Multiple memory systems: the power of interactions

Two relatively simple theories of brain function will be used to demonstrate the explanatory power of multiple memory systems in your brain interacting cooperatively or competitively to directly or indirectly influence cognition and behaviour. The view put forth in this mini-review is that interactions between memory systems produce normal and abnormal manifestations of behaviour, and by logical extension, an understanding of these complex interactions holds the key to understanding debilitating brain and psychiatric disorders.     

Chronic stress impairs recall of extinction of conditioned fear

Chronic restraint stress produces retraction of apical dendrites of pyramidal neurons in medial prefrontal cortex. To begin to examine the functional significance of this dendritic reorganization, we assessed the effects of chronic restraint stress on a prefrontally mediated behavior, extinction of conditioned fear. After bar press training to obtain a baseline of activity against which to measure freezing, rats were either unstressed or stressed via placement in a plastic restrainer (3 h/day for 1 week). After an additional day of bar press training, rats underwent fear conditioning and extinction. Rats received five habituation trials to a 30-s tone (4.5 kHz, 80 db) followed by seven pairings of tone and footshock (500 ms, 0.5 mA). One hour later, rats received tone-alone extinction trials to criterion. The next day, rats received 15 additional extinction trials. Percent freezing was assessed during all phases of training. Stress did not significantly affect unconditioned responding to tone, acquisition of conditioned fear, or initial extinction, but significantly increased freezing on extinction day 2. Thus, consistent with the regressive dendritic changes seen in medial prefrontal cortex, one week of restraint stress specifically impaired recall of extinction, a pattern of deficits typical of animals with impaired medial prefrontal function.     

Response-specific sources of dual-task interference in human pre-motor cortex

It is difficult to perform two tasks at the same time. Such performance limitations are exemplified by the psychological refractory period (PRP): when participants make distinct motor responses to two stimuli presented in rapid succession, the response to the second stimulus is increasingly slowed as the time interval between the two stimuli is decreased. This impairment is thought to reflect a central limitation in selecting the appropriate response to each stimulus, but not in perceptually encoding the stimuli. In the present study, it was sought to determine which brain regions are specifically involved in response selection under dual-task conditions by contrasting fMRI brain activity measured from a response selection manipulation that increased dual-task costs, with brain activity measured from an equally demanding manipulation that affected perceptual visibility. While a number of parieto-frontal areas involved in response selection were activated by both dual-task manipulations, the dorsal pre-motor cortex, and to a lesser extent the inferior frontal cortex, were specifically engaged by the response selection manipulation. These results suggest that the pre-motor cortex is an important neural locus of response selection limitation under dual-task situations.     

The neuronal basis of bimanual coordination: recent neurophysiological evidence and functional models

Recent physiological studies of the neuronal processes underlying bimanual movements provide new tests for earlier functional models of bimanual coordination. The recently acquired data address three conceptual areas: the generalized motor program (GMP), intermanual crosstalk and dynamic systems models. To varying degrees, each of these concepts has aspects that can be reconciled with experimental evidence. The idea of a GMP is supported by the demonstration of abstract neuronal motor codes, e.g. bimanual-specific activity in motor cortex. The crosstalk model is consistent with the facts that hand-specific coding also exists and that interactions occur between the motor commands for each arm. Uncrossed efferent projections may underlie crosstalk on an executional level. Dynamic interhemispheric interactions through the corpus callosum may provide a high-level link at the parametric programming level, allowing flexible coupling and de-coupling. Flexible neuronal interactions could also underlie adaptive large-scale systems dynamics that can be formalized within the dynamic systems theory approach.

The correspondence of identified neuronal processes with functions of abstract models encourages the development of realistic computational models that can predict bimanual behavior on the basis of neuronal activity.     

Explaining modulation of reasoning by belief

Although deductive reasoning is a closed system, one's beliefs about the world can influence validity judgements. To understand the associated functional neuroanatomy of this belief-bias we studied 14 volunteers using event-related fMRI, as they performed reasoning tasks under neutral, facilitatory and inhibitory belief conditions. We found evidence for the engagement of a left temporal lobe system during belief-based reasoning and a bilateral parietal lobe system during belief-neutral reasoning. Activation of right lateral prefrontal cortex was evident when subjects inhibited a prepotent response associated with belief-bias and correctly completed a logical task, a finding consistent with its putative role in cognitive monitoring. By contrast, when logical reasoning was overcome by belief-bias, there was engagement of ventral medial prefrontal cortex, a region implicated in affective processing. This latter involvement suggests that belief-bias effects in reasoning may be mediated through an influence of emotional processes on reasoning.     

