Synaptic Plasticity. Synaptic efficacy (strength) is changing with time. Many of these changes are activity-dependent , i.e. the magnitude and direction of change depend on the activity of pre- and post-synaptic neuron. Some of the mechanisms involved:.
Synaptic efficacy (strength) is changing with time.
Many of these changes are activity-dependent, i.e. the magnitude and direction of change depend on the activity of pre- and post-synaptic neuron.
Some of the mechanisms involved:
- Changes in the amount of neurotransmitter released.
- Biophysical changes in ion channels.
- Morphological alterations of spines or dendritic branches.
- Modulatory action of other transmitters.
- Changes in gene transcription.
- Synaptic loss or sprouting.
“When an axon of cell A is near enough to excite a cell B and repeatedly and persistently takes part in firing it, some growth process or metabolic change takes place in one or both cells such that A’s efficiency, as one of the cells firing B, is increased.”
Donald Hebb, “Organization of Behavior”, 1949
Long-Term Potentiation (LTP)
Cross-section of the hippocampus:
Brain slice preparation of the hippocampus:
Typical LTP experiment: record from cell in hippocampus area CA1 (receives Schaffer collaterals from area CA3). In addition, stimulate two sets of input fibers.
Typical LTP experiment: record EPSP’s in CA1 cells (magnitude)
Step 1: weakly stimulate input 1 to establish baseline
Step 2: give strong stimulus (tetanus) in same fibers (arrow)
Step 3: continue weak stimulation to record increased responses
Step 4: throughout, check for responses in control fibers (input 2)
LTP is input specific.
LTP is long-lasting (hours, days, weeks).
LTP results when synaptic stimulation coincides with postsynaptic depolarization (achieved by cooperativity of many coactive synapses during tetanus).
The timing of the postsynaptic response relative to the synaptic inputs is critical.
LTP has Hebbian characteristics (“what fires together wires together”, or, in this case, connects together more strongly).
LTP may produce synaptic “sprouting”.
Old(er) view: Associative requirement is mediated by the voltage-dependent characteristics of the NMDA receptor.
New discovery (1994): Active conductances in dendrites mediate back-propagation of AP’s into the dendritic tree.
Basic Idea: Change in synaptic strength depends on the precise temporal difference between pre- and post-synaptic neuronal firing (causality!).
Integrator or Coincidence Detector?
Synchronous inputs really matter!
Neuron in MT
Neuron in IT (object selective)
Desimone et al., 1984
Neurons in V1 (orientation selective)
PSTH (firing rate)
Engel et al., 1991
Rate coding versus temporal coding
One major mechanism of how neurons encode information is through their firing rate (number of AP’s per second). – Example: orientation selectivity.
Another major mechanism is synchronization (AP’s occurring together in time). – Example: perceptual grouping.
Synchrony could affect other neurons (e.g. through spatial summation – see unit 1).
Components of (most) neural models:
- Units and connections
- Inputs and outputs
- Activation function
- Learning rule
“Why is the mind in the head? Because there,
and only there, are hosts of possible
connections to be formed as time and
circumstance demand. Each new connection,
serves to set the stage for others yet to
come and better fitted to adapt us to the
world, for through the cortex pass the
greatest inverse feedbacks whose function
is the purposive life of the human intellect.”
Warren S. McCullogh, Hixon Symposium 1951.