CHAPTER 6
Vision
Outline and Lecture Notes
The Stimulus
Light – Continuous moving wave of electromagnetic energy or discrete particles of
energy called photons traveling through space.
Nanometer = a billionth of a meter
186,000 miles per second.
Travels at constant speed but differences in heights of waves.
Visible spectrum (380 -760 billionths of meter)
Three perceptual dimensions of light
Wavelength =hue/color
Brightness= intensity
Saturation = purity
Anatomy of the Visual System
The Eyes
Sclera – tough, opaque, white outer coating of eye
Conjunctiva –Thin membrane, attaches to the eye near the edge of the cornea
Cornea – transparent layer in front of eye – composed of living tissue
Anterior Chamber – filled with aqueous humor
Pupil – opening in iris – regulates amount of light that enters eye
Iris – pigmented ring of muscles behind the cornea
Lens – “lentil shaped” right behind the iris, consists of a series of transparent
onion-like layers. Shape can be altered by ciliary muscles
Posterior Chamber – filled with vitreous humor
Retina – performs the sensory functions of eye
Photoreceptors – receptor cell which transducer light into neural activity
Rods – 120 million – night vision
Cones – 6 million – daytime vision – color
Fovea – Where cones are most concentrated
Optic Disk – blind spot – where neurons form optic nerve and leave eye
Optic Chiasm – crossing over of impulses allows the brain to process two sets of
signals about an image and helps the human perceive form and depth.
Photoreceptors
Photopigment - Molecules embedded in lamellae made up of opsin (a protein)
and retinal (a lipid) Photon, or particle of light, strikes a photopigment, the
photopigment splits apart. This event starts transduction – the process by which
the sense organs convert energy from environmental events into neural activity.
Chain of events in Visual Perception – The primary retino-geniculo-cortical
pathway.
1. When a molecule of rhodopsin is exposed to a quantum of light it breaks into two
constituents; rod opsin and retinal
2. Ion Channels are open. Glutimate (neurotransmitter) flowing (dark current) Ion
channels now closes or hyperpolarize NO ACTION POTENTIAL
3. Reduction of glutamate causes bipolar cell to depolarize and the bipolar cell releases
more glutamate NO ACTION POTENTIAL.
4. Increase of glutamate causes the ganglion cell to be excited and creates an action
potential.
5. The action potential of ganglion cell sends info via optic nerve to DLGN (dorsal lateral
geniculate nucleus) of thalamus
6. The DLGN of thalamus has 6 layers. Layers 1 and 2 are magnocellular layers found in
all mammals –transmit B & W images and movement. Layers 3, 4, 5, and 6 are
parvocellular layers found in primates only – transmit color vision
7. Neurons of the DLGN send their axons to the primary visual cortex or striate cortex in
the occipital lobe
Analysis of Visual Information: Role of the Striate Cortex
Anatomy of the Striate Cortex
Six layers
Magnocellular, - B&W
Parvocellular - Red and Green cones
Koniocellular – blue cones
Modular Organization of the Striate Cortex
2500 modules, each containing approx. 150,000 neurons
CO blobs – Each half of a module receives information from one eye, but because
information is shared, most of the neurons respond to input from both eyes
Analysis of Visual Information: Role of the Visual Association Cortex
Two Streams of Visual Analysis
Dorsal - “Where things are”
Optic ataxia – can perceive it, but when reaching for it is often misdirected
Ocular apraxia – “without visual action” unable to make a systematic scan of
contents of room
Simultanagnosia – can perceive only one object at a time
Balint’s syndrome- all three – bilateral damage to dorsal stream
parieto-occipital region
Ventral – “What things are”
Visual agnosia – Cannot identify common objects by sight – inferior temporal
cortex
1. Apperceptive visual agnosia – cannot recognize objects by shape
Prosopagnosia- inability to recognize faces – fusiform face area in
extrastriate cortex of occipital lobe (a form of apperceptive visual
agnosia)
2. Associative visual agnosia – perceive normally, but cannot name what
they have seen
Perception of Color
“Achromatopsia” – vision without color – anterior to V4 – area V8
Perception of Movement
Area V5 – posterior parietal cortex – damage here disrupts ability to
perceive movement or spatial location of objects
“Akinetopsia” – inability to perceive movement caused by damage to
area V5 of the visual association cortex
Perception of Spatial Location
The parietal lobe is involved with spatial perception – Damage to the
parietal lobes disrupts a variety of tasks requiring perception and memory
of the location of objects.
Causes of Blindness and vision impairment
Cataract
Glaucoma
Macular Degeneration
Retinitis Pigmentosa