(Week 3) Sensation & Perception

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• Try to include all the important terms (such as terms that are focused on heavily or are said once but are clearly psychology related)

[Music] our sensus help us to take in the world around us by converting stimuli or sensory information into electrical signals that are processed by the brain in fact we're constantly bombarded by stimuli even though we're only aware of what our own senses can pick up we Define the absolute threshold of a sensation as the minimum stimulation needed to register a particular stimulus 50% of the time if I play a series of tiny beeps at different decb or levels of loudness randomly but repeatedly over a period of time and you tell me that you hear one of those beeps 50% of the times it was presented that would be the loudness considered to be your absolute threshold examples of absolute threshold that you'll find in almost any textbook include our ability to see a candle from 30 Mi on a dark clear night or to hear the tick of a watch at 20 ft under quiet conditions to taste one teaspoon of sugar in 2 gallons of water to smell one drop of perfume diffused into a three- room apartment or to feel the wing of a fly falling on your cheek from a distance of half an inch but picking up weak sensory signals isn't only about the strength of the stimulus it's also about your psychological State your alertness and expectations in the moment this is the basis of the signal detection Theory model for predicting how and when a person will detect a weak stimulus partly based on context for example when you walk to your car that is parked in an empty parking lot late at night all by yourself you might be much more aware of the noises around you because the situation is somewhat threatening you are primed and listening carefully to hear anything and everything in this case you might hear some slight noises that you might otherwise not hear if you were in a different situation that was not as threatening on the other hand if you've ever been expecting an important call that you didn't want to miss you probably had the experience where you think you hear your phone ringing or feel it vibrating in your pocket and then you realize no one was calling so it's important to remember that our ability to detect signals or Noises by any of our five senses is almost always affected by factors unrelated to the intensity of the stimuli itself I might go out at night and look up at the sky and while I know that no two stars could have exactly the same brightness some some may still seem to look exactly alike that is I know there must be a difference but I just can't detect the difference in their brightness the point at which one can tell the difference is known as the difference threshold but it's not linear instead if that tiny point of light was just a little bit brighter than another tiny point of light I would be more likely to detect the difference than if a larger light was brighter in exactly the same small amount this is known as Weber's law and it says that we perceive differences on a logarithmic rather than linear scale it's not the amount of change but rather the percent of change that matters so what are the biological processes involved in sensation and how are Sensations converted into perceptions we'll try to answer these questions in the context of one of our most powerful senses Vision your eyes taken energy from light and transform that energy into neural messages that your brain processes and organizes into what you actually see this process is called transduction but how do we transform light waves into meaningful information light itself what we humans see as light is only a small fraction of the full spectrum of the electromagnetic radiation that ranges from gamma to radio waves light travels in waves the waves length and frequency determines the Hue that is the color or shade that we see and the wave's amplitude determines its brightness or value its Purity what we perceive as saturation or richness of color is determined by the number of distinct wavelengths that make up the light for instance short waves with high frequency are registered by our eyes as bluish colors while we see long low frequency wavelengths as reddish Hues it's the eyes lens that is responsible for focusing these incoming light waves onto the retina But the lens must constantly change its shape so that the light is focused correctly this change in the curvature of the lens is also based on the wavelength with red light the long lowfrequency wavelengths requiring the greatest curvature and blue light with its short high frequency waves the least curvature this means that if pure blue and pure red Hues are intermixed the lens is constantly changing shape and the eye actually becomes tired as you can see from focusing on these circles for even a few seconds in the case of text the effect is even more pronounced with blue or green text on a red background here again two opposing wavelengths enter the lens forcing it to either focus on one or the other a related effect is called chromostereopsis which is that pure colors located at the same distance from the eye appear to be at different distances for example Reds appear closer and blues more distant if you put Violet next to yellow or green next to Orange the violet and green [Music] retreat in general the warm colors such as red orange and yellow come forward and the cool colors such as blue green and purple retreat in this sense the Chromo stereoptic effect gives shapes plasticity and allows for depth perception through simple color manipulation okay so as we said a color's Hue is determined by the length and frequency of the light waves but the brightness of a color is determined by the amplitude of those waves the contrast between the red of an apple and the red of a fire Tru is based on their amplitudes that is the amount of energy in a given light wave greater amplitude means higher intensity means brighter color so the length and frequency of a light wave determines the Hue and the wave's amplitude determines intensity or brightness at the same time a color's saturation or Purity depends on light complexity the range of wavelengths in light the color of a single wavelength is pure spectral color such lights are called fully saturated outside a laboratory light is rarely Pure or of a single wavelength light is usually a mixture of several different wavelengths the greater number of spectral colors in a light the lower the saturation light of mixed wavelengths looks duller or paler