1. Sensation

2. Sensation
The way by which the brain is informed about what is happening in the external environment and inside the body
Sensory receptors - specialized neurons that detect a variety of physical events
Stimuli impinge on them and alter their membrane potentials
Sensory transduction – process by which sensory stimuli are transduced into slow, graded receptor potentials

3. Receptor potential – a slow, graded electrical potential produced by a receptor cell in response to a physical stimulus

4. Visual sensation

5. The Visual Pathway Photoreceptors Retinal ganglion cells in the retina
Optic chiasm
Lateral geniculate nucleus (thalamus)
Visual cortex

6. Optic chiasma – where the nerve fibers in the nasal (medial) pathway crosses the midline
Contralateral (opposite side) – path of the nasal visual field
Ipsilateral (same side) – path of the lateral visual field

9. Anatomy of the Eye Orbit – the eye socket in the skull
Extraocular muscles – attachment and movement of the eyeballs
Conjunctiva – mucous membrane that lines the eyelid and folds back to attach to the eyes
Sclera – outermost tunic of the eyes
Cornea – transparent anterior portion of the sclera
Aqueous humor – clear liquid in the anterior chamber of the eye behind cornea but anterior to the lens

10. Anatomy of the Eye Iris – pigmented ring of muscles behind the cornea
Pupil – opening of the iris where light enters
Lens – transparent onion-like structure immediatelyn behind the iris and pupil
Ciliary muscles – contract to change the shape of the lens
Accomodation - changes in the shape of the lens for focusing images of near and distant objects in the retina

11. Aqueous humor
Ciliary muscles

12. Anatomy of the Eye
Vitreous humor – clear gelatinous substance behind the lens; the humors help maintain the shape of the eyeball
Retina – interior lining of the back of the eye, contains the photoreceptors
Photoreceptors – receptors for light

13. Anatomy of the Eye Cones – photoreceptors for light and color vision
Source of vision of the highest sharpness (acuity – from acus = needle)
Fovea – central region of the retina that mediates our most acute vision , contains only cones
Rods – photoreceptors for dark vision, more sensitive to light
Night vision color-blind and lack foveal vision

15. Anatomy of the Eye
Optic disk – where the axons conveying visual information gather together and leave the eye through the optic nerve
Blind spot – area of the optic disk in the retina because there are no photoreceptors located there

17. Anatomy of the Eye
Bipolar cells –neurons whose two arms connect the shallowest and deepest layers of the retina
Ganglion cells – neurons whose axons travel in the optic nerves (second cranial nerve) and carry visual information into the brain
Horizontal cells
Amacrine cells
These two combine messages from adjacent photoreceptors

19. Photoreceptors
Consist of an outer segment connected by cilium to the inner segment which contains the nucleus
Outer segment – contains lamellae or thin plates of membrane
Photopigment – molecules embedded in the membrane of the lamellae
2 parts : opsin and retinal
Opsin – protein
Retinal - lipid

20. Photoreceptors Rhodopsin – rod opsin + retinal Rhod = rose Pinkish hue
Retinal – synthesized from vitamin A

21. Photoreceptors
light rhodopsin (rosy pink) bleaching rod opsin + retinal (pale yellow) change in photoreceptor membrane potential change rate in photoreceptor release of NT glutamate

23. Connections between the eye and the brain
Axons of the retinal ganglion cells bring information to the rest of the brain
Optic nerves  dorsal lateral geniculate nucleus of the thalamus
Magnocellular layers – inner 2 layers
Parvocellular layers – outer 4 layers
Koniocellular layers – ventral
Optic radiations  primary visual cortex (striate cortex) – surrounding calcarine fissure in medial and posterior occipital lobe

25. Coding of light and dark
Receptive field – part of the visual field in which the presentation of visual stimuli will produce an alteration in the firing rate of a particular neuron
Periphery – many neurons converge on a single ganglion cell
Fovea – equal numbers of ganglion cells and cones
Foveal vision very acute but peripheral vision less precise

27. The role of retinal ganglion cells
Hubel and Wiesel 1979
each retinal ganglion cell responds to a pattern of light in a particular place in the visual field. The area in space that a cell responds to is called its receptive field.

