion and provides a unifying framework that includes both ecological and genomic factors. We conclude by suggesting research directions that would clarify the timing and manner in which associative learning evolved, and the effects it had on the evolution of nervous systems, genomes, and animal morphology.     

Neurophysiological changes and dynamics of bound calcium during the development of associative learning in Helix lucorum

Neurophysiological effects and the dynamics of the content of bound calcium (Ca-b) in command neurons (LP1 and RP1) of defensive behavior during food aversion conditioning are studied inHelix lucorum. In the case of associative learning, there arises in these cells both a response to the conditioned stimulus and a nonspecific facilitation of the reactions to sensory stimuli that is characteristic for the state of sensitization. A response to the presentation of a conditioned stimulus is detected approximately 30 min after the development of long-term sensitization. The use of three or more paired stimuli reveals the characteristic dynamics of the level of Ca-b, which correlates with the neurophysiological effects and differs from the changes in Ca-b content during the development of "pure" sensitization. It is thought that the command neuron of defense behavior exhibits inHelix differences of the molecular-cellular mechanisms lying at the basis of the development of associative learning and sensitization.P. K. Anokhin Institute of Normal Physiology, Russian Academy of Sciences, Moscow. I. P. Pavlov Institute of Physiology, Russian Academy of Sciences, St. Petersburg. Translated from Neirofiziologiya, Vol. 24, No. 6, pp. 691–701, November–December, 1992.     

Bartonella henselae infection in British Columbia: evidence for an endemic disease among humans

Human bartonellosis in North America is mainly associated with Bartonella henselae, and the availability of laboratory diagnostic tools has significantly heightened awareness of the spectrum of human disease that is caused by this bacterium. We detail herein examples of illness in a pediatric population which serve to confirm that B. henselae-associated disease exists in British Columbia. Seroprevalence studies among asymptomatic adults and among children with symptomatic respiratory illness of other causation demonstrated that 36.8% and 18.5% of sera, respectively, had IFA-IgG titres > or = 1:256. IFA-IgG titres did not vary significantly whether B. henselae ATCC 49793 or a local wild-type B. henselae isolate were used as substrate. An assessment of IgM response was consistent with the proposal that endemic seroprevalence is a function of past rather than recent exposure. Both clinical and serological studies are concordant in providing evidence that B. henselae is endemic in British Columbia.     

Stimulus coding in human associative learning: flexible representations of parts and wholes

An enduring theme for theories of associative learning is the problem of explaining how configural discriminations--ones in which the significance of combinations of cues is inconsistent with the significance of the individual cues themselves-are learned. One approach has been to assume that configurations are the basic representational form on which associative processes operate, another has tried in contrast to retain elementalism. We review evidence that human learning is representationally flexible in a way that challenges both configural and elemental theories. We describe research showing that task demands, prior experience, instructions, and stimulus properties all influence whether a particular problem is solved configurally or elementally. Lines of possible future theory development are discussed.     

Abnormal activity in the precuneus during time perception in Parkinson's disease: an fMRI study

Background

Parkinson''s disease (PD) patients are deficient in time estimation. This deficit improves after dopamine (DA) treatment and it has been associated with decreased internal timekeeper speed, disruption of executive function and memory retrieval dysfunction.

Methodology/Findings

The aim of the present study was to explore the neurophysiologic correlates of this deficit. We performed functional magnetic resonance imaging on twelve PD patients while they were performing a time reproduction task (TRT). The TRT consisted of an encoding phase (during which visual stimuli of durations from 5s to 16.6s, varied at 8 levels were presented) and a reproduction phase (during which interval durations were reproduced by a button pressing). Patients were scanned twice, once while on their DA medication (ON condition) and once after medication withdrawal (OFF condition). Differences in Blood-Oxygenation-Level-Dependent (BOLD) signal in ON and OFF conditions were evaluated. The time course of activation in the brain areas with different BOLD signal was plotted. There were no significant differences in the behavioral results, but a trend toward overestimation of intervals ≤11.9s and underestimation of intervals ≥14.1s in the OFF condition (p<0.088). During the reproduction phase, higher activation in the precuneus was found in the ON condition (p<0.05 corrected). Time course was plotted separately for long (≥14.1s) and short (≤11.9s) intervals. Results showed that there was a significant difference only in long intervals, when activity gradually decreased in the OFF, but remained stable in the ON condition. This difference in precuneus activation was not found during random button presses in a control task.

