primary visual pathway Flashcards
what can visual information from visual field stimulate
photoreceptors and areas in the retina
how does information travel in the primary visual pathway
from the retina info travels through optic nerve to primary visual cortex
- info from right half ends up in the left hemisphere and vice versa
what three things are in the primary visual pathway
- retina
- lateral geniculate body
- visual cortex
what is the experimental strategy to reveal mechanisms of visual perception
by studying the different neuronal responses at different stages of the visual pathway, one may gain understanding of the different stages of visual information processing that mediate visual perception
what is the experimental set up to record visual responses of neurons along the visual pathway
- electrodes at different parts of the visual pathway
- present light in controlled manner - measure with electrodes - see how neurons respond at different stages
what are some seminal contributions to our understanding of visual information processing
- David Hubel
- Thorston Wiesel
- nobel prize in physiology or medicine in 1981
what are rods
-more abundant
- no colour
- sensitive in low light
- higher density in periphery
- track high rate changes e.g. flicker in corner of the eye
what are cones
- less abundant
- 3 types discriminate wave lengths
- less sensitive to low light
- higher concentration in fovea
- cannot follow rapid changes
how do photoreceptors and bipolar cells differ from all subsequent cells
- vary their voltage as they are stimulated rather then very spike rate
what is photoreceptor detection of light translated into
- excitation or inhibition of retinal ganglion cells via bipolar cells
what are receptive fields of visual neurons
- the portion of the retina in which visual stimulation will evoke a change in the firing rate of a given visual neuron - not whole visual field excites all neurons
what is a substructure of a receptive field
- a description of how visual stimuli need to be presented in the receptive field of a visual neuron in order to evoke firing-rate changes
what do retinal ganglion neurons do
- receive input from multiple photoreceptors via bipolar cells
- ON-OFF centre-surround receptive fields
what does light presented in ‘ON’ regions do
- excites cell
what does light in ‘OFF’ regions do
- inhibits cell
how are ON and OFF regions organised
in ‘centre- surround’ fashion
what is the response rate of cell based on
- the sum of stimulation in ON region minus stimulation in OFF region
what is the functional significance of centre-surround fields
- responding only to changes and boundaries
- the luminance of features is represented relative to their surround
- helps preserve appearance of objects regardless of light levels in the environment
what do retinal ganglion cells and LGN cells receive and what are they sensitive to
- receive inputs from cone and are sensitive to colour
what do colour-sensitive ganglion and LGN neurons have
receptive fields that show centre-surround colour opponency
what are the 2 main types of orientation selective cells in v1
- simple cells
- complex cells
what are simple cells
- fields have inhibitory and excitatory regions
- can be thought of as combining inputs from ON and OFF cells
- feed into complex cells
what are complex cells
- fields have no discrete ON and OFF regions
- best response to moving stimuli
- can be thought as combining inputs from simple cells
what is the retino-topic map
- orderly mapping of retina/visual field onto visual cortex
what are modules in V1
V1 is divided into small columnar modules that combine neurons sensitive to different aspects of stimuli presented in a small part of the visual field
- each bit of retina responds to a specific part of the visual field
what does further processing of visual information do
- needed for perception and memory of the ‘holistic’ visual properties of whole objects and visual scenes
where does further processing of visual information take place
- in the visual association cortices - V2-V5
what is blindsight
- subjects with lesions to primary visual cortex and apparent ‘blindness’ can show appropriate responses to visual stimuli of which they are not conscious
what does blindsight highlight
- apart from the primary visual pathway that is critical for conscious vision, there area additional pathways
- the brain can perform visual information processing which can guide subjects behaviour without their conscious awareness
what did Schmid et al 2010 find about blindsight
- direct LGN projections to extrastriate cortex are critical for blindsight
