Transcript NS_eye
THE VISUAL SYSTEM Photons and the Randomness of Light Energy of a photon: J ch Photon inter-arrival time: P W t 1 e t Number of photons in a fixed time interval: pz t z e z! z 0,1, 2, direction of light Eccentricity 0° 5° 10° direction of light direction of light Midget ganglion cells (P cells) Small bistratified ganglion cells (K cells) Parasol ganglion cells (M cells) direction of light L and M cones Rods L and M cones S cones K cells P cells (midget) M cells (diffuse) Competing Goals for Visual System Design Maximize spatial resolution Maximize field of view Minimize neural resources Solution High resolution foveal vision Low resolution peripheral vision Eye movement system Types 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, microsaccades Relating Neuron Responses and Behavior Define the task and stimuli Measure behavioral responses Measure 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 hypothesis Optimal pooling hypothesis Sub-optimal pooling hypotheses Invariance hypothesis (read Parker & Newsome 1998) Retinal/LGN Model 1. 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 fovea b. Center diameter increasing in proportion to GC density c. Surround diameter 4-6 times larger than center d. Surround strength 50%-80% of the center 4. Response noise (Croner et al., 1993): a. Constant additive noise Light 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 level.