Conclusions/Significance

Our results show that differences in precuneus activation during retrieval of a remembered duration may underlie some aspects of time perception deficit in PD patients. We suggest that DA medication may allow compensatory activation in the precuneus, which results in a more accurate retrieval of remembered interval duration.     

Spatial arrangements and associative behavior of species in an in vitro oral biofilm model

The spatial arrangements and associative behavior of Actinomyces naeslundii, Veillonella dispar, Fusobacterium nucleatum, Streptococcus sobrinus, and Streptococcus oralis strains in an in vitro model of supragingival plaque were determined. Using species-specific fluorescence-labeled antibodies in conjunction with confocal laser scanning microscopy, the volumes and distribution of the five strains were assessed during biofilm formation. The volume-derived cell numbers of each strain correlated well with respective culture data. Between 15 min and 64 h, populations of each strain increased in a manner reminiscent of batch growth. The microcolony morphologies of all members of the consortium and their distributions within the biofilm were characterized, as were interspecies associations. Biofilms formed 15 min after inoculation consisted principally of single nonaggregated cells. All five strains adhered strongly to the saliva-conditioned substratum, and therefore, coadhesion played no role during the initial phase of biofilm formation. This observation does not reflect the results of in vitro coaggregation of the five strains, which depended upon the nature of the suspension medium. While the possibility cannot be excluded that some interspecies associations observed at later stages of biofilm formation were initiated by coadhesion, increase in bacterial numbers appeared to be largely a growth phenomenon regulated by the prevailing cultivation conditions.     

Disorders in learning and memory processes in the monkey model of Alzheimer's disease: the role of the cerebral cortex associative areas

Processes of novelty learning and keeping the results in Alzheimer's disease in two groups of rhesus-monkeys (three monkeys in each group), were studied: following neurotoxins administrati- on (I group) and saline administration (II group). In two months after the injections (the C1 stage), considerable differences between the groups were revealed in the task of differentiation among contour shapes. For the I group monkeys the learning was difficult: the correct decision making did not reach 85 %, and the probability of refusing to make a decision increased. For the II group monkeys the learning characteristics were not disturbed. In six months after the injections (the C3 stage) the differences between the groups in the task of differentiation among new stimuli (heads of two monkeys) remained at the same level. When studying characteristics of the operative memory associated with keeping the learning results achieved at the C1 stage, a considerable worsening of these characteristics was revealed: diminishing of the correct decision making probability at the C1 stage (actually to the level of 0.5), increase in the probability of refusing to make a decision. The structural-functional organization of interaction between sensory and cognitive processes in learning and keeping the information in the operative memory, is discussed in association with the control of motivation and attention system and the role of the cortex associative areas.     

Crossmodal processing of object features in human anterior intraparietal cortex: an fMRI study implies equivalencies between humans and monkeys

The organization of macaque posterior parietal cortex (PPC) reflects its functional specialization in integrating polymodal sensory information for object recognition and manipulation. Neuropsychological and recent human imaging studies imply equivalencies between human and macaque PPC, and in particular, the cortex buried in the intraparietal sulcus (IPS). Using functional MRI, we tested the hypothesis that an area in human anterior intraparietal cortex is activated when healthy subjects perform a crossmodal visuo-tactile delayed matching-to-sample task with objects. Tactile or visual object presentation (encoding and recognition) both significantly activated anterior intraparietal cortex. As hypothesized, neural activity in this area was further enhanced when subjects transferred object information between modalities (crossmodal matching). Based on both the observed functional properties and the anatomical location, we suggest that this area in anterior IPS is the human equivalent of macaque area AIP.     

