Device and Experiment

........In the last article, I just said that I got my first TvC function. But that was a pilot study, and I didn't control some environment factors like the environment luminance. I didn't calibrate the monitor before the pilot experiment. So after confirming the program worked, I began to do the calibration stuff. Unfortunately, it took me about a whole week. At the beginning, the measurement device, the lightmous, didn't work. After changing different monitor and PC, finally I found the offender, the USB keyboard. Because the driver of the lightmouse is the same with the USB mouse "and " USB keyboard, so if we plug the lightmouse and any one of the USB mouse or keyboard simultaneously, the Window system will get confused. Therefore, I replaced the new USB keyboard to an old PS2 keyboard.

........But the tragedy not end yet, the gamma table that I got was a little bit odd. The last 18 level of the gamma function was flat. So again, I change the monitors and computers. This time, I figured out the suspect was the video card then got a normal gamma function. And because I needed to use another video card, I had to use another computer. I installed the Psychophysics Toolbox of version 3.0 in the new one. Besides my boss, Dr. Chen, I am the only one voluntary pioneer to discover that. There are some routines changed in the version 3.0. The order of the arguments that should be passed into the Screen function, for example, is different:


Screen(win,'PutImage',...) become Screen('PutImage', win,...)


Therefore, all the old exp program are needed to be modified. And the biggest one is that, version 3.0 in Window system "CAN NOT" change the color lookup table!! Unless the lookup table we passed is a monotonically increasing lookup table. So if I want to present the colorful stimulus, the matrix of the image should be an n-by-m-by-3 one that each the third dimension indicates the intensity of different guns respectively. And also, we can't play Clut animation any longer...

........Anyway, yesterday I overcame all of the obstacles and everything finally went just right. Should I love this part in the research? It's quite hard to answer this question. Everyday I left the lab without figuring out the problem with frustrated mood, that is really bad. But yesterday I left the lab with hope and confidence, however, that is great. And a loss may turn out to be a gain.
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My first TvC function

........Last week, I got my first TvC function by myself. On the original purpose, I was testing the experiment program and debugging. I used Quest procedure to find subject's threshold in that experiment and this was quite new for me. I am not familiar with the Quest procedure actually, but in general, the Quest is based on the Bayesain probability model. It will use the accuracy subject makes in certain difficulty level to predict the correct rate in more difficult question. So the difficulty of the next question is depended on your answer is correct or not in current question. In conventional, the threshold of a given pedestal stimulus is defined by the intensity which subject has 75% probability to see the test stimulus. According to the Quest procedure, it takes about 32~40 trials to determine that intensity, said just-noticeable-difference (jnd). As long as changing different pedestal stimulus, we can get different threshold for different pedestal stimulus, then we can draw a function that is changed along the intensity of the pedestal stimulus. This function we call the TvC function, means threshold versus pedestal contrast function. My TvC fuction looks like that:

we call the function with such kinds of shape "dipper function", just because its shape likes a dipper. If the TvC function has this shape, it implies that the our visual system responses to the stimulus in the way that just like the neuron system does. As the intensity increasing, the neuron activation will first be increased very fast, then the increment speed will slow down, and finally the activation will meet to the saturated level. Therefore, When the intensity is became stronger and stronger, the threshold will be decreased at the beginning, then gradually increased. This kinds of the response pattern results in the dipper-shaped function as the figure shows above.

........When this figure popped out, I was so exciting. After several hours programming and repeating hundreds of boring trials, finally I got the function with dipper shape. This experience was so impressive and awesome. The feeling quite likes that I got my first fMRI activation image, even stronger than that. Honestly, I really love this part!
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