primary and visual pathway Flashcards
How does visual information come into the brain?
- main pathway
- photoreceptors from retina in back of eye
- information carried from retina to lateral geniculate nucleus via optic nerve
- this is the visual relay station for sensory thalamus
- information sent to occipital cortex, primary visual cortex of occipital lobe
- located in occipital lobe (back of brain)
what makes up total field of view?
- left visual field
- right visual field
- central field of view (where visual fields overlap)
when visual fields of both eyes overlap, which portion of the eye is hit?
Fovea
- central vision is what we use to see something in focus, something sharp, near vision for reading
- all of this involved the fovea
what is the lateral geniculate nucleus?
- relay station of sensory thalamus
how is sensory information relayed?
- via a specific nucleus of the thalamus
- to primary sensory cortex found in most modalities
where does information from right visual field end up in the brain?
the left side
where does information from left visual field end up in the brain?
the right side
identify a key principle of information processing
information processing is hierarchical
outline the stages of information-processing stages in primary visual pathway
RETINA
- photoreceptors
- bipolar cells
- retinal ganglion cells
-optic nerve-
LATERAL GENICULATE BODY
VISUAL CORTEX
describe an experimental set up to record visual responses of neurones along the visual pathway
- present visual information
- visual information can be controlled
- present visual information in field of view
- using micro electrodes, electrophysiological recordings of neural activity at these different stages
identify other ways of recording visual responses of neurones along the visual pathway
- fMRI
- EEG
- MEG
identify the two main types of photoreceptors contributing to vision
- rods
- cones
describe Rods
- cannot discriminate wave lengths (therefore cannot discriminate colour)
- 120 million in human retina (more abundant than cones)
- sensitive in low light
- higher density in periphery
- tracks high-rate changes
describe Cones
- 6 million in human retina
- can discriminate 3 types of wavelengths (small, medium, large)
- less sensitive in low light
- higher concentration in fovea
- cannot track high-rate changes
what are the photoreceptors detection of light translated into?
- excitation or inhibition of retinal ganglion cells
- via bipolar cells
what do bipolar cells do?
links photoreceptors to retina ganglion cells
what do photoreceptors and bipolar cells vary as they are stimulated?
voltage
(analogue signal)
what is the receptive field?
- refers to portion of retina/visual field
- this is where visual stimulation will evoke a change in firing rate
- the size of the receptive fields gets bigger as more information is combined from several photoreceptors
- very small receptive field at photoreceptor stage
what does a ‘substructure of a receptive field’ refer to?
- refers to the specific way visual stimuli/information is presented in the receptive field of a visual neurone
- this is so a firing-rate change can be evoked
outline the steps that take place from when retinal ganglion neurones receive visual input
- neurones receive input from multiple photoreceptors
- receive input via bipolar cells
- bipolar cell receptive fields become several times the size of photoreceptor field as more information comes in
- bipolar cell receptive fields have ‘on’ and ‘off’ and ‘centre’ surround structure
what are bipolar cells?
- one of the main retinal interneurons and provide the main pathways from photoreceptors to ganglion cells,
- i.e. the shortest and most direct pathways between the input and output of visual signals in the retina
what structures does bipolar cell receptive fields have?
- on structure
- off structure
- centre surround structure
what happens when light is presented in ‘on’ regions of bipolar cell receptive field?
- cells become excited
- increase in firing
what happens when light is presented in ‘off’ regions of bipolar cell receptive field?
- cells become inhibited
- firing rate decreases
explain the rebound effect
- when stimulus is taken back, firing rate shoots in the opposite direction to what light stimulus was
- following inhibition, neurone doesn’t go straight to baseline rate
what happens when the light information is in the centre?
- firing rate increases
- when light information taken away, rebound effect occurs
what is the response rate of the bipolar cells based on?
sum in ON stimulation - sum of stimulation in OFF region
when will neurones only react?
- when there is contrast and boundaries
why is it important for there to be contrast and boundaries?
- more efficient
world has many things that stay constant
therefore we don’t need to respond to it all the time
more efficient to respond to changes and boundaries - helps preserve appearance of objects regardless of environmental light levels
luminance of features = represented relative to their surround
can result in illusions
how is colour sensitivity perceived by retinal ganglion and lateral geniculate neurones?
- both receive input from cones
- you have different types of colour sensitive retinal ganglion and LGN
- both have receptive fields
- receptive fields have centre-surround organisation
- centre = stimulation of certain light property causes excitation
- surround = stimulation of certain wavelengths causes inhibition
what different types of colour-sensitive retinal ganglion and LGN are there?
(colour opponency)
- yellow on, blue off
- yellow off, blue on
- red on, green off
- red off, green on
what can colour opponency explain?
visual illusion
from the lateral geniculate body in the thalamus, where is information then passed on to?
first visual processing stage
primary visual cortex
where does the primary visual cortex (in humans) sit around?
Carlcarine fissure (between the hemispheres)
identify a key feature of the neurones in the primary visual cortex
they are all orientation selected
what is meant by ‘orientation-selective’ neurones?
- neurones have bigger receptive fields than the neurones at stages before
- they respond to light/elongated stimulation presented in a certain orientation
- stimuli = elongated
what are the type main types of orientation-sensitive primary visual cortex neurones?
- simple cells
- complex cells
explain what simple cells are
- they have receptive fields with inhibitory and excitatory regions
- only become excited if stimulus is presented in the right orientation on this excitatory region
- combines inputs from ON and OFF cells in LGN
- integrates information from several LGN
explain what complex cells are
- a step up in regards to processing compared to simple cells
- they combine information/inputs from several simple cells
- their fields have no discrete on and off regions
- but they respond best to moving stimuli (which reflects response adaptation - the neurone stops firing/reduces firing rate if stimulus doesn’t move)
what is the Retino-topic map?
- refers to the orderly mapping of retina/visual field onto visual cortex
- different points of the retina interact with specific points in the primary visual cortex
- neighbouring areas in the retina excite/interact with neighbouring areas in primary visual cortex
what are modules of the primary visual cortex?
- PVC divided into small columnar modules that combine neurones
- these are sensitive to different aspects of stimuli presented in small part of visual field
- some respond to stimuli of different orientation
- some respond to colour
what is ‘Blindsight’?
- the ability to respond to visual stimuli without consciously perceiving them
- a condition which can occur after certain types of brain damage
(lesions to primary visual cortex)
give examples of blindsight?
- looking (moving eyes) or pointing to visual stimuli
- but not being able to ‘see’ stimuli
what does ‘blindsight’ highlight?
- primary visual pathway = critical for conscious vision
- there are also additional visual pathways
- brain can perform visual information processing which can guide subjects’ behaviour without their conscious awareness