than pure light when a light is reflected from from the lens onto the retina it is not by means of a full image instead visual information is encoded as separate neural impulses from Tiny portions of the world so that eventually the brain can reconstruct in great detail fine visual differences from locations at which you're directly looking the retina contains over 200 million photo receptor cells called rods and cones rods give us sensitivity under dim lighting conditions and allow us to see at night cones allow us to see fine details in bright light and gives us the sensation of color cones are tightly packed around the fobia the central region of the retina and more sparsely elsewhere rods populate the periphery the region surrounding the phobia and are almost absent from the phobia when stimulated ated the rods and cones trigger chemical changes that spark neural signals which in turn activate the cells behind them called bipolar cells whose job it is to turn on the neighboring ganglion cells the long axon Tales of these ganglions braid together to form the ropey optic nerve which is what carries the neural impulses from the eye to the brain that information then slips through a chain of progressively complex levels as it travels from optic nerve to the thalamus and onto the brain's visual cortex the visual cortex sits at the back of the brain in the occipital lobe where the right cortex presses as input from the left eye and the left cortex processes input from the right eye what happens next well you might think that the eye would do something like record the amount of light at each location in the world and then send this information to the visual processing areas of the brain but in fact that is not what eyes do as soon as photo receptors capture light the nervous system gets busy analyzing differences in light and it is these differences that get transmitted to the brain the brain it turns out cares little about the overall amount of light coming from a specific part of the world or in the scene overall rather it wants to know does the light coming from this one point differ from the light coming from the point next to it as a demonstration place your hand on the table in front of you the Contour of your hand is actually determined by the difference in light the contrast between the light coming from your skin in your hand and the light coming from the table underneath to find the Contour of your hand we simply need to find the regions in the image where the difference in light between two adjacent points is maximal two points on your skin will reflect similar levels of light back to you as will two points on the table on the other hand two points that fall on either side of the boundary Contour between your hand and the table will reflect very different light we actually see objects by Edge detection where an edge can be created by a difference in color brightness or both edges formed by color differences alone with no brightness differences appear fuzzy and unfocused so we need to add changes in brightness to get sharp edges in addition to analyzing these differences in light your visual cortex also has specialized nerve cells called feature detectors that respond to specific features like shape angles movements and even faces some cells in a region May respond to just one type of stimulus like posture or movement or facial expression while other clusters of cells weave all that separate information together in an instant analysis of the situation this ability to process and analyze many separate aspects of the situation at once is called parallel processing in the case of visual processing this means that the brain simultaneously works on making sense of form depth motion and color it links together individual features into whole objects allows us to recognize what those objects are organizes objects into visual scenes and detects motion and change in those scenes wow that's a lot huh and that is where we need to begin to talk about the complicated and sometimes even ambiguous world of perception watch the image on your screen closely which of the two gray sections is darker the top one or the bottom one try holding up your hands and blocking this section of the image how can what appear to be two distinct Shades of Gray actually be the same color this happens because your brain uses Shadows to make decisions about what it's seeing take a look at this object how do you know what it is and where it's positioned in space your visual cortex is first cross referencing the information about this object with memories of past experiences stored throughout your brain after identifying the object your brain also has to determine how it is positioned in space and one of its most reliable ways of doing this is to use Shadows take a look at the basketball again now let's add a a shadow is it still moving in the same way what if we change the position of the Shadow now it looks as if the ball is rising and sinking let's tweak the shadow one more time now it appears as if the ball is bouncing in this world with a single natural light source your brain has learned to trust shadows as a near foolproof way to know the behavior of objects in space so what happened here the gradient and Shadow painted here have given your brain cues that the top is a well-lit gray surface and the bottom is a poorly lit light surface Clues Your Brain Trust because of past experience with shadows other than using Shadow to construct reality what other perceptual tools do we have available to us for one re anal disparity when viewing an object or scene allows our left eye and right eye to view slightly different images this binocular signal specifically provides information about distance and yet based on only two dimensional images that fall on our retinas we are still somehow able to see three-dimensional objects seeing objects in three dimensions also known as depth perception allows us to estimate distances between those objects and ourselves depth perception cues that don't require both eyes are called monocular cues these include occlusion relative size relative height texture gradient familiar size linear perspective aerial perspective and relative brightness let's consider each of these depth cues on this famous album cover of The Beatles first let's look for occlusion this is the idea that an object that blocks the view of another object must be in front of it so when looking at the White Volkswagen and George Harrison we know that George Harrison is in front of the Volkswagen because there's a portion of it that is being blocked by