Lecture 7 - Visual and auditory systems

Question 1

Structure of the retina

Answer 1

Structure of the retina
The retina is the sensory organ of vision
Three main layers:
Photoreceptor layer -> Rods and cones
Intermediate layer -> Bipolar, horizontal, and amacrine cells
Ganglion cell layer -> Retinal ganglion cells: midget & parasol

Question 2

Rods vs. cones

Answer 2

Cones= colour -> high levels light 
Rods= black and white, focus at low levels of light due to highly sensitive to light

Question 3

Fovea

Answer 3

Fovea is the main/ central part of the retina
Highest cell consentration
Most sensitive part of eye
Located in the centre

Question 4

Signal transduction in rods and cones

Answer 4

Rods and cones respond to light intensity
In darkness, rods & cones constantly release neurotransmitter (glutamate)
Light is absorbed by a pigment in rods and cones
Rhodopsin in cones, cone opsins in cones
Causes change in shape of photopigment that triggers a G-protein cascade that reduces glutamate release
So, paradoxically, photoreceptors are inhibited (deactivated) by light!

Question 5

Intermediate layer

Answer 5

Contains bipolar, horizontal, and amacrine cells
Bipolar cells transfer information from rods & cones to retinal ganglion cells
Site of lateral inhibition that creates opponent receptive fields
Transforms light (brightness) information into contrast information

Question 6

Two types of bipolar cells

Answer 6

ON and OFF bipolar cells differ in how they respond to input from photoreceptors:
ON bipolar cells are inhibited by input
OFF bipolar cells are excited by input
Opposite to intuition!

Question 7

Retinal gangliion cells

Answer 7

Parasol
Large dendritic trees
Combine inputs from many bipolar cells
Midget have fewer dentrites -> collect data from less place, have greater resolution as they take info from less cells
Small dendritic trees
Combine inputs from few bipolar cells
Dendritic trees larger in periphery for both
Different types of ganglion cells -> e.g. parasol
Dentrites collect the data -> lots means can collect from many bipolar cells (integrate the information) , more peripheral light -> less resolution knowing less detailed where the light is as it collects data from many cells

Question 8

Physiology of the retina

Answer 8

Photoreceptors translate light into neural signals for light intensity (signal transduction)
Signals for light intensity are then converted into signals for contrast (differences in light intensity) by bipolar and ganglion cells
Cells take in light -> retina translate to neural signals -> brain interprets that as vision

Question 9

Visual receptive fields

Answer 9

To understand how the retina works, we need to know about visual receptive fields (RFs)
RFs: the region of sensory space that evokes a response in a neuron
The part of the visual field where a stimulus causes a neuron to respond
RFs have a position and a size
RFs can have both excitatory and inhibitory subregions

ON and OFF receptive fields respond in opposite directions to contrast changes
ON RFs respond to an increase in light intensity
OFF RFs respond to a decrease in light intensity

Question 10

Retinal ganglion cell receptive fields

Answer 10

Most have “centre-surround organisation”
Opposite response to light in centre and surround
Most feilds have an on and off part -> create image by using contrast

Contrast at the lines make the lines seem more extreme
Contrast allows eye to define objects more clearly
Top cell excited by light -> gets more from right than left => dimmer due to suppression of darker

Question 11

Negative after image

Answer 11

Cells exited by stimuli -> for long period time (30 secs) -> experience fatigue -> seeing so much the colour they get tirred -> replace with blank, green cells that were responding -> overpowered by red cells (opposite) meaning see red when blank space shown

Question 12

Anatomy of the LGN

Answer 12

The LGN consists of six layers 
The layers differ in terms of: 
The kind of cells they contain 
What type of visual input they receive 
Which eye they receive input from

Question 13

Two main visual pathways in the LGN

Answer 13

Magnocellular (M) pathway
Inner two layers (1 & 2)
Receive input from parasol ganglion cells
Parvocellular (P) pathway
Outer four layers (3,4,5,6)
Receive input from midget ganglion cells
Receptive fields in LGN are similar to those of retinal ganglion cells (circular centre-surround)

Question 14

What is the function of the LGN

Answer 14

Relay station between eye and brain
Response properties similar to retinal ganglion cells
But receives massive feedback from cortex – 10x as many connections as from the eye!
First site of attentional gating/enhancement
Sleep-related gating of sensory input to cortex (reticular formation)

