Download Rods Cones

SENSATION & PERCEPTION MODULE
Neuroscience – Psych 129
2/23/2012
Dr. Kevin Jordan
[email protected]
What I hope to cover
•
•
Distinguishing sensation and perception
Visual processing in the eye
– Eye anatomy
– Retinal processes
•
•
•
Pathways from eye to brain
Orientation perception as distributed coding
Pathways for processing “what” and
“where”
Who is this?
1. Distinguishing sensation
and perception - two puzzles
•Eye as a camera
•The inversion problem
The eye as a camera
Is perception a reproduction of the
world much as a photo is a
reproduction?
Sensation & Perception Don’t “Just Happen”
Sensation
1. Light bounces off Dilbert
2. Light forms image on retina
3. Image generates electrical
signals in receptors
4. Signals travel along nerve
fibers to the brain...
Perception
Signals are processed and you “perceive” Dilbert
HE
SAW THE
THE BIRD
SITTING ON THE
THE BIRDHOUSE
HE WALKED THOUGH THE WOODS
The inversion problem
Descartes taught us that the
retinal image is inverted. So…
Etienne Condillac (1715-1780)
Where does all of this leave us?
• The eye is NOT a camera.
• Correction for inversion is highly-complex
and highly-interactive.
• Perhaps perception is best thought of as a
hypothesis test.
• We make our best guess as to what the
retinal image represents.
Sensation & Perception Don’t “Just Happen”
Sensation
1. Light bounces off Dilbert
2. Light forms image on retina
3. Image generates electrical
signals in receptors
4. Signals travel along nerve
fibers to the brain...
Perception
Signals are processed and you “perceive” Dilbert
Establishing a meaningful distinction between sensation and perception (i.e.,
sensation and perception are distinct processes.
Sensation – the process of gathering information from the environment.
Perception – the process of interpreting that information gathered from the
environment.
The opposing viewpoint is that there is no distinction to be made between
sensation and perception.
Naïve realism – we can directly know (perceive) the world via the sensory systems.
2. Visual processing in the eye
Why study the structure of the visual system when we are
interested in visual perceptual processes?
From Bruce Goldstein (1984): "the way in which neurons are wired
together in the nervous system influences our perception".
This quote justifies the study of functional visual neuroanatomy
Thus, we will be studying the relationship between visual system stucture
and visual system function.
examples:
cornea/lens system
retinal architecture
convergence of photoreceptor input onto retinal ganglion cells
lateral inhibitory connections among retinal neurons
Light is the Stimulus for Vision
• Electromagnetic spectrum
– Energy is described by wavelength
– Spectrum ranges from short wavelength
gamma rays to long wavelength radio
waves
– Visible spectrum for humans ranges from
400 to 700 nanometers
– Most perceived light is reflected light
Thompson Wadsworth
Figure 2.9 The electromagnetic spectrum, showing the wide range of energy
in the environment and the small range within this spectrum, called visible
light, that we can see.
Thompson Wadsworth
http://www.nei.nih.gov/photo/eyean/index.asp
Cornea
• The transparent dome which serves as the
window of the eye.
• The primary (most powerful) structure
focusing light entering the eye.
How does the cornea stay
transparent?
• No blood vessels.
• Transparent stroma with low level of fluids.
– Endothelium cells serves as a pump that
supply oxygen and remove fluids.
• Tear film also supplies oxygen and keep
corneal surface smooth and clean.
health.tau.ac.il/.../sylabus_b/year%20A/
Anatomy%20of%20the%20Eye%20and%20Orbit%20ripuy%20bisuk.ppt
The sphincter muscle lies around the
very edge of the pupil. In bright light,
the sphincter contracts, causing the
pupil to constrict. The dilator muscle
runs radially through the iris, like
spokes on a wheel. This muscle
dilates the eye in dim lighting.
http://www.stlukeseye.com/anatomy/Iris.asp
Pupil
The pupil is the opening in the
center of the iris. The size of
the pupil determines the amount
of light that enters the eye. The
pupil size is controlled by the
dilator and sphincter muscles of
the iris. Doctors often evaluate
the reaction of pupils to light to
determine a person's
neurological function.
http://www.stlukeseye.com/anatomy/Pupil.asp
The typical pupil is 3-4mm in diameter in normal room illumination,
whereas a dilated pupil is 7-8mm.
