Neuronal models of cognitive functions

Understanding the neural bases of cognition has become a scientifically tractable problem, and neurally plausible models are proposed to establish a causal link between biological structure and cognitive function. To this end, levels of organization have to be defined within the functional architecture of neuronal systems. Transitions from any one of these interacting levels to the next are viewed in an evolutionary perspective. They are assumed to involve: (1) the production of multiple transient variations and (2) the selection of some of them by higher levels via the interaction with the outside world. The time-scale of these "evolutions" is expected to differ from one level to the other. In the course of development and in the adult this internal evolution is epigenetic and does not require alteration of the structure of the genome. A selective stabilization (and elimination) of synaptic connections by spontaneous and/or evoked activity in developing neuronal networks is postulated to contribute to the shaping of the adult connectivity within an envelope of genetically encoded forms. At a higher level, models of mental representations, as states of activity of defined populations of neurons, are discussed in terms of statistical physics, and their storage is viewed as a process of selection among variable and transient pre-representations. Theoretical models illustrate that cognitive functions such as short-term memory and handling of temporal sequences may be constrained by "microscopic" physical parameters. Finally, speculations are offered about plausible neuronal models and selectionist implementations of intentions.     

Neuronal oscillations and visual amplification of speech

It is widely recognized that viewing a speaker's face enhances vocal communication, although the neural substrates of this phenomenon remain unknown. We propose that the enhancement effect uses the ongoing oscillatory activity of local neuronal ensembles in the primary auditory cortex. Neuronal oscillations reflect rhythmic shifting of neuronal ensembles between high and low excitability states. Our hypothesis holds that oscillations are 'predictively' modulated by visual input, so that related auditory input arrives during a high excitability phase and is thus amplified. We discuss the anatomical substrates and key timing parameters that enable and constrain this effect. Our hypothesis makes testable predictions for future studies and emphasizes the idea that 'background' oscillatory activity is instrumental to cortical sensory processing.     

Neuronal manifestation of two-way connections in conditioning

Neuronal characteristics of two-way conditional connections were studied in chronic experiments with alert cats during different types of alimentary conditional reflexes. Single unit and multi-unit activities were chronically recorded in the visual and motor cortex and the lateral hypothalamic nucleus. Cross-correlational analysis of neuronal impulse trains was used to characterize the organization of neuron groups in brain structures involved in the formation of a conditional reflex. The experimental data showed an increase in the number of neuronal two-way connections after learning in all three investigated coordinations: visual-motor, visual-hypothalamic, and motor-hypothalamic. With visual-motor interneuronal coordination, the strengthening of two-way connections was due to the enhancement of connections from the motor to the visual neuron (backward connections); with visual- and motor-hypothalamic coordination, the number of interneuronal connections was equal in both directions. In all investigated coordinations, the analysis of the temporal parameters of the interactions between the neurons of different groups showed a dependence of the conditioning procedure on delays of up to 30 ms and a dependence of the motivational state on the interval range of 90–100 ms. The polyfunctional and polycomponent character of two-way neuronal connections in conditioning can be inferred from these data. Evaluation of the activity of neurons of different brain structures suggests the specific organization of intracortical and cortical-subcortical integrity in learning.     

Neuronal assemblies: necessity, signature and detectability

The ease with which highly developed brains can generate representations of a virtually unlimited diversity of perceptual objects indicates that they have developed very efficient mechanisms to analyse and represent relations among incoming signals. Here, we propose that two complementary strategies are applied to cope with these combinatorial problems. First, elementary relations are represented by the tuned responses of individual neurons that acquire their specific response properties (feature selectivity) through appropriate convergence of input connections in hierarchically structured feed-forward architectures. Second, complex relations that cannot be represented economically by the responses of individual neurons are represented by assemblies of cells that are generated by dynamic association of individual, featureselective cells. The signature identifying the responses of an assembly as components of a coherent code is thought to be the synchronicity of the respective discharges. The compatibility of this hypothesis is examined in the context of recent data on the dynamics of synchronization phenomena, the dependence of synchronization on central states and the relations between the synchronization behaviour of neurons and perception.     

Induced Cross-Modal Synaesthetic Experience Without Abnormal Neuronal Connections

ABSTRACT— Are the kinds of abnormal cross-modal interactions seen in synaesthesia or following brain damage due to hyperconnectivity between or within brain areas, or are they a result of lack of inhibition? This question is highly contested. Here we show that posthypnotic suggestion induces abnormal cross-modal experience similar to that observed in congenital grapheme-color synaesthesia. Given the short time frame of the experiment, it is unlikely that new cortical connections were established, so we conclude that synaesthesia can result from disinhibition between brain areas.     

Neuronal effects of auditory distraction on visual attention

Selective attention in the presence of distraction is a key aspect of healthy cognition. The underlying neurobiological processes, have not, however, been functionally well characterized. In the present study, we used functional magnetic resonance imaging to determine how ecologically relevant distracting noise affects cortical activity in 27 healthy adults during two versions of the visual Sustained Attention To Response Task (SART) that differ in difficulty (and thus attentional load). A significant condition (noise or silence) by task (easy or difficult) interaction was observed in several areas, including dorsolateral prefrontal cortex (DLPFC), fusiform gyrus (FG), posterior cingulate (PCC), and pre-supplementary motor area (PreSMA). Post hoc analyses of interaction effects revealed deactivation of DLPFC, PCC, and PreSMA during distracting noise under conditions of low attentional load, and activation of FG and PCC during distracting noise under conditions of high attentional load. These results suggest that distracting noise may help alert subjects to task goals and reduce demands on cortical resources during tasks of low difficulty and attentional load. Under conditions of higher load, however, additional cognitive resources may be required in the presence of noise.     

