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lec 02

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Matthias Niemeier

Lecture 02 – First Step in Vision Example of Short Answer Question: Which kinds of psychophysical methods exist? Mention at least four. Describe one of them in more detail. A: Method of constant stimuli, method of estimation, method of limit, method of adjustment. Method of constant stimuli – where you have stimuli such as light with different intensity at fixed value. Randomly present the stimuli to the participant in repetitive manner by keep coming back to same intensity and asked them if they see it or not every time it’s shown. Through this data, for each level of intensity, could calculate probability of that person to yes, from which derive a curve with low intensity with low probability & high intensity with high probability. The curve is sigmoid function. Vision Retina (back of eye) first stage of vision processing Light Physics Light – a electromagnetic wave; a stream of photons, tiny particles that each consist of one quantum of energy o It is little piece out of a vast spectrum of electromagnetic wave starting from long wavelength (TV signal) to visible light from red to blue to very short wavelength to human visible light is very short spectrum of light o the shorter the wave length, the more powerful the wave is o light doesn't actually have a colour, it is our brain that assigns it in physic colours don’t exist colour is created by the visual system 5 ways of light can work: absorbed, diffracted, reflected, transmitted, or refracted: Absorbed: energy (e.g. light) is taken up by some substance and not transmitted at all o E.g. room w/ dust room (light in) will be reduced in brightness as opposed to when it 1 enter that room o E.g. Light on surface especially blac it will be observed o E.g. Filters also absorbe that picture of filter is red because it only lets out (transmit) light that has wavelength that we perceive as “red “lighall other wavelength are reduced & probably not completely filtered out Could have filter that are blue etc. Diffracted: Not directly do w/ eye it is a common phenomenon w/ waves of all sorts (it doesn’t need to be light – for e.g. it could happen to waves in water as well) o It occurs when wave of some sorts encounters some kind of obstcleswaves bent or having waves that are spread out o For e.g. – when waves pass through narrow aperture Reflected Energy is somehow redirected it’s path when it strikes a surface usually back toward its point of origin o When light hits the desk for e.g. --. So light is absorbed & some is itlected happens w/ glass surfaces, also happens w/ eye E.g. brown desk – some brown (dark orange) wavelengths is reflectedbackthus, view the desk as brown Purkinje reflections (is used in Purkinje eye (movement) tracking sstemthese reflection arise from the differential optical surfaces of the eyes o E.g. outer surface & inner surface of cornea that makes already the 2 reflective surfaces In total 4 images from the cornea & lens behind them o These reflection from front looks like little spot depending what kind of light source you have st th in the pic 1 & 4 reflection comes from the same light source & the position of b/w these two depends on eye position Could measure the position of lights spots in your camera and can determine the eye- position thus, determine where your eyes is looking at Transmitted Energy that is passed on through a surface (when it is neither reflected or absorbed by the surface ) o E.g. Solar Panel light energy is not just absorbed but transform into something else (electrical energy) o This is important when we talk about photoreceptors ( this where transmission happens) Refracted Probably the most important type of effect that happens to light Energy that is altered as it passes into another medium e.g. the pencil looked bent when you put into class of water (it is not in reality) o E.g. light entering water from the air o E.g. in the depth of swimming pool (looks shallow that it actually is ) o E.g. eye glass (or our entire eyes) that refracts light so can have focus image on what is the outside world o E.g. rainbow – light hitting droplet of water & is refracted it looks differently depending on the wavelength & thus, see all these different colours Part of an eye exam it which optometrist checks the patient’s description is often called a “refraction” because the doctor determines how much light must be bent by eyeglasses for it to be properly focused on the retina Refraction is necessary to focus light rays and this is done by the lens Why is Sensing light a “good idea”? Help us identify what we can eat or cannot eat OR find food in the 1 place Shape perception - differentiate b/w different shapes (face ) recognition - important for many reason including survival value o As result recognize danger Social significance – can perceive info about other people Light in some way determines whether we are getting sleeper or not Circadian rhythm - this “inner clock” (sleep & wake cycle) is adjusted by light o When travelling to another continent you get jet leg – this occurs because your circadian rhythm is off-sync w/ what time of day in your destination place ( sun rises up different times that your used to) it takes time to re-adjust rhythm w/ light info (sunlight) o there is special photoreceptor in the eye to provide the brain w/ info about what kind of light there is - where it is day or night / evening or morning For trees photosynthesis based on sunlight & also shedding and growing leaves w/ seasonal changes has to do w/ light could sense what time of the year based on light Can decide whether something is important to you in whatever ways whether you want to approach or avoid it Evolution of the Eye Know quite a bit of info even though don’t have fossil of it since eyes are not bone by looking at other animals’ eyes Eye spots – can already observe in single cellular organism the have little spot in their membrane (protein) that sense light o It just protein & is quite different from what we have in the eyes Different from single cell organism have region of photo sensitive cells (photoreceptors) which are responsible for sensing & have nerve fibres (single cell organism don’t have nerve – just one cell) o Photoreceptors often have light sensitive protein called opsin neural signal that is somehow converted & passed on to nerve fibre to some other areas such as central nervous system Next step to have these light sensitive cells somewhere in some kind of a fold/ depressed area (in a cup) o Why? It will limit the direction of the light bea thus, gets some more specific information in terms of where the light is coming from which crucial to perceive objects o This fold/ cup will help bit w/ spatial vision BUT still this aperture is very wi-e Next step – to get better focused vision the aperture in the front gets sma “Pinhole” o Huge advantage: helps focusing & get more accurate vision – thus, can get better optic perception essentially & better perception of directions But keep in mind the focusing is still limited for certain distances o BUT - What happens if you make the aperture even smaller - similar to what you get when you start closing your eyes the amount of light gets less Smaller the aperture = smaller the amount of light that enters the eyes - thus, needs more photosensitive receptors (OTHERWISE – won’t see much) NEXT step – has to do w/ hygiene the kind of eyes (animals) are prone to parasites attacks or bacteria or so Thus, It will better to have a membrane in front that is filled w/ some kind of fluid
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