The document summarizes key aspects of human visual system anatomy and physiology. It describes how light is detected by the retina and transmitted to the brain via the optic nerve. It explains the roles of the iris, pupil, lens and ciliary muscles in controlling the amount of light that reaches the retina and in focusing images. It discusses visual pathways from the retina to primary visual cortex and theories of visual processing in the brain related to motion, object recognition and conscious perception.

1. The Visual System…

2. Let there be Light….
 Light: waves of electromagnetic energy that
are between 380 and 760 nanometers.
 Wavelength: distance between one peak of a
light wave and the next peak. Plays an
important role in the perception of colour
 Intensity: plays a role in the perception of
brightness.

3. Let there be Light…
Irises: donut shaped bands of contractile tissue that
regulates the amount of light that reach the retinas.
Gives our eyes their characteristic colour.
Retina: a light sensitive tissue lining the inner surface
of the eye.
Pupil: Hole in the iris. This is where light enters.
Adjustment of pupil size in response to changes in
illumination represents a compromise between
sensitivity and acuity.

4. Let there be Light…
 Sensitivity: Ability to detect presence of dimly
lit objects.
 Acuity: Ability to see details of objects.
 Pupils constrict in high illumination because
sensitivity is not important. When constricted,
the image falling on each retina is sharper
and there is a greater depth of focus.

5. Let there be Light…
 Pupils dilate in low illumination to let in more light,
sacrificing acuity and depth of focus.
 Lens: located behind ach pupil. Focuses light on the
retina.
 Ciliary muscles: eye muscles that control the shape
of the lenses.
 Accommodation: process of adjusting the
configuration of the lenses to bring images into focus
on the retinas.

6. Let there be Light…
 When looking at an object that is near, the
tension on the ligaments holding each lens is
adjusted by the ciliary muscles, and the lens
assumes its natural cylindrical shape. This
increases the ability o the lens to refract light.
 When focusing on distant objects, the lens is
flattened.

8. Eye Position
Vertebrates have 2 eyes because they have two
sides. The left and the right…
Predators: have front facing eyes because this
enables them to accurately perceive how far
away a prey is.
Preys: have side facing eyes because it gives them
a wider field of vision, thus enabling them to see
approaching predators.

9. Eye Position
 Front facing eyes- an arrangement that is an
important basis for our visual system’s ability
to create three-dimensional retinal images.

10. Binocular Disparity
 It is the difference in the position of the retinal
image of the same object on the two retinas.
 Greater for close objects than for distant
objects.
 Visual system can use the degree of
binocular disparity to construct one three-dimensional
perception from two two-dimensional
retinal images.

11. The Retina and Translation of Light
into Neural signals…or the Boring Part
 The retina converts light to neural signals,
conducts them toward the CNS, and
participates in the processing of the signals.
 It is composed of 5 layers of different kinds of
neurons. Each of the five types of retinal
neurons comes in a variety of subtypes. Over
50 different kinds have been identified.

12. The Retina and Translation of Light
into Neural signals…or the Boring Part
 Receptors: cells that are specialized to
receive, mechanical, or radiant signals from
the environment; also proteins that contain
binding sites for particular neurotransmitters.
 Horizontal cells: type of retinal neurons
whose specialized function is lateral
communication.

13. The Retina and Translation of Light
into Neural signals…or the Boring Part
 Bipolar cells: bipolar neurons that form the
middle layer of the retina.
 Amacrine cells: type of retinal neurons
whose specialized function is lateral
communication.
 Retinal ganglion cells: retinal neurons whose
axons leave the eyeball and form the optic
nerve.

14. The Retina and Translation of Light
into Neural signals…or the Boring Part
 Blind spot: the area on the retina where the
bundle of axons of the retinal ganglion cells
penetrate the receptor layer and leave the
eye as the optic nerve.
 Fovea: Cone rich and central indentation of
the retina, which is specialized for high acuity
vision.

