Upload
others
View
8
Download
0
Embed Size (px)
Citation preview
1
VisionVision
[email protected]@cogsci.ucsd.edu
cogsci.ucsd.educogsci.ucsd.edu/~/~ksweeneyksweeney//psy260.htmlpsy260.html
Introduction to Introduction to
Physiological PsychologyPhysiological Psychology
Light-a part of the
spectrum of
Electromagnetic
Energy
(the part that’s
visible to us!)
2
In a vacuum, light travels at a constant speed of ~186,000
miles/sec. So if the frequency of the oscillation varies, the
distance between peaks (or wavelength) also varies.
(vertical)(vertical) Route within the retinaRoute within the retina
�� Rods and ConesRods and Cones
�� Bipolar CellsBipolar Cells
�� Ganglion CellsGanglion Cells
�� The axons of the The axons of the
ganglion cells form ganglion cells form
the optic nervethe optic nerve
3
Eye movementEye movement
�� Although each Although each
fixation generates a fixation generates a
different different sensationsensation at at
the level of the the level of the
retina, the brain retina, the brain
creates a single creates a single
perceptionperception
Yarbus, 1967
Why have two of them?Why have two of them?
�� ConvergenceConvergence ::–– eyes must turn slightly inward to eyes must turn slightly inward to focusfocus when when
objects are closeobjects are close
�� Binocular disparityBinocular disparity::–– difference between the images on the two difference between the images on the two
retinasretinas
�� Both are Both are greatergreater when objects are close when objects are close ––provides brain with a 3provides brain with a 3--D image and D image and distance informationdistance information
4
Sensory neurons for visionSensory neurons for vision
�� RODS and CONES:RODS and CONES:
–– Specialized neurons Specialized neurons
that respond to light that respond to light
with changes in their with changes in their
membrane potentialmembrane potential
Photoreceptors: Rods and ConesPhotoreceptors: Rods and Cones
�� RODSRODS::–– ~120 million rods~120 million rods
–– Scotopic Vision Scotopic Vision (skotos=darkness)(skotos=darkness)
–– Sensitive to brightness, but Sensitive to brightness, but
not color (shades of gray)not color (shades of gray)
5
Photoreceptors: Rods and ConesPhotoreceptors: Rods and Cones
�� RODSRODS::–– ManyMany rods converge onto rods converge onto one one
retinal ganglion cellretinal ganglion cell
–– Responsible for Responsible for lowlow--light light
visionvision
–– Not present Not present at all at all in foveain fovea
Photoreceptors: Rods and ConesPhotoreceptors: Rods and Cones
�� CONES:CONES:–– ~6 million cones~6 million cones
–– PhotopicPhotopic Vision Vision (photos=light)(photos=light)
–– Sensitive to colorSensitive to color
6
Photoreceptors: Rods and ConesPhotoreceptors: Rods and Cones
�� CONES:CONES:–– A A single retinal ganglion cell single retinal ganglion cell
receives signals from one (or receives signals from one (or
few) cones.few) cones.
–– Responsible for Responsible for high acuity high acuity
vision vision (fine detail)(fine detail)
–– Fovea contains Fovea contains only only conescones
Rods and ConesRods and Cones
�� The outer segment of a photoreceptor The outer segment of a photoreceptor
contains hundreds of contains hundreds of lamellae.lamellae.
�� Within the lamellae you find Within the lamellae you find
photopigmentsphotopigments-- molecules that contain molecules that contain
an an opsinopsin and a and a retinal.retinal. (E.g(E.g. . rhodopsinrhodopsin))
7
RhodopsinRhodopsin
�� Rhodopsin Rhodopsin is a receptor that responds to is a receptor that responds to
lightlight instead of to instead of to neurotransmitters neurotransmitters (photons bind to it)(photons bind to it)
�� When rhodopsin is exposed to light, it When rhodopsin is exposed to light, it
breaks down and the breaks down and the opsinopsin bleachesbleaches..
�� The effect of the bleaching is a change in The effect of the bleaching is a change in
the release of NTthe release of NT
–– Not the way you might think!Not the way you might think!
Strange but true…Strange but true…
�� The effect of light is to turn receptor The effect of light is to turn receptor
cells cells ““OFFOFF””… … darknessdarkness turns them turns them ““ONON””!!