This difference yields a three to seven times greater area through which
to examine the internal eye; this means that the entire retina can be
visualized through a dilated pupil with relative ease, while
examination of the entire retina is very difficult, at best, through
undilated pupils.
Focusing Images on the Retina
• The cornea, which is fixed, accounts for about
80% of focusing
• The lens, which adjusts shape for object
distance, accounts for the other 20%
– Accommodation results when ciliary
muscles are tightened which causes the
lens to thicken
• Light rays pass through the lens more
sharply and focus near objects on retina
Thompson Wadsworth
Accommodation
• Ciliary muscle
constrict > zonular
tension decreases >
lens becomes more
spherical > more
dioptric power that
converge light from
a near target onto
the retina.
Figure 2.11 Focusing of light rays by the eye. (a) Parallel rays from a light source further than 20 feet from
the eye. Focus point: on retina; (b) nonparallel rays from a light source closer to the eye. Eye relaxed.
Focus point: behind the retina; (c) non-parallel rays. Eye accommodated (indicated by fatter lens). Focus
point: on retina.
Image Formation on the Retina
Distant object => ~ parallel rays
Optical power:
Weak Convex Lens
Strong Convex
Lens
Focal length (f) = distance from focal plane to center of lens
for object at 
Power = 1/f (if f is in meters, then power is in diopters)
*Power and focal length are used to describe the effects of a
lens on light rays that are parallel (object at ). What
happens as the object moves closer?
Basics of focusing – demo
Accommodation - demo
Focusing Images on Retina - continued
• The near point increases when the lens can
no longer adjust for close objects
• Presbyopia - “old eye”
– Distance of near point increases
– Due to hardening of lens and weakening of
ciliary muscles
– Corrective lenses are needed for close
activities, such as reading
Thompson Wadsworth
Figure 2.12 Vertical lines show how the distance of the near point increases with increasing age (green
numbers). When the near point becomes further than a comfortable reading distance, corrective lenses
(reading glasses) become necessary.
Presbyopia
• With age, lens is less
elastic > muscle
constriction achieves
less accommodation.
Nearsightedness (Myopia)
Overview
Nearsightedness or myopia, occurs when light entering the eye focuses in
front of the retina instead of directly on it. This is caused by a cornea that
is steeper, or an eye that is longer, than a normal eye. Nearsighted
people typically see well up close, but have difficulty seeing far away.
http://www.stlukeseye.com/Conditions/myopia.asp
What happens in short sighted (myopic) eyes?