Attentional gating = focus attention on what we are looking out

Question 15

V1

Answer 15

Primary visual cortex
Also known as striate cortex (from Stria of Gennari – line of Gennari)
First site of visual processing in cortex
Posterior occipital lobe

Topographic (retinotopic) organisation
Contains a “map” of the visual field
Detailed maps of orientation, colour, spatial scale, motion direction, 3D depth
Projects to most higher visual areas in cortex
For each part of the visual scene, V1 computes: orientation, spatial frequency, motion, colour, depth

Question 16

Projection from LGN to V1

Answer 16

Most LGN neurons project to V1
V1 consists of six layers (like all of cortex) with several of the layers divided into sublayers
Layer 4 divided into 4A, 4B, 4Cα, 4Cβ
Axons from LGN terminate (synapse with) cortical neurons in layer 4 (IV) of V1

Question 17

P and M pathways project to different input layers in V1

Answer 17

Parvocellular (P) pathway:
Project to layer 4Cβ
Splits into two new pathways in upper layers:
P-B pathway: colour (blobs)
P-I pathway: orientation (interblobs)
Magnocellular (M) pathway:
Project to layer 4C and then onward to 4B
Cells in layer 4B are sensitive to movement
Some are binocular and disparity/depth sensitive

Question 18

Ocular dominance columns

Answer 18

Most cells in V1 are binocular (respond to stimulation in either eye)
Cells in layer 4 that receive input from LGN are monocular (respond only to one eye)
Most cells respond better to stimulation from one eye or the other
This is known as ocular dominance
Cells preferring each eye are clustered into ~1mm thick slabs called ocular dominance columns

Question 19

Temporal frequency selectivity

Answer 19

V1 cells respond best to limited range of temporal frequency (flicker rate; how quickly stimuli change over time)
Cells in the M pathway respond better to fast flicker
Cells in the P pathway respond best to slower flicker

Question 20

Projections from V1

Answer 20

Neurons in V1 project to higher visual cortical areas (extrastriate cortex): V2, V3, V3A, V4, V5…
Projections are topographic – each of these areas also contain a map of the visual field
Different higher visual cortical areas respond to different types of stimuli, e.g.:
V5 (MT): motion (M pathway)
V4: shape and colour (P pathway)
V3/V3A: motion boundaries and textures (M/P pathways)

Question 21

Area V2

Answer 21

Divided into multiple “stripes”: 
Thick stripes (M pathway) 
Sensitive to orientation and movement 
Sensitive to disparity (depth) 
Thin stripes (P pathway) 
Sensitive to colour 
Not orientation-selective 
Inter-stripes (P pathway) 
Orientation-selective

Question 22

V4

Answer 22

The P pathway projects to V4
Damage to human V4 impairs colour perception
But not clear if human V4 is same as in monkey!
Also involved in shape discrimination

Question 23

V3, V3A and V5

Answer 23

The M pathway projects to V3/V3A and V5 (MT)
Cells in V3/V3A
Selective for orientation
Respond to motion boundaries (dynamic form)
Cells in V5 (MT)
Selective for motion direction and speed
Process information on motion and stereoscopic depth

Question 24

Physics of sound

Answer 24

Sound is vibrations of medium (like air or water) – these vibrations are called sound waves
Sound waves have a frequency and amplitude
Frequency (=pitch) refers to the speed of vibrations (number of vibrations per second – Hertz, Hz)
Rapid vibrations = high frequency = high pitch sound
Slow vibrations = low frequency = low pitch sound
Amplitude (=loudness) refers to the size of the vibrations

Question 25

Frequency of sound

Answer 25

A sound at a single frequency is a pure tone A pure tone looks like a sine wave Any sound can be created by summing many pure tones (sine waves) at different frequencies and different amplitudes Conversely, any sound can be ”decomposed” (taken apart) into its pure tone components (component frequencies) The auditory system works by taking apart sounds into their component frequencies

Question 26

Sensitivity of human hearing

Answer 26

Humans can detect sounds between 20 – 20,000 Hz Most sensitive around 2000 – 5000 Hz (2-5 kHz) At 3000 Hz (3 kHz), humans can detect a sound corresponding to air vibrations no more than 0.01 nanometer (10-11 m) This is less than the diameter of a hydrogen molecule (H2) ! On a noiseless planet, a human could detect a 1W sound more than 450 km away!