him relative size in this photo also suggests depth this is the idea that smaller objects are further away and it can be seen in the many cars in the image not all cars are the same size but because the White Volkswagen is larger than all of the others we can conclude that the other cars are progressively further away similarly in texture gradient as a texture gets further away it forms smaller visual angles or pictures on the retina and is less noticeable if you look at the road at the bottom of the picture the texture is made up of larger shapes compared to the texture of the road in the middle of the photo in the middle the texture is less obvious suggesting it is further away relative height gives us the impression that objects that appear higher in our visual field are further away than objects object that appear lower notice that the Beetles themselves are lower in the visual field and appear much closer than the trees buildings and cars that appear higher in our visual field familiar size is the idea that knowledge of the normal size of certain objects can provide cues to depth for example we are familiar with the size of people in the picture there's a man standing on the sidewalk we know that this man is roughly the same size as the members of The Beetles because we have experience with the size of people this Clues us into the fact that the smaller looking man is probably just further away from the band members and is not actually a miniature person linear perspective which is the idea that parallel lines seem to converge as they move into the distance is also clear in this image this CU can be seen when looking at the way the edges of the road converge we know that the road isn't actually getting narrower it's just getting further away aerial perspective is actually a far subtler depth cue in this picture with aerial perspective objects that are further away also appear to be hazier and Bluer this is somewhat true though for the trees and cars in the middle of the picture finally relative brightness gives the impression that Brighter Images are closer and more shaded images are further away although John Lennon is probably directly in front of Ringo Star his bright white suit gives the impression that he is somewhat closer than the other members of the band I should mention that like Shadow and color depth cues can also sometimes provide misinformation take a good look at this image known as The Shepherd's Table which table is thinner and longer the one on theal cues do not generally work in this kind of isolation instead we combine information about shading and color and depth cues in an effort to build up an accurate perception perception is also Guided by many organizational principles originally laid out by gal psychologists for example we tend to group things together according to a principle of similarity meaning that all things being equal we group together figures that resemble each other so here we tend to group these dots into columns rather than rows by grouping dots of similar colors form is also commonly perceived in the simplest way possible for example we would see this form as two intersecting rectangles rather than as a single 12-sided irregular polygon we are also influenced by proximity the closer figures are to each other the more we tend to group them together perceptually here we perceive three groups linking the dots that are closer together and separating the dots that are farther apart perceptual organization is also Guided by the principle of good continuation a preference for organizing form in a way where Contours continue smooth smoothly along their original course for example we tend to see a continuous green bar rather than two smaller rectangles in another example this looks like this not like this we're also absolute masters of perceiving closure in the presence of incomplete information as you can see in this image though the edges of the circle are not defined entirely our minds continue the edges and it appears to be a white Circle surrounded by spikes of various sizes some theorists actually interpret these subjective Contours as a special case of good continuation in their view the Contour is seen to continue along its original path in either case human per receivers seem to have a very strong tendency to impose completeness on small amounts of input our brain unconsciously Works to construct meaning and patterns even in the absence of either for instance we often see images in clouds or rock formations this tendency to organize incomplete and even random images into meaningful images is called parolia an amazing graphic artist Kristoff nean who is a master at creating fun and compelling images has taken parolia in a particularly Charming Direction he uses art to make meaning and Whimsy out of everyday objects here are just a few [Applause] [Music] [Applause] [Music] [Applause] [Music] [Applause] [Music] [Applause] [Music] [Applause] [Music] it's not a coincidence that paradia predominantly affects the perception of animate figures like animals human bodies and human or human-like faces this shows the strength of these templates in our visual memory and the strong significance of seeing these objects in our environments in fact there is ample evidence that our tendency towards closure that is filling in information to complete perceptions is particularly powerful in the context of seeing human form particularly faces in this way surprisingly incomplete images are built by our brains into rich and complete images now let's look at closure in the context of movement take a look at these dots perceptual closure along with the fact that we are hardwired to recognize biological motion again helped to explain these impressive skills have these gal principles impressed you I hope so truly an endless number of examples of our brains tendency towards Simplicity coherence and wholeness as a final topic I will describe one particularly vigorous debate in the field of perception that came to inspire countless experimental studies across centuries the debate centered around the question of whether perception of the world is a set of innate abilities or instead a set of acquired or learned skills these opposing views have been termed nativism and empiricism also known as bottom up versus top- down processing it was psychologist Richard Gregory who first argued that perception is a constructive process which relies on top- down processing stimulus information from an environment is frequently ambiguous so to interpret it we require higher cognitive information either from past experience