Neuromodulation and cortical function: modeling the physiological basis of behavior

Cognitive functions of cortical acetylcholine: toward a unifying hypothesis

Previous efforts aimed at attributing discrete behavioral functions to cortical cholinergic afferents have not resulted in a generally accepted hypothesis about the behavioral functions mediated by this system. Moreover, attempts to develop such a unifying hypothesis have been presumed to be unproductive considering the widespread innervation of the cortex by basal forebrain cholinergic neurons. In contrast to previous descriptions of the role of cortical acetylcholine (ACh) in specific behavioral phenomena (e.g., mediation of the behavioral effects of reward loss) or mnemonic entities (e.g., working or reference memory), cortical ACh is hypothesized to modulate the general efficacy of the cortical processing of sensory or associational information. Specifically, cortical cholinergic inputs mediate the subjects' abilities to detect and select stimuli and associations for extended processing and to allocate the appropriate processing resources to these functions. In addition to evidence from electrophysiological and behavioral studies on the role of cortical ACh in sensory information processing and attention, this hypothesis is consistent with proposed functions of the limbic and paralimbic networks in regulating the activity of the basal forebrain cholinergic neurons. Finally, while the proposed hypothesis implies that changes in activity in cortical ACh simultaneously occur throughout the cortex, the selectivity and precision of the functions of cholinergic function is due to its coordinated interactions with the activity of converging sensory or associational inputs. Finally, the dynamic, escalating consequences of alterations in the activity of cortical ACh (hypo- and hyperactivity) on cognitive functions are evaluated.

Implicit and explicit memory in amnesia: some explanations and predictions by the TraceLink model

After a brief overview of some of the characteristics and neuroanatomy of amnesia, a new model of amnesia is described: the TraceLink model. One novel aspect of the model is that it makes specific and testable predictions regarding semantic dementia, a recently described disorder that is viewed here as being related to amnesia. The TraceLink model consists of: a trace system (roughly the neocortix), a link system (hippocampus and adjacent areas), and a modulatory system (certain basal forebrain nuclei). Different forms of learning in the TraceLink model are explained, followed by a discussion of implicit and explicit memory, prominence (ease of recall) and persistence (resistance to brain damage), consolidation, and Ribot gradients in retrograde amnesia. Patterns of recovery from retrograde amnesia are also discussed, and novel predictions are derived regarding implicit memory and various forms of amnesia.

Effects of transcutaneous electrical nerve stimulation on memory and affective behavior in a stroke patient

Although stroke is characterized by both a loss of mental and sensorimotor abilities, rehabilitation is particularly aimed at recovery from physical disabilities. In the case study reported in this article, it was hypothesized that a 30-min-a-day treatment with transcutaneous electrical nerve stimulation (TENS) might improve memory and affective behavior in a stroke patient. The hypothesis was derived from animal experimental studies in which peripheral stimulation resulted in activation of the hippocampus and hypothalamus. Although treatment could be started only 6 months after onset, the results suggest that TENS has a beneficial influence on visual short-term memory, verbal long-term memory, and verbal fluency. Moreover, [he patient's independent and social functioning in daily life improved. The conflicting effects of TENS on the patient's affective behavior are discussed.

Cognitive maps beyond the hippocampus

We present a conceptual framework for the role of the hippocampus and its afferent and efferent structures in rodent navigation. Our proposal is compatible with the behavioral, neurophysiological, anatomical, and neuropharmacological literature, and suggests a number of practical experiments that could support or refute it. We begin with a review of place cells and how the place code for an environment might be aligned with sensory cues and updated by self-motion information. The existence of place fields in the dark suggests that location information is maintained by path integration, which requires an internal representation of direction of motion. This leads to a consideration of the organization of the rodent head direction system, and thence into a discussion of the computational structure and anatomical locus of the path integrator. If the place code is used in navigation, there must be a mechanism for selecting an action based on this information. We review evidence that the nucleus accumbens subserves this function. From there, we move to interactions between the hippocampal system and the environment, emphasizing mechanisms for learning novel environments and for aligning the various subsystems upon re-entry into familiar environments. We conclude with a discussion of the relationship between navigation and declarative memory.

Psychobiological models of hippocampal function in learning and memory

We review current computational models of hippocampal function in learning and memory, concentrating on those that make strongest contact with psychological issues and behavioral data. Some models build upon Marr's early theories for modeling hippocampal field CA3's putative role in the fast, temporary storage of episodic memories. Other models focus on hippocampal involvement in incrementally learned associations, such as classical conditioning. More recent efforts have attempted to bring functional interpretations of the hippocampal region in closer contact with underlying anatomy and physiology. In reviewing these psychobiological models, three major themes emerge. First, computational models provide the conceptual glue to bind together data from multiple levels of analysis. Second, models serve as important tools to integrate data from both animal and human studies. Third, previous psychological models that capture important behavioral principles of memory provide an important top-down constraint for developing computational models of the neural bases of these behaviors.

