Contrast sensitivity was measured as a metric of form visual ability in a two-alternative forced-choice visual grating detection task (2AFC-VGDT) combined with a staircase method
43,51 that quantifies contrast sensitivity at the spatial frequency (SF) parameter without model fitting. We adopted an efficient training protocol for rapid learning of the 2AFC-VGDT.
44 Briefly, both the RCS and control rats were trained in three stages (see
Figs. 1A,
1B). In the first stage, the rats (3 weeks old) learned to obtain the fluid delivered from the spout lever by pulling up the choice lever. In the second stage, the rats (4–5 weeks old) learned the basic procedure of the 2AFC-VGDT, that is, how to initiate a trial and obtain fluid using the bright patch. This provides a highly salient and effective method for drawing an animal's passive attention. In the third stage (the grating patch detection training stage), the rats (6–11 weeks old) learned that the fluid supply was associated with the grating patch. The viewing distance of the stimulus center from the central lever was 13 cm. The parameters of the horizontal grating (contrast = 100%; spatial frequency = 0.1 cycles/degree; temporal frequency = 0 cycles/sec; diameter = 70 degrees) were determined according to our previous study.
43 The stimulus presentation continued until the rat pulled up the corresponding (right/left) choice lever. To measure the contrast sensitivity, the grating location was randomly changed from trial to trial. The stimulus contrast was initially set at 100%, and the level of contrast varied across trials according to the staircase method (1-up/1-down). Upon a correct choice, the rats received a reward of 2 to 3 µL of water, and the stimulus contrast was decreased from the current level in the next trial (
Fig. 1C). Upon an incorrect choice, the rats received an audible sound (500 hertz [Hz]) only, and the stimulus contrast was increased in the next trial. The stimulus contrast decreased or increased at 1%, 4%, and 10% contrast steps in the low- (1–10%), middle- (10%–50%), and high-contrast ranges (50%–100%), respectively. Each session lasted until the correct performance of the most recent 10 trials fell below 60% or the rats detected 1% of the stimulus contrast. The threshold contrast (C
threshold) was defined as the final stimulus contrast that the rats could choose correctly. The contrast sensitivity was calculated using the following equation: contrast sensitivity = 100/C
threshold. Measurements began after individual RCS rats achieved performance above the criteria in the third stage (>80%, see the blue dashed line in
Fig. 1B) and continued until they were 11 weeks old. For control rats, contrast sensitivity measurements were conducted at 9 to 12 weeks of age.