PSYX 250N: Test 2Vision: Part 1Sensation: the transformation of physical stimuli (e.g., light energy, mechanical energy, environmental chemicals) into signaling within the body (e.g., through neuron communication).Perception: The acquisition of knowledge through sensationSensory Transduction: The process by which energy in the environment is converted into cellular signaling - Happens in the eye Sensory Receptor: A cell that performs a sensory transduction (not to be confused with a protein receptor Photons: Units or waves of energy- Released when something (sun, lightbulb) expels energy - More energy -> higher wave frequency- Always travel at the same speedMost Essential Parts of Eye Pupil: Space within the iris that allows light to pass through the lens Lens: focuses lightVitreous Humor: Fluid inside the eye Retina: Layer of Photoreceptors and neurons lining the inner wall of the eye Optic Nerve: Bundle of axons carrying signals from the eye to the brain The RetinaRoute within retina: Messages from receptors at the back of the eyeBipolar CellsGanglion CellsGanglion Cells’ axons join together and travel back to the brain- Additional Cells called Amacrine cells get information from bipolar cells and send it to other bipolar, Amacrine, and ganglion cells. - Amacrine Cells refine the input to ganglion cells, allowing them to respond specifically to shapes,movements, etc. Optic Disk (Blind Spot): Where the axons of the retinal ganglion cells all converge to become the optic nerve. Area above the optic nerve where there are NO photoreceptors Most Essential Features of the RetinaFovea: Meaning pit, dense area of photoreceptors at the center of vision, specialized for acute, detailed vision3 Layers of Cells: PhotoreceptorsRods: Not color selective, more sensitive to light. Not useful in daylight b/c bright light bleaches them. Outnumber cones 20:1Cones: Color Selective, dense at fovea. Useful in bright light. Provides 90% of the brains input Shape Changeintracellular signalingreduced signaling to other cellsBipolar: “Relay” information to ganglion neuronsGanglion Cells: axons go to the brain Trichromatic Theory- All human vision is trichromatic (All colors we see are due to combinations f 3 types of color receptors R,G,B)Coding- Brain encodes information in a way that doesn’t resemble what you see- Brain codes information in terms of which neurons respond, their amount of response, and the timing of their responses - Which neurons are active- Amount of response- how many a/p’s a neuron sends per unit of time. Sensory coding depends on the frequency of firing Receptive Field: The area of space that a neuron codes for (doesn’t have to be physical space but often is)Center Surround Codes - Center activates cells- Surround inhibits cells - Depends on which cellsWhat is the mechanism of the center-surround receptive field? Lateral inhibitionWhat is the function of the center-surround receptive field? Increase spatial resolution of signalHerman Grid Explanation: This leads to a physiological mechanism called lateral inhibition, which has the effect of causing a bright surround to an area appear darker and, conversely, a dark surround will make an area appear lighter.The Physiological Explanation Your retina is partially composed of many small nerves, which function as receptors of light. These receptors are arranged in rows on the inside of your retina. A number of scientists have shown that it is possible to illuminate and record from a single receptor (A) without illuminating its neighboring receptors. It was discovered that if you illuminate a single receptor (A) you will get a large response; however, when you add illumination to A's neighbors, the response in A decreases. In other words, illumination of receptors "inhibits" of firing of neighboring receptors. This effect is called lateral inhibition because it is transmitted laterally, across the retina, in a structure called the lateral plexus. In the case of the Hermann grid, there is light coming from the four sides of the intersection, but fromonly two sides of a band going away from the intersection. The region viewing the intersection is more inhibited than the region of the band going away. Thus the intersection appears darker than the other section. You see dark spots at the intersections of the white bands, but not at the points away from the intersections. The effect is greater in your peripheral vision, where lateral inhibition acts over greater distances.Different types of cells are specialized for different functionsEach of 10-20 types of ganglion cells carries different types ofInformation to the brainTypes include:- color- light at large & small spatial scales- Movement! (in any direction)- texture- approaching movement- anticipated positionEach ganglion cell can be thought of as the output of a circuit.Each circuit computes different informationImmanuel Kant (1781) we don’t just take-in the world. We have pre-set structures (including structures for time and space) that interact with the world to generate our perception. Helmholtz (1821-1894) Unconscious inference: there is a gap between what information is available to us and the amount of information we can get from it. Our minds bridge this gap by making inferences using hidden assumptions about the world.What have we learned about the brain from just looking at the retina?1) Information processing is both serial and parallel2) Information processing involves the actions and Interactions of circuits with both excitatory andInhibitory neurons3) There is information about where in space Things are happening (i.e., there are maps ofExternal space) as well as information about what is in that spaceVision Part 2Color Blindness: - X linked Genes (affect males more than females)- Rod cones filled with opsins of green comes (protanopia)- Green cones filled with opsins of red cones (deuteranopia)- Retina lacks blue cones (trianopia—very rare)Light (photons) retinal photoreceptors (rods & cones) bipolar cells  ganglion cells  optic nerve inhibitory interneurons play important roles horizontal cells: lateral inhibition amacrine cells: help compute “features”- From the eye, retinal ganglion cells project their axons through the optic nerve.• Half of the axons cross to the other side of the brain at the optic chiasm, then land in thelateral geniculate nucleus of the thalamus • Neurons in the lateral geniculate