Intrinsic association fiber system of the piriform cortex: a quantitative study based on a cholera toxin B subunit tracing in the rat

By using retrograde and anterograde transport of the B subunit of cholera toxin (CTb), we examined quantitatively the association fiber systems, i.e., the collaterals of pyramidal cell axons, that reciprocally connect both the rostral and the caudal parts of the piriform cortex (PC). Well-defined CTb injections were obtained in layers Ib or II-III of the rostral and the caudal parts of the PC. Using precision counting, we determined the proportion of cellular profiles in layers II and III that gave rise to association fibers and thus demonstrated a predominance of rostrocaudal fibers over the caudorostral ones. Our data also support a precise laminar organization of the PC in which the rostrocaudal fibers originated mainly from layer II and the caudorostral fibers primarily from layer III. Cholera toxin injections into layer Ib produced a peak of labeled profiles 2 mm from the site, indicating that a large proportion of the association fibers from layer II travel for at least 2 mm and then synapse in layer Ib. At either end of the PC, the association projections with respect to olfactory processing, propagation of the activity within the PC, and the possible role of intrinsic fibers in olfactory memory.

Suppression of synaptic transmission may allow combination of associative feedback and self-organizing feedforward connections in the neocortex

Selective suppression of synaptic transmission during learning is proposed as a physiological mechanism for combining associative memory function at feedback synapses with self-organization of feedforward synapses in neocortical structures. A computational model demonstrates how selective suppression of feedback transmission allows this combination of synaptic function. During learning, sensory stimuli and the desired response are simultaneously presented as input to the network. Feedforward connections form self-organized representations of input, while suppressed feedback connections learn the transpose of the feedforward connectivity. During recall, suppression of transmission is removed, input activates the self-organized representation, and activity settles into a learned solution to the problem. This computational model can be used for learning of problems which are not linearly separable, including the negative patterning task (the XOR problem). Experiments in brain slice preparations of the rat somatosensory cortex tested whether the combination of self-organization and associative memory function could be provided by cholinergic suppression selective for feedback versus feedforward synapses. The cholinergic agonist carbachol selectively suppressed synaptic potentials elicited by stimulation of layer I (which contains a high percentage of feedback synapses), while having no effect on synaptic potentials elicited by stimulation of layer IV (with a high percentage of afferent and feedforward synapses).

Catecholamine innervation of the piriform cortex: a tracing and immunohistochemical study in the rat

In order to determine the origin of the catecholamine innervation of the rat piriform cortex (PC), we combined retrograde transport of the B subunit of the cholera toxin (CTb) with tyrosine hydroxylase (TH) immunohistochemistry. A substantial number of CTb retrogradely labeled cells was found in the parabrachial pigmented, paranigral and interfascicular nuclei of the ventral tegmental area and the dorsal part of the locus coeruleus, whereas nearly no labeling was noted in the substantia nigra. Following TH immunohistochemistry on the same sections, most if not all of the CTb labeled cells were also TH immunoreactive. Occasional double-labeled cells were also observed in the anterior part of the raphe dorsal nucleus. As visualized with dopamine beta-hydroxylase, dopamine or TH immunohistochemistry, the noradrenaline fibers were homogeneously distributed whereas the dopamine fibers showed rostro-caudal and latero-medial differences. The distribution of TH fibers overlapped both patterns. Our report suggests that the heterogeneous distribution of the DA fibers could support a differential centrifugal modulation of the olfactory information processing throughout the PC.

Intrinsic neuromodulation: altering neuronal circuits from within

There are two sources of neuromodulation for neuronal circuits: extrinsic inputs and intrinsic components of the circuits themselves. Extrinsic neuromodulation is known to be pervasive in nervous systems, but intrinsic neuromodulation is less recognized, despite the fact that it has now been demonstrated in sensory and neuromuscular circuits and in central pattern generators. By its nature, intrinsic neuromodulation produces local changes in neuronal computation, whereas extrinsic neuromodulation can cause global changes, often affecting many circuits simultaneously. Studies in a number of systems are defining the different properties of these two forms of neuromodulation.

TraceLink: a model of amnesia and consolidation of memory

A model of amnesia is introduced, called TraceLink, that consists of three systems: 1) a trace system (neocortex), 2) a link system (hippocampus), and 3) a modulatory system (hippocampus/fornix/basal forebrain). It aims to explain salient aspects of the neuropsychology of amnesia, such as Ribot gradients in retrograde amnesia, patterns of dissociation between anterograde and retrograde amnesia, recovery from amnesia, and a newly discovered form of amnesia (semantic dementia) that results from certain temporal lobe lesions that do not affect the hippocampus. The model, furthermore, offers a new explanation for the global neuroanatomy of the hippocampus and neocortex based on the assumption that the brain aims to minimize connectivity volume. It also offers various strategies for the consolidation of memory, the effects of which are explored in computer simulations. The paper concludes with ten, largely untested; predictions derived from the TraceLink model.

