Classical conditioning of kindled seizures

Motive control of unconscious inference: The limbic base of adaptive Bayes

Current computational models of neocortical processing, described as predictive coding theory, are providing new ways of understanding Helmholtz's classical insight that perception cannot proceed in a data-driven fashion, but instead requires unconscious inference based on prior experience. Predictive coding is a Bayesian process, in which the operations at each lower level of the cortical hierarchy are predicted by prior projections of expectancies from a higher level, and are then updated by error-correction with lower level evidence. To generalize the predictive coding model to the human neocortex as a whole requires aligning the Bayesian negotiation of prior expectancies with sensory and motor evidence not only within the connectional architecture of the neocortex (primary sensory/motor, unimodal association areas, and heteromodal association areas) but also with the limbic cortex that forms the base for the adaptive control of the heteromodal areas and thereby the cerebral hemisphere as a whole. By reviewing the current evidence on the anatomy of the human corticolimbic connectivity (now formalized as the Structural Model) we address the problem of how limbic cortex resonates to the homeostatic, personal significance of events to provide Bayesian priors to organize the operations of predictive coding across the multiple levels of the neocortex. By reviewing both classical evidence and current models of control exerted between limbic and neocortical networks, we suggest a neuropsychological theory of human cognition, the adaptive Bayes process model, in which prior expectancies are not simply rationalized propositions, but rather affectively-charged expectancies that bias the interpretation of sensory data and action affordances to support allostasis, the motive control of expectancies for future events.

Behavioral functions of the mesolimbic dopaminergic system: an affective neuroethological perspective

The mesolimbic dopaminergic (ML-DA) system has been recognized for its central role in motivated behaviors, various types of reward, and, more recently, in cognitive processes. Functional theories have emphasized DA's involvement in the orchestration of goal-directed behaviors and in the promotion and reinforcement of learning. The affective neuroethological perspective presented here views the ML-DA system in terms of its ability to activate an instinctual emotional appetitive state (SEEKING) evolved to induce organisms to search for all varieties of life-supporting stimuli and to avoid harms. A description of the anatomical framework in which the ML system is embedded is followed by the argument that the SEEKING disposition emerges through functional integration of ventral basal ganglia (BG) into thalamocortical activities. Filtering cortical and limbic input that spreads into BG, DA transmission promotes the "release" of neural activity patterns that induce active SEEKING behaviors when expressed at the motor level. Reverberation of these patterns constitutes a neurodynamic process for the inclusion of cognitive and perceptual representations within the extended networks of the SEEKING urge. In this way, the SEEKING disposition influences attention, incentive salience, associative learning, and anticipatory predictions. In our view, the rewarding properties of drugs of abuse are, in part, caused by the activation of the SEEKING disposition, ranging from appetitive drive to persistent craving depending on the intensity of the affect. The implications of such a view for understanding addiction are considered, with particular emphasis on factors predisposing individuals to develop compulsive drug seeking behaviors.

Neurobiologic processes in drug reward and addiction

Neurophysiologic processes underlie the uncontrolled, compulsive behaviors defining the addicted state. These"hard-wired"changes in the brain are considered critical for the transition from casual to addictive drug use. This review of preclinical and clinical (primarily neuroimaging) studies will describe how the delineation between pleasure, reward, and addiction has evolved as our understanding of the biologic mechanisms underlying these processes has progressed. Although the mesolimbic dopaminergic efflux associated with drug reward was previously considered the biologic equivalent of pleasure, dopaminergic activation occurs in the presence of unexpected and novel stimuli (either pleasurable or aversive) and appears to determine the motivational state of wanting or expectation. The persistent release of dopamine during chronic drug use progressively recruits limbic brain regions and the prefrontal cortex, embedding drug cues into the amygdala (through glutaminergic mechanisms) and involving the amygdala, anterior cingulate, orbitofrontal cortex, and dorsolateral prefrontal cortex in the obsessive craving for drugs. The abstinent, addicted brain is subsequently primed to return to drug use when triggered by a single use of drug, contextual drug cues, craving, or stress, with each process defined by a relatively distinct brain region or neural pathway. The compulsive drive toward drug use is complemented by deficits in impulse control and decision making, which are also mediated by the orbitofrontal cortex and anterior cingulate. Within this framework, future targets for pharmacologic treatment are suggested.

Conditioned effects of kindling

Mild periodic electrical stimulation to any one of many brain sites leads to the development and progressive intensification of elicited motor seizures. Since its discovery in 1969, this kindling phenomenon has been widely studied both as a model of epileptogenesis and as a form of neuroplasticity, and recently there has been increasing interest in kindling as a model of the interictal (i.e. between-seizures) changes in emotionality that accompany certain forms of epilepsy. Despite the extensive use of the kindling model, little consideration has been given to the role played by the cues regularly associated with the delivery of the kindling stimulations. However, we have recently demonstrated that cues associated with the standard kindling protocol (e.g. the stimulation environment) produce conditioned effects on both the motor seizures and interictal behavior of rats and that some kindling sites, such as the amygdala, produce conditioned interictal behaviors that are defensive in nature. The implications that these findings have for the study of interictal behavioral changes in particular and to kindling research in general are discussed.

