Toad +
Stool
Severed Corpus Callosum •  “Bell + Music” example –  “Bell” registers in Joe’s right hemisphere •  Joe then points to bell picture with his right arm •  There is ipsilateral control of gross arm movements –  His leD brain sees his arm poinEng to bell picture •  Explains his choice in terms of what it knows •  The word “Music,” plus a memory of hearing bells Some methods for localizing brain funcEon •  Transcranial magneEc sEmulaEon (TMS) –  Creates temporary (!) lesions –  Allows experimental designs •  FuncEonal magneEc resonance imaging (fMRI) –  Measures blood flow, which increases to Essue where neurons are firing faster –  Inferences about funcEon depend on subtrac.ve method +
Some methods for localizing brain funcEon •  Lesions –  E.g., severed corpus callosum –  Inference: the damaged Essue supported the impaired funcEon –  Are these experiments? •  No; there’s no manipulaEon SubtracEve method Looking at a s.mulus Looking at blank screen -
Difference =
Inference: Looking at the sEmulus acEvates posterior corEcal areas Good internal validity requires careful design of control condiEons 1
The neuron Structure: FuncEon: Dendrites Soma Axon Inputs ComputaEon Output Neurons SensaEon -­‐ -­‐ ++ Polariza.on: Imbalance of electrical charges across cell membrane ComputaEon: How polarized am I? If depolarized, then fire: Propagate depolarizaEon down axon This propagaEon is an ac.on poten.al (“spike”) AcEon potenEal Synapses Synapse: Small gap between axon terminal and dendrite Bridged by neurotransmiders released when a spike arrives Some neurotransmiders add +’s, some add -­‐’s Excitatory synapses add +’s to the soma + + -­‐ -­‐ -­‐ ++ Each spike travels at constant speed The firing rate (# spikes/second) determines signal strength Traffic analogy: strong signal = more cars (same speed) Signal strength Inhibitory synapses add -­‐’s to the soma Signal strength 2
SensaEon •  SensaEon begins with transduc.on: –  Light/sound/pressure to neural signals –  Vision: Photoreceptor (rods, cones) •  Ends with percepEon of objects –  Somewhere in the cortex –  Many stages in between RecepEve field of a ganglion cell Dim
Bright
•  Which ganglion cell(s) are firing at baseline? –  B and D •  Which ganglion cell(s) are firing above baseline? –  A •  Which ganglion cell(s) are firing below baseline? –  C 3
Dim
Bright
The Hermann grid
•  Ganglion cells project topographically to primary visual cortex •  There, the brain detects rows of cells firing above or below baseline –  So detects edges in the world –  Edges are important for percepEon The Hermann grid
-
-
+
-
-
+
-
-
+
-
-
Ganglion cells at
intersections send weaker
signals than their neighbors
The brain interprets the
weaker signal as less light
hitting the “+” region
The Hermann grid
Why do the spots go away
when you foveate them?
The fovea has higher visual
acuity (ganglions there have
smaller receptive fields)
4