�� Remember that receptors have a Remember that receptors have a
spontaneous firing rate:spontaneous firing rate:
–– They do NOT fire action potentials, but They do NOT fire action potentials, but
graded graded potentialspotentials
–– The effect of receptors firing is The effect of receptors firing is inhibition inhibition of of
the bipolar cellsthe bipolar cells
8
Transduction: how light becomes Transduction: how light becomes
neural signalsneural signals
�� A cone or rod actually releases A cone or rod actually releases
LESSLESS neurotransmitter when neurotransmitter when
stimulated by light!stimulated by light!
–– RhodopsinRhodopsin molecules are bleached molecules are bleached
by light, causing by light, causing hyperpolarization hyperpolarization
of rods. of rods.
–– Thus, inhibition: Thus, inhibition: lessless release release
of neurotransmitterof neurotransmitter (glutamate)(glutamate)
–– Result is: Result is: depolarizationdepolarization of of
bipolar cell (= more release of bipolar cell (= more release of
neurotransmitter)neurotransmitter)
–– Ganglion cell is Ganglion cell is more more likely to likely to
fire (generally)fire (generally)
The effect of a bleached The effect of a bleached
photopigment…photopigment…
�� … is that a the photoreceptor… is that a the photoreceptor’’s s membrane potential changes.membrane potential changes.
�� ReceptorReceptor’’s membrane potential affects s membrane potential affects
release of NT onto bipolar cells.release of NT onto bipolar cells.
�� Bipolar cells Bipolar cells ‘‘speakspeak’’ to ganglion cells, to ganglion cells,
which bring information to the brain. which bring information to the brain.
9
So in the So in the darkdark……
�� Photoreceptors release enough NT to Photoreceptors release enough NT to
prevent prevent bipolar cells from triggering bipolar cells from triggering
ganglion cells.ganglion cells.
–– Ganglion cells, by NOT firing, report to the Ganglion cells, by NOT firing, report to the
brain: brain: ““no lightno light””
�� And in the light?And in the light?
Cone and Rod VisionCone and Rod Vision
�� Only cones are Only cones are
found at the found at the
fovea!!fovea!!
Distribution of rods and cones
10
Cone and Rod VisionCone and Rod Vision
�� Less convergence Less convergence
in cones, in cones,
increasing increasing acuityacuity
while decreasing while decreasing
sensitivitysensitivity
�� More convergence More convergence
in rod system, in rod system,
increasing increasing
sensitivitysensitivity while while
decreasing decreasing acuityacuity
So we have a response from a So we have a response from a
ganglion cell… now what?ganglion cell… now what?
�� Bundle of ganglion cell axons exiting the Bundle of ganglion cell axons exiting the
eye: blind spot eye: blind spot
�� No receptors where No receptors where
information exits information exits
the eye:the eye:–– Visual system uses Visual system uses
information from cells information from cells
around the blind spot for around the blind spot for
““completion,completion,”” filling in filling in
the blind spotthe blind spot
11
From the Eyes to the Visual CortexFrom the Eyes to the Visual Cortex
Lateral Geniculate NucleusLateral Geniculate Nucleus
�� A nucleus within the thalamus A nucleus within the thalamus ((““relay centerrelay center””) )
–– receives information from the retina and receives information from the retina and
projects to primary visual cortex.projects to primary visual cortex.
�� Contains six Contains six ““layerslayers”” of neuronsof neurons
–– each layer receives information from only 1 each layer receives information from only 1
eye.eye.
�� First two layers: magnocellularFirst two layers: magnocellular
�� Next four layers: parvocellularNext four layers: parvocellular
12
M and P channelsM and P channels
�� MagnocellularMagnocellular
–– Larger cell bodiesLarger cell bodies
–– Responsive to movementResponsive to movement
–– Input primarily from rodsInput primarily from rods
�� ParvocellularParvocellular
–– Small cell bodiesSmall cell bodies
–– Responsive to color, fine Responsive to color, fine
detailsdetails
–– Input primarily from conesInput primarily from cones
M and P channelsM and P channels
�� Layers 1, 4, 6Layers 1, 4, 6-- contracontra
�� Layers 2, 3, 5Layers 2, 3, 5-- ipsiipsi
13
From the Eyes to the Visual CortexFrom the Eyes to the Visual Cortex
�� The visual system is The visual system is
organized organized
retinotopically: retinotopically:
–– The left hemiretina of each The left hemiretina of each eye (right visual field) eye (right visual field) connects to the right lateral connects to the right lateral geniculate nucleus (LGN)geniculate nucleus (LGN)
–– the right hemiretina (left the right hemiretina (left visual field) connects to the visual field) connects to the left LGN left LGN
Coding of Visual Coding of Visual
InformationInformation
14
Coding of information in the retinaCoding of information in the retina
�� For any sensory neuron, a receptive field For any sensory neuron, a receptive field
is the is the ‘‘placeplace’’ in which a stimulus will in which a stimulus will
cause the neuron to fire.cause the neuron to fire.
