VisionVision

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