http://www.patient.co.uk/showdoc/23069162/
Retinal Processing - Rods and Cones
• Differences between rods and cones
– Shape
• Rods - large and cylindrical
• Cones - small and tapered
– Distribution on retina
• Fovea consists solely of cones
• Peripheral retina has both rods and
cones
• More rods than cones in periphery
health.tau.ac.il/.../sylabus_b/year%20A/
Anatomy%20of%20the%20Eye%20and%20Orbit%20ripuy%20bisuk.ppt
http://www.macula.org/anatomy/
http://www.google.com/imgres?imgurl=http://webvision.med.utah.edu/imageswv/retina.jpeg&imgrefurl=http://webvision.med.utah.edu/sretina.html&h=515&w=621&sz=56&tbni
d=X1SrEFJePe-a9M::&tbnh=113&tbnw=136&prev=/images%3Fq%3Dpictures%2Bof%2Bretina&usg=__B6GgVWA2J2WIkL-i2lwCmc-A_VQ=&ei=CjycSc2gEZKWsQOXqC1Ag&sa=X&oi=image_result&resnum=2&ct=image&cd=1
Figure 4.7. The retina of the human eye
http://www.google.com/imgres?imgurl=http://webvision.med.utah.edu/imageswv/retina.jpeg&imgrefurl=http://webvision.med.utah.edu/sretina.html&h=515&w=621&sz=56&tbnid=X1SrEF
JePe-a9M::&tbnh=113&tbnw=136&prev=/images%3Fq%3Dpictures%2Bof%2Bretina&usg=__B6GgVWA2J2WIkL-i2lwCmc-A_VQ=&ei=CjycSc2gEZKWsQOXqC1Ag&sa=X&oi=image_result&resnum=2&ct=image&cd=1
http://www.google.com/imgres?imgurl=http://webvision.med.utah.edu/imageswv/retina.jpeg&imgrefurl=http://webvision.med.utah.edu/sretina.html&h=515&w
=621&sz=56&tbnid=X1SrEFJePe-a9M::&tbnh=113&tbnw=136&prev=/images%3Fq%3Dpictures%2Bof%2Bretina&usg=__B6GgVWA2J2WIkL-i2lwCmcA_VQ=&ei=CjycSc2gEZKWsQOX-qC1Ag&sa=X&oi=image_result&resnum=2&ct=image&cd=1
direction of light
From light to electricity
light sensitive
pigment
located in
outer
segment
opsin
retinal
Note that change in electrical potential is graded!
How did Selig Hecht show that rod receptors could be excited by a single photon?
Duplex retina theory (Schultze, 1866) –
There are two kinds of photoreceptors and each
has a different function
Evidence
•
•
•
•
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Cross species
Differential distribution
Differential rate of dark adaptation
Differential spectral sensitivity
Differential convergence onto retinal ganglia
Distribution of rods and cones on the retina
• ~5 million cones; ~120 million rods
Rods
• Fovea: ~50,000 (1%) cones; no rods
• Periphery: rods outnumber cones by 20-to-1
Cones
ratio
The two-stage dark adaptation
Rod
curve
Rod-Cone
Break
Cone
How are these data obtained using psychophysical methods?
Spectral Sensitivity for Rods and Cones
Rods
(peripheral)
Cones
(fovea)
Convergence
The second major functional property of the retina
Horizontal cell
Bipolar cell
Amacrine
cell
Ganglion
cell
Convergence: 126 million photoreceptors => 1 million ganglion cells
Consequence of convergence #1:
Sensitivity
Receptors
2
2
2
Bipolar
cell
spatial
summation
2
2
2
2
2
2
2
∑10
∑10 ∑10 ∑10 ∑10 ∑10
Consequence of convergence #2:
Acuity
periphery
fovea
DIHCNRLAZIFWNSMQPZKDX
DIHCNRLAZIFWNSMQPZKD
Summary: Rods vs. cones
Rods
Cones
Number
~120 million
~5 million
Distribution
All in periphery
Most dense in fovea,
but also in periphery
Pigment
regeneration
Slow (~30 min)
Fast (~6 min)
Spectral sensitivity
(max sensitivity)
505 nm
550 nm (S, M, L cones)
Convergence
Many:1
Fewer:1
Sensitivity
High (better vision in
dark)
Low (poor vision in
dark)
Acuity
Poor
Good
Lateral inhibition
The second major functional property of the retina
Hartline, H. K., Wagner, H. G., and Ratliff, F. (1957). Inhibition in the eye
of Limulus. Journal of General Physiology, 39, 651-673.
This study reveals the second major process in retinal organization, lateral
inhibition.
•Single unit recording of individual facets in the compound eye of Limulus.
•Definition: a reduction in the activity of a neuron resulting from activity in a
neighboring neuron.
•Integration of convergence and lateral inhibition results in receptive field.
