Download Vision - Dave Brodbeck

Vision
Biology/Psychology 2606
Some introductory thoughts
• Sensory world in general is basically a
representation of the real world
• So, we have a rich visual world, not much
of a smelly one
• Different parts of the brain do different bits
of processing
• This all comes together basically
seamlessly to form our visual world
Vision
• Like any sensory process, vision converts
some energy to neural messages
• In this case, light
• Light is just a form of electromagnetic
radiation
• So are x rays, micro waves, infra red, UV
cosmic rays etc
I wish to hell I could see
better….
• Wavelength of light determines hue
• Intensity determines brightness
• Light enters the eye through the cornea
and the pupil
• Pupil size regulated by iris
• Behind pupil, lens, which accommodates
• Light hits the retina
• Oh ya, it is upside down….
Acuity
• Acuity is affected by
the shape of the eye
• Nearsighted, eye too
long, or cornea too
curved
• So far away stuff is
blurry
• Image is in front of
the retina
• Farsighted, opposite
The retina
• There are two kinds of
receptors in the retina,
rods and cones
• Rods for night, brightness
• Cones for day, colour
• When a photon hits a
receptor it sends a
message via the optic
nerve to the brain
• Because of this, we have
a blind spot!
Gotta love the retina
•
•
•
•
•
Cones are for fine detail and colour
Cones only really work in the light
Concentrated in the fovea
Rods are more evenly distributed
Many rods to one bipolar cell, so you can
see in dim light, but only in black and white
• One cone, one bipolar cell
• About 130 000 000 receptors per retina
Follow the optic nerve
• Connection is next to the ganglion cells in
the optic nerve
• Cross at the optic chiasm
• Left visual field to right half of brain, right
to left
• Next the path goes to the LGN
• Geniculostriate system
• Some to occipital lobe some to parietal
And there is another pathway….
• Superior colliculus in the tectum
• Pulvinar in thalamus
• Whole thing is called the tectopulvinar
system
• Medial pulvinar, parietal
• Lateral pulvinar, temporal
Dorsal and ventral streams
• Temporal -> ventral
• Parietal -> dorsal
Occipital lobe
• V1 – V5
• Blobs and interblobs in V1
• Blobs do colour, interblobs do motion and
form
• Info goes on to V2
• Now the dorsal and ventral streams are
produced
Receptive Fields
• The region of the retina that when stimulated will
cause a cell to fire
• This is coded on to layers in V1
• So top is top layer, etc
• Cortical cells have receptive fields too
• Receptive field in cortex relates to much bigger
area that receptive field in retina, so , many
ganglion cells
• Only adjacent areas of visual field in centre have
colossal connections
• So, for example, we can see shape by the
nervous system analyzing luminance
contrast between receptive fields
• Simple cells, orientation
• Complex cells, orientation and movement
• Hypercomplex cells, orientation,
movement and inhibition on one side
V1 – V5
• V1 and V2 are basically like
clearinghouses into which different bits of
information are deposited and then routed,
post offices if you will
• Connections from blobs in V1 to V4 (so V4
does some things with colour)
• V4 does some stuff about form too
• V5 only does motion
• V3 does ‘dynamic form’ (shape in motion)
V was a bad early 80s sci fi show
•
•
•
•
Damage to V4 -> only see shades of grey
Damage to V5 -> motion detection is gone
V1 damage, blind, yet they can see!
So V1 probably is where the brain makes
sense of visual input
Case studies can be useful
• DB -> V1 Damage and blindsight
• JL -> V4 Damage and colour
• LM -> V5 Damage and movement
perception
• DF -> General occipital damage and visual
agnosia
• D and T -> Higher level visual processing
Conclusions
•
•
•
•
Visual system is way complex
It is, frankly, counter intuitive
It is not all occipital
Vision is clearly important to humans, as
much of our brain is devoted to it
• There MAY be asymmetries