Chapter 4 I The Visual Cortex and Beyond following the signals from the retina to the cortex a The Visual System i Most signals from the retina travel out of optic nerve in the eye to the lateral geniculate nucleus LGN in the thalamus ii Signals then travel to the primary visual receiving striate cortex area in the occipital lobe of the cortex iii Signals are then transmitted along two pathways 1 One to temporal lobe 2 Other to parietal lobe 3 Visual signals also reach areas of frontal lobe iv Superior Colliculus an area involved in controlling eye movements and other visual behaviors that receives about 10 of the fibers from the optic nerve b Processing in the LGN i Receptive fields of LGN neurons ii Information flow in LGN 1 Center surround configuration 2 Neurons in LGN respond best to small spots of light on the retina 3 Major function of LGN to regulate neural information as it flows from the retina to the visual cortex 1 LGN does not only receive signals from the retina 2 LGN receives more input back from the cortex than it receives from the retina 3 Smallest signal is from the LGN to the cortex a Organizes and regulates information flowing from the retina to the cortex iii Organization by the left and right eyes 1 LGN bilateral one LGN in right hemisphere one LGN in left hemisphere 2 Six layers each layer receives signals from only one eye a 2 3 5 red receive signals from ipsilateral eye eye on the same side of the body as the LGN b 1 4 6 blue receive signals from contralateral eye eye on the opposite side of the body from the LGN Information from left and right eyes is kept separated in the LGN c iv Organization as a spatial map 1 Retinotopic Map a map in which each point on the LGN corresponds to a point on the retina a Mapping can be determined by recording from neurons encountered along an oblique electrode track 2 Fibers carrying signals from the same area of the retina end up in the same area of the LGN 3 Occur perpendicularly in each of the layers 4 Result aligned overlapping retinotopic maps in each o the c Striate Cortex LGN s six layers i More than 80 of the cortex responds to visual stimuli most of the cortex responds when the retina is stimulated ii Hubel and Wiesel 1959 described receptive field properties organization of neurons in the striate cortex iii Simple Cortical Cells the striate cortex has receptive fields that are arranged side by side rather than in the center surround configuration 1 Respond best to vertical bars that illuminate only the excitatory area to increase firing iv Orientation Tuning Curve the relationship between orientation and firing v Hubel and Wiesel 1 Curve is determined by measuring the responses of simple cortical cells to bars with different orientations 1 Discovered that many cortical neurons respond best to moving barlike stimuli with specific orientations 2 Complex Cells a Like simple cells respond best to bars of a particular orientation b Unlike simple cells respond only when a correctly oriented bar of light MOVES across entire receptive field thus don t respond to stationary flashes of light 3 End Stopped Cells fire to moving lines of a specific length or to moving corners or angles 4 Discovered that neurons respond selectively to stationary and moving lines important step in determining how neurons respond to more complex objects vi Feature Detectors simple complex and end stopped cells fire in response to specific features of the stimulus such as orientation or direction of movement d Feature detectors and their role in perception i Feature detectors neurons respond best to specific types of stimulus ii Selective Adaptation if the neurons fire for long enough they become fatigued or adapt causing 2 physiological effects 1 The neuron s firing rate decreases 2 The neuron fires less when that stimulus is immediately presented again iii Adaptation is selective because only the neurons that respond to verticals or near verticals adapt and other neurons do not iv Psychophysical selective adaptation experiment 1 Grating Stimuli alternating bars 2 Basic assumption adaptation of neurons that respond to verticals should result in the perceptual effect of becoming selectively less sensitive to verticals but not to other orientations 3 Contrast Threshold the difference in intensity at which the bars can just barely be seen the contrast of gratings 4 Adaptation selectively affects only some orientations just as neurons selectively respond to only some orientations v Selective Rearing 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 prevalent 1 Neural Plasticity Experience Dependent Plasticity the idea that the response properties of neurons can be shaped by perceptual experience 2 Example rearing an animal in an environment that contains only vertical lines should result in the animal s visual system having neurons that respond predominantly to verticals 3 Contradicts selective adaptation 4 Use it or lose it e Maps and Columns in the Striate Cortex i Maps 1 Nearby points on the cortex receive signals from nearby locations in the retina 2 Retinotopic mapping indicates that information about objects near each other in the environment is processed by neurons near each other in the cortex 3 Cortical Magnification Factor the area on the cortex representing the cone rich fovea is much larger than one would expect from the fovea s small size a Brain Imaging refers to a number of techniques that result in images that show which areas of the brain are active used to measure cortical magnification b Positron Emission Tomography PET changes in the activity of the brain are accompanied by changes in blood flow and monitoring the radioactivity of the injected tracer provides a measure of this blood flow c Subtraction Technique brain activity is measured in 1 an initial condition before the stimulus of interest is presented and 2 a test condition in which the stimulus of interest is presented i Subtracting the activity record in the initial condition from the activity in the rest condition d Functional Magnetic Resonance Imaging fMRI determines relative activity of various areas of brain by detecting changes in the magnetic response of the hemoglobin that occurs when a person perceives a stimulus or engages in a specific behavior i Subtraction technique also used ii Main method used today 4 Connection between cortical area and acuity found through