Lecture 3- Topographic and Tonographic Organisation

Question 1

Where do 90% of fibres originating in the retina terminate?

Answer 1

Travel by the optic nerve and terminate at the lateral geniculate nucleus of the thalamus

Question 2

What do responses of cells in the LGN allow for?

Answer 2

Allow mapping of the visual field

Question 3

What type of cell are the LGN cells? i.e. what type of receptive field do they have and how does this respond to light?

Answer 3

• Have on-centrer, off-surround receptive fields
• This means when light falls on the center the cell is activated (increase in firing), when light falls on the surround the cell is inhibited (decrease in firing).
• Note: if light falls in equal amounts on the center and surround then there is no change in firing rate. This is known as diffuse illumination.

Question 4

What does the term retinotopic organisation mean?

Answer 4

• Describes how the visual world maps on to retina and then to the LGN
• The means that spatial relationships are maintained in the nervous system

Question 5

Where do cells from the LGN project to?

Answer 5

The primary visual cortex (V1, striate cortex, Brodmann’s area 17)

Question 6

How does a simple cell respond in V1?

Answer 6

• Respond best to elongated bars or edges
• Are orientation selective.
• Have separate ON and OFF subregions
• Can be monocular or binocular

Question 7

How does a complex cell respond in V1?

Answer 7

• Are orientation selective.
• Have spatially homogeneous receptive fields (no separate ON/OFF subregions)= THIS IS WHAT SEPERATES THEM FROM SIMPLE CELLS
• Nearly all binocular.

Question 8

What is the ordering of different cell types in the visual system? i.e. what feeds into what?

Answer 8

• Retinal ganglion cells (front of retina output into optic nerve)
• LGN
• Simple cells
• Complex cell (multiple simple cells feed into)

Question 9

What kind of ‘architecture’ does V1 have? What does this mean?

Answer 9

-Has a columnar architecture
-As an electrode is moved perpendicular to the cortical surface receptive fields and orientation preference of cells are
maintained (orientation columns). i.e. all cells that vertically align in V1 respond to stimulus in a similar way
-As an electrode is moved parallel to the cortical surface receptive fields and orientation preference of cells shift i.e. cells across respond differently to stimulus

Question 10

What does it mean when we say V1 has retinotopic maps?

Answer 10

• Each point of the visual field maps onto a local group of neurons in V1
• That is what retinotopy means: the remapping of retinal image onto cortical surface

Question 11

What region of the retina corresponds to more space in V1?

Answer 11

• The fovea i.e a lot more of the cortex is devoted to interpreting the high resolution information in the center of the visual field as opposed to the periphery.
• This creates an interesting effect where the center of the retinal image is magnified in the cortical map

Question 12

What is retinotopic mapping an example of?

Answer 12

Topographic organisation : An ordered representation of the sensory environment where spatially adjacent surfaces are represented in adjacent positions in the brain i.e. spatial positioning is preserved

Question 13

Why does our visual system operate topographically? What are the benefits?

Answer 13

Reduces axon volume:
-Brain volume is driven largely by axon volume (evolutionary pressure to keep the brain small due to child birth constraints)
-Reducing axon length provides space for more neurons and
conserves metabolic resources
-Axon length is shortest when neurons that share dense
connectivity are clustered together
-In the peripheral visual system much processing is local
(orientation - contrast – movement). Objects are often
clusters.

Facilitates processing e.g. lateral inhibition (opposing processes)

Question 14

What is sound? What is often used as a measurement for it?

Answer 14

• Pressure pulses: oscillations of high and low pressure which travel at ~340 metres/sec
• Frequency: Cycles per second which is measured in Hertz

Question 15

What is the range of sounds that humans can hear?

Answer 15

20-20,000 Hz

Question 16

What does a 10dB decrease correspond to?

Answer 16

A halving of perceived loudness

Question 17

What happens to our hearing as we age? How does it effect different genders?

Answer 17

• As we get older our ability to detect high frequency sounds worsens
• Worse for men than women

Question 18

What tonotopic representation occurs within the cochlear?

Answer 18

• Detect low frequencies at the end (apex)

- Detect higher frequencies at the base

Question 19

What is transduction in the auditory system?

Answer 19

Sound frequency to spatial representation

Question 20

What pathway does auditory information take?

Answer 20

• Cochlea
• Brainstem
• Midbrain (inferior colliculus)
• Medial Geniculate Nucleus of Thalamus (forebrain)
• Auditory Cortex

Question 21

How does tonotopic representation in the auditory cortex occur?

Answer 21

-Specific areas of the cochlear correspond to specific regions of the cortex

Question 22

Why do we have tonotopic organisation in the auditory system?

Answer 22

• Reduces axon length? Sounds close in frequency are processed together
• Facilitates processing (lateral inhibition)
• Allows sounds to be encoded on the basis of time/frequency changes
• Scene analysis: frequency separation relates to objects in the environment
• Detection of beat promotes social cohesion?

Question 23

In music (present in all cultures) what different features are detected by what?

Answer 23

In music, higher frequencies process pitch and timbre while lower frequencies carry rhythm and beat. The beat is often used to synchronise activity. Synchronised activity has been shown to promote social cohesion.

Some medial geniculate neurons respond to the low frequency components – modulate firing threshold in A1 according to the underlying rhythm. As a result responses that occur out of beat are weakened

Question 24

What is the difference between top-down and bottom- up sensory processing?

Answer 24

• Top-Down processing: Using information from higher-level mental processes and prior experience
• Bottom-Up processing: Processing that begins with the sense receptors

Question 25

How much of an influence does top-down processes have on the visual system?

Answer 25

Approximately 6% of inputs to visual cortex are top-down

Question 26

How much of an influence does top- down processing have on the audition system?

Answer 26

Primary auditory cortex is larger in macaques than in humans
However in humans, the surrounding belt and parabelt areas are
about 10x larger.

Most connectivity into auditory cortex is top down –
around 66% is from other cortical areas

-Top down processing in the audition system allows context and procedural demands to bias perception. This allows for integration of multisensory input and means greater predictive capacity i.e use prior info (Priming) to modulate response to sensory input allowing fast response

Question 27

What is the McGurk effect?

Answer 27

• Visual info overriding auditory info
• VA versus BA while the auditory info actually remains the same what we see visually is different and that causes a difference in our perception
• Shows the top-down influence of vision on the audition system

Question 28

Do both the visual and audition system respond primarily to one type of processing?

Answer 28

No, they both respond to some top down and some bottom up
However, the audition system has a particularly strong top down component because of the evolution of language and the need to rapidly decipher what things we are hearing mean.