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Lecture 4

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University of Toronto St. George

Lecture 4 Slide 2 hemi means half, an means doesnt work/not, opia means vision half not vision -half of visual field is missing relative to your fovea, -heteronymous hemianopia means that the hemianopia doesnt have to be in both eyes heteronymous stands for in one eye having visual field defect in left visual field and in other eye its visual field defect in right visual field.. which eye has what defect?... in left eye, left part is missing and right eye its the right side thats missing, thats why its heteronymous (meaning its different).. homonymous means both eyes the same side would be defected -the fact that its heteronynmous means its indicative of which part of the primary visual pathway is disrupted optic chiasm is whats disrupted (part where path of visual info is crossing over) what eyes see is left and right visual fieldboth eyes see both visual fields more or less -what happens after optic chiasm? Visual fields are being sorted, whatever is in left visual field ends up in right hemisphere and vice versa when this crossing is disrupted, it is the optic chiasm that is missing/lesioned -looks like an inverted U, the contrast sensitivity function, horizontal axis is spatial frequencies e.g. cycles per degree -on vertical axis is the sensitivity contrast (not a physical measure) but rather the ability of a person to perceive itlower spatial frequencies (lower cpd) and lower sensitivity.. if SF gets too high we cant resolve it because our photoreceptors dont have receptive fields small enough so thats why it goes down after a point -exam will not cover lecture 6 Slide 4 important distinction b/w recognition and perception which explain diff b/w middle vision and high level vision early level vision is about breaking objects apart Slide 6 point of this slide is object recognition, we have pixels on a screen and we assign the arrangement in space of these pixels to a certain class of object (house) matching up visual input with memoryspecial kind of object recognition is object identification, panel C has same house as panel A so can identify it to be same object as panel A even though its rotated view -object naming = word you can assign to perceptual impression of what you see -object perception = steps that lead to object recognitionneeds to happen at some point, need to put together the visual input so that an object becomes a coherent thing, not a given, at the moment we are at area v1 where things are atomistic, everything is broken down to individual lines/edges, nothing coherent yet object perception is process that will bring all these lines together Slide 8 gestalt laws = visual rules that system adheres to help us see coherently -perceptual committees, gestalt laws have certain rules of extracting info from visual input, certain committees sort out what info is correct -ground perception goes back to gestalt laws as well, about identifying an object as something that im looking at as something in the foreground vs the background -global images like forest and trees, joining visual info together to create the global images Slide 9 even computers cant simulate our visual system can perform the algorithm for luminance and different pixel contrasts very simplistic, still pixels, not lines at this point in visual system , V1 does this for us neurons respond to local patches of edginess and at least locally, we can identify something of a line the smooth transition on left pic, the gray level is identical so you still have gaps b/c of the exact same shade, no contrasts for computer to detect Slide 10 circles in panel b show neurons from v1 in visual field that analyze those orientation sensitivities those circles are just 3 individual neurons but in reality there are way more, panel b is to illustrate that the info from that house is very disjointed panel c, v1 itself has no way of telling the difference between the snowman and house most of its receptive field shows a vertical line and ignores the squiggle from the snowman same case with the car these are problems that v1 faces cant just join together lines in order to perceive structuralist idea was just bottom up, identify lines, and then join them and finally you have a house (does not work) Slide 11 gestalts pointed this out, gestalt means whole, the gestalt psychology major statement is that the whole is greater than sum of its parts gestalt psychologist most imp contribution is formulating set of gestalt laws that help middle vision to put things together Slide 12 panel a:N1 and N2 could be grouped together (high degree of co-linearity) lines after that become less and less collinear (less smooth) panel b: you still see a loop but area 1cannot perceive this loop need something else, gestalt law helps us join these lines togetherhave some kind of neuron that has receptive field on one of these lines with a certain orientation tuning wont be as interested in a line with a totally different orientation because of the location of n1 and n2 receptive fields, if they respond together, we have two collinear line segments.. neurons have on avg 10 000 connections with other neurons, so probably a connection between n1 and n2 neurons if this part of v1 is really good at gestalt law, there will be a positive connection b/w these two neurons, if n1 sees a line with this kind of orientation it should encourage n2 to see that kind of line as well.. n2 probably does the same thing and activates n1 both encourage each other to look for the line.. mutually enhance each other they will also bully any other neuron responsive to lines that are not collinear e.g. n3 would inhibit n3have to have this balance of power b/w inhibition and activation.. balance of power is brought into imbalance b/c of collinearity, so that tips balance of power in one way.. making certain lines more salient -so gestalt law can even be implemented in V1 through lateral connections Slide 13 what people perceived as being smooth versus not smooth is simply a breaking point slide b is the better continuation of the two lines, this way the lin
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