3 - Sensation and Perception Flashcards
Sensation-Perception relationship
- sensory signal is converted to an electrical signal via transduction in sensory receptors
- signal is sent via the spinal cord to the brain
- a percept is formed in the brain
- the perception is interpreted in light of past experience and attention to form a Perceptual Understanding
- off of which a behavioural response is based
Sound waves
- longitudinal waves in the air caused by vibration
- frequency indicated pitch
- intensity (amplitude) indicates loudness
Protection in the middle and outer ear
- in the middle ear, skin cells migrate outwards to prevent dead cell debris
- middle ear is slightly acidic to prevent bacterial growth
- ear wax has anti-fungal and antibacterial properties as well as being waterproof
- shape and depth of the middle and outer ear prevent damage to the Tympanic Membrane (ear drum)
- outward pointing hairs prevent objects getting in
Amplification by the outer ear
- the outer ear acts like a closed tube resonator, causing amplification of sounds with a frequency of 2-5 kHz (due to its length)
- 2-5 kHz is the standard human speech frequency
- length of a closed tube resonator is equal to 1/4 of the wavelength of the sounds it amplifies
- this works because the nodes (no total difference in amplitude) concentrate at the Tympanic Membrane
Protection by the middle ear
- reflex reaction to a loud sound causes Stepedius muscle and Tendor Tympani muscle to contract, locking the Ossicles, so sound isn’t transmitted through the middle ear
Acoustic Impedence in the middle ear
- Middle ear starts at the Tympanic Membrane and ends at the footplate
- footplate has 17x less area so pressure increases 17x
- this helps the sound travel through the fluid-filled inner ear
Function of the Inner Ear
- three chambers
- inner chamber contains organs of corti which are mechanoreceptors, which detect movement of surrounding fluids or membranes (sound)
Organ of corti
- many hair cells on it with cilia on the exterior, decreasing in size, the largest of each cell is the Kinocilium
- these hair cells can not produce action potentials, but synapse onto neurons that can
- extracellular fluid in the ear has a high [K]+ concentration (opposite to other cells), so [K]+ diffuses in at resting
- the ion channels in the stereocilia are mechanically gated so open due to sound wave vibration in the fluid of the inner ear
Transduction
receptor potential to action potential
- sound wave bends stereocilia towards the kinocilium
- mechanically-gated ion channels open and allow Potassium to flood into the cell, down its concentration causing a receptor potential
- depolarising receptor potential spreads to the rest of the cell, opening voltage-gated [Ca]2+ channels
- Calcium floods in causing Glutamate release into the synapse
- Glutamate binds to AMPA receptors on the post-synaptic neuron and may cause an action potential in the auditory nerve to the brain
Hearing Pathway
- Spiral ganglion (in the cochlea)
- to the Cochlear Nuclear Complex (via Cochlear nerve)
- to the Superior Olivary Complex (via Trapezoid Body)
- to the Inferior Colliculus (via Lateral Lemniscus)
- to the Medial Geniculate Nucleus
- to the Auditory Cortex
Frequency coding of sound information
Place Coding:
- in the Cochlea, different hair cells activate different afferent neurons, and are activated by different frequency sounds
> highest frequency at the beginning (widest) part of the cochlea, and lowest frequency at the end (smallest) part
- Place Coding is poor for low frequencies but is good for frequencies above 1000Hz, and is the only coding method for frequencies about 300Hz
Temporal Coding:
- determining the frequency of the sound by matching that with the frequency of nerve impulses fired in the afferent neuron
- the theoretical limit for frequency of neuron firing is 1000Hz but in reality, sustained activation can only fire as fast as 300Hz
- useful with lower frequencies
Intensity coding of sound
By firing rate:
- the faster a neuron is firing, the louder the sound
By number of neurons:
- a neuron firing at high frequency will recruit nearby neurons to amplify the message
Beyond the Auditory Cortex
From the auditory cortex via:
- Posterodorsal Stream
> to the Posterior Parietal Cortex (PPC) to process ‘where’ information
- Anteroventral Stream
> to the Superior Temporal Region (ST) in the temporal lobe, processing ‘what’ information - Both of these converge on the prefrontal cortex
Superior Olivary Complex
- is involved in location coding
- the first structure to receive biaural input
Hearing Loss
Two types according to the area of damage:
- Conductive Hearing Loss
> damage to the middle or outer ear
+ impact on hearing threshold but nor normally discrimination (can hear okay, but not quiet sounds)
- Sensorineural Hearing Loss (2)
> Cochlear hearing loss (if effecting the inner ear)
> Retrocochlear hearing loss (if effecting the auditory nerves)
+ impact on discrimination but not necessarily threshold (all sounds become muddy)