15. The Retina and Translation of Light
into Neural signals…or the Boring Part
 Retinal neurons communicate chemically
through synapses and electrically through
gap junctions.
 Completion: the visual system’s automatic
use of information obtained from receptors
around the blind spot, or scotoma, to create
a perception of the missing portion of the
retinal image.

16. The Retina and Translation of Light
into Neural signals…or the Boring Part
 Surface Interpolation: process by which the
visual system perceives large surfaces, by
extracting information about edges and from
it, inferring the appearance of adjacent
surfaces.

17. Cone and Rod Vision
 Cone: visual receptors in the retina that
mediate high acuity colour vision in good
lighting.
 Rods: visual receptors in the retina that
mediate achromatic, low-acuity vision under
dim light.
 Duplexity theory: theory that cones and rods
mediate photopic and scotopic vision,
respectively.

18. Cone and Rod Vision
 Photopic Vision: cone mediated, predominates when
lighting is good.
 Scotopic: rod mediated, predominates in dim light.
 Difference between phopic and scotopic vision is
how they are “wired”
 In scotopic vision, the output of several hundred rods
converge on a single retinal ganglion cell.
 In the photopic system only a few cones converge
on each retinal ganglion cell to receive input from
only a few cones.

19. Spectral Sensitivity
 Photoptic spectral sensitivity curve: graph of the
sensitivity of cone-mediated vision to different
wavelengths of light.
 Scoptic spectral sensitivity curve: graph sensitivity of
rod-mediated vision to different wavelengths of light.
 Purkinje effect: in intense light, red and yellow
wavelengths look brighter than blue or green
wavelengths of equal intensity; in dim light, blue and
green wavelengths look brighter than red and yellow
wavelengths of equal intensity.

20. Eye Movement
 Fixational eye movements: involuntary
movements of the eyes that occur when a
person tries to fix their gaze on a point.
 Tremor
 Drifts
 Saccades: rapid eye movement of the eyes
between fixations

21. Visual Transduction: Conversion of
Light to Neural ignals
 Transduction: conversion of vone form of
energy to another.
 Visual Transduction: conversion of light to
neural signals by the visual receptors.
 Rhodopsin: photopigment of rods. Loses
colour when exposed to light.

22. Primary Visual Cortex
 Retina-geniculate-striate pathway: major visual
pathway from each retina to the striate cortex [PVC]
via the lateral geniculate nuclei of the thalamus.
 Primary Visual Cortex: area of the cortex that
receives direct input from the lateral geniculate
nuclei
 Lateral geniculate nuclei: 6 layered thalamic
structures that receive input from the retinas and
transmit their output to the primary visual cortex.

23. Primary Visual Cortex
 To simplify things, all signals from the left
visual field reach the right primary visual
cortex either ipsilaterally from the temporal
hemiretina of the right eye or contralaterally
(cia the optic chiasm) from the nasal
hemiretina of the left eyes.

24. Primary Visual Cortex
 Retinotopic: organized according to the map
of the retina [retina-geniculate-striate]
 Parvocellular layers: layers of the lateral
geniculate nuclei that are composed of
neurons with small cell bodies; the top four
layer.Particularly responsive to colour, fine
pattern details, and to stationary or slow
moving objects. Cones provide the majority
of input.

25. Primary Visual Cortex
 Magnocellular layers: layers of the lateral
geniculate nuclei that are composed of
neurons with large cell bodies; the bottom
two layers. Particularly responsive to
movements. Rods provide the majority of
input.

26. Seeing Edges
 Story of the horse shoe crab and machbands
from 135
 Lateral Inhibition: inhibition of adjacent
neurons or receptors in a topographic array.
 Receptive field: the area of the visual field
within which it is possible for the appropriate
stimulus to influence the firing of a visual
neuron.

