Lecture 3- Topographic and Tonographic Organisation Flashcards
Where do 90% of fibres originating in the retina terminate?
Travel by the optic nerve and terminate at the lateral geniculate nucleus of the thalamus
What do responses of cells in the LGN allow for?
Allow mapping of the visual field
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?
- 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.
What does the term retinotopic organisation mean?
- 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
Where do cells from the LGN project to?
The primary visual cortex (V1, striate cortex, Brodmann’s area 17)
How does a simple cell respond in V1?
- Respond best to elongated bars or edges
- Are orientation selective.
- Have separate ON and OFF subregions
- Can be monocular or binocular
How does a complex cell respond in V1?
- Are orientation selective.
- Have spatially homogeneous receptive fields (no separate ON/OFF subregions)= THIS IS WHAT SEPERATES THEM FROM SIMPLE CELLS
- Nearly all binocular.
What is the ordering of different cell types in the visual system? i.e. what feeds into what?
- Retinal ganglion cells (front of retina output into optic nerve)
- LGN
- Simple cells
- Complex cell (multiple simple cells feed into)
What kind of ‘architecture’ does V1 have? What does this mean?
-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
What does it mean when we say V1 has retinotopic maps?
- 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
What region of the retina corresponds to more space in V1?
- 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
What is retinotopic mapping an example of?
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
Why does our visual system operate topographically? What are the benefits?
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)
What is sound? What is often used as a measurement for it?
- Pressure pulses: oscillations of high and low pressure which travel at ~340 metres/sec
- Frequency: Cycles per second which is measured in Hertz
What is the range of sounds that humans can hear?
20-20,000 Hz
What does a 10dB decrease correspond to?
A halving of perceived loudness
What happens to our hearing as we age? How does it effect different genders?
- As we get older our ability to detect high frequency sounds worsens
- Worse for men than women
What tonotopic representation occurs within the cochlear?
- Detect low frequencies at the end (apex)
- Detect higher frequencies at the base
What is transduction in the auditory system?
Sound frequency to spatial representation
What pathway does auditory information take?
- Cochlea
- Brainstem
- Midbrain (inferior colliculus)
- Medial Geniculate Nucleus of Thalamus (forebrain)
- Auditory Cortex
How does tonotopic representation in the auditory cortex occur?
-Specific areas of the cochlear correspond to specific regions of the cortex
Why do we have tonotopic organisation in the auditory system?
- 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?
In music (present in all cultures) what different features are detected by what?
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
What is the difference between top-down and bottom- up sensory processing?
- Top-Down processing: Using information from higher-level mental processes and prior experience
- Bottom-Up processing: Processing that begins with the sense receptors
How much of an influence does top-down processes have on the visual system?
Approximately 6% of inputs to visual cortex are top-down
How much of an influence does top- down processing have on the audition system?
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
What is the McGurk effect?
- 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
Do both the visual and audition system respond primarily to one type of processing?
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.
