Information processing stages
Photo receptors - bipolar cells - retinal ganglion cells ALL IN RETINA - lateral geniculate body - visual cortex
How do you reveal mechanisms of visual perception?
By studying the different neuronal responses at different stages of the visual pathway you can understand different stages of visual information processing that mediate visual perception
David Hubel and Thorsten Wiesel Nobel prize…
1981 for discoveries concerning information processing in the visual system
Photo receptors - rods
More abundant (120 million) No colour discrimination Sensitive in low light levels Higher density in periphery Track high rate changes (flicker of monitor from corner of eye)
Photo receptors - cones
Less abundant (6 million) Colour discrimination for different wavelengths Less sensitive in low light levels Higher concentration in fovea Cannot follow rapid changes
Photo receptor and bipolar cells…
Vary their voltage as they are stimulated - analogue
All other cells vary spike rate - digital
Photo receptors detection of light is translated…
Into excitation or inhibition of retina ganglion cells via the bipolar cells
Receptive fields of visual neurons are..
A portion of the retina or visual field where visual stimulation will evoke a change in firing rates of the visual neurons
The substructure of a receptive field is the description of how visual stimuli must be presented to evoke firing rate changes
Retina ganglion neurons receive…
Input from multiple photoreceptors via bipolar cells
They have ON/OFF centre surround receptive fields
ON/OFF centre around receptive fields…
Light in the ON region excited the cell
Light in the OFF region inhibits the cell
The response rate is based on enhancement of contrast and boundaries - the sum of stimulation in ON minus stimulation in OFF regions
The functional significance of centre surround fields is..
Only responding to changes in contrast and boundaries is efficient as lots of things stay constant
Luminance of an object is presented relative to their surround which helps to preserve the appearance of objects regardless of light levels
From the eyes to the primary visual cortex…
Through the occipital lobe.
Right field of view = left side of brain
Left field of view = right side of brain
Colour sensitivity in retinal ganglion and the lateral geniculate neurons …
They receive input from cones and are sensitive to colour.
They have a receptive field that show centre surround colour opponent. Functional significance of this not known
This can explain negative after images.
The primary visual cortex V1 is found…
Between two hemispheres. Between calcarine fissure
Simple cells in V1…
Their fields have inhibitory and excitatory regions
This can be thought of as combining inputs from ON and OFF cells
Complex cells in V1…
Their fields have no discrete ON and OFF regions
The pay respond best to moving stimuli
They can be thought of as combining inputs from simple cells
Most V1 neurons rejoins to…
Elongated stimuli with specific orientation
A retino-topic map is…
Orderly mapping of retina and visual field onto the visual cortex
The V1 is divided into..
Smaller columnar modules that combine neurons sensitive to different aspects of stimuli presented in a small part of the visual field
Further processing of memory and holistic visual properties takes place in…
The visual association cornices (V2-V5, the inferior temporal cortex and posterior parietal cortex)
Blindsight occurs with…
Lesions to primary visual cortex
Blind can show appropriate responses to visual stimuli of which they are not consciously aware
This highlights that there are additional visual pathways - direct LGN neuron projections to extrastriate cortex