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Lecture 7

PSY270H1 Lecture 7: Perception & Attention

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David Chan

Our own body (egocentric) representation can influence our perception of tactile information. This egocentric representation operates at all times regardless of any environmental stimulus -- you have a sense of where your body is at all times. Ex. You can touch your nose while closing your eyes without having to look into a mirror. P’s brought into lab and given two cubes to hold in their hands. The cubes can be pushed down like buttons. The cubes are vibrating. The task is called a temporal order judgement task where the cubes will vibrate one after the other and the P must push down which cube vibrated first. To make this task harder you can alter the duration between the first buzz and second buzz (it’s easy if there’s a lot of time in between). If you reduce the duration, P’s start to produce errors. Doing this with your hands in a regular anatomical position, the P’s are accurate. But, if you get the P’s to cross their hands, the accuracy of the P’s decreases. The y-axis is called “Just noticeable difference” which is the amount of time required in between the two buzzes for you to have a 75% chance of accurately discriminating which one came before the other (white bar is uncrossed hands and black is crossed). The reason why this crossed-hands deficit occurs is because when you are receiving information to the hands, we have information coming from our egocentric representation (that our left hand is receiving information and our right hand is receiving information). Naturally we want to assume our left hand is on our left side of space and right hand is on our right side of space. At the same time we are also getting visual information (allocentric/environmental representation) matches our egocentric representation. When you cross your hands your egocentric representation still wants to represent the left hand on the left side and right hand on the right side. But your visual information is telling you that is not the case (your left hand is now on the right side and vice versa). This conflict causes the deficit. If this assumption is true that if these two representations are in conflict then that must mean if we close our eyes it will remove the visual information and thus the black bar should be the same as the white bar. But…even when the P’s are blindfolded they still produce this effect. Why? When you walk into a room, your visual system will create a mental representation of the room that you’re in. So even when you close your eyes you have a mental representation of the room (even if you were only here for 5 mins). So theoretically you still have some access to a visual representation of the room. To test this out they did a subsequent experiment where they brought P’s to the lab and kept them outside in a waiting room. They then blindfolded them prior to entering the room and got them to do the task. Blindfolding P’s before going into the room does in fact remove this effect (so you must prevent the visual representation from being created). Helmholtz Theory of Unconscious Inference: Some of our perceptions are a result of unconscious inferences or assumptions based on likelihood (probability). The more likely something is, the more we assume something is. This happens in order to reduce cognitive load (prevents you from having to make a new construct each time). The last two images would create the same retinal image in a bottom-up process. Most of us will perceive (a) as what (b) is because we know based on our experiences that objects behind another object are continuous and not missing a piece (even though theoretically c can exist). This is how Helmholtz argued how we perceive things. Bottom up  if ambiguous, make inferences based on past experiences  whichever one more likely, you perceive it as that. Gestalt school of psychology – came up with Laws of Perceptual Organization – Rules to explain how small elements of a scene are grouped to form larger units that allow us to perceive the environment. Based on these rules they came up with the idea that “the whole is different than the sum of its parts” (i.e. if you take each of the bottom-up stimulation and add it up, it doesn’t give you the same experience of what we naturally experience  because our top-down processes provide us with more information then what is there) Good figure – We tend to group things into the easiest figure possible (ex. most people perceive that object as a square and a triangle rather than a square, triangle, trapezoid) Common fate – We tend to group things together if we think they’re bound to a common fate (ex. Those rectangles are moving to the bottom left and we group all those things together. The one in the middle is moving to a different path and we perceive that as different). Good continuation – We tend to connect smooth lines even when the line is broken (makes sense because based on our experience, we know that when something isn’t visible we know it continues) Proximity – Things that are close together we group together Similarity – Things that look similar we tend to group together (ex. We say 3 rows of circles and 2 rows of squares rather than 4 columns of alternating) Familiarity – We tend to group things into familiar objects (ex. We see those individual shapes but we perceive that object as a house) Note that these laws are not mutually exclusive and every scene that we see has multiple laws acting on it (but of course some can be more obvious than others within a scene) Regularities (further rules on how we group information) Remember, the brain is lazy and wants to make predictions in order to reduce the amount of cognitive work it has to do. One of things your perception does is it relies on physical regularities (Regularly occurring physical properties) to make assumptions (we will talk about two). 1. In order world there are more vertical & horizontal orientations than oblique orientations in the environment (physically easier due to gravity). Because of this we as humans have what’s called the oblique effect – We are faster at processing stimuli when they are oriented vertical or horizontal compared to oblique. Example: Give people stimuli like that picture just above and change the angles of the lines. P’s are faster at indicating if a line is horizontal or vertical rather than oblique (even if it’s very obvious oblique). As a whole, people are more likely to say something is horizontal/vertical regardless whether they are or not. In other words, we assume something is horizontal or vertical until our minds have processed it as oblique. 