Topic 3- Signals and Perception Flashcards
Distinguish between sensation and perception.
What the sensory system detects is not interpreted in only one way to give us a perception.
Sensation is input about the physical world obtained by our sensory receptors, and perception is the process by which the brain selects, organises, and interprets these sensations.
How does perception occur from sensory signals?
Sensory signals are the first part of the process- this could be light waves falling on the retina or sound waves hitting the ear. These signals must be received by specific receptors and converted into nerve impulses to travel through the nervous system.
From the receptors, that signal reaches the brain. For hearing and vision its a very direct pathway- there is a nerve travelling from the back of the ear or eye directly into the brain through one of our cranial nerves.
For some of our other senses, e.g. touch, most signals will reach the brain through the spinal cord.
Signals in the brain are:
Combined with previous experience and attention (so bottom up and top down information). Top down information is our prior knowledge and experiences of the world and bottom up is the information from the sensory stimulus.
The brain combines our prior knowledge, attention and sensory inputs to create a percept- an idea of what something is.
The percept is then used in our perceptual understanding of the environment around us and used to determine an appropriate behavioural response. Sometimes we make different responses based off of our perceptual understanding.
Explain the characteristics of auditory waves.
Sound is a form of longitudinal wave in air created by the vibration of objects. Wave properties can therefore be used to understand sound:
Sound waves have amplitude – the size of the fluctuations indicates this. Greater amplitude is generally associated with a louder sound.
Sound waves have a frequency– the time over which the cycle repeats indicates the frequency with quicker repeats giving a higher frequency.
Humans can here sounds from 0-140 dB.
Humans can here sounds from 20 Hz to 20,000 Hz.
Explain the structure of the ear.
The structure of the ear is highly specialised to support our sense of hearing:
It can be divided into outer, middle, and inner.
The outer ear consists of the part we can easily see and touch: pinna, auricle, external auditory canal.
The tympanic membrane separates the outer ear from the air-filled middle ear cavity/chamber which contains three tiny bones called the ossicles- they are the malleus, incus and stapes. They move in a lever type action to transmit information from the ear drum to the inner ear/cochlea.
The stapes contacts the entrance to the cochlea, which is where the auditory hair cells are, and so sound waves have to get all the way into the inner ear before they can be sensed. There are inner hair cells, which are the main cells involved in detecting auditory stimuli, and there is a set of cells called the outer hair cells which have a role in amplifying signals.
From the back of the cochlea you can see the cochlear nerve and the vestibular nerve which combine at one point to form one of the cranial nerves carrying information into the brain.
What are the functions of the outer ear?
The outer ear:
Funnels sound inwards
Amplifies the sound by acting as a tube (providing a tube resonator) for it to echo in
Helps us to localise sounds. Because we can’t move our outer ear, this is limited to vertical localisation in humans but can be across other planes in other animals.
Protection: Ear wax is water-resistant and has antibacterial and anti-fungal properties. The whole environment is acidic, which prevents the growth of bacteria, and the location and direction of the hairs prevent insects and other things from getting in and move debris out.
What are the functions of the middle ear?
Protection: the middle ear reflex responds to very loud noises and can lock the three little bones in a particular position to prevent them transferring information from the eardrum to the cochlea to protect the cochlea.
Acoustic impedance matching: The inner ear/cochlea is filled with fluid, whereas the middle earis filled with air. Due to this, a lot of information would be lost at the point of the middle ear becoming the inner ear as sound waves are unable to travel effectively through fluid. The middle ear amplifies the pressure applied to the inner ear to counteract the loss of signal, making sure the sound wave is transmitted effectively.
Explain the structure of the inner hair cells and how auditory transduction occurs within them.
Th inner hair cells have hair (stereocylia) of slightly different lengths sticking out of the top of it.
On the left there is a very long stereocylia and they gradually get shorter. The taller one is called the kinocilium and when the hair cells bend in the direction of the kinocilium this generally results in the cells becoming excited and when it bends away it results in the cell being inhibited.
