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Lecture 4

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University of Toronto Scarborough
Matthias Niemeier

PSYB51 October 8 , 2010 Lecture 4: - Visual field defects - Hemianopia: half of the visual field is missing [relative to your fovea] in both eyes or one eye - Heteronymous hemianopia: one eye has visual field defect in the left visual field while the other eye has a defect in the right visual field [for left eye: the left visual filed is missing; for right eye: the right visual field is missing] - Optic chiasm is disrupted: crossing over of visual field info. is disrupted [so visual field info from left visual field eventually goes to the right hemisphere and vice versa = this process is disrupted] = sorting of info occurs in the optic chiasm Contrast sensitivity function: [Contrast sensitivity is a measure of the ability to discern between luminances of different levels in a static image .]  Y-axis = contrast sensitivity X-axis = spatial frequency - Perception: inverted U-shape! = lower spatial frequency means a lower contrast sensitivity While contrast sensitivity rises at intermediate spatial frequencies & it comes to 0 at 60 cycles/degree [on graph] At 60 cycles/degree = can’t see anything anymore Contrast Sensitivity comes to 0! PSYB51 October 8 , 2010 - In terms of neurons: higher spatial frequencies [60 degree/cycle]– we can‟t simply resolve that b/c our photoreceptor fields aren‟t small enough and near 1 degree/cycle there aren‟t enough neurons to represent these lower spatial frequencies Object recognition -has to do with matching visual input [object of a certain kind] with memory -special kind of object recognition = object identification e.g. in (c) so you can still identify that it‟s the same house in (c) that is in (a) So for example – you don‟t want to just enter any kind of house [so you recognize an object being a house] but you want to enter your OWN HOUSE [specific identification!] Object Perception encompasses processes that: (perceive an object as a house) Object naming = being able to express verbally what you‟ve seen [so you see an object and use a word to assign to it]  so assigning a verbal tag to your „visual perception‟ Object perception CONTINUED = are these steps that actually lead to object recognition; it‟s the process that brings all the visual info together as a „coherent‟ thing for us to be able to recognize [=object recognition] PSYB51 October 8 , 2010 - Remember: most neurons in V1 are orientation-sensitive In (b): So V1 with receptive fields will be localized to the top, left side of house, and bottom [& way more of course; these 3 are just examples]. So neurons would have some kind of orientation- sensitivity (for horizontal or vertical) In (c) – >V1 has no way of telling the difference between two objects apart [e.g. car and the house] So these are the problems that V1 faces! [So structuralism failed! –> which was strictly bottom up processing from the bare structure] – so you first identify lines in the house, then you join them together & you finally have a house THIS DOES NOT REALLY WORK! – THIS IS WHERE THE GESTALT STUFF COMES IN! PSYB51 October 8 , 2010 - neuron 1 has some receptor field somewhere with a certain orientation tuning [the line in the picture] – it wouldn‟t be interested in any other orientation - Neuron 2 = same kind of orientation tuning but for a different part of the visual field that‟s pretty close by - So connection between these 2 neurons! = these neurons will mutually enhance each other! They’ll also bully any other neurons that are NOT co- linear! [inhibit = bully LOL]  inhibition is probably more indirect - So neurons will inhibit or activate each other. Visual system hates coincidences so you want a balance in these two things! b/c it tends to miss them! In the picture (c): turning point that‟s NOT smooth  it‟s actually 2 turning points [2 diff. Lines!] - They found that what ppl perceive as being smooth versus not smooth is simply a breaking point [at which point you say it‟s smooth or vice versa] - In nature: So the point at which you perceive when things are smooth versus not smooth  if the curve is too disrupted –> then we perceive it as two different things [like twigs for example]  basically what lines actually belong together [continuation!] So, the same twig versus 2 twigs PSYB51 th October 8 , 2010 - The luminance is the same yet we tend to perceive the foreground as LIGHTER! - So all these things are made up by our visual system b/c it makes sense in normal life - When we look at this picture –> our visual system says that it can’t be a coincidence [the seemingly white arrow] –> so we say that there must be something with a contour in front of it that is OCCLUDING what actually are the circles and lines that we think appear in the background. So basically the visual system tried to find the EASIEST EXPLANATION FOR WHAT IT SEES! [Visual input = ambiguous as there is lack of info; There‟s lack of visual info but that is compensated by the fact that coincidences are very unlikely] IC = Illusory contours - Lateral occipital complex = needed for object perception - There is research that suggests that IC perception is the function of very late stages of object perception and then probably feeds back into V1 and modulates activity in those earlier areas - many objects have some kind of info for texture - 2 types of features: shapes and colours = they are somewhat independently represented in the brain - So to bring them together you need to work your memory [“Oh so it’s a diamond and it was red!] - THIS IS SOMETHING WHERE GESTLAT DOESN’T WORK – so gestalt is for middle vision and not for working memory! PSYB51 October 8 , 2010 - so it‟s easier to group together line 2 and 3 instead of line 1 & 2 - But it’s a weaker gestalt law = not a strong effect but it is little bit of an advantage! - Things that change together probably belong together = can‟t be a coincidence! Common region = part of the same region! - committees of neuro-mechanisms (groups of neuron) specialized for one or the other thing - So one member of that committee might be responsible for proximity while another could be responsible for common region [so neurons will have a discussion and one will win in that meeting LOL] E.g. pandemonium model: diff ranks of demons [feature demons at the bottom, then middle-ranking cognitive demons and decision demon at the TOP] PSYB51
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