The Brain PDF

The Brain  Marr’s Three Levels What are the goals What information is What physical objects and properties of manipulated, in what in the world carry out this system? format, and using this process? what rules?  What are...

The Brain  Marr’s Three Levels What are the goals What information is What physical objects and properties of manipulated, in what in the world carry out this system? format, and using this process? what rules?  What are the basic units of information processing in the brain? How can learning happen biologically? How is the brain organized at multiple scales? 17th century BC: Egyptians recognized mental symptoms as coming from brain injury 4th century BC: Aristotle argues that the heart is the center of sensation Ø Only some animals have brains, all have hearts Ø Only heart affected by emotion Ø Brain not sensitive to pain Ø Heart more centrally located Santiago Ramón y Cajal (late 1800s) Ø Other neurons that synapse to this neuron influence its voltage Ø If voltage rises above a threshold, this causes a sudden spike Ø Spikes travel down the axon, and can influence other neurons Gross et al., 2013 Berning et al., 2012  Presynaptic neurons 1 Postsynaptic neuron 2 3 Passing signals between neurons: 1. Presynaptic spikes temporarily change the voltage in postsynaptic neuron 2. Synapses have weighted strengths – some presynaptic spikes matter more than others 3. Synapses can be inhibitory – presynaptic spikes can suppress postsynaptic spikes So a neuron spikes only when recent excitatory inputs are larger than recent inhibitory inputs Retinal bipolar cells “Center-surround” receptive field - - + - - https://nba.uth.tmc.edu/neuroscience/m/s1/introduction.html  Building a line segment detector - - + - - - - + - -- - + - - - - + - - What about cognitive functions learned from experience? How to set synapses weights and signs? Many kinds of learning rules have been proposed, and the answer may differ in different neurons Common theme: a neuron should a) detect an “interesting” pattern in its inputs b) signal as quickly as possible when it sees it Spike Timing Dependent Plasticity (STDP) Ø If a presynaptic spike comes just before a postsynaptic spike, increase synaptic weight Ø If a presynaptic spike comes just after a postsynaptic spike, decrease synaptic weight Zhang et al., 1998 Intuition: Postsynaptic spike means something interesting happened, and we want to be sensitive to the earliest signal that this interesting thing happened STDP is an example of unsupervised learning Ø Just trying to learn some structure across inputs and/or across time Ø Unrelated to goals or mistakes To “program” the brain to perform a cognitive task, we also need supervised learning Ø System is told what the desired output is for each set of inputs Ø Goal: wire up synapses to achieve a desired output Ø We know how to do this in connectionist models (see 9/18 lecture), unclear how the brain does this Orientation map in visual cortex Afgoustidis 2015 Jung et al. 2022 Retinotopic maps in visual cortex Heeger, 2006  ry tos r/ so ma oto en so M Vi o ry l su di t a al Au V isu [Middle surface of [Side view of left hemisphere] left hemisphere] Sydnor et al., 2021 Sydnor et al., 2021 Sydnor et al., 2021 Sydnor et al., 2021  Hippocampus Eye movement control Object representation Motion processing First cortical region (V1) Retina Felleman & Van Essen, 1991 Perino et al. 2021 ? “This set of brain regions: 1. creates temporal memories for events 2. by tagging events with a temporal code 3. defined by oscillating activation levels” “This circuit: 1. tries to identify edges in the world 2. by combining center/surround detectors 3. connected to visual cortex neurons” Next week: Studying the Mind by Measuring the Brain How do we measure the brain? What can brain measurements tell us about cognition? How can brain measurements go beyond behavior? Presynaptic 1 Presynaptic 2 Presynaptic 3 Presynaptic 4 Presynaptic 5 Presynaptic 6 Postsynaptic Time STDP Update: Ø Increase synaptic strength for 2,3,4,5 Ø Decrease for 6 Presynaptic 1 Presynaptic 2 Presynaptic 3 Presynaptic 4 Presynaptic 5 Presynaptic 6 Postsynaptic Time STDP Update: Ø Increase synaptic strength for 3,4,5 Ø Decrease for 2 Presynaptic 1 Presynaptic 2 Presynaptic 3 Presynaptic 4 Presynaptic 5 Presynaptic 6 Postsynaptic Time Ø Solution will stabilize around here Ø STDP took us from a weak detector of this pattern (3+4+5 firing together) to a fast, tuned detector Ø This learning can be carried out right in the synapse Does reduced thickness of the cortex predict psychopathology? Association regions more often implicated in psychopathology Sydnor et al., 2021