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Facerecognition

Fusiformfacearea

BothhemispherescanrecognizeobjectsOnlytherighthemisphererecognizesfaces(andfacesarespecial)

+

+Flowers

Face

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+FaceFruit

+ToadStool

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+

Somemethodsforlocalizingbrainfunction

•  Lesions– E.g.,severedcorpuscallosum– Inference:thedamagedtissuesupportedtheimpairedfunction

– Aretheseexperiments?• No;there’snomanipulation

Somemethodsforlocalizingbrainfunction

•  Transcranialmagneticstimulation(TMS)– Createstemporary(!)lesions– Allowsexperimentaldesigns

Somemethodsforlocalizingbrainfunction

•  Functionalmagneticresonanceimaging(fMRI)– Measuresbloodflow,whichincreasestotissuewhereneuronsarefiringfaster

–  Inferencesaboutfunctiondependonsubtractivemethod

Subtractivemethod

Brainactivationwhilelookingatastimulus

Brainactivationwhilelookingatblankscreen

Difference

- =

Lookingatstimulusactivatesposteriorcorticalareas

Somekeyideas

•  Localizationoffunction– Differentbrainareasprocessdifferentinformation

•  Projectionareas– Specificcorticalareasthatreceivesensoryinputsorsendoutputstomuscles

•  Contralateralorganization– Projectionareascommunicatewiththeoppositesideofthebodyfromwheretheyarelocated

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Somekeyideas

•  Topographic(map-like)organization– Neighboringbraintissuemapstoneighboringareasofspaceorthebody Neurons

Theneuron

Inputs Computation OutputFunction:

Computation:AmIdepolarized?Ifso,then“fire”:PropagatedepolarizationdownaxonThispropagationisanactionpotential(“spike”)

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++ Polarization:Imbalanceofelectricalchargesacrosscellmembrane

AxonSomaDendritesStructure:

Actionpotential

EachspiketravelsatconstantspeedThefiringrate(numberofspikes/second)determinessignalstrengthAnalogy:Trafficonthehighway

Strongsignal=manycars,weaksignal=fewcarsInbothcases,allcarsgothesamespeed

Synapses

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++

-

+

ExcitatorysynapseHelpsdepolarizethesoma

InhibitorysynapseHelpspolarizethesoma

Synapse:SmallgapbetweenaxonterminalanddendriteBridgedbyneurotransmittersreleasedwhenaspikearrivesSomesynapsesareexcitatory,someareinhibitory

Signalstrength

Assumethisisthebaselinesignalstrength.(Neuronsarealwaysfiring.They’realwaysreceivinginputsandgeneratingoutputs.)

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Signalstrength

…increasestheoutputsignalabovebaseline

Astronginputsignalfromanexcitatoryneuron…

Signalstrength

…depressestheoutputsignalbelowbaseline

Astronginputsignalfromaninhibitoryneuron…

Sensation

Sensation

•  Sensationbeginswithtransduction:– Cellsconvertastimulustoneuralsignals– E.g.,vision:Photoreceptorcells(rods,cones)

•  Endswithperceptionofobjects– Somewhereinthecortex– Manystagesinbetween

Keythings:RetinaFoveaOpticnerve

Thefoveaisaregionofhighacuity

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ReceptivefieldsontheretinaAreceptivefieldreceivesthelightforaneuronThatneuronforeachfieldsendsitsoutputtotheprimaryvisualcortex

Receptivefieldsonsurfaceofretina

Receptivefieldofaganglioncell

RefertothisinsteadofFig.4.31inthebook

WhenlighthitsanR,theRsendsastrongersignaltoitstargetneuron(horizontalorbipolarcell)

Dimly lit Brightly lit

•  Whichganglioncell(s)arefiringatbaseline?–  BandD

•  Whichganglioncell(s)arefiringabovebaseline?–  A

•  Whichganglioncell(s)arefiringbelowbaseline?–  C

•  Ganglioncellsprojecttopographicallytoprimaryvisualcortex

•  Thevisualcortexdetectsrowsofcellsfiringaboveorbelowbaseline–  Sodetectsedgesintheworld–  Edgeshelpdefineobjects

Dimly lit Brightly lit

The Hermann grid

+ -- - + --+ -- -

The ganglion cell at the intersection sends a weaker signal than its neighbors The brain has to guess why the signal is weaker: less light hitting “+” or more light hitting “-” Guesses less light hitting “+”

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The Hermann grid

Why do the spots go away when you foveate them? Foveal ganglion cells have smaller receptive fields