Reminder
•  ClassiscanceledTuesday9/20
+
Whichwordwouldtheypointto?
Face
Fruit
+
Whichwordwouldtheypointto?
Face
Fruit
Toad + Stool
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Somemethodsforlocalizing
brainfuncBon
•  Lesions
+
–  E.g.,severedcorpuscallosum
–  Inference:thedamagedBssuesupportedthe
impairedfuncBon
–  Aretheseexperiments?
•  No;there’snomanipulaBon
Somemethodsforlocalizing
brainfuncBon
•  TranscranialmagneBcsBmulaBon
(TMS)
SubtracBvemethod
Lookingat
as(mulus
–  Createstemporary(!)lesions
–  Allowsexperimentaldesigns
•  FuncBonalmagneBcresonance
imaging(fMRI)
–  Measuresbloodflow,whichincreases
toBssuewhereneuronsarefiringfaster
–  InferencesaboutfuncBondependon
subtrac(vemethod
Lookingat
blankscreen
-
Difference
=
EvidencethatlookingatthesBmulusacBvates
posteriorcorBcalareas
Goodinternalvalidityrequirescarefuldesign
ofcontrolcondiBons
Theneuron
Structure:
FuncBon:
Dendrites
Soma
Axon
Inputs
ComputaBon
Output
Neurons
SensaBon
--
++
Polariza(on:Imbalance
ofelectricalcharges
acrosscellmembrane
ComputaBon:AmIdepolarized?
Ifso,then“fire”:PropagatedepolarizaBondownaxon
ThispropagaBonisanac(onpoten(al(“spike”)
2
AcBonpotenBal
Synapses
Synapse:Smallgapbetweenaxonterminalanddendrite
Bridgedbyneurotransmieersreleasedwhenaspikearrives
Someneurotransmieersadd+’s,someadd-’s
Excitatorysynapsesadd+’stothesoma
+
+
- --
++
Eachspiketravelsatconstantspeed
Thefiringrate(#spikes/second)determinessignalstrength
Trafficanalogy:strongsignal=samespeed,lotsofcars
Signalstrength
Inhibitorysynapsesadd-’stothesoma
Signalstrength
SensaBon
•  SensaBonbeginswithtransduc(on:
–  Light/sound/pressuretoneuralsignals
–  Vision:Photoreceptorcells(rods,cones)
•  EndswithpercepBonofobjects
–  Somewhereinthecortex
–  Manystagesinbetween
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RecepBvefieldofa
ganglioncell
Dim
Bright
•  Whichganglioncell(s)arefiringatbaseline?
–  BandD
•  Whichganglioncell(s)arefiringabovebaseline?
–  A
•  Whichganglioncell(s)arefiringbelowbaseline?
–  C
Dim
Bright
The Hermann grid
•  Ganglioncellsprojecttopographicallyto
primaryvisualcortex
•  Thevisualcortexdetectsrowsofcells
firingaboveorbelowbaseline
–  Sodetectsedgesintheworld
–  Edgeshelpdefineobjects
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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)
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