Lecture 7 - Visual and auditory systems Flashcards
Structure of the retina
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
Rods vs. cones
Cones= colour -> high levels light Rods= black and white, focus at low levels of light due to highly sensitive to light
Fovea
Fovea is the main/ central part of the retina
Highest cell consentration
Most sensitive part of eye
Located in the centre
Signal transduction in rods and cones
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!
Intermediate layer
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
Two types of bipolar cells
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!
Retinal gangliion cells
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
Physiology of the retina
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
Visual receptive fields
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
Retinal ganglion cell receptive fields
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
Negative after image
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
Anatomy of the LGN
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
Two main visual pathways in the LGN
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)
What is the function of the LGN
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
V1
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
Projection from LGN to V1
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
P and M pathways project to different input layers in V1
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
Ocular dominance columns
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
Temporal frequency selectivity
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
Projections from V1
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)
Area V2
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
V4
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
V3, V3A and V5
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
Physics of sound
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
Frequency of sound
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
Sensitivity of human hearing
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!
Anatomy of the ear
Divided into outer, middle, and inner
Auditory sensory neurons are located in the inner ear
Processing of sound by the outer ear
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
Processing of sound in the middle ear
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
Processing of sound in the inner ear
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
Organ of Corti
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
Hair cells
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
How hair cells respond to sound
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
Signal transduction by hair cells
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
Coding of sound frequency by hair cells
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
Different sound frequencies are coded by location in the cochlea - tonotopy
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
What are outer hair cells for
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
Deafness and cochlear implants
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
Auditory neural pathways
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
Sound localisation
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
Tonotopy in the auditory cortex
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
