Lecture 8

Question 1

Auditory pathways

Answer 1

• Afferents from the cochlea travel via the vestibulocochlear nerve
  (cranial nerve VIII) to the cochlear nucleus in the brain stem
• From here, strong contralateral projections run to the superior olivary
  nucleus (SON); there is a small amount of ipsilateral information sent
  from cochlear nucleus to the SON
• Remaining projections are ipsilateral:
• SON TO inferior colliculus TO medial geniculate nucleus (MGN) TO
  cortical regions important for auditory processing

Question 2

There are 2 main “streams” of auditory processing in higher cortical
regions after A1:

Answer 2

1. dorsal path – projects to parietal cortex – focuses on spatial
   localization and coordinating head movements (what/where is sound)
2. ventral path – projects to temporal cortex – interprets what complex
   sounds are, such as voices or speech (context)

Question 3

Basics of visual system

Answer 3

1) Light reflects off objects in all directions
- If the eye took in all light, the photoreceptors would
respond to EVERYTHING – creating a giant blur
2) Instead, an aperture (pupil) limits the amount of passing
light

Question 4

Eye Anatomy

Answer 4

Light enters the eye through an opening in the center of the iris called the pupil
* Light is focused by the lens and the cornea onto the rear surface of the
eye known as the retina
* Retina - forms the receptive field for detection of light, and is lined with
rods and cones

Question 5

Retina

Answer 5

*Processing in retina is complex; many aspects of visual processing can be
explained at this level alone

*Five cell types in the retina: rods/cones, horizontal cells, bipolar cells, amacrine cells, ganglion cells

Question 6

RODS –

Answer 6

most abundant in the periphery of the eye and respond to faint light (~100 million per retina)

Question 7

scotopic system –

Answer 7

system in retina that operates on dim light and comprised of rod

Question 8

CONES –

Answer 8

differentiate between wavelengths of light and they reside in and around the fovea (~25 million per retina)

Question 9

photopic system –

Answer 9

system in retina that operates in bright light, shows sensitivity to color, and involves cones

Question 10

Retinal Receptors

Answer 10

• Photopigments - chemicals contained by both rods and cones that release
  energy when struck by light; e.g., rhodopsin
• Mechanism: light induces a conformational (structural) change in retinal molecule, activating the second messenger cGMP, and closing of Na+ channels

Question 11

Retinal Processing - Steps

Answer 11

-Rods and cones relay
messages to bipolar cells,
located closer to the center
of the eye

-Bipolar cells send messages to ganglion cells

-The axons of ganglion cells join one another to form the optic
nerve that travels to the brain
Rod/ cone TO bipolar TO ganglion cell

Question 12

Fovea –

Answer 12

highest acuity and concentration of cones (i.e., phototopic system)

Question 13

_____ is the center of the human retina.

Answer 13

Macula.

• The central portion of the macula is the fovea and allows for acute and detailed vision
  – Contains the highest concentration of cones
  – Nearly free of ganglion cell axons and blood vessels

Question 14

The ……… consists of the axons of ganglion cells that bundle together and exit through the back of the eye and travel to the brain

Answer 14

Optic nerve

Question 15

Contrasting patterns of innervation:
Cones vs. Rods - Cones

Answer 15

CONES
* Mostly in fovea
* Each receptor in the fovea attaches to a single bipolar cell and a single
ganglion cell
* Each cone in the fovea has a direct line to the brain which allows the
registering of the exact location of input
* 25 million cones
* Forms photopic system
- MORE INFO FROM CONES THAN RODS

Question 16

Contrasting patterns of innervation:
Cones vs. Rods - Rods

Answer 16

RODS
* In the periphery of the retina, has a greater number of receptors
converging onto ganglion cells
* Lacks detail but allows for the greater perception of much fainter light
involving peripheral field of vision
* 100 million rods
* Forms scotopic system
- MORE INFO FROM CONES THAN RODS

Question 17

Lateral inhibition –

Answer 17

phenomenon whereby interconnected neurons inhibit their neighbor, producing greater contrast of edges in visual objects

-in the retina, the closer a ganglion cell is to its neighbor, the greater
amount of inhibition that it may receive

In humans, lateral inhibition is mediated by horizontal cells and
amacrine cells –these are cell types in the retina that modify activity of
information flow from rod/ cone en route to the ganglion cell to promote
the edge enhancement effect

Question 18

Two major interpretations of color vision include the
following:

Answer 18

1. Trichromatic theory
2. Opponent-process theory

Question 19

Lateral Inhibition Conceptual basis

Answer 19

1. No stimulation, low inhibition
2. No stimulation, medium inhibition
3. Stimulation, medium inhibition
4. Stimulation, high inhibition