Hebbian learning reconsidered: Representation of static and dynamic objects in associative neural nets

According to Hebb's postulate for learning, information presented to a neural net during a learning session is stored in the synaptic efficacies. Long-term potentiation occurs only if the postsynaptic neuron becomes active in a time window set up by the presynaptic one. We carefully interpret and mathematically implement the Hebb rule so as to handle both stationary and dynamic objects such as single patterns and cycles. Since the natural dynamics contains a rather broad distribution of delays, the key idea is to incorporate these delays in the learning session. As theory and numerical simulation show, the resulting procedure is surprisingly robust and faithful. It also turns out that pure Hebbian learning is by selection: the network produces synaptic representations that are selected according to their resonance with the input percepts.     

Collagen deposition in the subcutaneous tissue during wound healing in humans: a model evaluation

Wound healing encompasses coagulation, inflammation, angiogenesis, fibroplasia, contraction, epithelialisation and remodeling. A granulation tissue is produced following incision of tissue such as skin, abdominal wall or the gastrointestinal tract, and the strength of the wound is determined primarily by the collagen content early in the healing course. Few models are available to study wound healing in man. The percutaneous insertion of expanded poly-tetrafluoroethylene tubes (ePTFE) into the subcutaneous tissue has been an established model for 20 years. The procedure is performed using a local anesthesia. The model has a diameter of 2.5 mm, a length of 5-10 cm and a pore size of 90-120 microns which is substantially more than that of vascular grafts. The polymer accumulates granulation tissue, the architecture of which resembles that of a normal surgical wound. Previous studies on the use of the ePTFE model in wound healing research are summarized in detail. Histological and immunohistochemical analyses of the granulation tissue deposited in the model were undertaken. The content of amino acids following hydrolysis of the granulation tissue was determined applying spectrophotometric or HPLC assays. Collagen amounts accumulated in the model are expressed as hydroxyproline per length of ePTFE or per total protein. Following a study in rats we examined 85 healthy volunteers and 158 surgical patients in the studies. Higher contents of hydroxyproline were found 10 days after implantation as compared to 5 days with considerable inter-person variation. Regarding median values there was a 25% difference between two measurements performed on two distinct ePTFE tubes from the same person, and a 12% difference between values obtained from two different pieces of the same ePTFE. Higher accumulation levels of hydroxyproline did not result in higher variability. Deposition of proline in the model correlated closely to total protein content. The ePTFE and a modified PVA model were compared in surgical patients. No reproducible measurements of hydroxyproline deposition were obtained with the PVA model as opposed to the ePTFE model. It is concluded that the modified PVA model is inadequate for determination of collagen deposition in subcutaneous granulation tissue. We found no correlation between collagen deposition levels obtained with placement of the ePTFE model in the subcutaneous tissue of the arm and in an uncomplicated surgical wound of the groin in the same patient, respectively. Significantly higher collagen deposition levels in the model were found in the surgical wound. Conversely, there was a significant correlation between protein deposition levels obtained at the two sites. Patients undergoing minor surgery (groin hernia repair) did not differ from healthy non-traumatized volunteers as regards deposition of collagen in subcutaneous tissue of the arm, whereas patients subjected to major general surgery demonstrated a significant decline during the postoperative phase compared to a preoperative evaluation. This decline was enhanced in patients who had infectious complications. Non-smoking volunteers were found to specifically accumulate more collagen (median value 82%) than smokers matched for age and gender. Irrespective of the smoking status women accumulated significantly more collagen in the model than men. These findings were re-tested in a prospective series leading to the same conclusion. Matrix metalloproteinases (MMP-2 and MMP-9) were determined in wound fluid obtained from the subcutaneous cavities of herniotomy wounds 24 and 48 h after operation. A significant and inverse correlation was demonstrated between MMP-9 after 24 h and accumulation levels of collagen in the ePTFE tube 10 days after implantation in the wound. Finally, it was demonstrated that local application of granulocyte-macrophage colony-stimulating factor into the ePTFE model during implantation specifically and dose-dependently reduced the number of fibroblasts and deposition of collagen. The doses chosen for the experiments resulted in both a local and a systemic effect. It is concluded that the minimally invasive ePTFE model, despite a certain level of variability, presently provides one of the best possibilities of evaluation of the wound healing potential in both volunteers and patients under various conditions. We found the model convenient for the assessment of both matrix deposition during wound healing and the influence of several factors including demographic characteristics, trauma, tobacco smoking, drugs and tissue degrading components of the wound.