ISALET OTERO PROFESSOR GROSE FIFER Lab HW 2 due 2 26 14 Homework 2 due 2 26 14 5 Contrast sensitivity Use method of adjustment From the cd rom Chapter 4 The visual cortex and beyond You should sit 22 inches 57cm from your computer and do the experiment with the lights on Do the practice first Make sure that you check the circle for method of adjustment You will see a striped pattern grating which will fade out at some point in the rectangle You will move the cursor to the height where it is difficult to distinguish the pattern of stripes i e the stripes have disappeared Then press Set contrast to record your results and to present the next grating Now repeat for the experiment REMEMBER TO USE METHOD OF ADJUSTMENT Print out your results it should be a graph What spatial frequency stripe size are you able to see at the lowest contrast level 8 CPD was the spatial frequency I was able to see at the lowest contrast level which was 3 59 8 3 59 What do you notice about your sensitivity to contrast is it greater for narrow stripes or wide stripes My sensitivity to contrast is greater for narrow stripes You used the method of adjustment what is the advantage of this method Disadvantage An advantage of this type of method is that it is easy and fast to conduct On the other hand one major disadvantage in this technique is that it produces unreliable results that are not measurable compared to that of the method of limits What is the classic pattern for CSF When it peaks in the middle around 3 4 c degree and decreases at higher and lower spatial frequencies In a graph the pattern looks as the following there is the contrast sensitivity function which is represented by the solid black line of a CSF graph and several of its principal channels which are represented by the dashed lines These networks each of which is sensitive to a narrow range of frequencies all together create CFS Why might your CSF not look like the classic pattern One common reason why a participants CSF might not look like the classic pattern is doing the task viewing the stimuli at an erroneous distance Where would you expect maximum sensitivity Around the middle between 3 4 c degree Selective Adaptation No need to print anything from the screen for these two experiments 6 Orientation after effect What do you experience Is this the expected result What is the expected result and why does it occur Think about physiology and psychophysics What I experienced during this task was that after looking at the fixed bar in the middle of the two moving bars once these stop the bars seemed to be tilted in the opposite direction to what they were originally were but also as the these dissipated the center of the lines seemed to be distorted sort of like wavy right in the middle of each line Yes with an exception of the part were as I kept on looking after the lines appeared to be in the opposite direction as they adjusted the middle of it appeared wavy The exact expected result in this type of task is that after the adapting period finishes the gratings will be substituted by two perpendicular gratings where each vertical grating should look to be tilted slightly in the direction opposite to that in which it was original tilted This can be explained by orientation sensitivity neurons during this task when we stare at the middle neurons that respond to the left oriented lines on top become fatigued and the ones that are still firing are the neurons that respond to right oriented lines thus that is why once we stop staring at the middle and back at the vertical lines for a brief moment the lines in the top that were tilted to the left now appear to be tilted to the right and the bottom lines that were originally tilted to the right now appear to be tilted to the left 7 Size After effect What do you experience Is this the expected result What is the expected result and why does it occur Think about physiology and psychophysics What I experienced was that the top bars LSF appeared narrower and the bottom bars HSF appeared wider Yes this is the expected result The expected result is that after adapting the stimulus that is being tested appears to be different than it normally would This occurs as counter effect mechanism to fatigue When the eye is overworked by the stimulus it weakens and even though it recovers within a few minutes however throughout that time lapse the visual system is still not working at its full potential and even other parts are working harder than what they normally would Similar to selective adaption where if neurons fire for long enough they become fatigued thus they adapt This adaptation causes two physiological effects 1 The neurons firing rate decreases and 2 The neurons fires less when that stimulus is immediately presented again In other words when we look at the medium grating and LSF neurons have adapted become fatigued on top our perception becomes bias towards slightly higher SFs because LSF neurons are tired Therefor only MSF and HSF neurons are working when LSF are adapting On the other hand when we look at the medium grating and HSF neurons have adapted becomes fatigued our perception becomes bias towards slightly lower SFs because HSF neurons are tired Therefore only MSF and LSF neurons are working when HSF are adapting