Prof. Greg Francis 5/23/081Receptive fieldsIIE 269: Cognitive PsychologyGreg FrancisLecture 06How do you recognize your grandmother?Action potential With enough excitatory input, a cellproduces an action potential that sends asignal down its axon to other cells If the input stays present, the cellproduces another, and another,… The number of action potentials in acertain length of time determines the firingrate of the cellFiring rate You can watch the firing rate over time0204060801001 2 3Time (seconds)# spikes / 500 millisecondsSpecificity A key question in cognitive neuroscience is What stimulus (or stimuli) make a cell fire at astrong rate? e.g.» something red?» a pen?» your grandmother?Receptive field A receptive field consists of the set ofstimuli that reliably change a cell’s firingrate. A stimulus could excite the cell above normal firing rate Or inhibit the cell below normal firing rateReceptive field In the inferior temporal cortex of monkeys some cellsappear to havereceptive fieldsthat respond tomonkey faces, inprofileProf. Greg Francis 5/23/082Grandmother cells Could there be a cell in your head that firesrapidly whenever you see your grandmother? What would that tell us about cognition andneural activity? It is difficult to test for grandmother cellsbecause there are billions of neurons!Receptive field Identifying receptive fields is particularly usefulfor neuron responses to spatial senses touch, visionskindendritesReceptive field Receptive fields are very useful for studies of visual perception Vision involves spatial patterns of lightReceptors The back of the eyecontains tightlypacked sensorscalled rods andcones that detectlight at a particularlocationLightReceptors Light sensors (rods and cones) respond to lightat a particular location in the back of the eye produces a neural responseLightReceptors A receptor has a simple receptive field it responds to light of the right wavelength (color) and the rightpositionProf. Greg Francis 5/23/083Blind spot Where nerves leavethe back of the eye,there are no lightreceptors light that hits this spot isnot visibleBlind spot In Coglab you mapped your blind spot mydatalookslikethisBlind spot Here’s the average data for the whole class 139 participantsNetwork Light receptors do not just pass informationto the brain Neurons are hooked together in an inhibitorywayLightEvidence of inhibition Stimulation of a center region alonegives a strong responseLight0510152025303540TimeFiring rateEvidence of inhibition Adding light to a surrounding regioncan reduce the responseLight0510152025303540TimeFiring rateProf. Greg Francis 5/23/084Receptive field The receptive field of this cellincludes any place on the retinawhere light excites the cell and anyplace where light inhibits the cell On-center, off-surround+-On-center, off-surround The structure of these receptive fieldsmean they respond best to small spotsof light they do not respond well to homogeneousfields of light they respond OK to some edgesSimple cells On-center, off-surround cells send actionpotentials to simple cells in parts of visual cortex,which have more complicated, oriented,receptive fieldsSimple cells Simple cells come in a variety of types,but all are sensitive to bars or edges of apreferred orientation at a particularlocationSimple cells Look at howa simple cell(left)responds tobars ofdifferentorientations The data onthe right arefor anunorientedcellInformation processing For simple cells, an image like thisProf. Greg Francis 5/23/085Information processing For simple cells, an image like this is coded something like thisStrongresponsesat edges!Complex cells Simple cells feed intocomplex cells, which areinsensitive to direction ofcontrast Often these cells are alsosensitive to directions ofmotionHigher order cells Complex cells feed into hypercomplex cells,which are sensitive to some types of curvesand visual forms Eventually, in monkeys, this processproduces cells that respond to monkeyfaces Is there a grandmother cell?Grandmother cells It could be that a cell has a receptive fieldtuned to respond to the image of yourgrandmother but it is unlikely not enough cells cell death Receptive fields become less useful formore complicated situationsLaboratory You have to complete a CogLab thatdemonstrates receptive fields Some hints on how to go about it watch meConclusions Receptive fields any stimulus that affects a cell’s firing rate excitatory inhibitory Very useful for studies of the visualnervous system Lots of issues left unresolvedProf. Greg Francis 5/23/086Next time Networks of neurons Connections between cells Feedback -- resonance CogLab on Receptive fields due! Seeing things that are not