•Phenomenon consistent with this interpretation: Simultaneous lightness
contrast, Mach Bands, The Hermann grid.
http://www.mbl.edu/animals/Limulus/index.html
http://www.unc.edu/depts/oceanweb/hscpix/hsc4.jpg
The ommatidia is shaped like a vase. At
the top is a cup-shaped space. Covering
this is the cornea, which is part of the
exoskeleton. It is translucent and forms a
lens which directs light into the central
interior of the cup.ht is intensified, the
frequency of the discharge of the nerve
impulses increases.
http://www.mbl.edu/animals/Limulus/vision/ommatidia/parts1.html
from Cornsweet, 1970
from Cornsweet, 1970
Figure 3.5 A demonstration of lateral inhibition in the Limulus. The records on the right show the response
recorded by the electrode in the nerve fiber of receptor A: (a) when only receptor A is stimulated; (b) when
receptor A and the receptors at B are stimulated together; (c) when A and B are stimulated, with B at an
increased intensity. (From Mach Bands: Quantitative Studies on Neural Networks in the Retina, by F. Ratliff,
1965, figure 3.25, p. 107. Copyright © 1965 Holden-Day, Inc. Reprinted with permission.)
from Cornsweet, 1970
http://www.yorku.ca/eye/machband.htm
“You can take a complex thing
and make it understandable, but
you can’t make it simple.”
John Madden
2/24/2009
Remember the Bruce Goldstein quote…
• "the way in which neurons are wired
together in the nervous system influences
our perception".
http://www-unix.oit.umass.edu/~phy139/L15_PDF_act.pdf#search='lateral%20inhibition'
What is the consequence of lateral inhibition?
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•
•
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Enhances contrast
With weak edges…or with strong edges
Thus, enhances edge detection
Edges define the boundaries of objects
We perceive a world of objects
Putting it all together
The RECEPTIVE FIELD: The functional unit of the retina
Kuffler, S. W. (1953). Discharge patterns and functional organization
of mammalian retina. Journal of Neurophysiology, 16, 37-68.
This study represents the (perhaps) definitive approach to determining
the “scheme” of convergence of photoreceptor input and lateral
inhibitory connections within the retina.
Single unit recording of the activity of retinal ganglion cells in cats.
The central construct that emerged can be thought of as the functional
unit of the retina, the receptive field.
Definition: region of the retina that produces a response on a given
neuron.
http://www.cis.rit.edu/people/faculty/montag/vandplite/pages/chap_10/ch10p2.html
http://psych.hanover.edu/Krantz/receptive/beststim.html
Examples of four different stimuli on a receptive field.
Which is the best stimulus for this receptive field?
http://psych.hanover.edu/Krantz/receptive/beststim.html
http://www.yorku.ca/eye/recepfld.htm
Hermann Grid Illusion
Figure 4.17. Retinal ganglion cells exaggerate edges
3. Pathways from Eye to Brain
http://www-medlib.med.utah.edu/calendar/block4/ppt_cortex_normann/sld004.htm
A. The pathway:
Vision is generated by photoreceptors in
the retina, a layer of cells at the back
of the eye.
The information leaves the eye by way of
the optic nerve, and there is a partial
crossing of axons at the optic chiasm.
After the chiasm, the axons are called
the optic tract.
The optic tract wraps around the midbrain
to get to the lateral geniculate
nucleus (LGN), where all the axons
must synapse.
From there, the LGN axons fan out
through the deep white matter of the
brain as the optic radiations, which
will ultimately travel to primary visual
cortex, at the back of the brain.