�� The receptive fields in the fovea are The receptive fields in the fovea are
smaller than in the rest of the retina. smaller than in the rest of the retina.
Receptive Receptive
FieldsFields�� Many ganglion cells Many ganglion cells
have have receptive receptive fields fields with a with a centercenter--surround surround organization: organization: excitatory and excitatory and inhibitory regions inhibitory regions separated by a separated by a circular boundarycircular boundary
�� Some cells are Some cells are ““onon--centercenter”” and some and some are are ““offoff--centercenter””
15
What does color get us?What does color get us?
What does color get us?What does color get us?
16
�� Why can you visualize red (imagine a fireWhy can you visualize red (imagine a fire--
truck)… and you can imagine a reddish truck)… and you can imagine a reddish
yellow… but it is difficult (impossible?) to yellow… but it is difficult (impossible?) to
imagine a reddish green…imagine a reddish green…
�� … or a bluish… or a bluish--yellow?!yellow?!
Color Mixing vs. Pigment MixingColor Mixing vs. Pigment Mixing
17
Color vision theoriesColor vision theories
�� Trichromatic theory:Trichromatic theory: there are 3 different there are 3 different
receptors (types of cones) in the eye, each receptors (types of cones) in the eye, each
sensitive to a single hue (red, green, blue)sensitive to a single hue (red, green, blue)
Color vision theoriesColor vision theories
�� Trichromatic theory:Trichromatic theory: there are 3 different there are 3 different
receptors (types of cones) in the eye, each receptors (types of cones) in the eye, each
sensitive to a single hue (red, green, blue)sensitive to a single hue (red, green, blue)–– Because Young noted that any color could be accounted for by Because Young noted that any color could be accounted for by
mixing just 3 mixing just 3 lightslights in various proportionsin various proportions
18
Trichromatic TheoryTrichromatic Theory
�� At the level of the retina, At the level of the retina, conescones code for code for three wavelengths of light (different opsins): three wavelengths of light (different opsins):
�� ShortShort (S), (S), MediumMedium (M), (M), LongLong (L): blue, green, red(L): blue, green, red
19
20
�� The precise The precise
distribution of distribution of
cones varies from cones varies from
person to person, person to person,
but generally but generally
speaking speaking ““blueblue””
sensitive cones are sensitive cones are
less common than less common than
““redred”” and and ““greengreen””
conesconesImage from David Williams, U of Rochester
Color BlindnessColor Blindness
� Protanopia: no red cones
– see yellow and blue, red and green
hues confused
� Deuteranopia: no green cones
– red and green hues confused
� Tritanopia: blue cones lacking
or faulty
– world seen in reds and greens, no
blue
21
�� Those with normal color Those with normal color vision should read the vision should read the number 8. number 8.
�� Those with redThose with red--green color green color vision deficiencies vision deficiencies (protanopia, deuteranopia) (protanopia, deuteranopia) should read the number 3. should read the number 3.
�� Total color blindness should Total color blindness should not be able to read any not be able to read any numeral.numeral.
The trichromatic theory The trichromatic theory
doesndoesn’’t tell the whole story…t tell the whole story…
22
The trichromatic theory The trichromatic theory
doesndoesn’’t tell the whole story…t tell the whole story…�� The retinal ganglion cells code for The retinal ganglion cells code for
complementary colors.complementary colors.
