visual and auditory Flashcards
what is the 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
what are the functions of the different cells and layers of the retina?
Cones and rods take in light and respond to different wavelengths of light
Intermediate layer takes infor from mainly cones
Horizontalcells take info from both cones and rods
Biopolar cells
Ganglion layer- information being sent.
what is the fovea?
he fovea centralis is a small, central pit composed of closely packed cones in the eye. It is located in the center of the macula lutea of the retina.
what happens when a photon of light hits a cone or rod?
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 rods, cone opsins in cones, iodopsin
Causes change in shape of photopigment that triggers a G-protein cascade that reduces glutamate release
So, paradoxically, photoreceptors are inhibited (deactivated) by light!
S cones respond to lower wavelgenths of light < rodes < “red cones(L)” < green cones(M)
what does the intermediate layer do?
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
what do bipolar cells do?
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!
what are retinal ganglion cells?
Parasol Large dendritic trees Combine inputs from many bipolar cells Midget Small dendritic trees Combine inputs from few bipolar cells Dendritic trees larger in periphery for both
What is the 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
what about 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
what are on and off recpetive 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
what is lateral inhibition?
Neighbouring bipolar cells inhibit each other through horizontal cells (lateral inhibition)
summerise retina lphysiology
Photoreceptors respond to light intensity Lateral inhibition transforms this response to code for contrast (light differences) Retinal ganglion cell RFs code for Differences in light and dark over time (flicker) Differences in light and dark over space (contrast) Differences in colour Retinal ganglion cell RFs are circular No information about direction or orientation
what is the LGN?
Lateral geniculate nucleus
what is the 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
what are the two main visual pathways of 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!
Eye
First site of attentional gating/enhancement Sleep-related gating of sensory input to cortex (reticular formation)
what is V1
primary visual cortex
Also known as striate cortex (from Stria of Gennari – line of Gennari) First site of visual processing in cortex
the 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
projections 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
what do the P and M pathways do 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
what are ODc’s
Ocular dominance columns (ODCs)
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
How does V1 temprorally select frequency
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
What does V1 do with the signal it sends out
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)
what does V2 do?
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
outline area s 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
outline areas V3, 3a and 5
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
outline the 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
what is a pure tone?
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
what is the human sensitivity of 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!
So in principle you could be at Royal Holloway and
Outline the anatomy of the ear
Divided into outer, middle, and inner Auditory sensory neurons are located in the inner ear
what is the processing like of 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
what is the processing like of 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
what are the processing like of 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
what is the Organs or 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
what are 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 do 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
outline signal transduction of 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 and fire
Outline the coding of 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
how are sound frequencies coded in the Cochlea?
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 the 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
Outline 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
outline the auditory neural pathway
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 Auditory information then passes through the MGN in the thalamus to
outline 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
explain tonotopy of 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