http://thalamus.wustl.edu/course/basvis.html
RECEPTIVE FIELDS FROM EYE TO BRAIN
RECEPTIVE FIELD - region of the retina which produces a response
in a particular neuron
1. Optic nerve fibers (retinal ganglion neuron)
circular center-surround (antagonistic) receptive field due to
convergence and lateral inhibition
optimal stimulus: a small spot of light covering center of
receptive field (for on-center neuron)
2. Lateral geniculate nucleus of the thalamus (LGN)
circular center-surround (antagonistic) receptive field due to
convergence and lateral inhibition
optimal stimulus: a small spot of light covering center of receptive
field (for on-center neuron)
3. Area 17 of visual cortex (occipital lobe of cerebrum)
Hubel and Wiesel functional architecture - hierarchical organization
a. Simple cell
elongated center-surround receptive fields perhaps due to
summation of input from several LGN cells whose receptive fields are aligned
along a particular axis
optimal stimulus: lines of a particular orientation (orientation and
position specificity)
Figure 4.13. Schematic neuronal circuit involving neurons
in LGN and area V1
Primate lateral geniculate nucleu
of the thalamus (LGN)
6 layers
3 from contralateral eye (C)
3 from ipsilateral eye (I)
The upper 4 layers = parvocellular
medium-sized cell bodies
center/surround receptive
fields (sustained)
color, fine texture, pattern,
depth perception
The lower 2 layers = magnocellular
large cell bodies
receptive fields sensitive to
timing (transient “on”, “off”)
motion sensitive
Laminar (layered) structure of LGN
http://www.nfos.org/degree/opt41/3
http://www.nfos.org/degree/opt41/3
4. Orientation perception as distributed coding
Please see slide #77 to understand the shapes of the distributions above.
5. Pathways for processing “what” and “where”
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•
•
•
Describe origins of each pathway
Trace pathways from eye to brain
Distinguish processing within each stream
Integrate research findings
The “What” and “Where”
Pathways
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Ungerleider & Mishkin (1982)
Dorsal stream = “where” pathway = “where it is”
Analyzes location, motion of visual stimuli
Ventral stream = “what” pathway=“what it is”
Analyzes form, color, identity of objects
where
www.cnbc.cmu.edu/~mgilzen/85219/slides/Week8a.ppt
what
Processing streams: What vs. where
processing stream
ventral
dorsal
function
what
where/how
brain areas
occipital, temporal
occipital, parietal
modules
IT (form)
MT (motion)
response specialization
orientation, length, color
spatial layout, direction
of motion
Fig. 4-15, p. 79
Cellular responses in dorsal stream
• V5 (a.k.a. MT) motion and direction selective...
– cells which “prefer” a direction of motion cluster
together
– sensitive to local and global motion
– stimulation can change perception of motion
– disparity selective (role in stereopsis?)
– ignore color
– input from M pathway via superior colliculus & V1-3
– projects to brainstem eye movement centers
www.cnbc.cmu.edu/~mgilzen/85219/slides/Week8a.ppt
Effects of lesions to dorsal stream
• Animal studies
– Damage to V5 (MT) affects smooth pursuit and
saccadic eye movements
– Diffuse damage to posterior parietal cortex disrupts
maze performance
• Human patients
– Balint’s syndrome (optic ataxia, ocular apraxia and
simultanagnosia)
– Spatial hemineglect
• Patients ignore “neglect” part of visual space
www.cnbc.cmu.edu/~mgilzen/85219/slides/Week8a.ppt
Balint's syndrome, identified by Rezső (Rudolf) Bálint in
1909, is characterized by optic ataxia (the inability to
accurately reach for objects), optic apraxia (the inability
to voluntarily guide eye movements/ change to a new
location of visual fixation), and simultanagnosia (the
inability to perceive more than one object at a time, even
when in the same place).
Balint's syndrome has been found in patients with
bilateral damage to the posterior parietal cortex.