When the cell is at rest, the stereocylia are largely vertical and there are tiny channels on the ends of the stereocylia which are attached to the next channel with a tip link. These channels are shut, so no ions can move into the stereocylia at this point.
The stereocylia are on the apical end of the hair cell, the basal end contains all the other key organelles the cells need.
A post-synaptic neuron that is part of the auditory/cochlear nerve forms a synapse with the hair cell. This neuron expresses AMPA receptors or an ionotropic glutamate receptor.
When the fluid around the stereocylia is moved by the arrival of a sound wave, this causes the stereocylia to move. As they move in the direction of the kinocylium, this physically opens their mechanically gated ion channels by lifting the lid on them. The area surrounding the stereocilia has a very high concentration of potassium, which drives potassium into the cell down it’s concentration and electrostatic gradient. Once the positively charged potassium ions move in, depolarisation occurs which results in the opening of voltage-gated calcium channels. Calcium ions flood in to the hair cell and cause interactions with the vesicles and the snare proteins/docking proteins, resulting in the release of glutamate.
Glutamate is then released into the synapse with the post-synaptic neuron and binds to the AMPA receptors to excite the cell to the point where it can elicit an action potential in the auditory/cochlear nerve. This action potential does not occur in the nerve- the nerve has receptor potential, meaning an electrical change found in a receptor.
Explain the structure of the inner hair cells and how auditory transduction occurs within them.
Th inner hair cells have hair (stereocylia) of slightly different lengths sticking out of the top of it.
On the left there is a very long stereocylia and they gradually get shorter. The taller one is called the kinocilium and when the hair cells bend in the direction of the kinocilium this generally results in the cells becoming excited and when it bends away it results in the cell being inhibited.
When the cell is at rest, the stereocylia are largely vertical and there are tiny channels on the ends of the stereocylia which are attached to the next channel with a tip link. These channels are shut, so no ions can move into the stereocylia at this point.
The stereocylia are on the apical end of the hair cell, the basal end contains all the other key organelles the cells need.
A post-synaptic neuron that is part of the auditory/cochlear nerve forms a synapse with the hair cell. This neuron expresses AMPA receptors or an ionotropic glutamate receptor.
When the fluid around the stereocylia is moved by the arrival of a sound wave, this causes the stereocylia to move. As they move in the direction of the kinocylium, this physically opens their mechanically gated ion channels by lifting the lid on them. The area surrounding the stereocilia has a very high concentration of potassium, which drives potassium into the cell down it’s concentration and electrostatic gradient. Once the positively charged potassium ions move in, depolarisation occurs which results in the opening of voltage-gated calcium channels. Calcium ions flood in to the hair cell and cause interactions with the vesicles and the snare proteins/docking proteins, resulting in the release of glutamate.
Glutamate is then released into the synapse with the post-synaptic neuron and binds to the AMPA receptors to excite the cell to the point where it can elicit an action potential in the auditory/cochlear nerve. This action potential does not occur in the nerve- the nerve has receptor potential, meaning an electrical change found in a receptor.
How does auditory information travel from the cochlea to the brain?
Once information has been coded in the cochlea in the form of a receptor potential and then an action potential in the cochlear nerve, it has to travel from the cochlea to the brain.
Information leaves the cochlea and travels to the cochlear nuclear complex in the brain stem. It then continues to stay in the brain stem as it reaches the superior olivary complex. From there, it travels to the midbrain and a structure called the inferior colliculus, and then into a particular area of the thalamus called the medial geniculate nucleus. From there it reaches the primary auditory cortex.
At some point, information has to be combined from both ears and this happens at the level of the superior olivary complex- information from both ears is found in each superior olive and then that information is still there when it travels up to higher structures.
What key features of a sound wave must the brain be able to deduce in order to understand what’s going on in the environment?
There are 2 things the sensory system needs to do:
It needs to understand what something is and it needs to understand where something is- how far away it is and which direction its coming from.