2. Light-from-above heuristic: The assumption that light is coming from above. This is what creates the illusion that some of the bubbles are popping out and some of the bubbles are going in to the page. It’s not just physical properties that govern these top-down inferences but also meaning – Semantic regularities – If a characteristic associated with a scene matches, you will be faster to process that than if it mismatches Ex: Flash a picture of a kitchen (and the kitchen had mittens, pots, etc.) you will be faster to detect that it’s a kitchen. Hollingsworth gave P’s 20 seconds to study a scene where a target wither present or absent (see pic on the left). The target is the dumbbell. It is present in A and absent in B. After the 20 seconds, the screen went blank and you saw a picture of a dumbbell. You are asked to place the dumbbell where it should go. Group A obviously you know it went on the carpet on the left. Group B they never got the dumbbell but because it’s a gym there are clues on where it should go (most likely near the mat on the left). The twist is that people in Group C (who just got a picture of a dumbbell) and were told to imagine there was a gym and place where the dumbbell would go. Results are below. The right picture shows circles which correspond to the labels that say which group those people are in. The results for Present and Absent make sense (Absent they know meaningfully where it should go). What’s amazing is that people in Group C (the big circle) are still accurate in where they’re putting the dumbbell (choosing the left-middle side of the screen even though there are infinite possibilities). This shows that people all have some commonalities in their schemas. In a gym there are only a certain number of arrangements a gym could be in and so that governs how you represent a gym scene in your head (those in Group C). Dual Visual Pathways In our visual systems we know that visual information hits our retina and goes to the occipital lobe. The information then breaks off into two distinct visual pathways. One is a path to the parietal lobe and helps us determine where an object is in space (functional name is where/how pathway; anatomical name is dorsal pathway). The other path is to the temporal lobe which helps us determine what an object is (functional name is what pathway; anatomical name is ventral pathway). Experiment is on the left (done with monkeys). People with their temporal lobe removed could still perform the landmark discrimination task and people with their parietal lobe removed could still do the object discrimination task. Note the object discrimination he explained in lecture was “the goal in this task was to recall which item was previously presented”. This is a double dissociation. We represent space for the purpose of interacting with that space. For example, I don’t need to know that my water bottle is black in order to interact with it. I do need to know where it is in space to interact with it (i.e. how to interact with it) and so that is why is called where/how and not just where. Visual agnosia – A condition in which a person can see but cannot recognize or interpret visual information due to damage to the temporal lobe. The man in the video could not recognize the picture as a lock but he knew with hands how to operate it. Another person who suffered from visual agnosia is patient DF. The experimenters made a mail slot in which the experimenters could rotate its orientation. In the first task DF was told to match the orientation of the paper with the mailbox. DF had a hard time doing this and could not match the orientation as seen in the results. Control is people who do not have damage. Orientation has to do with object identification (what) in this way: We can easily identify the letter X but when we orient it in a new way eventually it will look like a + sign. As a result, orientation can tells us more about what an object is rather than where it is (orientation isn’t changing where an object is in space). And so this tests DF’s ability to figure out the ‘what’ of an object. The other task was for her to push the card through the slot and told her not to worry about the orientation. DF is just as good as control participants. This suggests that her how stream (aka dorsal aka action stream) was still intact but her what stream (aka ventral aka perceptual stream) was compromised (lost her ability to match the orientation of the slot with her card because this is how we infer what the object is). To get a double dissociation we have Optic ataxia – Patients have deficits in their action stream (how), while their perceptual stream (what) stay intact. This patient can still identify the object but they are not able to interact with it (i.e. can’t reach out for the object and she doesn’t know where it is in space like which object is closer to her). They don’t have motor deficits just an inability to know where it is (she knows the general direction though). Mirror Neurons They are group neurons in the premotor cortex that respond to actions that an observer performs. The pattern of activation in our mirror neurons corresponds to the action we are observing (they fire in a similar pattern as if we are performing that action we are observing). E: Stick electrodes in monkey, have the monkey pick up food and observe the firing rate (activation) of the mirror neurons. If now the monkey watched a human pick up this piece of food you will also get activation in the mirror neurons (not as much but still activation). This phenomenon only works if the monkey also knows how to do this action. If the monkey sees the human pick up the piece of food with pliers, there is no activation of the mirror neurons (the monkey sees the object, doesn’t know what it is or how to use it). If the monkey is given time to learn how to use the pliers, then when you have the human use the pliers, the mirror neurons will become activated. Therefore, mirror neurons help us to encode and respond to actions that we are observing. The use of having these mirror neurons is that they help us understand (infer) another person’s action and respond to them appropriately. (Ex. Your friend is walking towards you in a very angry way. Because you have prior knowledge as to how an angry walk looks like, your mirror neurons fire and allow you to recognize that your friend is coming at you in an angry way and allows you to appropriately respond (in this case, you appropriately respond by running away). Question: Do mirror neurons encode action or intention? E: Bring P’s into a lab and put into an fMRI machine. Show them pictures of scenes like the example on the left. In the first scene it looks like the person is looking like he will have a cup of the tea (the drinking context). In the second scene it looks like the person is going to be cleaning up and that they are done their cup of tea (because the cookies look like they’ve been eaten and the place looks messy – the cleaning context). Another piece of background info: There are two ways to hold a cup. You hold the cup either using a
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