http://www.answers.com/topic/balint-s-syndrome
Neuropsychology and visual awareness
•
•
•
•
•
Dr. Li Li
Balint’s syndrome: a severe attentional deficit that results in an almost complete
inability to see anything except a single fixated visual object
patients are known to stare at inconsequential objects for extended periods of
time and take very little interest in events occurring around them (ocular apraxia)
they are functionally blind
must use conscious strategies (e.g., closing their eyes) to break fixation from one
object
inability to perceive more than one object at a time during a single fixation even
when two objects occupy the same location in the visual field (simultagnosia)
– patient can see a person’s face, but cannot tell whether person is wearing
glasses
Neuropsychology and visual awareness
• Unilateral neglect (hemineglect): systematic failure to
notice objects on the side of the world opposite the
brain injury
•
•
•
•
•
Dr. Li Li
commonly associated with damage to the parietal lobe of the right
hemisphere often following a stroke
often orient toward one side by keeping head turned in that direction
fail to look at people on the neglected side
fail to eat food on the neglected side of their plate, even when hungry
when asked to copy a figure (or draw from memory), will only draw one
side
Processing streams
What are the functional characteristics of these two processing streams?
temporal lobe
parietal lobe
Fig. 4-12, p. 78
Cellular responses in ventral stream
• V2-V4 especially orientation & color sensitive
– V4 contains color sensitive cells
– some have complex preferred stimuli
• Inferior temporal cortex (IT) is form sensitive
– big receptive fields (up to entire visual field)
– face selective cells in some regions
– others respond best to complex, 3D stimuli (Tanaka)
www.cnbc.cmu.edu/~mgilzen/85219/slides/Week8a.ppt
Effects of lesions to ventral stream
• Animal studies
– Damage to P pathway inputs abolishes color
perception
– Damage to IT impairs discrimination between
objects and identification
• Human patients
– Achromatopsia
• Loss of color perception
• Associated with damage to lingual and fusiform gyri
– Agnosia
• Apperceptive
• Associative
• Prosopagnosia
www.cnbc.cmu.edu/~mgilzen/85219/slides/Week8a.ppt
Processing streams (cont.)
Patient D.F. with temporal lobe damage
Figure 4.16 Performance of D.F. and a person without brain damage for two tasks: (a) judging the
orientation of a slot; and (b) placing a card through the slot. See text for details. (From the Visual Brain in
Action by A. D. Milner and M. A. Goodale. Copyright ©1995 by Oxford University Press. Reprinted by
permission.)
What and How Pathways - Further Evidence
• Rod and frame illusion
– Observers perform two tasks: matching
and grasping
• Matching task involves ventral (what)
pathway
• Grasping task involves dorsal (how)
pathway
– Results show that the frame orientation
affects the matching task but not the
grasping task
Thompson Higher Education, 2007
Figure 4.17 (a) Rod and frame illusion. Both small lines are oriented vertically. (b) Matching task and
results. (c) Grasping task and results. See text for details.
Processing streams
What are the functional characteristics of these two processing streams?
temporal lobe
parietal lobe
Table 4-1, p. 79
Cellular responses in ventral stream
• V2-V4 especially orientation & color sensitive
– V4 contains color sensitive cells
– some have complex preferred stimuli
• Inferior temporal cortex (IT) is form sensitive
– face selective cells in some regions
– others respond best to complex, 3D stimuli (Tanaka)
www.cnbc.cmu.edu/~mgilzen/85219/slides/Week8a.ppt
Fig. 4-20, p. 83
Modularity: Structures for Faces, Places, and
Bodies
• Module - a brain structure that processes
information about specific stimuli
– Inferotemporal (IT) cortex in monkeys
• One part responds best to faces while
another responds best to heads
• Results have led to proposal that IT
cortex is a form perception module
– Temporal lobe damage in humans results
in prosopagnosia
Thompson Higher Education, 2007
Fig. 4-21, p. 84
Prosopagnosia
Faceblind!
Fig. 4-18, p. 83
Fig. 4-13, p. 78
Fig. 4-28a, p. 88
Fig. 4-28b, p. 88
Fig. 4-28, p. 88
Fig. 4-29, p. 89
Fig. 4-25, p. 86
Sensation & Perception Don’t “Just Happen”
Sensation
1. Light bounces off Dilbert
2. Light forms image on retina
3. Image generates electrical
signals in receptors
4. Signals travel along nerve
fibers to the brain...
Perception
Signals are processed and you “perceive” Dilbert