Possible involvement of neuromodulatory systems in cortical Hebbian-like plasticity

Plasticity of neuronal covariances (functional plasticity) is controlled by behavior (Ahissar et al (1992) Science 257, 1412-1415). Whether this behavioral control involves neuromodulatory systems was tested by examining the effect of acetylcholine (ACh) and noradrenaline (NE) on functional plasticity in anesthetized animals and by comparing the effects of these neuromodulators in an anesthetized preparation to that of behavior in awake animals. Local ionotophoretic applications of these drugs during manipulations of activity covariance in guinea pig auditory cortex did not mimic the behavioral control of functional plasticity that was previously observed in awake monkeys. Thus, the hypotheses according to which these neuromodulators control functional plasticity independent of their concentration and time of release were not supported by our data. The significant plasticity induced nevertheless, by some of the conditionings in the presence of ACh and NE, suggests that factors, other than those that were experimentally controlled, could regulate this plasticity. These factors could be among others the timing of drug(s) applications relative to the conditioning time, the local concentrations of the drug(s) and/or the site of application with respect to the relevant synapses.

Encoding and retrieval of episodic memories: role of cholinergic and GABAergic modulation in the hippocampus

This research focuses on linking episodic memory function to the cellular physiology of hippocampal neurons, with a particular emphasis on modulatory effects at cholinergic and gamma-aminobutyric acid B receptors. Drugs which block acetylcholine receptors (e.g., scopolamine) have been shown to impair encoding of new information in humans, nonhuman primates, and rodents. Extensive data have been gathered about the cellular effects of acetylcholine in the hippocampus. In this research, models of individual hippocampal subregions have been utilized to understand the significance of particular features of modulation, and these hippocampal subregions have been combined in a network simulation which can replicate the selective encoding impairment produced by scopolamine in human subjects.

Distinct memory circuits composing the hippocampal region

The very different anatomical designs of the adjacent circuitries of the cortico-hippocampal pathway, along with their somewhat different synaptic plasticity mechanisms, suggest a nearly serial pathway of distinct memory circuits each contributing its own specialized processing operation to overall hippocampal function. Modeling and formal theoretical analysis of the prominent anatomical design features of particular circuits (piriform/entorhinal cortex; hippocampal field CA3; hippocampal field CA1) are found to identify potential emergent function not readily arrived at in the absence of these formal models, and yet which once derived can be seen potentially to confer unique capabilities to an integrated hippocampal mechanism for processing memories during behavior.

Intrinsic and extrinsic neuromodulation of motor circuits

Neuromodulation of motor circuits by extrinsic inputs provides enormous flexibility in the production of behavior. Recent work has shown that neurons intrinsic to central pattern-generating circuits can evoke neuromodulatory effects in addition to their neurotransmitting actions. Modulatory neurons often elicit a multitude of different effects attributable to actions at different receptors and/or through the release of co-transmitters. Differences in neuromodulation between species can account for differences in behavior. Modulation of neuromodulation may provide an additional level of flexibility to motor circuits.

Acetylcholine: a neurotransmitter for learning and memory?

The cholinergic hypothesis claims that the decline in cognitive functions in dementia is predominantly related to a decrease in cholinergic neurotransmission. This hypothesis has led to great interest in the putative involvement of the cholinergic neurotransmission in learning and memory processes. This review aims to assess the data of studies in which the role of acetylcholine (ACh) in cognitive functions was investigated. For this purpose, studies from three different fields of research, namely: (1) behavioral pharmacology (effects of drugs on behavior); (2) behavioral neuroscience (effects of brain lesions on behavior); and (3) dementia, are discussed separately. The experimental tools that have been used in pharmacological studies may appear to be inadequate to enable conclusions to be drawn about the involvement of ACh in learning and memory processes. Especially, the use of scopolamine as a pharmacological tool is criticized. In the field of behavioral neuroscience a highly specific cholinergic toxin has been developed. It appears that the greater and more specific the cholinergic damage, the fewer effects can be observed at the behavioral level. The correlation between the decrease in cholinergic markers and the cognitive decline in dementia may not be as clearcut as has been assumed. The involvement of other neurotransmitter systems in cognitive functions is briefly discussed. Taking into account the results of the different fields of research, the notion that ACh plays a pivotal role in learning and memory processes seems to be overstated. Even when the role of other neurotransmitter systems in learning and memory is taken into consideration, it is unlikely that ACh has a specific role in these processes. On basis of the available data, ACh seems to be more specifically involved in attentional processes than in learning and memory processes.