INTRODUCTION TO THE PHYSIOLOGY OF PERCEPTION Chapter 2

Transcription

1 Intr t the Physilgy f Perceptin 1 Perceptin (PSY 4204) Christine L. Ruva, Ph.D. INTRODUCTION TO THE PHYSIOLOGY OF PERCEPTION Chapter 2 RECEPTORS & NEURAL PROCESSING Theme f Chapter: We d nt just perceive what is ut there. We perceive what is ut there as filtered thrugh the prperties f ur visual system. OVERVIEW OF QUESTIONS Hw are physilgical prcesses invlved in perceptin? Hw can electrical signals in the nervus system represent bjects in the envirnment? BASIC BRAIN STRUCTURE The brain has mdular rganizatin The sensry mdalities have primary receiving areas Visin - ccipital lbe Auditin - tempral lbe Tactile senses - parietal lbe Frntal lbe crdinates infrmatin received frm tw r mre senses STRUCTURE & FUNCTION OF NEURONS Dendrites: frm neighbring neurns and frm envirnmental stimuli. Axn: extends frm bdy and t ther neurns. Synapses: structure mediating chemical r electrical cmmunicatin between neurns. Terminal Buttns r Synaptic Vesicles: small knbs at the end f the axnal branches that. Neurtransmitters: chemical transmissin f infrmatin released frm neurn. Myelin: fatty prtein insulatin surrunding an axn that imprves the f actin ptentials. Myelinatin f neurns is nt cmplete until? Ndes f Ranvier: Multiple Sclersis:

2 Intr t the Physilgy f Perceptin 2 STRUCTURE & FUNCTION OF NEURONS (cntinued) Nerves: are made up f many (axns). These axns are called nerve fibers. Purpse f a neurn is t transmit infrmatin t ther neurns. Receptrs - specialized neurns that respnd t specific kinds f energy. Neurns cme in different shape and sizes depending n lcatin and functin in the NS. Recrding Neural Signals Micrelectrdes are used t recrd frm single neurns. Recrding electrde is inside the nerve fiber. Reference electrde is utside the fiber. Difference in charge between them is -70 mv when the cell is at rest. This negative charge f the neurn relative t its surrundings is the resting ptential (plarized). ACTION POTENTIAL OF NEURONS Neurns are bathed in liquid. Thus, the neural impulse is a wet electrical signal. Ins: small electrically charged particles. The 2 types f ins (psitive and negatively charged) resemble the 2 ples n a battery. Sdium ins (Na + ) psitive charge Chlrine ins (CL - ) negative charge Ptassium ins (K + ) psitive charge Membranes have selective permeability The permeability f the membrane changes as a result f actin ptentials allwing sdium and ptassium t crss the membrane. Actin Ptential (Nerve Impulse): this is a rapid increase in psitive charge (deplarizatin) that is caused by changes in the membrane s permeability. Na+ flws the fiber making the neurn mre (rising phase). K+ flws f the fiber making the neurn mre (dwnward phase) bringing the charge back t resting level. Sdium-Ptassium Pump: cntinuusly pumps sdium t the f the axn and ptassium t the, thus maintaining sdium and ptassium at their.

3 Intr t the Physilgy f Perceptin 3 Basic Prperties f Actin Ptentials prpagated respnse : the actin ptential remains the as it travels dwn the axn n matter hw far it travels. Changing the intensity f a stimulus des nt affect the size f the actin ptential, but des effect the Refractry Perid: recvery perid after actin ptential which lasts apprximately 1 millisecnd (max rate f firing = impulses per secnd). Spntaneus Activity: many axns fire withut any. Chemical & Electrical Events at the Synapse The actin ptential des NOT crss the synapse. It triggers the release f neurtransmitters. Presynaptic neurn Pstsynaptic neurn Lck and Key System: Types f Neurtransmitters A neurn s charge is plarized when at rest because the slutin inside neurn is mre negative than slutin n the utside. Excitatry transmitters - Neurn becmes mre the likelihd f an actin ptential Inhibitry transmitters - Neurn becmes mre the likelihd f an actin ptential NEURAL PROCESSING BY EXCITATION & INHIBITION Neural Circuits are grups f neurns cnnected by excitatry and inhibitry synapses. A linear circuit has cnvergence and nly excitatry inputs. Input int each receptr has effect n the utput f neighbring circuits. Each circuit can nly indicate single spt f stimulatin. Firing receptr B prvides n infrmatin n length f line.

