Publications

1996

1. Redistribution of synaptic efficacy: A mechanism to generate infinite synaptic input diversity from a homogenous population of neurons without changing absolute synaptic efficacies

   Markram H. & Tsodyks M. (1996) Journal of Physiology Paris. 90, 3-4, p. 229-232  Abstract

   Changing the reliability of neurotransmitter release results in a change in the efficacy of low frequency synaptic transmission and in the rate of high frequency synaptic depression thus it can not cause an uniform change in strength of synapses and instead results in a change in the dynamics of synaptic transmission referred to as 'redistribution of synaptic efficacy' (RSE). Since the change in synaptic transmission associated with RSE depends on the history of action potential activity it is concluded that RSE serves as a mechanism to generate a potentially infinite diversity of synoptic input.
2. Plasticity of neocortical synapses enables transitions between rate and temporal coding

   Tsodyks M. V. & Markram H. (1996) Artificial Neural Networks ICANN 96. Sendhoff B., von der Malsburg C., Vorbrüggen J. C. & von Seelen W.(eds.). p. 445-450  Abstract

   The transmission across neocortical synapses changes dynamically as a function of presynaptic activity [1]. A switch in the manner in which a complex signal, such as a burst of presynaptic action potentials, is transmitted between two neocortical layer 5 pyramidal neurons was observed after coactivation of both neurons. The switch involved a redistribution of synaptic efficacy during the burst such that the synapses transmitted more effectively only the first action potential in the burst. A computational analysis reveals that this modification in dynamically changing transmission enables pyramidal neurons to extract a rich array of dynamic features of ongoing activity in network of pyramidal neurons, such as the onset and amplitude of abrupt synchronized frequency transitions in groups of presynaptic neurons, the size of the group of neurons involved and the degree of synchrony. These synapses transmit information about dynamic features by causing transient increases of postsynaptic current which have characteristic amplitudes and durations. At the same time, the ability of synapses to signal the sustained level of presynaptic activity is limited to a narrow range of low frequencies, which become even narrower after synaptic modification.

3. Population dynamics and theta rhythm phase precession of hippocampal place cell firing: A spiking neuron model

   Tsodyks M. V., Skaggs W. E., Sejnowski T. J. & McNaughton B. L. (1996) Hippocampus. 6, 3, p. 271-280  Abstract

   O'Keefe and Reece have observed that the spatially selective firing of pyramidal cells in the CA1 field of the rat hippocampus tends to advance to earlier phases of the electroencephalogram theta rhythm as a rat passes through the place field of a cell. We present here a neural network model based on integrate-and-fire neurons that accounts for this effect. In this model, place selectivity in the hippocampus is a consequence of synaptic interactions between pyramidal neurons together with weakly selective external input. The phase shift of neuronal spiking arises in the model as a result of asymmetric spread of activation through the network, caused by asymmetry in the synaptic interactions. Several experimentally observed properties of the phase shift effect follow naturally from the model, including 1) the observation that the first spikes a cell fires appear near the theta phase corresponding to minimal population activity, 2) the overall advance is less than 360°, and 3) the location of the rat within the place field of the cell is the primary correlate of the firing phase, not the time the rat has been in the field. The model makes several predictions concerning the emergence of place fields during the earliest stages of exploration in a novel environment. It also suggests new experiments that could provide further constraints on a possible explanation of the phase precession effect.

4. Redistribution of synaptic efficacy between neocortical pyramidal neurons

   Markram H. & Tsodyks M. (1996) Nature. 382, 6594, p. 807-810  Abstract

   EXPERIENCE-dependent potentiation and depression of synaptic strength has been proposed to subserve learning and memory by changing the gain of signals conveyed between neurons. Here we examine synaptic plasticity between individual neocortical layer- 5 pyramidal neurons. We show that an increase in the synaptic response, induced by pairing action potential activity in pre- and postsynaptic neurons, was only observed when synaptic input occurred at low frequencies. This frequency-dependent increase in synaptic responses arises because of a redistribution of the available synaptic efficacy and not because of an increase in the efficacy. Redistribution of synaptic efficacy could represent a mechanism to change the content, rather than the gain, of signals conveyed between neurons.