Vision
“El ojo que ves no es
ojo porque tu lo veas,
es ojo porque te ve”
Antonio Machado
“The eye you see
is not an eye due to you seeing it,
It’s an eye because it sees you”
Vision: Outline
• Light
• Eye
• Visual Path
• Visual Cortex
Perceptual Dimensions of Light
UV rays
Wave
frequency
Wave
amplitude
Purity of the wave
The Eye
• Cornea
• Pupil/Iris
• Lens
• Retina
– Cones
– Rods
– Fovea
– Optic disc (blindspot)
4
Eye anatomy: Functions
transparent medium
air
(cornea, aqueous humor, pupil,
lens, vitreous humor)
lens
iris
retina
a focal point
lens
diaphragm
film
a focal point
Similarity btw eye
& camera known
since 1600’s
Eye Anatomy:
Abnormalities
Near-sightedness
(Myopia ):
Lasik Changes the shape
image falls too short of retina
of the cornea
(eyeball too long)
(Laser-Assisted In Situ
Keratomileusis)
newborns
Far-sightedness:
focal point of light falls
beyond retina
(Eyeball is too short)
Near-Sightedness
nearby things are on focus
Cataracts
• Reduced illumination, acuity, and color saturation
• Deposits in the lens
• Common in older adults
Eye Anatomy: Retina
Red eye in photos due
to dilated pupils
Retina of diabetic patient
• fovea: center of the retina, high concentration of cones
• optic disk (blindspot) & direct view of arteries (clinical importance)
• photorreceptors: cones (color vision) and rods
Eye Anatomy: Retina
Concentration of
Cones & Rods
in Retina
Visual
Acuity
Eye Anatomy: Retina
One Cones --> one ganglion cell
Many Rods --> ganglion cells.
high acuity (fovea)
High sensitivity (periphery)
(e.g, night vision)
Eye Anatomy: Optic disc (blindspot)
Lateral visual field
Medial
Retina
The eye is a device 'designed' to see, but the ‘blindspot’
reveals it is not perfect
Receptive field (RF)
• is that portion of the visual field (outside world) in which
the presentation of visual stimuli will produce an
alteration in the firing rate of a particular neuron
COLOR VISION
Tri-chromatic theory
– Blue, red, & green “color”
receptors
Peak sensitivities of the three cones
Most people who are color blind can see colors
No ‘green’ cones
Test for Deuteranopia: Name number: (‘5’ or ‘2’)
If you see a 2: Red/Green Color blindness (male)
Vision: Outline
•
•
•
•
Light
Eye
Visual Path & its deficits
Visual cortex
Visual Paths
Hemianopia – objects are bisected with
½ obscured experiencing the obscured
part as “blank” or “void”
Scotoma: A small blindspot in the visual
field caused by a small lesion, usually
in the occipital lobe
Vision: Outline
•
•
•
•
Light
Eye
Visual Path & its deficits
Visual cortex
• V1: Orientation sensitive
– Ventral Pathway
– Dorsal Pathway
Visual Cortex
V1:
primary
visual
cortex
Primary visual cortex (V1)
• V1 cells respond to lines
– of particular orientations
– of particular widths.
Vision: Outline
•
•
•
•
Light
Eye
Visual Path & its deficits
Visual cortex
• Orientation sensitive
– Ventral Pathway
• Area MT (motion), Object Recognition, Area V4 (color)
• synesthesia
– Dorsal Pathway
• Spatial Attention
• Hemispatial Neglect
Cortical Connections of Visual areas
• Complex & with multiple
connections
• Over-simplified version:
dorsal & ventral paths
Ventral & Dorsal Paths
¼ of the brain is involved in visual
processing, more than for all other senses
Ventral & Dorsal Paths
& how
Ventral & Dorsal Paths
Ventral Path: Object recognition
Lesion of ventral pathway
Agnosia
fMRI:
Object recognition
Ventral Pathway (V4): Color perception
Cerebral Achromatopsia: bilateral damage to V4
Color is more important of ‘what’ than for ‘where’
Synesthesia
Ventral Path: Objects vs. Faces
Are faces very difficult objects or
special ones (i.e., specific process)
Neuroimaging of face, bird and car experts
Fusiform
Gyrus
Cars-Objects
Birds-Objects
Car
Experts
“Face Experts”
Fusiform
Gyrus
Bird
Experts
Gauthier et al., 2000
Fusiform
Gyrus
Children with autism as face “novices”
Faces
Control
Group
Fusiform
Gyrus
Autism
Group
Fusiform
Gyrus
Hypoactivation of fusiform face area
Schultz, et al. 2000
Area MT:
motion
perception
In sum, different parts of the visual cortex are
specialized in the processing of specific features
• For example,
•
•
•
•
•
movement,
color.
Objects
Faces
Location
Binding problem: If the brain processes features
separately, how does it bind those features into
a single conscious representation:
Answer: Attention