Explain frequency coding.
One of the key features of sounds is the frequency of it.
Frequency is coded in two different ways in the auditory system and starts in the cochlea. Once the cochlea has decided a code, that code is kept throughout the pathway auditory information travels down.
Explain place theory/place coding.
The first is called place code or place theory. This is derived from the layout of the cochlea itself- if you were to unroll the cochlea you would find that hair cells responding to the high-frequency sounds are found at the start of the tube- the end nearest to the middle ear- and those responding to low frequency sounds are found at the end. This is called a tonotopic relationship. The frequencies gradually change and neighbouring hair cells will have similar frequencies to which they respond. That frequency is called the characteristic frequency. From here, each hair cell is connected to a specific axon in the auditory nerve, so if the brain can tell which axon has been activated it can tell which hair cell has been activated and so can tell the approximate frequency because it knows the hair cells are arranged in order of frequency.
This is a very good method for higher frequencies and works from about 1000 hz upwards.
Explain temporal/rate coding.
The hair cells oscillate the release of neurotransmitter at the same frequency of the sound wave. Every time the sound wave creates movement of the fluid, this creates activation of the hair cells- this is called phase locking.
This method is mainly helpful for the low frequencies that place code can’t do but can code up to 3000 hz. It does this through neurons working together- one neuron will first fire and then the next. This is called the volley principle- it allows multiple hertz cells to release their neurotransmitter and result in multiple axons firing in relation to a specific sound wave.
Explain intensity coding.
The first way the brain can deduce intensity is from the frequency of firing in the auditory nerve. This is also encoded in two different ways.
Explain intensity coding through the frequency of firing.
The first way the brain can deduce intensity is from the frequency of firing in the auditory nerve:
Action potentials can’t get bigger but can be produced more often- a more intense sound results in greater movement in the cochlea, bending the stereocilia more. This results in the opening the ion channels for longer and allowing more potassium to enter, thus allowing more calcium in and releasing more neurotransmitter. More neurotransmitter released means greater activation of the post-synaptic cell and more action potentials.
Explain intensity coding through the number of neurons firing.
The second way in which the brain can deduce intensity is from the number of neurons firing.
Although specific hair cells are activated by a specific frequency, if you have lots of movement in the fluid to do a very intense sound, the neighbouring hair cells will also have some activation and so there will be a greater number of axons activated in the auditory nerve.
Higher intensities cause more movement in the cochlea so the hair cell with the appropriate CF is depolarised but so are some of its neighbours to a lesser extent.
The neighbouring cells also release some glutamate and activate their auditory nerve cell so more cells overall are activated.
Vertical location is limited in humans as we cannot move our pinna. It is instead informed by the way in which sound waves bounce off of the lumps and bumps of our outer ear. For other species, they move their pinna.
Horizontal location is carried out by the superior olivary complex. It is carried out here as it requires information from both ears and this is the first in which that information is combined.
Unless a sound is directly in front or behind us, there is a difference between the signals received depending on where the sound is coming from. There are two ways in which they differ:
Sound intensity- a sound coming from the right side will be greater at the right ear than the left ear. This creates an inter oral intensity difference which gives you a perception of where the stimulus is coming from.
Speed- Sounds reach the era closest to the source slightly quicker- if the sound is on the right then the signal from the right cochlea will reach the super olivary complex quicker than the signal from the left cochlea. These are called interaural time delays and are informative for us to understand location.
There are also some extra cues- e.g. if we expect something to be very loud but it is very quiet then that indicates it is a long way away e.g. a train.
What happens to the auditory information we code?
All information we code about frequency, intensity and location is transmitted to the inferior colliculus, the thalamus and the primary auditory complex.
Information from the primary auditory complex passes through a posterior dorsal stream which goes up to the parietal lobe and onwards to a frontal lobe- this processes ‘where’ information. It also passes through an anteroventral stream which goes to the superior temporal region in the temporal lobe and processes ‘what’ information.
