25

26

27

iCl

ECl��60mV

GABAAgGABAa

iK

EK��80mV

GABABgGABAb

iNa /K

ENa /K��10mV

AMPA

g AMPA

iCa

ECa��50mV

NMDA

g NMDA

28

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Examples of EPSCS recorded in response to paired-pulse stimulation of Schaffer collateral axons in a pyramidal cell (A), interneurone with facilitation (B) and interneurone with depression (C). Each trace is the average of 10 responses; traces are overlaid for paired-pulse intervals of 30, 50, 80, 100, 150 and 200 ms. D, group results for paired-pulse ratios (mean ± s.e.m.) from pyramidal cells (squares, n = 32), interneurones with facilitation (circles, n = 57) and interneurones with depression (triangles, n = 9).

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Schematic drawing of a paradigm of LTP induction. A. A weak test pulse (left) evokes the postsynaptic response sketched on the right-hand side of the figure. B. A strong stimulation sequence (left) triggers postsynaptic firing (right, the peak of the action potential is out of bounds). C. A test pulse applied some time later evokes a larger postsynaptic response (right; solid line) than the initial response. The dashed line is a copy of the initial response in A (schematic figure).

Figure from: Gerstner and Kistler Spiking Neuron Models. Single Neurons, Populations, Plasticity Cambridge University Press, 2002

31

Cooperativity in the induction of LTP. Synapses at the W channel are strengthened only if both the presynaptic site is stimulated via the W electrode and the postsynaptic neuron is active due to a simultaneous stimulation of the S pathway

Figure from: Gerstner and Kistler Spiking Neuron Models. Single Neurons, Populations, Plasticity Cambridge University Press, 2002

32Eric Hargreaves: http://homepages.nyu.edu/~eh597/ltp.htm

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Hebbian synaptic modification and spike-timingdependence.

A: Activity-dependent synaptic strengthening as proposed by Hebb.Triangles = synapses formed between neurons. Arrows indicate external stimulation. Traces next to synapses represent evoked postsynaptic current (EPSC), thesize of which is a measure of synaptic strength. Inset depictsone episode of repetitive pre- and postsynaptic activity during``Hebbian learning''. Note that the postsynaptic spike istriggered after the onset of postsynaptic potential (EPSP)evoked by the presynaptic spike. The thickness of connecting lines represents synaptic strength. After repetitive activation, the synapse is strengthened.

B: Critical temporal window for synaptic modifications. LTP/LTD were induced by correlated pre- and postsynaptic spiking at synapses between hippocampal glutamatergic neurons in culture. The percentage change in the EPSC amplitude at 20±30 minutes after repetitive correlated spiking (60 pulses at 1 Hz) was plotted against spike timing, which is defined as the time interval (Dt) between the onset of the EPSP and the peak of the postsynaptic action potential during each pair of correlated spiking, as illustrated by the traces above. LTP and LTD windows are each fitted with an exponential function with time constants ~17ms and 35ms.

� �For LTP and LTD, respectively, A 0.78 and À 0.27; t 16.8and À 33.7 ms. Scales: 50 mV, 10 ms. (Adapted from Bi andPoo, J. Neurosci. 18:10464±10472).

36

Some important properties of LTP

For LTP to occur, both the synapse and the post-synaptic neuron must simultanously be depolarized beyond a threshold.

LTP is input specific: LTP can enhance the synaptic effectiveness of a synapse without affecting other synapses in the cell. This specificity is largely due to the compartmentalization offered by dendritic spines, and greatly increases the storage capacity of individual neurons.

LTP is cooperative: weak stimulations in a single pathways can cooperatively induce LTP.

LTP is associative: weak stimulation in pathways, when coupled with strong stimulation in other pathways, can induce LTP.

Stimuli must be delivered at high frequency, because the post-synaptic cell must be depolarized past a certain threshold for LTP to occur

LTP has a transient early (lasting 1-3 hours) phapse, followed by a consolidated later phase (at least 24 hours). In the early phase the conductance of the post symaptic site is enhanced without the need for new protein synthesis. The later phase does require new protein and RNA synthesis, which results in the cosntruction of new presynaptic active zones and postsynaptic receptors.

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A. Definition of the mean firing rate via a temporal average.

B. Neuronal gain function (FI curve). The output spike rate is given as a function of the total somatic input current I_0 .

Neuronal Rate Codes

Figure from: Gerstner and Kistler Spiking Neuron Models. Single Neurons, Populations, Plasticity Cambridge University Press, 2002

38

Spike density in the Peri-Stimulus-Time Histogram (PSTH) as an average over several runs of the experiment.

Figure from: Gerstner and Kistler Spiking Neuron Models. Single Neurons, Populations, Plasticity Cambridge University Press, 2002

39

Example of the MUA and LFP responses of one site (ckbfa12) to repeated presentations of 3 different images (food, a hand and a masked human face). The MUA responses (middle part) are shown as raster plots where each row shows a separate repetition and each tick indicates a spike. The gray box denotes the stimulus presentation period (100 ms). There is a clear enhancement in the spike rate approximately 110 ms after appearance of the hand picture. The LFP responses (arbitrary units) are shown in the bottom part, the gray traces show the individual repetitions and the red trace shows the average. (Kreiman etal, 2006)

40

A. A postsynpatic neuron receives spike input from the population m with activity Am.

B. The population activity is defined as the fraction of neurons that are active in a short interval [t, t + t] divided by t.

Figure from: Gerstner and Kistler Spiking Neuron Models. Single Neurons, Populations, Plasticity Cambridge University Press, 2002

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Neuronal Event Codes

Time-to-first spike.

The spike train of three neurons are shown. The third neuron from the top is the first one to fire a spike after the stimulus onset (arrow). The dashed line indicates the time course of the stimulus.

Figure from: Gerstner and Kistler Spiking Neuron Models. Single Neurons, Populations, Plasticity Cambridge University Press, 2002

42

Phase. The neurons fire at different phases with respect to the background oscillation, eg local field potential (dashed). The phase could code relevant information.

Figure from: Gerstner and Kistler Spiking Neuron Models. Single Neurons, Populations, Plasticity Cambridge University Press, 2002

43

Local Field Potential (LFP) signals for 5 different objects Visual stimuli (example images of dog, hand, airplane, cat, and a different dog) are shown, and neural signals in the monkey inferotemporal (IT) cortex appear 100 msec later. Local field potential signals (LFPs, in red) are emitted by the web of input activity, mostly from the neurons' dendrites (the gray, tree-like branches). The researchers also recorded the neurons' output activity in the form of spikes (not shown) produced at the cell bodies (triangles). The signals vary in strength for different objects. Comparing the input (LFP) and output (spiking) signals can help to understand how object shapes are processed in the brain. (Hung & Kreiman, 2007)

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Synchrony. The upper four neurons are nearly synchronous, two other neurons at the bottom are not synchronized with the others.

Figure from: Gerstner and Kistler Spiking Neuron Models. Single Neurons, Populations, Plasticity Cambridge University Press, 2002