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Introduction to Neurobiology Yonatan Loewenstein [email protected] Introduction to Neurobiology • Introduction to computational neuroscience: Applications in the study of learning, memory and decision making Delayed match-to-sample task Delayed match-to-sample task Delayed match-to-sample task Delayed match-to-sample task 16 sec 0.2 sec 0.2 sec time Activity of a single neuron in the temporal cortex Miyashita (1988) Activity of a single neuron in the temporal cortex The visual system “photoreceptors” “ganglion cells” “LGN” “V1” “temporal ctx” F-I curve of single neurons Berger, EPFL, unpublished Neurons and synapses Nelson, Brandeis Characteristic time of neurons and synapses is typically ~1-100 msec “photoreceptors” “ganglion cells” “LGN” “V1” “temporal ctx” “photoreceptors” “ganglion cells” “LGN” “V1” “temporal ctx” “photoreceptors” “ganglion cells” “LGN” “V1” “temporal ctx” “photoreceptors” “ganglion cells” “LGN” “V1” “temporal ctx” “photoreceptors” “ganglion cells” “LGN” “V1” “temporal ctx” “photoreceptors” “ganglion cells” “LGN” “V1” “temporal ctx” “photoreceptors” “ganglion cells” “LGN” “V1” “temporal ctx” “photoreceptors” “ganglion cells” “LGN” “V1” “temporal ctx” “photoreceptors” “ganglion cells” “LGN” “V1” “temporal ctx” “photoreceptors” “ganglion cells” “LGN” “V1” “temporal ctx” “photoreceptors” “ganglion cells” “LGN” “V1” “temporal ctx” “photoreceptors” “ganglion cells” “LGN” “V1” “temporal ctx” “photoreceptors” “ganglion cells” “LGN” “V1” “temporal ctx” “photoreceptors” “ganglion cells” “LGN” “V1” “temporal ctx” “photoreceptors” “ganglion cells” “LGN” “V1” “temporal ctx” “photoreceptors” “ganglion cells” “LGN” “V1” “temporal ctx” “photoreceptors” “ganglion cells” “LGN” “V1” “temporal ctx” “photoreceptors” “ganglion cells” “LGN” “V1” “temporal ctx” “photoreceptors” “ganglion cells” “LGN” “V1” “temporal ctx” Activity of a single neuron in the temporal cortex Activity of a single neuron in the temporal cortex Questions: • How can we explain activity that persists for 16 sec after the disappearance of the external stimulus • Why some images and not others? “photoreceptors” “ganglion cells” “LGN” “V1” “temporal ctx” “photoreceptors” “ganglion cells” “LGN” “V1” “temporal ctx” most inputs to cortical neurons are recurrent, coming from the same cortical region • How can we explain activity that persists for 16 sec after the disappearance of the external stimulus • The recurrent connections in the cortex enable persistent activity in time scales that are much longer than the time-scale of neurons and synapses • Why some images and not others? Activity of a single neuron in the cortex one of the learned stimuli new stimulus Neurons and synapses Nelson, Brandeis Synapses are plastic • What rules underlie changes in synaptic efficacies? Coincidence-based plasticity Coincidence-based plasticity Coincidence-based plasticity Coincidence-based plasticity Coincidence-based plasticity Coincidence-based plasticity Coincidence-based plasticity Hebbian plasticity “When an axon of cell A is near enough to excite cell B and repeatedly or 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, 1949 “Neurons that fire together wire together” A cortical network A cortical network A cortical network A cortical network A cortical network A cortical network Hebbian plasticity A cortical network Network can sustain activity even in the absence of input Specificity of sustained activity Specificity of sustained activity Specificity of sustained activity Specificity of sustained activity Specificity of sustained activity Activity of a single neuron in the cortex one of the learned stimuli new stimulus Difference in response can be explained as resulting from Hebbian plasticity creating strong recurrent connections A model of associative memory A model of associative memory A model of associative memory A model of associative memory A model of associative memory ‘Biological’ memories • Memories are stored in the efficacies of synapses in the brain • Recalled memories are represented in the pattern of activity of neuron new stimulus Learned stimulus Miyashita (1988) Is Hebbian plasticity, mediated by NMDA synapses, important for associative memory? Henry Gustav Molaison (1926 – 2008) Associative memory in the CA3 region of the hippocampus • Ample evidence for the involvement of the hippocampus in associative memory Neurons in area CA3 of the hippocampus form a recurrent network in which each neuron receives ~10,000 collateral inputs (2% of the network) • Synapses between strengthened cells are easily Requirement for Hippocampal CA3 NMDA Receptors in Associative Memory Recall Nakazawa et al., Science (2002) Morris water maze test day 13 day 14 Conclusion: Hippocampal CA3 NMDA receptors are required for associative memory recall Can we store several patterns in this framework? A model of associative memory A model of associative memory A model of associative memory A model of associative memory A model of associative memory A model of associative memory A model of associative memory A model of associative memory A model of associative memory A model of associative memory A model of associative memory A model of associative memory Question: If neurons A and B are grouped by a memory pattern and neurons B and C are grouped by another memory pattern, will neuron C be activated as a result of the activation of neuron A? • What is the memory capacity of the network? ‘Biological’ memories • Associative: recall is based on content rather than on the address • A transient cue induces a sustained recall • Robust to minor failures of the hardware • Distributed • Fast dynamics relative to the intrinsic timescale of the neuron parallel computing