1. The visual system
Part I

2. In general, our visual system represents the world:
Imperfectly
Accurately
Better than reality
Distinction between:
-- transduction
-- coding

3. Light Photons – discrete particles of energy
travel through space at 300,000 kilometers/sec (186,000 miles/sec)
Waves of electromagnetic energy
380 to 760 nanometers in length

4. Electromagnetic spectrum
nm = nanometer

5. What other animals see…
Honeybees can see Ultraviolet light
Rattlesnakes can see infrared light

6. Rats can see ultraviolet light

7. Properties of light and perception
In general:
Wavelength – color (hue) perception
Intensity – brightness perception
Saturation – purity perception

8. Light enters the eye through the pupil size of the pupil
is regulated by the iris
The lens focuses light
on the retina
Note: that the
retinal image
is upside down.

9. Pupil size
Adjusted in response to changes in illumination, which is a tradeoff between:
Sensitivity – ability to detect the presence of dimly lit objects
Acuity – ability to see the details of objects
When illumination is high, pupils are constricted allowing a greater depth of focus of the image falling on the retina
When illumination is low, pupils dilate in response to low activation of receptors allowing more light to enter the eye but sacrificing acuity and depth of focus

10. Ones to know
Anatomy of the eye
Ligament

11. Accomodation
Process of adjusting the configuration of the lens to bring images into focus on the retina
Focus on a near object
ciliary muscles contract
putting less tension on the ligaments
allowing the lens to take its natural cylindrical shape
thus increasing its ability to refract (bend) light
Focus on a distant object
Ciliary muscles relax
Increasing tension on the ligaments
flattens the lens
thus decreasing its ability to refract (bend) light

12. Binocular disparity
The difference in the positions of the same image on the two retinas
Is greater for close objects (eyes must converge or turn slightly inward)
The degree of binocular disparity enables the visual system to construct 3-D perception from two 2-D retinal images

13. The retina Composed of 5 layers of neurons Receptors (photoreceptors)
1 rod
3 cones
Horizontal cells (2 subtypes)
Bipolar cells (10 subtypes)
Amacrine cells (25-30 subtypes)
Ganglion cells (10-15 subtypes)

15. The cellular structure of the retina
Appears to be inside-out
Light passes through the 4 cell layers before reaching the receptors
After receptor activation, signals are transmitted back out to the ganglion cells whose axons project across inside surface of the retina, gathering at the optic disk where the optic nerve begins as the ganglion cell axons leave the eye.

16. Two visual problems result from the inside out arrangement:
Incoming light is distorted as it passes through the cell layers
There is a blind spot (no receptors or cells) at the optic disk where the axons gather to exit the eye

17. Solutions
The fovea is an area (0.33 cm diameter) in the center of the retina where there is a thinning of the retinal ganglion cell layer.
Less distortion of light
Specialized for high-acuity vision (seeing details)
Completion – the visual system uses information from receptors around the blind spot to fill in the gap in the retinal image.

18. photopic and scotopic vision
The two systems are “wired” differently
Cones – low degree of convergence (a single ganglion cell receives signals from a few cones).
Rods – high degree of convergence (a single ganglion cell receives signals from hundreds of rods).

20. photopic and scotopic vision
Cones are concentrated in the fovea, which contains no rods.
Rods are concentrated 20 degrees from the fovea and in the nasal hemiretina (retina half of both eyes near the nose).

21. Spectral sensitivity curve
In general, more intense light appears brighter. However, wavelength also has an effect on the perception of brightness.
A graph of the relative brightness of lights of the same intensity but at different wavelengths is called a spectral sensitivity curve (see Pinel p. 138).

22. Spectral sensitivity curves
There are two spectral sensitivity curves.
The photopic spectral sensitivity curve has a peak brightness at 555 nm (yellow-green)
The scotopic spectral sensitivity curve has a peak brightness at 507 nm (green-blue)
The Purkinje effect – walking through his garden, Purkinje noticed that his yellow and red flowers were brighter than the blues ones just before dusk; just a few minutes later the trend was reversed (blue flowers appeared as brighter greys).

23. Transduction - Conversion of one form of energy to another.
Visual transduction – conversion of light to neural signals.
Rhodopsin – the red pigment in rods becomes bleached when exposed to light.
It is a G-protein-linked receptor that responds to light.

25. Light activation of rods:
Light bleaches rhodopsin molecules.
cGMP is broken down, closing sodium channels
Sodium ions cannot enter the rod resulting in hyperpolarization.
Glutamate release is reduced
Transduction of light by rods demonstrates that signals can be transmitted through neural systems by inhibition.

26. signal transduction Light bleaches rhodopsin
Opsin separates from retinal molecule
Transducin (G-protein) activated
α-subunit breaks away and activates PDE6
PDE breaks down cGMP
With cGMP broken, the Na+ channel closes

27. From retina to primary visual cortex
Pathway: retina  lateral geniculate nucleus (LGN)  primary visual cortex
~90% of axons of retinal ganglion cells make up this pathway
LGN channels
Parvocellular (P layers) run through top 4 layers of LGN – responsive to color and fine detail (input from cones)
Magnocellular (M layers) run through bottom 2 layers of LGN – responsive to movement (input from rods)
Most LGN neurons that project to primary visual cortex (V1, striate cortex) terminate in the lower part of cortical layer IV

28. Is retinotopic – each level is organized like a map of the retina
Temporal hemiretina
does not cross
Nasal hemiretina
crosses
Right visual fields of
Both eyes
Left visual fields of
Both eyes
Bottom of visual field
to dorsal cortex
Top of visual field
to ventral cortex
Note that 25% of V1 is dedicated to fovea

29. Seeing Edges A visual edge is “nothing”
Where two different areas of an image meet.
A perception of contrast between two adjacent areas of the visual field

30. Mach bands

31. Lateral inhibition of ommatidia receptors in lateral eye of horseshoe crab
A, B & C fire at the same rate (same high level of light stimulation and same degree of lateral inhibition)
D fires more (same high level of stimulation from light but less lateral inhibition from E)
F, G & H fire at the same rate (dim light)
E, fires less because of greater inhibition from D.

32. Saccades
The eye continually scans the visual field and makes a series of brief fixations (3/sec) connected by quick eye movements called saccades.
The fixations are integrated to produce greater color and detail than the restricted foveal region can produce if it remained stationary
stabilized retinal images, projected from a contact lens that moves with the eye; image disappears in a few seconds.

33. Brief fixations associated with saccades while a person views different pictures
Making visual saccades to items of interest is a function of the superior colliculus

34. Retina-geniculate-striate pathways
Visual fields
Hemi-retinas
Optic chiasm
LGN
P layers
M layers
Optic radiations
Striate (primary visual) cortex

35. LGN