�� This is known as This is known as opponentopponent--process codingprocess coding
�� Another type of ganglion cell only Another type of ganglion cell only
encodes brightness: encodes brightness: ‘‘blackblack--whitewhite’’
23
Opponent Process Theory
�� Ganglion cellsGanglion cells
–– Three typesThree types
�� Red/green, yellow/blue, black/whiteRed/green, yellow/blue, black/white
–– Each cell represents an Each cell represents an opponent process opponent process
systemsystem
�� Resting behavior in red/green cells is midResting behavior in red/green cells is mid--level level
rate of responserate of response
�� For R+GFor R+G--, rate , rate increasesincreases when red is present, when red is present,
decreasesdecreases when green is present when green is present (opposite for R(opposite for R--G+)G+)
�� Yellow/blue (Y+BYellow/blue (Y+B--) ) increasesincreases when both red and when both red and
green are present, green are present, decreasesdecreases when blue is when blue is
presentpresent
Opponent Process TheoryOpponent Process Theory
Opposing retinal processes enable color visionOpposing retinal processes enable color vision
““ONON”” ““OFFOFF””
red red greengreen
green green red red
blue blue yellow yellow
yellowyellow blue blue
black black whitewhite
whitewhite blackblack
24
�� ReddishReddish--green? Bluishgreen? Bluish--Yellow?Yellow?
�� You canYou can’’t imagine them because ganglion t imagine them because ganglion
cells that signal red or green (or yellow cells that signal red or green (or yellow
or blue) can only or blue) can only increase increase or or decrease decrease
rate of firing, they canrate of firing, they can’’t do both at t do both at
once!!once!!
25
�� The The complementary aftercomplementary after--effect effect is caused by the fact is caused by the fact
that after adaptation, locations stimulated by green that after adaptation, locations stimulated by green
light will be less sensitive to green than to red, and light will be less sensitive to green than to red, and
vice versa. Since vice versa. Since white lightwhite light contains all colors and contains all colors and
stimulates all photoreceptors equally, those that have stimulates all photoreceptors equally, those that have
been been ““green adaptedgreen adapted”” will fire will fire ‘‘redred’’ to white light to white light
(and vice versa)(and vice versa)-- a larger a larger ““redred”” than than ““greengreen”” signal will signal will
be generated. Itbe generated. It’’s the s the locallocal imbalance between the red imbalance between the red
and green inputs to the opponent mechanism that and green inputs to the opponent mechanism that
generates the (relatively weak) color aftergenerates the (relatively weak) color after--effects.effects.
We havenWe haven’’t even reached the t even reached the
cortex yet!cortex yet!�� Primary visual cortex (Striate Cortex, V1)Primary visual cortex (Striate Cortex, V1)
�� Visual Association cortex (extrastriate)Visual Association cortex (extrastriate)
26
Primary Visual Cortex (V1, Striate Cortex)Primary Visual Cortex (V1, Striate Cortex)
~140 million neurons just in V1!
Retinotopic OrganizationRetinotopic Organization
�� Information received at adjacent portions Information received at adjacent portions of the retina remains adjacent in V1.of the retina remains adjacent in V1.
�� More cortex is devoted to areas of high More cortex is devoted to areas of high acuity. acuity. (Just like the disproportionate representation of (Just like the disproportionate representation of sensitive body parts in somatosensory cortex!)sensitive body parts in somatosensory cortex!)
�� About 25% of primary visual cortex is About 25% of primary visual cortex is dedicated to processing input from the dedicated to processing input from the fovea.fovea.
27
Striate CortexStriate Cortex
�� Six principal layers of striate cortexSix principal layers of striate cortex
Processing in Striate CortexProcessing in Striate Cortex
�� Layers 2 and 3 receive information from Layers 2 and 3 receive information from
the parvocellular layers and koniocellular the parvocellular layers and koniocellular
layers of the LGN.layers of the LGN.