27. Seeing Edges
 Simple cells: neurons in the visual cortex that
respond maximally to straight-edge stimuli in
a certain position and orientation.
 Complex cells: neurons in the visual cortex
that respond optimally to straight-edge
stimuli in a certain orientation in any part of
their receptive field.

28. Seeing Colour
 Component Theory (trichromatic)
 Proposed by Thomas Young in 1802 and
refined by Hermann von Helmholtz in 1852
 The relative amount of activity produced in
three different classes of cones by light
determine its perceived colour

29. Seeing Colour
 Opponent process Theory
 Ewald Herring 1878
 A visual receptor or a neuron signals one colour
when it responds in one way and signals the
complementary colour when it responds in the
opposite way.
 Complementary colours: colours that produce white
or gray when combined in equal measure.

30. Seeing Colour
 Colour Constancy: tendency of an object to
appear the same colour even when the
wavelengths of light that it reflects changes.
 Retinex theory: colour of an object is
determined by its reflectance, which the
visual system calculates by comparing the
ability of adjacent surfaces to reflect short ,
medium and long wavelengths.

32. Seeing Colour
 Cytochrome Oxidase: an enzyme present in
particularly high concentrations in the
mitochondria of dual-opponent color cells of
the visual cortex.

33. Cortical Mechanisms of Vision And
Conscious Awareness….

34. Cortical Mechanisms of Vision And
Conscious Awareness….
 Secondary visual cortex: areas of cerebral
cortex that receive most of their input from
primary visual cortex
 Visual association cortex: areas of cerebral
cortex that receive input from areas of
secondary visual cortex as well as from
secondary cortex of other sensory systems.

35. Cortical Mechanisms of Vision And
Conscious Awareness….
 Prestriate cortex: band of tissue in the occipital lobe
that surrounds the primary visual cortex
 Inferotemporal cortex: cortex of inferior temporal
lobe.
 Posterior parietal complex: area of association
cortex that receives input from the visual, auditory,
and somatosensory systems and is involved in the
perception of spatial location and guidance of
voluntary behaviour.

36. Cortical Mechanisms of Vision And
Conscious Awareness….
 Scotoma: an area of blindness produced by
damage to, or disruption of, an area of the
visual system.
 Perimetry Test: procedure used to map
scotomas.
 Hemianospic: scomota covering half of the
visual field

37. Cortical Mechanisms of Vision And
Conscious Awareness….
 Blindsight: ability of some patients who are
blind as a consequence of cortical damage to
unconsciously see some aspects of their
visual environments [outlines/foggy
images/etc]

38. Cortical Mechanisms of Vision And
Conscious Awareness….
 Dorsal stream: group of visual pathways that flows
from the primary visual cortex to the dorsal prestriate
cortex to the posterior parietal cortex; according to
one theory, its function is the control of visually
guided behaviour.
 Ventral stream: group of visual pathways that flows
from the primary visual cortex to the ventral
prestriate cortex to the inferotemporal cortex;
according to one theory, its function is conscious
visual perception

39. Cortical Mechanisms of Vision And
Conscious Awareness….
 Where vs What theory: Dorsal S specializes
in visual spatial perception; Ventral S
specializes in visual pattern recognition
 Damage to DS disrupts visual spatial
perception
 Damage to VS disrupts visual pattern
recognition

40. Cortical Mechanisms of Vision And
Conscious Awareness….
 Control of Behaviour vs Conscious
Perception Theory
 DS specializes in visually guided behaviour
 VS specializes in conscious visual perception

41. Prosopagnosia
 Visual agnosia for faces
 Agnosia: inability to consciously recognize
sensory stimuli of a particular class that is not
attributable to a sensory deficit or to verbal or
intellectual impairment
 Visual agnosia: failure to recognize visual
stimuli that is not attributable to sensory,
verbal, or intellectual impairment

42. Akinetopsia
 Deficiency in the ability to perceive motion,
which often results from damage to dorsal
visual pathway.