Time course of brain activation during graphemic/phonologic processing in reading: an ERP study

The present study sought to investigate the time course of brain activation during grapheme-to-phoneme conversion and phonologic processing of legal Italian syllables. To this goal, we monitored a group of right-handed native Italian volunteers performing a phonologic decision task (same/different judgments). ERPs were recorded from 28 scalp sites during silent reading. Analysis of ERP latency showed faster responses to same syllables than to different ones as early as 105 ms at left temporal electrode site and 160 ms at frontal sites. The first effect of phonologic/graphemic incongruity on the amplitude of ERPs was observed at lateral occipital/posterior temporal sites (N185), where physiologic responses were enhanced to incongruous syllables.     

Role of cholinergic system on the construction of memories: taste memory encoding

There is a large body of evidence suggesting that cholinergic activity is involved in memory processes. It seems that cholinergic activity is essential to learn several tasks and recent works suggest that acetylcholine plays an important role during the early stages of memory formation. In this review, we will discuss the results related to taste memory formation, focusing particularly on the conditioned taste aversion paradigm. We will first give evidence that nucleus basalis magnocellularis is involved in taste memory formation, due to its cholinergic projections. We then show that the cholinergic activity of the insular (gustatory) cortex is related to the taste novelty, and that the cholinergic signals initiated by novelty are crucial for taste memory formation. Then we present recent data indicating that cortical activation of muscarinic receptors is necessary for taste trace encoding, and also for its consolidation under certain circumstances. Finally, interactions between the cholinergic and other neuromodulatory systems inducing intracellular mechanisms related to plastic changes will be proposed as important processes underlying gustatory memory trace storage.     

Contribution of acetylcholine to visual cortex plasticity

Acetylcholine is involved in a variety of brain functions. In the visual cortex, the pattern of cholinergic innervation varies considerably across different mammalian species and across different cortical layers within the same species. The physiological effects of acetylcholine in the visual cortex display complex responses, which are likely due to cholinergic receptor subtype composition in cytoplasm membrane as well as interaction with other transmitter systems within the local neural circuitry. The functional role of acetylcholine in visual cortex is believed to improve the signal-to-noise ratio of cortical neurons during visual information processing. Available evidence suggests that acetylcholine is also involved in experience-dependent visual cortex plasticity. At the level of synaptic transmission, activation of muscarinic receptors has been shown to play a permissive role in visual cortex plasticity. Among the muscarinic receptor subtypes, the M(1) receptor seems to make a predominant contribution towards modifications of neural circuitry. The signal transduction cascade of the cholinergic pathway may act synergistically with that of the NMDA receptor pathway, whose activation is a prerequisite for cortical plasticity.     

The awareness of thirst: proposed neural correlates

The neural and endocrine bases of the generation of thirst are reviewed. Based on this review, a hierarchical system of neural structures that regulate water conservation and acquisition is proposed. The system includes primary sensory-receptive areas; secondary sensory structures (circumventricular organs), which detect levels of hormones, including angiotensin II and vasopressin, which are involved in generating thirst; preoptic and hypothalamic structures; and an area within the ventrolateral quadrant of the periaqueductal gray matter. Hodological and other data are used to determine the hierarchical organization of the system. Based on studies of the effects of lesions to various structures within the hierarchy of the system, it is proposed that the awareness of thirst in rodents is either entirely or predominantly due to neuronal activities in a subsection of the ventrolateral periaqueductal gray matter. It is also hypothesized that the awareness of thirst in primates is due to neuronal activities in both the ventrolateral periaqueductal gray and in a region within the medial prefrontal and anterior cingulate cortex.     

Regional grey matter volume in the posterior cingulate cortex predicts positive risk-taking: A moderation model

Positive risk-taking is a crucial element of individual creativity and social development. However, little is known regarding the relation between individual neural differences and positive risk-taking. In addition, critical thinking (CT) and gender have been proven to be two important individual-specific factors associated with risk-taking behaviour, and different levels of CT and gender may have diverse effects on the relationship between brain structure and positive risk-taking. The present study examined the relationship between positive risk-taking and regional grey matter volume (rGMV) in 292 healthy participants. The results showed that positive risk-taking was significantly positively associated with the rGMV of the posterior cingulate cortex (PCC). In addition, this study investigated individual differences in critical thinking and found that it moderated the relationship between rGMV and positive risk-taking. Individuals with lower CT had a stronger association between rGMV and positive risk-taking. Further analysis showed that for males, a greater rGMV was significantly linked to higher positive risk-taking tendency. These findings suggest that PCC evaluates risk and serves as a behavioural adaptation hub for positive risk-taking. This study thereby contributes to the literature on individual differences in brain structure and risk-taking by elucidating the moderating effects of CT and gender in healthy adults.