Chapter 4 - Sensation and perception 2390
Chapter 4 - Following the signal from the retina and cortex
The visual system
Most of the signals from the retina travel out of the eye, in the optic nerve to the LGN
(lateral geniculate nucleus) in the thalamus, then to the striate cortex area (primary
visual receiving area) in the occipital lobe, then the signals are transmitted along
two pathways, one to the temporal lobe, and one to the parietal lobe. Visual signals
also reach the frontal lobe of the brain. Superior colliculus is an area involved in
control of eye movements and other visual behaviors that receive about 10% of the
fibres from the optic nerve
Processing in the lateral geniculate nucleus (LGN)
Major function of the LGN is to regulate neural information as it flows from the
retina to the visual cortex. 90% of the fibres out of the optic nerve arrive at the LGN,
and also receives signals from the cortex, the brain stem, from other neurons in the
thalamus and from other neurons in the LGN. For every 10 nerve impulses the LGN
receives from the retina it sends only 4 to the cortex. Organizes information before
sending it out to the cortex.
The LGN is a bilateral structure, which means there is one LGN in the left
hemisphere and one in the right hemisphere, a cross section reveals 6 layers, each
layer receives signals from only one eye. Layers 2,3 and 5 receive signals from the
ipsilateral eye (the eye on the same side of the body as the LGN) and layers 1, 4 and
6 receive signals from the contralateral (the eye on the opposite side of the body
from the LGN).
Spatial maps, retinotopic map on the LGN is a map in which each point on the LN
corresponds to a point on the retina, in each of the 6 layers.
Receptive fields of neurons in the striate cortex
Now from the LGN to the visual cortex.
Simple cortical cells – cells with side by side receptive fields, excitatory and
inhibitory areas arranged side by side, responds best to bars of a particular
orientation
Complex cortical cells - responds best to movement of a correctly oriented bar
across the receptive field, in a particular direction of movement
End-stopped cortical – responds best to corners, angles or bars of a particular length
moving in a particular direction
Retinal ganglion cells respond best to spots of light whereas cortical end stopped
cells respond best to bars of a certain length that are moving in a particular
direction Chapter 4 - Sensation and perception 2390
Do feature detectors play a role in perception?
Selective adaptation and feature detectors
Selective adaptation – the is that if the neurons fire for long enough, they become
fatigued or adapt, which causes two physiological effects 1) neurons firing rates
decrease and 2) neurons fire less when that stimulus is immediately presented
again
Grating stimulus and contrast threshold – pg. 80 fig 4.9
Selective rearing and feature detectors
Following neural plasticity or experience-dependent plasticity, Selective rearing – is
that if an animal is reared in an environment that contains only certain types of
stimuli, then neurons that respond to these stimuli will become more dominant, for
example rearing an animal in an environment that contains only vertical lines
should result in the animals visual system having neurons that respond
predominantly to verticals
Selective rearing and adaption seem to contradict each other, however rearing effect
occurs over long periods of time and is strongest in young animal (ex, use it or loose
it kitty)
Maps and columns in the striate cortex
Maps in the striate cortex
Retinotopic mapping indicates that information about objects near each other in the
environment is processed by neurons near each other in the co
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