Prof. Greg Francis 8/14/12 PSY 200: Intro. to Cognitive Psychology 1 Purdue University Receptive fields PSY 200 Greg Francis Lecture 06 How do you recognize your grandmother? Purdue University Action potential  With enough excitatory input, a cell produces an action potential that sends a signal down its axon to other cells  But a single action potential has little effect  If the input stays present, the cell produces another, and another,…  A rapid series of action potentials can influence other cells  The number of action potentials in a certain length of time determines the firing rate of the cell Purdue University Firing rate  8 spikes (action potentials) during 300 ms  Firing rate of 27 Hz (27 spikes per second) Purdue University Specificity  Two key questions in cognitive neuroscience are  What stimulus (or stimuli) makes a given cell fire at a strong rate? » something red? » a pen? » your grandmother?  What does it mean when a given cell fires strongly? » You are thinking of something? » Seeing something? » Remembering something? Purdue University Receptive field  The set of stimuli that reliably changes a cell’s firing rate.  A stimulus could excite the cell  above normal firing rate  Or inhibit the cell  below normal firing rate Purdue University Receptive field  Receptive fields are very useful for studies of spatial perception  Touch involves sensitivity to pressure on skin  The loops indicate the regions where a single neuron responds to pressureProf. Greg Francis 8/14/12 PSY 200: Intro. to Cognitive Psychology 2 Purdue University Receptive field  Receptive fields are very useful for studies of visual perception  Vision involves spatial patterns of light Purdue University Receptors  The back of the eye contains tightly packed sensors called rods and cones that detect light at a particular location Light Purdue University Receptors  Light sensors (rods and cones) respond to light at a particular location in the back of the eye  produces a neural response Light Purdue University Receptors  A receptor has a simple receptive field  it responds to light of the right wavelength (color) and the right position Purdue University Blind spot  Where nerves leave the back of the eye, there are no light receptors  light that hits this spot is not visible Purdue University Blind spot  In CogLab you mapped your blind spot  my data looks like thisProf. Greg Francis 8/14/12 PSY 200: Intro. to Cognitive Psychology 3 Purdue University Blind spot  Here’s the average data for the whole class  141 participants Purdue University Network  Light receptors do not just pass information to the brain  Neurons are hooked together in an inhibitory way Light Purdue University Evidence of inhibition  Stimulation of a center region alone gives a strong response Light 0510152025303540Time Firing rate Purdue University Evidence of inhibition  Adding light to a surrounding region can reduce the response Light 0510152025303540Time Firing rate Purdue University Receptive field  The receptive field of this cell includes any place on the retina where light excites the cell and any place where light inhibits the cell  On-center, off-surround + - Purdue University Spatial responses  The spatial pattern of excitation (center) and inhibition (surround) means the cell is sensitive to the location of a small spot of lightProf. Greg Francis 8/14/12 PSY 200: Intro. to Cognitive Psychology 4 Purdue University Spatial responses  You have many different such cells with receptive fields that are centered at different locations  They respond differently to an edge Purdue University Simple cells  On-center, off-surround cells send action potentials to simple cells in parts of visual cortex, which have oriented receptive fields Purdue University Simple cells  On-center, off-surround cells send action potentials to simple cells in parts of visual cortex, which have oriented receptive fields Purdue University Simple cells  Simple cells come in a variety of types, but all are sensitive to bars or edges of a preferred orientation at a particular location Purdue University Information processing  For simple cells, an image like this Purdue University Information processing  For simple cells, an image like this  is coded something like this Strong responses at edges!Prof. Greg Francis 8/14/12 PSY 200: Intro. to Cognitive Psychology 5 Purdue University Complex cells  Many simple cells feed into a complex cell, which is insensitive to direction of contrast and responds to an oriented bar in many different places Purdue University Complex cells  Many simple cells feed into a complex cell, which is insensitive to direction of contrast and responds to an oriented bar in many different places  Often these cells are also sensitive to directions of motion Purdue University Receptive field hierarchy  Receptive fields inherit some properties from “lower-level” cells  But they also gain new selectivity by interacting with each other (and across levels) Purdue University Higher order cells  Complex cells feed into hypercomplex cells, which are sensitive to some types of curves and visual forms  Receptive fields seem to get ever more complex  What does this mean?  Is there a grandmother cell? Purdue University Receptive field  In the inferior temporal cortex of monkeys  some cells appear to have receptive fields that respond to monkey faces, in profile Purdue University Grandmother cells  It could be that a single cell has a receptive field selectively tuned to respond to the image of your grandmother  but it is unlikely  not enough cells  cell death  Receptive fields become less useful as we search for neural representations of non-sensory concepts  What is the receptive field of a neuron that codes “love” or “trust”?Prof. Greg Francis 8/14/12 PSY 200: Intro. to Cognitive Psychology 6 Purdue University Conclusions  Receptive fields  any stimulus that affects a cell’s firing rate  excitatory  inhibitory  Very useful for studies of the visual nervous system  Lots of issues left