Neurosteroids in the Hippocampus: Neuronal Plasticity and Memory

The hippocampus, which is critically involved in learning and memory processes, is known to be a target for the neuromodulatory actions of steroid hormones produced by the adrenal glands and gonads. Much of the work of B.S. McEwen and collaborators has focused on the role of glucocorticosteroids and estrogen in modulating hippocampal plasticity and functions. In addition to hormones derived from the endocrine glands, cells in the hippocampus may be exposed to locally synthesized neurosteroids, including pregnenolone, dehydroepiandrosterone and their sulfated esters as well as progesterone and its reduced metabolites. In contrast to hormones derived from the circulation, neurosteroids have paracrine and/or autocrine activities. In the hippocampus, they have been shown to have trophic effects on neurons and glial cells and to modulate the activity of a variety of neurotransmitter receptors and ion channels, including type A gamma-aminobutyric acid, N-methyl-D-aspartate and sigma receptors and N- and L-type Ca2+ channels. There is accumulating evidence that some neurosteroids, in particular pregnenolone sulfate, have strong influences on learning and memory processes, most likely by regulating neurotransmission in the hippocampus. However, the hippocampus is not the only target for the mnesic effects of neurosteroids. Associated brain regions, the basal nuclei of the forebrain and the amygdaloid complex, are also involved. Some neurosteroids may thus be beneficial for treating age- or disease-related cognitive impairments.     

Neuronal changes in normal human aging and Alzheimer's disease

This article discusses age-related changes in brain weight, total number of cortical neurons, cortical dendrites, spine, and synapse density. The conclusion is that the present outlook is less grim than it was 30 years ago. Age-related reduction appears to be specific to brain region and cortical layer rather than a general feature. In addition, we describe a different pattern of changes that occur in Alzheimer's disease patients. This review concludes that the association cortices in particular are affected in aging and Alzheimer's disease and that the primary visual and somatosensory cortices are relatively spared.     

Neuronal deactivation is equally important for understanding emotional processing

In their analyses of the neural correlates of discrete emotionality, Lindquist et al. do not consider the numerous drawbacks to inferring psychological processes based on currently available cognitive neurometric technology. The authors also disproportionately emphasize the relevance of neuronal activation over deactivation, which, in our opinion, limits the scope and utility of their conclusions.     

智能发育与退化的神经网络基础

智能是人类最有特色的行为,正是通过智能活动,人类对自然环境进行了杰出的控制。智能与大脑皮层中大量的神经回路有关。本文讨论了智能赖以存在的基础-神经系统的发育,形成构筑和退化的过程,及其与智能的关系。神经生长锥的延长,分支,迁移到射靶运动,导致突触的形成和可塑性到神经联系的建立,关注局部神经回路到功能神经网络的形态构筑和功能特点,以及神经网络损伤对智能的影响。     

The association of perimenopausal mood disorders with other reproductive-related disorders

Data regarding the increased incidence of psychiatric illness during midlife in women are still conflicting. However, there is a growing consensus that certain groups of women may in fact be at higher risk for mood symptoms and psychiatric disorders during the perimenopausal transition. Mood symptoms during the perimenopause may be related to mood disorders during other periods of hormonal fluctuation throughout a woman's reproductive lifecycle. Elucidating these associations may advance the understanding of mood disorders during the perimenopausal transition. The epidemiology and treatment of perimenopausal mood symptoms compared with the epidemiology and treatment of mood disorders during the late luteal phase of the menstrual cycle, pregnancy, and postpartum. Common risk factors associated with mood disorders during these periods of hormonal changes or instability include poor lifestyle habits, a history of hormonally related mood disorders, stress and negative life events, ethnicity, and comorbidity. Reproductive-related mood disorders also are subject to an improvement in symptoms in response to treatment with selective serotonin reuptake inhibitors. As the morbidity associated with mood disorders during midlife may be quite significant, and as life expectancy continues to increase, recognition, prevention, and treatment of perimenopausal affective illness is becoming increasingly essential.     

Neuronal functional diversity and collective behaviors: a scientific case

A major issue in today’s neuroscience is how the brain complex and highly flexible organization emerges from its individual components. Robustness of neuronal properties with weak linkages between regulatory processes are suggested to account for the adaptive, tunable, multistable dynamics, the coding schemes and the complexity of neuronal functional (sub)systems. Interneurons and neurotransmitter diversity, resonance phenomena due to properties of the cell or network, time/frequency-dependent activation of dedicated neuronal assemblies, code- and frequency-specific oscillations interact in determining the brain functional setup and operations. Despite the scientific relevance, comprehensive theories are not yet available, but the scenario—however incomplete and incompletely characterized—is promising and warrants further investigation.     

Neuronal functional diversity and collective behaviors: a scientific case

A major issue in today’s neuroscience is how the brain complex and highly flexible organization emerges from its individual components. Robustness of neuronal properties with weak linkages between regulatory processes are suggested to account for the adaptive, tunable, multistable dynamics, the coding schemes and the complexity of neuronal functional (sub)systems. Interneurons and neurotransmitter diversity, resonance phenomena due to properties of the cell or network, time/frequency-dependent activation of dedicated neuronal assemblies, code- and frequency-specific oscillations interact in determining the brain functional setup and operations. Despite the scientific relevance, comprehensive theories are not yet available, but the scenario—however incomplete and incompletely characterized—is promising and warrants further investigation.