4 NEURAL CIRCUITS CONTINUED Intr t the Physilgy f Perceptin 4 Cnvergent circuit with nly excitatry cnnectins. Input frm each receptr int the next neurn in the circuit. Output frm cnvergent system varies based n input. Cnvergent circuit with excitatry and inhibitry cnnectins: Inputs frm receptrs summate t determine utput f circuit Summatin f inputs result in: fr single inputs & lng stimuli fr medium length stimulus (size inf) RECEPTIVE FIELDS Receptive Fields: the area f the retina that, when stimulated, influences the firing rate f ganglin cells. Receptive fields are determined by mnitring single cell respnses. Stimulus is presented t retina and respnse f cell is measured by an electrde. Bth excitatry areas and inhibitry areas make up the neurn s receptive field. Excitatry r On respnse Inhibitry r Off respnse - Center-surrund receptive fields are arranged with a center area that respnds ne way and a surrund area that acts anther. Excitatry-center-Inhibitry surrund Inhibitry-center-Excitatry surrund Center-Surrund Antagnism Output f center-surrund receptive field changes depending n area stimulated: Highest respnse when is stimulated Lwest respnse when is stimulated Intermediate respnses when are stimulated Sensry Cde: Representatin f Envirnment Sensry cde - representatin f perceived bjects thrugh neural firing Specificity cding - specific neurns respnding t specific stimuli Leads t the hypthesis Recent research shws cells in the that respnd t cncepts such as Halle Berry.

5 Intr t the Physilgy f Perceptin 5 Sensry Cde: Representatin f Envirnment cntinued Prblems with specificity cding: t assign specific neurns Mst neurns respnd t. Distributed cding - Large number f stimuli can be cded by a few neurns. Mind-bdy Prblem Hw d physilgical prcesses becme transfrmed int perceptual experience? Easy prblem f cnsciusness Neural crrelate f cnsciusness (NCC) - hw physilgical respnses crrelate with experience Hard prblem f cnsciusness Hw d physilgical respnses cause experience? FEATURE DETECTORS Sme neurns in the visual crtex respnd nly t certain types f visual infrmatin. Fr example, a vertical line. These cells are called feature detectrs Simple crtical cell Cmplex crtical cell End-stpped crtical cell These are measured in a similar way as visual fields. Feature Detectrs are determined by mnitring single cell respnses. Stimulus is presented t retina and respnse f cell is measured by an electrde. TO WHAT EXTENT DO WE LEARN TO PERCEIVE? Adults wh were brn blind, but gained sight later in life (Gregry, 1978; vn Senden, 1932). Example: Surgery invlved transplanting stem cells nt the surface f the eye t replace the scar tissue. Michael May, age 46, was blind fr 43 years lst his visin at age 3 ½ years) and nw 3 years after surgery his right eye functins perfectly, but unless his brain catches up there is still a lt he cannt see. Sept 2003 Nature Neurscience Similar experiments were cnducted with cats (Weisel, 1982).

6 Selective Rearing Studies Selective Rearing Studies: Animals reared in specific envirnment Limits type f stimuli present Blakemre & Cper, 1970: Selective Rearing Study Intr t the Physilgy f Perceptin 6 Kittens were reared in the dark except fr a perid each day when they were expsed t lines f ne rientatin (hrizntal r vertical). Kittens expsed t hrizntal lines had few, if any, cells that maximally respnded t rientatins ther than hrizntal. Cells that were nt stimulated became. Their eyes had nt degenerated and their retinas still relayed signals t their visual crtex, but lacking stimulatin, the crtical cells had nt develped nrmal cnnectins. Thus, the animals remained. Visual Develpment Severe cngenital astigmatism creates a visual envirnment similar t that f the. crnea is distrted, causing the image t be ut f fcus in. permanent neural changes ccur - visin is impaired in ne rientatin. must be crrected befre age Strabismus the eyes are, resulting in excessive disparity between the tw eyes' input t the brain. ne eye's input is. lss f bincularly driven crtical cells impaired: critical perid fr bincular visin after age little develpment ccurs