29. The role of retinal ganglion cells
For an on-center field, the retinal ganglion cell maximally increases its firing to a stimulus that is a circle of light that completely covers the on center and does not at all cover the ring surround.
A ring of light that completely covers the surround and does not at all cover the inner circle will maximally turn the cell off.

30. The role of retinal ganglion cells
For off-center receptive fields the firing patterns are exactly the reverse of on-center cells.

31. The role of retinal ganglion cells
X cells seem to respond to stationary points of light and can distinguish fine grain detail.
Y cells respond best to changes in illumination or moving stimuli.
W cells have more complicated receptive fields than simple on-center or off-center fields.
Their field centers respond to either light or dark rather than to just one or the other.

32. The role of the lateral geniculate nucleus
organizational relay station.
the first place for which the receptive fields all relate to the opposite side of the body.
LGN receptive fields are also made up of a circle surrounded by a ring and which is either on-center or off-center.
both X cells and Y cells are also seen in the LGN.

33. The role of the lateral geniculate nucleus
The cells of the four P-cell layers have X-cell properties and respond to fine grain detail. They also respond to lights of different colors.
The cells of the two M-cell layers have Y-cell properties in that they respond most to moving stimuli.

34. Coding of color
Thomas Young and Hermann von Helmholtz 1802: Young-Helmholtz Theory or Trichromatic Color Theory
Color mixing vs. Pigment fixing
Three different types of cones responsible for color vision
in order to make any color people only need three color receptors: red, green and blue.
Different absorption characteristics controlled by the particular opsin the photoreceptor contains

35. Coding of color
Different opsins absorb particular wavelengths more readily
Young–Helmholtz theory could not account for negative afterimages.
if you stare at a green square and then look at a white square, the white square appears red.

36. Coding of color The opponent-process theory proposed by Hering in 1878
coding for color occurs as three opponent processes.
red and green, blue and yellow, and black and white were all opposing colors and that individual neurons code for a pair.

37. Coding of color
Evidence for this opponent process has been found in the LGN.
two types of opponent cells in the LGN
red–green and yellow–blue opposition
Red–green opponent cells register either red and not green or green and not red
saying that we see green negates our ability to simultaneously say we see red, and vice versa.

38. Coding of color
The blue–yellow opponent cells work in a similar fashion.
Yellow is an opponent color because red and green cones firing together give us the perception of yellow.

42. Genetic defects in color vision
Anomalies in one or more of three types of cones
X-linked (more manifest in males)
Protanopia – first color defect – confuse red and green, see mostly yellow and blue
Red cones filled with green opsins
Deuteranopia –second color defect
Green cones filled with red opsins
Tritanopia – M=F
See the world in greens and reds
Retina lacks blue cones

44. Coding in the Visual Cortex
Simple cells (Hubel and Wiesel) - The cells of layer 4 of visual area V1
the receptive fields of these cells all code for simple visual features such as lines and edges.

45. Coding in the Visual Cortex
they can have an on-center or an off-center but this does not have to be central within the field.
Alternatively, the field can be split in half so that it registers an edge.

46. Simple Cells
simple cells have a preference for lines and edges of a particular orientation.

47. Simple Cells
All respond best to a stimulus of a particular orientation and a stimulus positioned so that it borders the on- and off-zones.
the on- and off-zones of all simple cells exactly cancel each
other out.

48. Coding in the Visual Cortex
complex cells - Layers 2, 3 and 6 of visual area V1
Receive input from the simple cells similar to simple cells in that they require the specific orientation of a light/dark boundary.
Light over the whole receptive field leads to no overall response
Exact position of the light/dark boundary within the receptive field is not important

49. Complex Cells
As long as the boundary falls somewhere within the receptive field, the cell will alter its firing rate.
cells also respond to boundaries that are moving across the field.

