Disconjugate memory-guided saccades to disparate targets: evidence for 3D sensitivity

The saccadic system has been traditionally regarded as two-dimensional (horizontal, vertical) and basically conjugate in the two eyes. However, saccades to disparate targets (e.g., targets in real three-dimensional space that are located in different directions and at different distances) are naturally disconjugate. We report here that memory-guided saccades to a disparate target flashed 1 s earlier become disconjugate following repeated trials. After 15 min of repetition, the disconjugacy persists even when the target to be remembered is no longer disparate. This suggests fast memory-based learning. Learning, however, fails to occur if, during the repetition trials, the memory delay is 2 s. These findings suggest that the saccadic system has access to a 3D representation of targets and is gifted with 3D short-term memory and learning capacity. Received: 7 July 1997 / Accepted: 25 May 1998     

Organization and physiology of thesubstantia nigra

Summary A comprehensive review of the literature on the anatomy, electrophysiology and pharmacology of thesubstantia nigra is presented. A diagram is developed taking into account the interneuronal interactions of neurotransmitters and receptors that control firing rates and neurotransmitter releases. The central features of the diagram are a positive dopaminergic feedforward process and a positive feedback mechanism mediated by extrasynaptic substance P diffusing from striatal terminals to dopaminergic dendrites of thezona compacta neurons. This loop can enhance the transmission of information from thestriatum through thepars reticulata output neurons. The loop is controlled at the level of zona compacta neurons by a negative feedback supported by the dendritic release of dopamine and boosted by pedunculopontine activation mediated by muscarinic receptors. The output of the loop is controlled by two negative feedforward processes, both involving GABAergic striatonigral afferents. Application of the model to pharmacological studies of diverse behaviors including seizures, turning, and conditioned behaviors reveals unforseen relationships and may offer insights into, and directions for, further analysis of the mechanisms and functions involved.     

Immediate saccade amplitude disconjugacy induced by unequal images

We tested the ability of normal subjects to make changes in the conjugacy of their saccades. Subjects dichoptically viewed a grid the size of which was 10% larger in one eye. The grids were centred onto a flat screen at 57 cm or 1 m from the subject. Horizontal saccades immediately became larger in the eye viewing the larger grid. For some subjects this disconjugacy persisted even under subsequent monocular viewing. Such persistent changes occurred mainly in the field where the required disconjugacy was divergent for centrifugal saccades, convergent for centripetal saccades. Vertical saccades also developed compensatory disconjugacy; its amplitude was smaller but less variable. To explain these results we propose a fast associative learning mechanism that pairs peripheral disparity with saccades and is capable of producing saccade disconjugacy even in the absence of disparity. For horizontal saccades a secondary conditioning of monocular depth cues by the disparity would also be involved.     

Analysis of the avoidance learning deficit induced by the serotonin releasing compound p-chloroamphetamine

The effects of the serotonin-releasing compound p-chloroamphetamine (PCA, 2.5 mg/kg) on avoidance acquisition, retention and memory retrieval were examined in male Sprague-Dawley rats using a one-way active avoidance and a one-trial passive avoidance task. The drug was injected IP prior to training, following acquisition and prior to the retention test 24 hr after training using a state-dependent design. In the normal context situation pretraining administration of PCA markedly impaired active avoidance acquisition, but PCA-treated rats did not differ from controls in their retention performance when tested 24 hr after training. In the dark/light box test pretraining administration of PCA caused a dose-dependent impairment of both active and passive avoidance retention which could not be explained in terms of changes in locomotor activity or behavioural disinhibition at the time of testing or state-dependent retention. Post-training administration of PCA failed to affect avoidance retention in both tasks. The drug was found to impair memory retrieval in a dose- and time-dependent fashion in the one-way active but not in the passive avoidance test. Pretraining administration of PCA produced a progressive loss of passive and active avoidance performance at increasingly longer retention intervals. The present results suggest that serotonin has dual effects on processes underlying learning and memory involving effects on both associative and non-associative learning processes in the rat. The time-dependent loss of memory retention following 5-HT release indicates that serotonin has a role in the way information is processed in the brain.     

The locus coeruleus, norepinephrine, and memory in newborns

Use of learned odor cues by newborn rats is critical for pup survival. Rat pups acquire approach responses to maternal odors through an associative conditioning mechanism. This learned behavioral response is accompanied by a modification of olfactory bulb neural response patterns to the learned odor. Both the behavioral and neural reponse changes involved and require norepinephrine release in the olfactory bulb. The source of this norepinephrine is the locus coeruleus. It is proposed that the unique response properties of the locus coeruleus during the early postnatal period in the rat may facilitate acquisition of these critical early memories.     