Antecedent tone presentation significantly delays rate of amygdala kindling

External stimuli may contribute to seizure occurrence in at least two ways. First, the aberrant neuronal activity that precipitates a seizure could be elicited by certain external events; and second, external events could cue the organism to an impending seizure and result in a compensatory response that is, in effect, 'anti-convulsant'. While previous research has been aimed at addressing these issues, the results have been inconclusive. The present study was conducted to clarify and extend this prior work. Adult male Long-Evans hooded rats were chronically implanted with a kindling electrode and randomly assigned to one of two groups. The Tone group was presented with a 2-s auditory stimulus (Tone) beginning one second prior to and overlapping with the 1-s kindling stimulus. Animals in the No Tone group received only the kindling stimulus. Antecedent tone presentation significantly delayed the rate of amygdala kindling. The Tone group required significantly more stimulations to reach a Stage 5 seizure than did the No Tone group. The possibility of this phenomenon providing a means to study the mechanisms underlying anti-epileptogenesis is discussed.

Clinical and biochemical manifestations of depression. Relation to the neurobiology of stress (2)

Thousands of studies have been conducted of the functioning of the many neurotransmitter systems in order to explore the biologic basis of major depressive disorder. Instead of reviewing this literature exhaustively, we have attempted to propose a model that accommodates the clinical observation that chronic stress early in life in vulnerable persons predisposes them to major depression with contemporary observations of the potential consequences of repeated central nervous system exposure to effectors of the stress response. This model accords with current clinical judgment that major depression is best treated with a combination of psychopharmacologic agents and psychotherapy. Accordingly, whereas psychopharmacologic intervention may be required to resolve an active episode of major depression and to prevent recurrences, psychotherapy may be equally important to lessen the burden of stress imposed by intense inner conflict and counterproductive defenses.

Differential conditioning of environmental cues with amygdala kindling

Recent evidence suggests that the pairing of environmental cues with kindling stimulation can affect the rate at which seizures develop. In the present study, the effect of differential conditioning was evaluated. Rats were kindled in either a black box or a highly illuminated white box. Half of the subjects in each of these groups (discrimination groups) was placed in the opposite box on separate days without receiving kindling stimulation. The remaining subjects (control groups) were placed only in the box in which they received stimulation. Subjects kindled in the white box developed stage 5 (clonic) seizures significantly faster than those kindled in the black box. Those subjects that received discrimination training with the white box positive kindled faster than all other groups. However, after reaching stage 5, both discrimination groups, regardless of which box was positive, had significantly shorter afterdischarge (AD) durations during threshold testing as compared with the control groups. Seizure thresholds did not differ for the different groups. No evidence for conditioned seizures was found. The results were discussed in terms of the potential facilitory and inhibitory effects of environmental cues on seizures.

Amygdala kindling in the classical conditioning paradigm

The hypothesis was tested that epileptiform alterations of brain excitability may be elicited by a conditioning stimulus (CS) in a classical conditioning paradigm. Fifteen male albino rats were chronically implanted with electrodes in the amygdala and over the visual cortex. A train of six stroboscopic flashes served as the CS and the unconditioned stimulus was the amygdaloid electrical stimulation, delivered together with the sixth flash in the course of the kindling procedure. Rats were randomly assigned to two groups. Rats in Group 1 were exposed to one or two kindling sessions per day, and those in Group 2 were exposed to 10 such sessions every day. All rats in Group 1 showed fully mature seizures within 15 +/- 5.5 sessions, whereas none of those in Group 2 displayed kindled seizures. There was no evidence of amygdaloid afterdischarges or enhanced spiking as a response to the CS. A gradual selective suppression of the visual evoked potentials (VEP) secondary components, i.e., slow negative wave and sensory afterdischarge, paralleled kindling in Group 1. This effect was weak in Group 2. There was no change in the primary response of VEPs in either group. No recovery of VEPs followed the extinction of conditioning.

Conditioning and the kindling model of epilepsy

Since the development of the kindling preparation there has been much speculation on the contribution of conditioning mechanisms in accounting for the phenomenon. Two recent reports have described attempts to establish conditioned responses utilizing the kindling model. We suggest that the conclusion of uncondition-ability asserted by Wyler & Heavner (1979) is not warranted because of serious misunderstandings relating to the concepts and techniques that are central to both neurophysiological and conditioning theory. We further suggest that the results of their retardation effect can be better interpreted as positive evidence for the successful establishment of conditioning. With respect to the findings by Janowsky et al. (1980), we regard their sample size (2 animals undergoing simultaneous kindling and conditioning and one animal beginning conditioning following kindling) as inadequate for a conclusive demonstration of the success they report. Both studies, however, are seen as substantive contributions, insofar as they introduce this important perspective in kindling research, that can be expected to occupy a major role in the future developments of basic and clinical epileptology.