Transcript lens
Psychophysical theories • Signal detection theory: A psychophysical theory that quantifies the response of an observer to the presentation of a signal in the presence of noise – There are four possible stimulus/response situations in signal detection theory: • Hit: Stimulus is presented and observer responds “Yes” • Miss: Stimulus is presented and observer responds “No” • False alarm: Stimulus is not presented and observer responds “Yes” • Correct rejection: Stimulus is not presented and observer responds “No” Detecting a stimulus using the signal detection theory (SDT) approach (Part 1) Detecting a stimulus using the signal detection theory (SDT) approach (Part 2) Sensitivity to a stimulus: The separation between the distributions of response to noise alone and to signal plus noise For a fixed d’, all you can do is change the pattern of your errors by shifting the response criterion Summary: Goal was to connect physical world with psychological states Limits of our senses: thresholds Thresholds are not all-or-nothing, they are probabilistic Depending on attention, intention etc. we can shift perception that comes with different patterns of errors A Little Light Physics • Light: A wave; a stream of photons, tiny particles that each consist of one quantum of energy Light: The stimulus for vision Electromagnetic radiation structured in waves over space energy Wavelength amplitude = Intensity distance same amplitude different wavelengths same wavelength different amplitude multiple wavelengths (vs. pure) source How is light structured? Why light? Vision as a distance sense reflected scattered absorbed surfaces, substances Light travels straight fast far •good forabout image-production •we can can know know them immediately •we far objects Some light gets to eye The eye captures light reflected from objects and forms an image on the back of the eye. How “should” the image be formed? Problem: Spatial ordering of rays reflected from the object have to be recovered from the divergent light. Solution #1: Allow one ray from each part of the object into the eye. box with a pinhole as the eye Camera Obscura (Alhazen) Limitation: Doesn’t let in much light—blurry image Problem: Spatial ordering of rays reflected from the object have to be recovered from the divergent light. …while letting in enough light for a clear image. Solution #2: Use a lens that refracts light so that rays from the same point on the object converge. Allows larger hole more light sharper image Limitation: Clear focus depends on the power of the lens and angle of divergence of light rays. Problem: Spatial ordering of rays reflected from the object have to be recovered from the divergent light …while letting in enough light for a clear image. Solution #2: Use a lens that refracts light so that rays from the same point on the object converge. Limitation: Clear focus depends on the power of the lens and angle of divergence of light rays. Different distance of object from eye changes angle of light rays Out of focus for that lens. Problem: Spatial ordering of rays reflected from the object have to be recovered from the divergent light …while letting in enough light for a clear image …of objects at varying distances. Solution #3: Lens with variable optical power changes shape to accommodate the distance of the object to the size of the eye. The eye as a chamber for capturing light What properties should the eye have? Optical parts Structures for gathering and focusing light cornea pupil iris lens fovea Translating parts Structures for copying light and sending signals Light is focused on light-sensitive photoreceptors of retina that convert or transduce physical energy into neural activity •Lens shape can be changed to accommodate to the distance of an object for focusing on the back of the eye—the retina—centering on the fovea •Fluid in eye keeps its shape from changing so that focusing properties are reliable Focusing-relevant features highlight the importance of the retinal image The human right eye in cross section (viewed from above) Eyes That See Light • Refraction is necessary to focus light rays and this is accomplished by the lens – Accommodation: The process in which the lens changes its shape, thus altering its refractive power – Emmetropia: The happy condition of no refractive error So what can go wrong….? Astigmatism: A visual defect caused by the unequal curving of one or more of the refractive surfaces of the eye, usually the cornea So what can go wrong….? • Retinitis pigmentosa (RP): A family of hereditary diseases that involves the progressive death of photoreceptors and degeneration of the pigment epithelium – Many people may not notice the onset of retinitis pigmentosa at first because it primarily affects peripheral vision healthy patient with RP – Visual field tests for someone with normal vision compared with someone with retinitis pigmentosa Next time on Sensation and Perception: Rods and cones Dark adaptation Blind spot Receptive fields, Mach bands