Biomedical subjects

M A Idiart

Publications and source records attributed to M A Idiart.

3 recordsLinked to original sources

PubMed1995

Storage of 7 +/- 2 short-term memories in oscillatory subcycles.

Psychophysical measurements indicate that human subjects can store approximately seven short-term memories. Physiological studies suggest that short-term memories are stored by patterns of neuronal activity. Here it is shown that activity patterns associated with multiple memories can be stored in a single neural network that exhibits nested oscillations similar to those recorded from the brain. Each memory is stored in a different high-frequency ("40 hertz") subcycle of a low-frequency oscillation. Memory patterns repeat on each low-frequency (5 to 12 hertz) oscillation, a repetition that relies on activity-dependent changes in membrane excitability rather than reverberatory circuits. This work suggests that brain oscillations are a timing mechanism for controlling the serial processing of short-term memories.

Computer Simulation↗

PubMed1994

A theoretical framework for quantal analysis and its application to long-term potentiation.

1. We present a new mathematical description of the complete distribution of electrical responses to stochastic synaptic activity (quantal analysis) that is intended as a model of experiments on central neuronal synapses. Unlike previous treatments, this distribution is calculated for each instant after the release of transmitter into the cleft. 2. We follow the traditional description of probabilistic presynaptic vesicle release. On the postsynaptic side, however, we assume that channel fluctuations are important and we take them into account. The probability of finding a given channel open after a certain amount of transmitter is released is calculated from detailed receptor/channel and neurotransmitter clearance kinetics. This approach allows us to naturally include the nonlinear dependence of open probability on the amount of transmitter released, with saturation for large transmitter doses. The distribution of open channels is calculated from this probability. 3. We also allow the possibility that multiple synaptic inputs to a target neuron may be active in a typical experiment. We have not treated cable effects. We explore the implications of multiple synapses for the nonlinearities of the system. The most important of these is that vesicles in different synapses have independent responses, and therefore their effects add linearly. 4. The resulting distributions depend heavily on what region of the nonlinear dose-response curve the synapses are in. Far from saturation, peaks in the distribution are due to vesicles, and close to saturation they are due to active synapses. Peak widths are due to channel fluctuations and instrumental noise, which we introduce to make closer contact with experiments.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMedDate not supplied

Physiologically realistic formation of autoassociative memory in networks with theta/gamma oscillations: role of fast NMDA channels.

Recordings from brain regions involved in memory function show dual oscillations in which each cycle of a low-frequency theta oscillation (5-8 Hz) is subdivided into about seven subcycles by high frequency gamma oscillations (20-60 Hz). It has been proposed (Lisman and Idiart 1995) that such networks are a multiplexed short-term memory (STM) buffer that can actively maintain about seven memories, a capability of human STM. A memory is encoded by a subset of principal neurons that fire synchronously in a particular gamma subcycle. Firing is maintained by a membrane process intrinsic to each cell. We now extend this model by incorporating recurrent connections with modifiable synapses to store long-term memory (LTM). The repetition provided by STM gradually modifies synapses in a physiologically realistic way. Because different memories are active in different gamma subcycles, the formation of autoassociative LTM requires that synaptic modification depend on N-methyl-D-aspartate (NMDA) channels having a time constant of deactivation that is of the same order as the duration of a gamma subcycle (15-50 msec). Many types of NMDA channels have longer time constants (150 msec), as for instance those found in the hippocampus, but both fast and slow NMDA channels are present in cortex. This is the first proposal for the special role of these fast NMDA channels. The STM for novel items must depend on activity-dependent changes intrinsic to neurons rather than recurrent connections, which have not developed the required selectivity. Because these intrinsic mechanisms are not error-correcting, STM will become slowly corrupted by noise. This limits the accuracy with which LTM can become encoded after a single presentation. Accurate encoding of items in LTM can be achieved by multiple presentations, provided different memory items are presented in a varied interleaved order. Our results indicate that a limited memory-capacity STM model can be integrated in the same network with a high-capacity LTM model.

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