Question 20

Trichromatic Theory -

Answer 20

• Idea behind this theory comes from the fact that artists have known for years that
  many hues can be created from a small number of colors
• Predicts that color perception occurs through the relative rates of response by three kinds of cones: short, medium, and long-wavelength (Population or cross-fiber coding)

Question 20

Color “blindness” -

Answer 20

is an impairment in perceiving color differences

• Gene responsible is on the X chromosome and this is inherited in a mendelian manner, i.e., sex-linked and recessive
• Most common form is difficulty distinguishing between red and green

– This results from the long/red and medium/green wavelength cones having the same photopigment

Question 21

opponent-process theory -

Answer 21

suggests that we perceive color
in terms of paired opposites (e.g., blue vs. yellow, green vs. red, black vs. white)

Question 22

Color constancy –

Answer 22

is the ability to recognize color despite
changes in lighting
- **this is not easily explained by either theory

Question 23

Retinex theory -

Answer 23

suggests the brain compares information from
various parts of the retina to determine the brightness and color
for each area
- ***Retinex theory explains color constancy better

Question 24

Lateral geniculate nucleus (LGN)

Answer 24

– the part of the thalamus specialized for visual perception – most ganglion cell axons project to LGN – LGN relays to other parts of the thalamus and to the visual areas of the occipital cortex

Question 25

receptive field - in retina and LGN

Answer 25

part of the visual field that either excites or inhibits the cell *In the retina: ON-center and OFF-center fields are created by different kinds of bipolar cells (will draw an example) *In the LGN: Ganglion cells and LGN cells also have ON- center and OFF-center fields

Question 26

The ..... receives information from LGN and is the first level of cortical processing of visual information

Answer 26

primary visual cortex (area V1)

Question 27

Hubel and Wiesel (1959) distinguished various types of cells in the visual cortex...

Answer 27

Simple cells – Respond to bars or edges with a particular width, location, and orientation V1 Cortex Complex cells – Have elongated receptive fields, can not be tied to location – Respond to a pattern of light in a particular orientation and most strongly to a stimulus moving perpendicular to its axis

Question 28

V1 Cortex – Hierarchical model

Answer 28

more complex events are built up based upon inputs from simpler ones

Question 29

V1 Cortex – Spatial frequency filter model

Answer 29

Spatial frequency filter model – involves Fourier analysis of frequency patterns in a visual scene ***The spatial frequency model is more accurate than the hierarchical model since it doesn’t require assembling of visual space

Question 30

“Where” and “What” Streams

Answer 30

Dorsal stream - Where Ventral stream - What

Question 30

dorsal stream -

Answer 30

visual pathway in the parietal cortex -helps the motor system to find objects and move towards them -processes information relevant to motion -”where”

Question 31

V1 has 4 separate representations measured by neuronal activity...

Answer 31

1. location in visual field – there is larger, more detailed mapping in center vs. peripheral field of view 2. Ocular dominance – some neurons fire more strongly to one eye vs. the other, i.e., think of handedness but for eyes (visual info is overlapping) 3. Orientation - lines, edges, objects, etc. (e.g., simple and complex cells) 4. Color

Question 32

ventral stream –

Answer 32

visual pathway in the temporal cortex -processes more complex features, color and brightness -”what”

Question 33

Ventral “What” Pathways - From V1, go to...

Answer 33

* The secondary visual cortex (area V2) receives information from area V1, processes information further, and sends it to other areas * V2 TO... V4 TO... ITC/ PTC

Question 34

Ventral “What” Pathways - V4 region –

Answer 34

receives input from V2 –responds to sinusoidal frequencies; also responds to concentric and radial stimuli (processing more abstract features)

Question 35

Ventral “What” Pathways - PTC (posterior temporal cortex)

Answer 35

– contains cells that respond to color and brightness; important for color constancy (e.g., retinex theory in prev. slides) - Recieves input from V4

Question 36

Ventral “What” Pathways - ITC (inferior temporal cortex)

Answer 36

contains cells that respond selectively to complex shapes but are insensitive to distinctions that are critical to other cells - Recieves input from V4

Question 37

Ventral “What” Pathways - Shape constancy

Answer 37

is the ability to recognize an object’s shape despite changes in direction or size * ITC activity ignores changes in size and direction, and thus likely contributes to our capacity for shape constancy

Question 38

Ventral “What” Pathways - Visual agnosia

Answer 38

is the inability to recognize objects despite satisfactory vision – Caused by damage to the pattern pathway usually in the temporal cortex * Damage to ITC may cause prosopagnosia -the inability to recognize faces