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Sensation Psychology 3196
Introduction • To catch a ball, we have to see it coming • How does the external get internalized – That in essence, is what sensation is • Bottom up vs. Top down processing – Sensation is bottom up – Perception is top down
Basic Principles • Thresholds – We sense some things and not others – Faintest stimuli • Absolute threshold – Difference thresholds or jnds • Proportion • Stimuli must differ by a constant proportion to be seen as different – Weber’s Law
Sensory adaptation • Getting used to something • If you stop your eyes from moving, everything would go grey! • Same thing if you give them constant stimulation, the ping pong ball trick • Ever notice how everybody else’s house smells funny and yours has no smell at all?
Vision • Like any sensory process, vision converts some energy to neural messages • In this case, light • Light is just a form of electromagnetic radiation • So are x rays, micro waves, infra red, UV cosmic rays etc
I wish to hell I could see better…. • Wavelength of light determines hue • Intensity determines brightness • Light enters the eye through the cornea and the pupil • Pupil size regulated by iris • Behind pupil, lens, which accomodates • Light hits the retina • Oh ya, it is upside down….
Acuity • Acuity is affected by the shape of the eye • Nearsighted, eye too long, or cornea too curved • So far away stuff is blurry • Image is in front of the retina • Farsighted, opposite
The retina • There are two kinds of receptors in the retina, rods and cones • Rods for night, brightness • Cones for day, colour • When a photon hits a receptor it sends a message via the optic nerve to the brain • Because of this, we have a blind spot!
Gotta love the retina • • • Cones are for fine detail and colour Cones only really work in the light Concentrated in the fovea Rods are more evenly distributed Many rods to one bipolar cell, so you can see in dim light, but only in black and white • One cone, one bipolar cell • About 130 000 receptors per retina
OK, now let’s bring in the evolution • Why are we only sensitive to ‘visible light? ’ • UV tends to be affected by atmospheric conditions • Indeed, not much UV reaches the ground • UV and IR go through walls – Ok as cool as that would be…
What we can see • The light we can see has cool properties • Differentially reflected by different objects • So easy to recognized different objects
The eye is poorly designed • Would you build in a blind spot? • Would you make only one bit of the retina really sensitive? (the fovea) • Would you install the retina backwards? • OK so how would an eye evolve? • Easy, and it has happened about 60 times
Other senses • • • Taste and smell Smell is old Smell is good About 1 part per 10 billion Body odor and symmetry Mothers recognize their own baby’s poo
Taste • • • Salt Sweet Sour Bitter MSG I wonder why snacks are so tasty?
coevolution • Moths and Bats • Guppies and prawns • The key point here is that evolution is affected by others’ abilities too • It is an arms race
Modularity • • • Perception is modular Colour and brightness Movement and shape Speech and everything else? Not surprising from an evolutionary perspective