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Sensation and Perception lecture 2.docx

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Western University
Psychology 2115A/B
Anthony Skelton

Sensation and Perception – Lecture 2 Dark adaptation - Differences in light sensitivity in rods and cones - Rods are much more sensitive to darkness, while cones are less sensitive - In dark adaptation, the cones are used to start the adaptation, after about 7-10 min the rods sensitivity occurs and creates a lower threshold, that slows the peak sensitivity - Vitamin a – produces retinal which is essential to our vision - Stage 1 , cones adapt - Stage 2, rods adapt Methods of dark adaptation - Start with an observer in good light adaptation, and once it is dark you adjust a light until you can just barely see it to measure sensitivity in dark - Observer looking at fixation point, but still have light in the periphery, this light deals with both lights and cones - To find cones sensitivity, put a light that stimulates only cones. There is a spot on the retina that you can use to focus on that will give you your sensitivity to cones - To find rod sensitivity, need to find a rod monochromat, which is someone who has a deficiency of cones. Visual pigment regeneration - Retinal molecules changes shape - Opsin molecule separates from retinal - The retina shows pigment bleaching o Cannot see well at this bleaching state - Retinal and opsin must recombine to respond to light - Cone regeneration 6 min , while rod regeneration is about 30 minutes - Isomerizing (reacting with light), bleaching and regeneration is continuously occurring Rods and cones sensitivity to the visual spectrum - Most sensitive to wavelengths in the middle of the spectrum (green/yellow) - Sensitivity =1/threshold - You need less light to see wavelengths in the middle of the spectrum, and more light to see wavelengths in the outsides of the spectrum - Sensitivity of cones 550nm – yellow - Sensitivity of rods 500 nm – green - Cones tend to be better on the right side of the spectrum, and rods on the left side of the spectrum Absorption spectra - Difference in spectral sensitivity is due to absorption spectra of visual pigments - Rod pigments absorbs best at 500nm - Cone pigments absorb best at 419, 532 and 558 nm o Absorption of all cones equals peak of about 560 nm in th
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