�� Cells are grouped together in Cells are grouped together in ““blobsblobs””
–– Cells Cells within within blobs are sensitive to colorblobs are sensitive to color
–– Cells Cells outsideoutside blobs are sensitive to blobs are sensitive to
orientation, movement, binocular disparityorientation, movement, binocular disparity
28
Orientation and MovementOrientation and Movement
�� Most neurons in V1 are Most neurons in V1 are sensitive to orientation: sensitive to orientation: –– if a line or edge appears in if a line or edge appears in
their receptive field, they their receptive field, they respond best when it is at a respond best when it is at a certain anglecertain angle
Receptive Fields in Striate Cortex Receptive Fields in Striate Cortex
�� Most neurons in V1 are eitherMost neurons in V1 are either–– Simple Simple –– receptive fields are rectangular with receptive fields are rectangular with ““onon”” and and ““offoff”” regions, orregions, or
–– Complex Complex –– also rectangular, larger receptive also rectangular, larger receptive fields, respond best to a particular stimulus fields, respond best to a particular stimulus anywhere in its receptive fieldanywhere in its receptive field
29
Receptive Fields in Striate CortexReceptive Fields in Striate Cortex
SIMPLESIMPLE
�� RectangularRectangular
�� ““OnOn”” and and ““offoff””regions, like cells regions, like cells in layer IVin layer IV
�� Orientation and Orientation and location sensitivelocation sensitive
�� All are monocularAll are monocular
COMPLEXCOMPLEX
�� RectangularRectangular
�� Larger receptive Larger receptive fieldsfields
�� Do not have static Do not have static ““onon”” and and ““offoff””regionsregions
�� Not location sensitiveNot location sensitive
�� MotionMotion sensitivesensitive
�� Many are binocularMany are binocular
Orientation and MovementOrientation and Movement
�� Simple cells: receptive fields are Simple cells: receptive fields are rectangular with rectangular with ““onon”” and and ““offoff”” regions, regions, organized in an opponent fashionorganized in an opponent fashion
30
Orientation and MovementOrientation and Movement
�� Complex cells: also Complex cells: also rectangular, larger rectangular, larger receptive fields, respond best to a receptive fields, respond best to a particular stimulus particular stimulus anywhereanywhere in its in its receptive field, especially if there is receptive field, especially if there is movement movement in the right direction in the right direction (no (no inhibitory surround)inhibitory surround)
Orientation and MovementOrientation and Movement
�� HypercomplexHypercomplex cellscells-- respond best to a respond best to a particular orientation, but have inhibitory particular orientation, but have inhibitory region: they code for region: they code for endsends of lines!of lines!
31
Beyond Striate CortexBeyond Striate Cortex
�� Fundamentally, the coding in striate Fundamentally, the coding in striate
cortex is for cortex is for featuresfeatures: color, orientation, : color, orientation,
spatial frequency, retinal disparityspatial frequency, retinal disparity
�� PerceptionPerception requires the combination of requires the combination of
these features into an integrated whole!these features into an integrated whole!
�� This occurs in This occurs in extrastriate cortexextrastriate cortex
Dorsal and Ventral StreamsDorsal and Ventral Streams
�� Dorsal streamDorsal stream: : striate cortex � dorsal prestriate
cortex � posterior parietal cortex
– The “where” pathway (location and movement), or
– Pathway for control of behavior (e.g. reaching)
�� Ventral streamVentral stream: striate : striate cortex � ventral prestriate
cortex � inferotemporal cortex
– The “what” pathway (color and shape), or
– Pathway for conscious perception of objects
32
6363
PET study of where/what dichotomyPET study of where/what dichotomy
33
Not a fixed feedNot a fixed feed--forward system!forward system!
Image from Wagner and Kline
34
Visual AgnosiaVisual Agnosia
�� Deficits in visual form perceptionDeficits in visual form perception
�� NOT NOT blindness!blindness!
�� Caused by damage to visual association Caused by damage to visual association
areas in ventral streamareas in ventral stream
�� Video….Video….
ProsopagnosiaProsopagnosia
�� Damage to the fusiform face area (FFA) Damage to the fusiform face area (FFA)
results in results in prosopagnosia. prosopagnosia.
Diffusion tensor imaging (DTI) tractography reveals a reduction in the volume of the inferior longitudinal fasciculus in the brains of 6 patients with congenital prosopagnosia (top). (From Thomas et al 2008)
35
�� The lateral occipital complex is activated The lateral occipital complex is activated
in response to a wide variety of objects.in response to a wide variety of objects.
�� It seems possible that different It seems possible that different
categories of objects are processed at categories of objects are processed at
least in part in different subregions. least in part in different subregions.
�� Also in the ventral stream is the Also in the ventral stream is the
extrastriate body areaextrastriate body area
–– Seems to be particularly responsive to body Seems to be particularly responsive to body
partsparts
36
Perception of MovementPerception of Movement