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Chapter 4

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PSYC 2390
Lana Trick

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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