Chapter 10 Visual Perception Combined Theory Hurvich Jameson For example when red light shines on a red cone the ganglion cell is inhibited This signals the brain that the eye is seeing the color red But if green light shines on the red cone nothing will really happen The green light will excite the green cone and ganglion cell and notify the eye that green is being seen Same thing with blue as above cone ganglion cell excitation Yellow light is between green and red so both the red and green cone will be somewhat activated We don t see red green because the effects are cancelled out when it goes to the red green ganglion cell But both cones send inhibitory signals to the yellow blue ganglion cell which indicates to the brain that you re seeing yellow Receptive fields of color opponent ganglion cells picture in textbook Center of the receptive field is excitatory periphery of the receptive field is inhibitory Types where red is middle green outside green is middle red outside yellow in middle blue outside blue in middle yellow outside Yellow circles are a mixture of red green ganglion cells Receptive fields look like concentric circles because it s allows for more efficient discrimination of wavelengths and enhanced info on color contrast in objects Form Perception Perception of Edges Sensory system enhances perception of borders Ganglion cells present especially to extract the edges of something Edge a sudden change in the luminosity of nearby objects or spaces Mach band illusion Gray bands close to the edges one side seems lighter than the other side Caused by lateral inhibition Lateral inhibition Ganglion cells inhibit and are inhibited by neighboring cells Ex receptor 9 sends excitatory signal to ganglion cell 9 but also sends inhibitory signals to ganglion cells 8 and 10 Signals closer to the lighter side are firing slower than the side farther away Ex ganglion cell 7 is firing at 2 5 above resting state 10 5 2 5 2 5 while ganglion cell 3 is firing at 5 10 2 5 2 5 5 above resting state At edges ganglion cells get differential amounts of inhibition from darker edge and brighter edge makes edge stand out perceptually On center Off center ganglion cells Edge detectors Antagonistic arrangement of receptive fields detection of light dark contrast On center shine light on center excitatory effect shine light on periphery inhibitory effect Off center shine light on center inhibitory effect shine light on periphery excitatory effect If the light is uniform over these cells whether light or dark no activation change in rate of firing will happen If not uniform then an edge has been detected and it will fire at a changed rate whether excitatory or inhibitory depends on where the light falls and what type of ganglion cells it falls on Receptive Fields Hubel Wiesel Retina circle Lateral Geniculate Nuclei LGN of Thalamus circle Visual Cortex rectangular bars Farther from retina bigger receptive field and more complicated Simple cells in visual cortex respond to edges at a specific orientation and place on retina Simple cells receive info from ganglion cells that are on top of each other form a strip column only receives info when all of them are excited at once this allows the simple cell to perceive exactly that edge Have much larger receptive fields than ganglion cells in retina Complex cells respond to edges moving across retina Takes information from simple cells pattern of activity across the simple cells it receives info from inform it how the edge is moving Smudging the results of simple cell data gives a better approximation of how something actually looks Some neurons perform a Fourier analysis of the luminosity variations of a scene Neurons in visual cortex do not detect only edges Visual world is combination of high and low spatial frequencies Needs neurons sensitive to both feature detectors filter info from simple cells which will eventually be put together into a uniform picture Spatial Frequency Theory Visual Pathways Parvocellular system P ganglion cells project to ventral stream what system help you figure out what you re looking at that travels through inferior temporal lobe from areas V1 V2 V4 of visual cortex Color vision V4 color constancy and detailed object recognition inferior temporal lobe Magnocellular system M ganglion cells projects to dorsal stream where system that travels to posterior parietal lobe from areas V1 V2 V5