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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