or stored information in order to make inferences about what we perceive for Gregory perception is a hypothesis which is based on our prior knowledge in this way we are actively constructing our perception of reality based on our environment and stored information for example understanding difficult handwriting is easier when reading complete sentences than when reading single and isolated words this is because the meaning of the surrounding words provides a context to Aid understanding read the following quote out loud notice anything odd while you were reading the text in the Triangle did you notice the secondth if not it's likely because you were reading this with a top- down processing approach having a secondth doesn't make sense our brain knows this and it doesn't expect there to be a second one so we have a tendency to skip right over it your past experience has changed the way you perceive the writing in the Triangle a beginning reader one who is using a bottomup approach by carefully attending to each piece would be less likely to make this error Gregory argued perception involves a lot of constant unconscious hypothesis testing to make meaning of the information presented to our senses and there are several types of evidence that seem to support this idea of how perceptions are created first highly unlikely objects tend to be mistaken for likely objects Gregory famously demonstrated this with a Hollow Mask of a face such a mask is generally seen as normal even when you know and can feel the real mask there seems to be an overwhelming need to reconstruct the face based on past experience we also see objects as whole and complete even when parts of them are obscured for example a person walking toward us carrying let's say a tuba does not look fractured a top a bottom and a gap in the middle caused by the tuba we see the whole person but the outline of the person is incomplete as it is in this example Gregory would argue that we must be using knowledge of people that is they can't be in parts to go beyond the sensory evidence delivered from this visual image additional evidence for this top- down view is supported by ambiguous fig figures during continued observation of an ambiguous figure sudden perceptual reversals occur while the stimulus itself stays unchanged a good example is the Necker cube when you stare at the crosses on the cube the orientation can suddenly change or flip it becomes unstable and again shows that a single sensory pattern can produce two perceptions if you are a proponent of top- down processing you might say that this object appears to flip between orientation s because the brain develops two equally plausible hypotheses and is unable to decide between them when the perception changes though there is no change of sensory input the change of appearance cannot be due to only the static pattern that we're sensing it must be built top down by our current prevailing perceptual hypothesis of what is near and what is far in other words if one physical display can induce two different perceptions then this is an example of when perception cannot be tied directly to stimulation alone there is also however evidence to refute a top- down hypothesis testing view of perception first if perceptions make use of hypothesis testing the question can be asked what kind of hypotheses are they are we as perceivers also able to modify our hypothesis in some cases it would seem the answer is yes for example this probably looks like a random arrangement of black shap Apes in fact there's a hidden figure in there can you see it what if I had shown you this first now can you see it once the dog is discovered very rapid perceptual learning takes place and the ambiguous picture now obviously contains a dog each time we look at it while this is arguably a very good example of a direct relationship between modifying hypothesis and perception in other cases this process is not evident for example Illusions persist even when we have full knowledge of them remember the gray squares in inverted face one would expect that the knowledge we have gained from covering the center line between the squares or touching the mask would modify our hypothesis but in this case learning does not change our perception another perplexing question for The empiricist Who propose perception is essentially topd down is how can newborns ever perceive in fact newborn infants as soon as 5 minutes after birth already show a preference for normal facial features rather than scrambled features infants also show shape constancy that is the ability to perceive the shape of a rigid object as constant despite differences in the viewing angle taken together this evidence has been put forward in support of a nativist or bottomup view of perception in which nothing more is needed beyond the stimulus set within the rich visual context of the world world around us first articulated by James Gibson bottomup processing suggests that perception involves innate mechanisms forged by Evolution and that no learning is required Gibson argued that perception is direct and not subject to hypothesis testing as Gregory had proposed there is enough information in our environment to make sense of the world in a direct way for Gibson sensation is perception what you see is what you get there there's no need for processing or interpretation as the information we receive about size shape distance Etc is sufficiently detailed for us to interact directly with the environment put simply he proposed that this is possible due to something called affordances affordances for humans include such things as surfaces that are standable or sit on objects that are graspable or Thor objects that afford hitting substances that afford pouring in short affordances are meanings that an environment has as meanings they guide Behavior telling the Observer what is and what is not possible Gibson would say that there has in fact been learning in the perception of affordances but that this has taken place over the course of millions of years of evolutionary history not the course of particular lifetimes he believes that individual learning is only involved in terms of figuring out which affordances to attend to neither top down or bottom up theories of perception on their own seem capable of explaining all perception all of the time perceptual research over the last several decades has in fact shown convincing evidence for both types of processing top down and bottom up processes are interacting with each other to produce the best interpretation of the world around us [Music] he [Music]