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PSYC 2390
Lana Trick

1. Processing in the retina and beyond a). What is the difference between an after-image and an after-effect? Give one example of an after-image and one example of after-effect to clarify the distinction. An after effect is when something floats in front of your eyes or disappears as an ‘effect’ of looking at an object too long, and after image is a specific type of after effect in which case the photoreceptors use up a lot of photochemical (action potentials) and when you look away they send out a weak signal but the photoreceptors that weren’t being used send out a strong signal and the area with the weak photoreceptors appears to have the opposite colour (white/black, green/red, yellow/blue) b). Explain why after-images occur (e.g., use a simple black/white afterimage as your example—Hint: We had one in class!). when you see something that is white, the photoreceptors use up a lot of photochemical but for the parts of the eye that were on the surrounding black visual field the cells aren’t using a lot of photo pigment (due to the lack of light) so not many action potentials are being used; when the stimulus is removed, the parts that looked black seem bright and the object that was white seem black because the parts of the retina responsible for white are fatigued whereas the black parts are not c). What is lateral inhibition? Describe 2 phenomena that are thought to be caused by lateral inhibition, and describe how lateral inhibition produces each of those effects. Side by side cells inhibit one another and this has the effect of highlighting edges of objects (from white to dark); we perceive an exaggeration. Mach bands are a black area beside a white area in which we see an exaggerated banded effect in between the white and black areas (looks very white beside the black and dark black beside the white) this is based on the number of action potentials per second. The Hermann grid is a 4x4 grid and at the intersections there appears to be a grey circle even though there isn’t, this is due to collector cells from some photoreceptors and more action potentials per second and inhibitory neurotransmitters from all four sides d). What is a receptive field? Compare the receptive fields of retinal ganglia with those of the photoreceptors. Receptive fields are areas on a retina which cells respond to; cells have either an on response or an off response and the collector cells and brain cells indicate whether the cell receives information Area on a neuron which affects the rate of firing on neurons; center surround receptive field is the areas of the receptive field arranged in a center region that responds one way and a surrounding region responds the opposite way; this effect is called center-surround antagonism Receptive fields on cells can have an on or off response; on an ‘on’ response, cells have more action potentials per second and on ‘off’ responses cells have fewer action potentials 2. Orientation and spatial frequency a). What is contrast? Give an example of a high contrast and low contrast stimulus (draw it!) and also indicate how contrast is measured. Contrast is the difference in brightness/sum in brightness. A high contrast is a larger difference in brightness and a low contrast is a small difference in brightness If a high contrast was 10 and a low contrast was 2 it would be 10-2/10+2 = 8/12 = 0.67 High contrast: low contrast: Gratings are a bunch of lines in any orientation, but they are basically repreated patterns of lines b). What is spatial frequency? How is spatial frequency measured? What is the difference between spatial frequency and orientation? (Hint: Drawing things may help here). Spatial frequency is how close together lines are in an image; measured in cycles/degree visual angle Cycles are measured in a unit of degrees visual angle which is the amount of space in the back of the eye that the image takes up (0-360 degrees) high spatial frequency: ////////// Low spatial frequency: / / / when you squint you lose high spatial frequency information, caused a distorted image to become normal; spatial frequency is measured using gratings, and is measured by the cycles/degree of visual angle (a cycle is a light bar beside a dark bar) this is measured using trigonometry and in terms of degrees of how much space a stimulus takes up in the eye out of 360 degrees difference between spatial frequency and orientation is that spatial frequency deals with vertical lines and the distance between them and orientation is the different types of lines involved, including vertical, horizontal and diagonal c). How is sensitivity to spatial frequency measured in adult humans? (Hint: This involves testing people and asking what they see – It does not involve doing brain surgery or implanting electrodes.) measure the contrast threshold of the individuals (method of adjustment) which is the smallest difference in contrast that you can see (e.g. 4-2/4+2 0.33); this contrast threshold is then converted to contrast sensitivity by dividing 1 by the contrast threshold (1/0.33=3) d). Describe (using an example) how you would measure the strength of an after-effect using contrast sensitivity. You would use the constrastto measure sensitivity to spatial frequency Use the method of limits by manipulating the contrast of the flag and you record if the person can see an after image or not based on the
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