Memory-related Processes Research Articles

Memory-related processing of complex sound patterns in human auditory cortex: a MEG study.

Responses of the human brain to a complex sound pattern were recorded with a 24 channel magnetometer. The sound pattern consisted of 9 successive 50 ms segments, each with a different frequency. An infrequent change in the frequency of one of the segments elicited a magnetic mismatch response (MMNm) which peaked at about 200 ms after the deviant segment onset and resembled the electrical mismatch negativity (MMN). The equivalent current dipole which best explained the MMNm was located in the supratemporal auditory cortex, suggesting that a memory trace for the sound pattern was stored in that region.

Activation of multifunctional Ca 2+/calmodulin-dependent kinase in intact hippocampal slices

In vitro phosphorylation of multifunctional Ca 2+/calmodulin-dependent protein kinase (CaM kinase) converts it to a form that is independent of Ca 2+. We demonstrate that significant Ca 2+-independent CaM kinase activity is present in untreated hippocampal slices. Two manipulations that produce a long-lasting enhancement of neuronal activity in hippocampal slices, elevated extracellular Ca 2+ or depolarization with high K +, generate additional Ca 2+-independent activity. This increase is dependent on extracellular Ca 2+ and is correlated with an increased phosphorylation of CaM kinase. In contrast, CaM kinase in posterior pituitary, a brain structure that is not thought to be involved in memory-related processes, is not modulated by depolarization. These results suggest that the Ca 2+-independent form of CaM kinase may modulate neuronal activity in the hippocampus.

Dynamics of activation in semantic and episodic memory.

Spreading-activation models for the structure of semantic and episodic memory postulate a network of interconnected nodes in which activation spreads from a source node to recipient nodes. These models account for a broad range of memory-related processes, including word recognition, sentence verification, prose comprehension, and sentence production. A fundamental question regarding this account concerns the nature of activation growth at each node in the network. Two mutually exclusive possibilities are (a) that activation grows in a discrete fashion, making abrupt transitions between two or more distinct states and (b) that activation grows continuously from a resting level to an asymptotic level. In the present article, we characterize this dichotomy with examples from the literature, and we apply an adaptive priming procedure for testing discrete versus continuous activation models. Our procedure involves the presentation of prime stimuli at various moments before a test stimulus; subjects are required to make a lexical (word/nonword) decision about the test stimulus. The duration of the interval between the prime and test stimuli is varied adaptively on the basis of subjects' performance. Reaction times are recorded as a function of this duration. According to discrete activation models, there is a unique reaction-time distribution associated with each possible state of node activation. The distribution of reaction times observed when the test stimulus appears near the moment of transition between discrete states should therefore constitute a finite mixture of the underlying basis distributions associated with the individual discrete activation states. The mixture proportion will depend on the relation between the priming interval and the distribution of state-transition times. Continuous activation models assert instead that activation grows continuously over time and that there is a unique reaction-time distribution associated with any given degree of intermediate priming. Such models predict that no finite mixture distribution will emerge when the priming interval has a fixed intermediate duration. Two experiments with the adaptive priming procedure are reported to test these alternative predictions. In Experiment 1, the prime and test stimuli were semantically associated words (e.g., bread-butter). In Experiment 2, episodic associations between the prime and test stimuli were established through paired associate learning. For both cases, the mixture prediction failed, and two-state discrete activation models were rejected.(ABSTRACT TRUNCATED AT 400 WORDS)

Dynamics of activation in semantic and episodic memory.

Spreading-activation models for the structure of semantic and episodic memory postulate a network of interconnected nodes in which activation spreads from a source node to recipient nodes. These models account for a broad range of memory-related processes, including word recognition, sentence verification, prose comprehension, and sentence production. A fundamental question regarding this account concerns the nature of activation growth at each node in the network. Two mutually exclusive possibilities are (a) that activation grows in a discrete fashion, making abrupt transitions between two or more distinct states and (b) that activation grows continuously from a resting level to an asymptotic level. In the present article, we characterize this dichotomy with examples from the literature, and we apply an adaptive priming procedure for testing discrete versus continuous activation models. Our procedure involves the presentation of prime stimuli at various moments before a test stimulus; subjects are required to make a lexical (word/nonword) decision about the test stimulus. The duration of the interval between the prime and test stimuli is varied adaptively on the basis of subjects' performance. Reaction times are recorded as a function of this duration. According to discrete activation models, there is a unique reaction-time distribution associated with each possible state of node activation. The distribution of reaction times observed when the test stimulus appears near the moment of transition between discrete states should therefore constitute a finite mixture of the underlying basis distributions associated with the individual discrete activation states. The mixture proportion will depend on the relation between the priming interval and the distribution of state-transition times. Continuous activation models assert instead that activation grows continuously over time and that there is a unique reaction-time distribution associated with any given degree of intermediate priming. Such models predict that no finite mixture distribution will emerge when the priming interval has a fixed intermediate duration. Two experiments with the adaptive priming procedure are reported to test these alternative predictions. In Experiment 1, the prime and test stimuli were semantically associated words (e.g., bread-butter). In Experiment 2, episodic associations between the prime and test stimuli were established through paired associate learning. For both cases, the mixture prediction failed, and two-state discrete activation models were rejected.(ABSTRACT TRUNCATED AT 400 WORDS)

