Slide 1Slide 2Slide 3Slide 4Slide 5Slide 6Slide 7Slide 8Slide 9Slide 10Slide 11Slide 12Slide 13Slide 14Slide 15Slide 16Slide 17Slide 18Slide 19Slide 20Slide 21Slide 22Slide 23Slide 24Slide 25Slide 26Slide 27Slide 28Slide 29Slide 30Slide 31Slide 32Slide 33Slide 34Slide 35Slide 36Slide 37Slide 38Slide 39Slide 40Slide 41THE VISUAL SYSTEMP h o t o n s a n d t h e R a n d o m n e s s o f L i g h t E n e r g y o f a p h o t o n : J c h P h o t o n i n t e r - a r r i v a l t i m e :  1tP W t e   N u m b e r o f p h o t o n s i n a f i x e d t i m e i n t e r v a l :  0 , 1 , 2 ,!zep z zz  L t direction of light0°5°10°Eccentricitydirection of lightdirection of lightMidget ganglion cells (P cells)Small bistratified ganglion cells (K cells)Parasol ganglion cells (M cells)direction of lightL and M conesRodsL and M conesS conesP cells (midget) M cells (diffuse)K cellsCompeting Goals for Visual System DesignMaximize spatial resolutionMaximize field of viewMinimize neural resourcesSolutionHigh resolution foveal visionLow resolution peripheral visionEye movement systemTypes of Eye Movements•Saccadic Movements:–Scanning movements where the gaze is abruptly shifted from one point to the next: conjugate, ballistic, no visual feedback•Vergence Movements:–Cooperative movements that keep both eyes fixed on the target; converge or diverge•Pursuit movements:–Smooth tracking movements that keep an object’s image fixed in place on the retina•Vestibulo-Ocular Reflex (VOR)–Stabilize image during head and body movements•Micro movements–Tremor, drift, microsaccadesDefine the task and stimuliMeasure behavioral responsesMeasure responses of single neurons or populations to same stimuli (preferably in same species and while behaving the task)Test linking hypotheses; for example,Most sensitive neuron hypothesisOptimal pooling hypothesisSub-optimal pooling hypothesesInvariance hypothesis(read Parker & Newsome 1998)Relating Neuron Responses and Behavior1. Optical point-spread function (Campbell & Gubisch, 1966)2. Ganglion cell sampling lattice (Curcio & Allen, 1980)3. Receptive field properties of P cells (Derrington & Lennie,1984; Croner & Kaplan, 1995):a. Center diameter of one cone in foveab. Center diameter increasing in proportion to GC densityc. Surround diameter 4-6 times larger than centerd. Surround strength 50%-80% of the center4. Response noise (Croner et al., 1993):a. Constant additive noiseRetinal/LGN ModelLight and Dark Adaptation: Solving the Dynamic Range Problem -The pupil opens up at low light levels, closes down at high light levels. -There are two photoreceptor systems: the rod system (for low light levels) and the cone system (for high light levels). -The photoreceptors adjust their individual sensitivities based upon the ambient light level. -The other retinal neurons adjust their sensitivities based upon the ambient light