Prefrontal unit activity during associative learning in the monkey

Summary Single unit activity was recorded from the dorsolateral prefrontal cortex of two monkeys which were trained on a stimulus-reward association task. The monkeys were trained on a reaction time task overlapped with a classical conditioning paradigm. The sequential events of the task were as follows: (1) lever pressing to start the trial; (2) presentation of a visual cue for 1 s; (3) delay period of 1 s; (4) imperative stimulus presentation; and (5) release of the lever by the animal. The visual cue signaled whether or not a drop of fruit juice would be given (its associative significance) for the animal's release response instead of signaling what response the animal should perform (its behavioral significance). In this task, the animal had to release the lever even on the trial where no juice was given in order to advance to the next trial. A total of 423 units showed activity changes in relation to one or more of the task events, such as the cue presentation, delay, release response and reward delivery. Among 313 units which showed cue-related activity changes, 179 units showed differential activity in relation to the different cues. A majority of them (Type M; n = 120) showed activity changes in relation to whether the cue indicated juice delivery or not, independent of its physical properties. The activity of 13 units (Type P) was related to the physical properties of the stimulus, and the activity of the remaining 46 units (Type MP) appeared to be related to both aspects of the stimulus. Sustained activity changes during the delay period were observed in 68 Type M, in 3 Type P and in 24 Type MP units. The results suggest that the prefrontal cortex plays important roles in the stimulus-reward association and that prefrontal units are involved in higher order information processing, extracting and retaining the associative significance of the stimulus independent of its physical properties.     

The electrophysiological correlates sustaining the retrieval of face-name associations: an ERP study

An ERP study on 9 healthy participants was carried out to temporally constrain the neural network proposed by Campanella et al. (2001) in a PET study investigating the cerebral areas involved in the retrieval of face-name associations. Three learning sessions served to familiarize the participants with 24 face-name associations grouped in 12 male/female couples. During EEG recording, participants were confronted with four experimental conditions, requiring the retrieval of previously learned couples on the basis of the presentation of name-name (NN), face-face (FF), name-face (NF), or face-name (FN) pairs of stimuli. The main analysis of this experiment consisted in the subtraction of the nonmixed conditions (NN and FF) from the mixed conditions (NF and FN). It revealed two main ERP components: a negative wave peaking at left parieto-occipital sites around 285 ms and its positive counterpart recorded at left centro-frontal electrodes around 300 ms. Moreover, a dipole modeling using three dipoles whose localization corresponded to the three cerebral areas observed in the PET study (left inferior frontal gyrus, left medial frontal gyrus, left inferior parietal lobe) explained more than 90% of the variance of the results. The complementarity between anatomical and neurophysiological techniques allowed us to discuss the temporal course of these cerebral activities and to propose an interactive and original anatomo-temporal model of the retrieval of face-name associations.     

Altered EphA5 mRNA expression in rat brain with a single methamphetamine treatment

Methamphetamine is a potent and indirect dopaminergic agonist which can cause chronic brain dysfunctions including drug abuse, drug dependence and drug-induced psychosis. Methamphetamine is known to trigger molecular mechanisms involved in associative learning and memory, and thereby alter patterns of synaptic connectivity. The persistent risk of relapse in methamphetamine abuse, dependence and psychosis may be caused by such alterations in synaptic connectivity. EphA5 receptors constitute large families of tyrosine kinase receptor and are expressed almost exclusively in the nervous system, especially in the limbic structures. Recent studies suggest EphA5 to be important in the topographic projection, development, and plasticity of limbic structures, and to be involved in dopaminergic neurotransmission. We used in situ hybridization to examine whether methamphetamine alters EphA5 mRNA expression in the brains of adult male Wister rats. EphA5 mRNA was widely distributed in the medial frontal cortex, cingulate cortex, piriform cortex, hippocampus, habenular nucleus and amygdala. Compared to baseline expression at 0 h, EphA5 mRNA was significantly decreased (by 20%) in the medial frontal cortex at 24 h, significantly increased (by 30%) in the amygdala at 9 and 24 h, significantly but transiently decreased (by 30%) in the habenular nucleus at 1 h after a single injection of methamphetamine. Methamphetamine did not change EphA5 mRNA expression in the cingulate cortex, piriform cortex or hippocampus. Our results that methamphetamine altered EphA5 mRNA expression in rat brain suggest methamphetamine could affect patterns of synaptic connectivity, which might be responsible for methamphetamine-induced chronic brain dysfunctions.     

HOPFIELD网络的联想记忆性质

本文首先给出了有关联想记忆的一系列概念，通过计算机模拟验证了网络的某些固有性质，同时指出了它的固有问题，并仿真研究了Ｈｏｐｆｉｅｌｄ网络中参数间的关系。