Thalamic mediodorsal nucleus and memory: A critical evaluation of studies in animals and man

Following a general description of the anatomical organization of the thalamic mediodorsal nucleus (MD) of animals and man, the involvement of this nucleus in the processing of memory related information has been evaluated by reviewing stimulation, electrophysiological, and lesion studies in animals, and by reviewing research on induced lesions, degenerative changes and vascular damage of MD in humans. Neither the results from animal experiments nor those from studies on humans provide clear-cut evidence for a specific, memory related role of MD. However, the findings here presented do support the theory that MD is one of several, possible memory related relay stations. While therapeutically induced and circumscribed lesions of MD rarely result in long-lasting memory deficits, pathological processes in MD are more likely to be followed by severe memory disturbances if one or more particular structures in addition to MD are included in the lesioned regions. Consequently, it is emphasized that only the disruption of more than one site along memory related pathways will result in severe and enduring memory deficits. To account for apparent inter-species differences in the involvement of MD in memory related processes, it has been argued that MD and its principal cortical target region might basically be involved in arousal and emotional processes, but that for primates and especially for man the phylogenetically young parvocellular sector of MD and its cortical projection region, the dorsolateral prefrontal cortex, are furthermore involved in memory functions, which are modulated by emotional factors via the rest of MD and the prefrontal cortex.

Neurohypophyseal peptide influences on ethanol tolerance and acute effects of ethanol

The neurohypophyseal hormone, arginine vasopressin (AVP), was previously shown to prolong the duration of ethanol tolerance in mice. Since drug tolerance and certain memory-related processes are examples of CNS adaptation, these phenomena have been proposed to share underlying mechanisms. We investigated the effects on ethanol tolerance of two other neurohypophyseal peptides, both of which modulate memory consolidation or retrieval of information. (Des-9-glycinamide, 8-lysine) vasopressin (DGLVP), like AVP, maintained ethanol tolerance in C57Bl mice, while cyclo(Leu-Gly) (cLG), at an equimolar dose, was ineffective. Thus, various neurohypophyseal peptides may differentially influence CNS adaptive phenomena. Direct peptide effects on ethanol-induced hypothermia and "sleep time," the parameters used to evaluate ethanol tolerance, were also determined. AVP per se caused hypothermia in mice, but neither AVP nor cLG affected ethanol-induced hypothermia. Both peptides, however, increased "sleep time" after acute ethanol administration. Although these direct peptide-ethanol interactions do not account for the observed peptide effects on tolerance, the findings emphasize the importance of using several parameters to assess ethanol tolerance.

Arginine vasopressin maintains ethanol tolerance.

THE neurohypophyseal hormone, [Arg8]vasopressin (AVP), like some of its analogues and smaller peptides of related structure, has the capacity to influence memory-related processes through direct action in the central nervous system1–4. These peptides have relatively long-lasting effects on memory consolidation or on retrieval of information1,5. It has been suggested that development of functional tolerance to drug effects involves an adaptive mechanism in the CNS similar to that which occurs during learning and/or memory consolidation6,7. We therefore wished to extend our studies4 of the central actions of the neurohypophyseal hormones to include an investigation of the influence of these peptides on various aspects of functional tolerance to ethanol. One of the most consistently demonstrable effects of the neurohypophyseal peptides with respect to memory is the ability to inhibit the extinction of a conditioned avoidance response1–3,5. We now present evidence that treatment of animals with AVP during and after chronic ethanol administration leads to long-term maintenance of tolerance to the hypothermic and sedative effects of ethanol.