Question 27

Anatomy of the ear

Answer 27

Divided into outer, middle, and inner | Auditory sensory neurons are located in the inner ear

Question 28

Processing of sound by the outer ear

Answer 28

The shape of the outer ear (pinna) serves two main functions: Amplifying (30x-100x) sounds around 3 kHz (corresponding to frequency of speech) To help determining the direction of a sound (by allowing through more high frequencies from a high than a low sound

Question 29

Processing of sound in the middle ear

Answer 29

``` The middle ear amplifies sounds so they can pass from air to water (inside the inner ear) Two mechanisms: The eardrum (tympanic membrane) is much larger than the oval window, giving a proportional amplification The ear bones ```

Question 30

Processing of sound in the inner ear

Answer 30

The inner ear consists of the cochlea ('snail') and the semicircular canals Semicircular canals: part of the vestibular system The cochlea is a hollow spiral tube (like a snail) Actually two tubes joined at tip (apex) of

Question 31

Organ of Corti

Answer 31

The organ of Corti runs along the length of the cochlea | It sits between the two liquid-filled tubes (scala vestibuli and scala tympani) of the cochlea

Question 32

Hair cells

Answer 32

Hair cells are the sensory neurons of hearing – the neurons that respond to sound vibrations There are two types of hair cells, outer hair cells (OHCs) and inner hair cells (IHCs) IHCs and OHCs form two sets of rows along the length of the cochlea

Question 33

How hair cells respond to sound

Answer 33

Sound vibrations cause the basilar membrane (BM) to vibrate This causes a ”shearing” motion of the BM relative to the tectorial membrane (TM) This causes hair cells that sit between the BM and TM to bend back and forth

Question 34

Signal transduction by hair cells

Answer 34

Bending of the ”hairs” (stereocilia) of hair cells pulls filaments (strings) connecting stereocilia These filaments (tip links) are believed to connect mechanically to ion channels in the hair cells, opening them (like lifting a lid by a string) This causes the hair cells to depolarize

Question 35

Coding of sound frequency by hair cells

Answer 35

Hair cells respond very fast – less than 10 μs Allows hair cells to fire in synchrony with sound vibrations but only up to about 3 kHz – not enough Different frequencies are instead coded by hair cells preferring different frequencies at different locations in the cochlea

Question 36

Different sound frequencies are coded by location in the cochlea - tonotopy

Answer 36

Each location along the basilar membrane is most sensitive to one sound frequency – high frequencies near base, low frequencies near tip (apex) This is called ”tonotopy” Different sounds cause different patterns of activity along the membrane

Question 37

What are outer hair cells for

Answer 37

Only the inner hair cells send out axons – these are the neurons that respond to sound The outer hair cells (3x as many!) instead receive neural input from the auditory nerve In response to stimulation, outer hair cells can contract and modify stiffness of basilar membrane – allowing fine tuning of sound sensitivity This can even create sounds – otoacoustic emissions One possible cause of tinnitus

Question 38

Deafness and cochlear implants

Answer 38

Hair cells are easily damaged by strong sounds, one cause of deafness Cochlear implants are electrodes placed inside cochlea that directly stimulate auditory nerve, mimicking function of hair cells

Question 39

Auditory neural pathways

Answer 39

Axons from the inner hair cells join to form the auditory nerve This connects the cochlea with the olive (olivary nucleus) in the brainstem Olive is involved in sound localisation

Question 40

Sound localisation

Answer 40

Sounds are localised by two mechanisms: Time differences between the ears (<3 kHz) Processed in the medial superior olive (MSO) Intensity differences between the ears (>3 kHz) Processed in the lateral superior olive Time between the two ears picking it up -> closer to the ear gets it first Further to the side = greater difference Intensity difference -> louder in the closer ear

Question 41

Tonotopy in the auditory cortex

Answer 41

Sound information is processed in the primary auditory cortex, located in the superior temporal lobe Like in the cochlea, each sound frequency is represented in a different location – tonotopy Higher auditory areas process complex sounds