50. Coding in the Visual Cortex
Hypercomplex cells
responds best to a line that stops within the receptive field or to a corner placed within the field.
Now labeled as further varieties of complex cell.
All of these varieties of cortical cell suggest that the visual system builds up a pattern of lines and edges from the ‘dots’ of the retinal ganglion cells.

52. Coding for Stereoscopic Depth
Retinal disparity – source of the most important form of depth information in humans
the images from two objects that are at different distances do not fall on the same parts of the retinas of the two eyes.
use the degree of disparity as a relative marker of the distance between the two objects.

53. Coding for Stereoscopic Depth
The question that many have asked is how we can match the two images to be able to measure disparity.
there may be some cells within the visual cortex that are especially for this function
layer 4 of the cortex (the input layer) the cells are monocular
Strabismus in which the binocular cells in the visual cortex are not simultaneously activated by the same stimulus.

54. Coding for Stereoscopic Depth
Strabismus in which the binocular cells in the visual cortex are not simultaneously activated by the same stimulus.
The result is that these cells become monocular and the child permanently loses stereopsis.
Three types of disparity detector in the visual cortex.
One type of cell is active when the images are displaced outward (close neurons),
a second type is active when the images are displaced inwards (far neurons)
the third type are active when there is no retinal disparity (in focus neurons).

55. Coding for Color layers 2 and 3 of area V1
cells called blobs and interblobs that code for color rather than for orientation
receive their input from the parvocellular layers of the lateral geniculate nucleus.
Blobs are also opponent-color cells that are either red–green or blue–yellow
area V4. - give us the perceived color
V4, information
is sent to the temporal lobe for further color processing.

56. Coding for movement
Complex cells of area V1 – some detection of motion
cells of area V5 (also called area MT) - main processing of motion information
Organized such that the cells in a column respond to movement in a particular direction
cells respond to the direction of movement irrespective of the orientation of the object.
involves opponent mechanisms
similar to those involved in color perception

57. TWO VISUAL SYSTEMS

58. Why two distinct pathways?
distinction between what an object is and where in space that object is

59. The dorsal stream
the ‘where’ stream because it deals mainly with information about where an object is.
starts in the magnocellular cells of the lateral geniculate nucleus (LGN)
Fibers from these cells project to layer 4 of area V1 and then project to area V2.
Dynamic form information is processed by area V3
motion information is processed by area V5.

60. Information in the dorsal stream leaves the visual cortex and goes to the parietal cortex.
projection to areas of the frontal cortex that control movement.
combination of dynamic form and motion information that allows us to determine where an object is.
Capabilities like reaching and grasping critically require information about where an object is.

61. Ventral stream
the ‘what’ stream because it deals mainly with information about what an object is.
processing of what information was cortical in origin
starts with the parvocellular cells of the LGN.
project to areas 2 and 3 of visual area V1.
Sent to the blobs and interblobs of this area
Information about form then goes to area V2 and on to area V4.
Information about color goes to area V3 and then to area V4.

62. From area V4, information is sent to the temporal cortex, presumably to allow the ‘what’ information to be integrated with other information processing
likely that the cells of the temporal cortex code for the building blocks of whole objects

63. Damage to the Visual Cortex
Agnosia – “failure to know”
Inability to perceive or identify a stimulus by a particular sensory modality even though its details can be detected by means of that sensory modality and the person retains relatively normal intellectual capaciy
Visual agnosia – deficits in visual perception in the absence of blindness
Apperceptive visual agnosia – failure to perceive objects even though visual acuity is relatively normal

64. Prosopagnosia – failure to recognize particular faces (damage to the fusiform face area)
Associative visual agnosia – inability to identify objects that are perceived visually, even though the form of the perceived object can be drawn or matched with similar objects
Akinetopsia – inability to perceive movement caused by damage to area V5 (MST) of the visual association cortex