Sensory processing Flashcards
Also recap Sem 1 RC for hearing, visual pathways, chemical senses
What are the 3 key stages of sensory perception?
Info enters cortex from cranial/spinal nerves that connect at brainstem and sensory thalamus, entering at unimodal primary sensory cortices (crossover occurs at top of spinal cord just before brainstem)
Hierarchical processing occurs in unimodal association cortices
Highly processed unimodal signals enter multimodal association cortex where sensory integration begins
What are the main pathways for the 5 senses?
TOUCH - skin via spinal cord to somatosensory cortex
SIGHT - eye via optic nerve to visual cortex
HEARING - ear via vestibulocochlear nerve to auditory cortex
TASTE - tongue via facial nerve, glossopharyngeal, and vagus, to somatosensory cortex (tastes perceived over full sensory area of tongue)
SMELL - nose via olfactory nerve to olfactory cortex (only sense where pathway doesn’t go through thalamus)
What is Area V1 of the visual system responsible for?
This is the primary visual cortex, first level of input to the visual cortex (input from LGN)
Cells respond differently to different aspects of the visual signal e.g. orientation, size, colour
Involved in characterisation not analysis
Sends independent outputs to other areas, and damage here leads to total or partial blindness depending on extent of damage
What is the role of area V3?
First stage of building object form
Codes for component aspects of objects e.g. edges, orientation, spatial frequency
Feeds info to V4, V5, TEO (temporo-occipital), TE, STS )superior temporal sulcus) and to parietal cortex
What is V4 responsible for?
Colour recognition - individual neurones here respond to a variety of wavelengths
PET studies show activation in V4 to coloured patterns but not to greyscale
What is the role of the temporal lobe in the visual system?
Highest level of processing of visual information - recognition of objects dependent on form, independent of scale(distance), orientation, illumination; visual memory
Face recognition occurs here - subject specific facial features, facial expressions, gaze direction etc
What is V5 responsible for?
Movement perception - individual neurones respond to movement
Activation to moving patterns but not stationery ones
What is the role of the posterior parietal cortex?
Analysis of spatial location of visual cues, building of an image of multiple objects in space
Coordinates visually directed movements such as reaching and receives info from all areas of visual cortex
What is blindsight?
Subjects blind due to damage to area V1, but can “guess” direction of travel of a moving object or colour; movement and colour not analysed in V1, implies that info can bypass V1 to reach secondary visual cortex even without conscious awareness
What is Achromatopsia?
Damage to V4 so inability to perceive colour (inc inability to imagine or remember colour)
What is the difference between Associative and Aperceptive visual agnosia?
Both are damage to temporal lobe
Associative - Normal visual acuity but cannot name what see
Aperceptive - Normal visual acuity but cannot visually recognise objects by shape
What is impaired motion perception?
Damage to area V5, so unable to perceive continuous movement but just see successive snapshots of positions
Unaffected in colour, perception, object recognition etc, able to judge movement of tactile and auditory stimuli
What is Balint’s syndrome?
Damage to posterior parietal cortex
Optic ataxia - deficit in reaching for objects, misdirected movement
Ocular apraxia - deficit in visual scanning
Difficulty fixating on an object
Visual simultagnosia: an inability to see the whole picture, unable to perceive location of objects in space
No difficulty in overall perception or object recognition
What are the key stages of auditory processing?
Sound waves to vibration in basilar membrane
Hair cells in organ of corti transduce movement of basilar membrane into elec signal
High freq sound transduced at base, low at top
How does auditory processing occur in the brain?
Originally thought to be in auditory cortex, with the intermediate stages only stepping stones to get there
But auditory discrimination possible even in absence of auditory cortex e.g. direction, pitch, tune etc
So initial processing actually occurs in pons and thalamus
Auditory cortex responsible for analysing more complex aspects of sound - dorsal (parietal) stream is spatial analysis, ventral (temporal) stream is component analysis
How can location of sound be analysed?
Depends on different characteristics of the sound arriving at each ear:
Intensity difference -
Latency - phase shift between the two ears due to slightly different distance to reach each ear
Duplex theory - sound location depends on a combination of latency and intensity
What are the 3 main causes of deafness?
Conduction - disorders of outer/middle ear which prevent vibrations reaching cochlea
Sensorineural - inability of auditory nerve fibres to be excited normally
Central - damage to auditory brain centres (seldom simply a loss of hearing)
How do the semi-circular canals detect head rotation
They tilt around three axes, and head movement causes endolymph –> displacement of capula, stimulation of hair cells, activation of CN8, and information is transmitted to brain
What can cause motion sickness?
Mismatch between visual info and info from vestibular organ
What are the 2 classes of sensory subsystems in the vestibular system?
3 x Crista - encode angular acceleration; located in ampulla at terminus of each of 3 fluid-filled semi-circular canals (in same plane of rotation), inertial force of fluid provides basis for moving hair bundles
Otolith organs - encode linear acceleration; Utricle (horizontal), Saccule (vertical, found in the utricle). Inertial force of otoliths provides basis for moving hair bundles
What are otoliths?
Crystals that move in response to gravitational forces - increase sensitivity of the stereocilia (receptors) to movement and the gelatinous membrane shifts as the crystals shift
What are the 3 semi-circular canals?
Superior - Like nodding, rotation of head front to back
Posterior - Tilt of head to either shoulder, left to right
Horizontal - Rotation left or right, shaking head for no
What happens when stereocilia activated by movement?
Activation of CN8 fibres and information passes to brain to compute the movement occurring
What is the orientation of the stereocilia like?
Within the cupula (crista) orientation is precise and determines direction of mechanical force to which they are sensitive. Movement in one axis sets up a relative flow of fluid in canal in same plane, deflecting stereocilia in ampulla and signalling head movement
What is the vestibulo-ocular reflex?
As the head rotates, eyes move to compensate, maintaining a relative position
Not reliant on input from eyes
Direct, not involving cortex - eyes automatically move in response to head movement
Where do nerve fibres from the vestibular system receptors go?
Enter lower levels of brainstem and synapse in vestibular nuclei
Some fibres bypass this to go straight to cerebellum to contribute to motor functions there
Outputs from nuclei go to motor nuclei of eye muscles (e.g. oculomotor and abducens nuclei), thalamus and motor cortex (e.g. for control of postural muscles)
What are the roles of the oculomotor and abducens nuclei in the Vestibulo-ocular reflex
Abducens nucleus (pons) - fibres from here innervate contralateral lateral rectus muscles directly Oculomotor nucleus (midbrain) - fibres from here innervate the medial rectus muscles
What actually happens during the vestibulo-ocular reflex?
If the head rotates to anti-clockwise, for example, the fluid in the semi-circular canal rotates clockwise and we also want the eyes to rotate in a clockwise direction to maintain position
So we want the right medial and left lateral rectus muscles to contract (inhibition of extraocular muscles on the other side) - these are both activated via the right abducens and oculomotor nuclei
What are the functions of the medial and lateral vestibulospinal tracts?
Medial - neck muscles and head orientation
Lateral - Peripheral muscles and postural muscles for balance
What is the sensory conflict theory of motion sickness?
We feel bad when we receive contradictory signals from senses - such discrepancies could normally signal neurological impairment, thus triggering dizziness and vomiting to get rid of, for example, potentially toxic food that could be a cause
What is the false climb illusion?
Upward head tilt and linear acceleration produce same stimulation of otolith organs
In low visibility, acceleration may be misinterpreted as upward climb, so pilot forces plane into compensatory dive, but this causes further acceleration, heightening the illusion
