Brightness-discrimination test

FIGURE 67.5 A three-choice sunultaneous brightness discrimination test. The rats (deprived of water) learn that they receive water from the well in the positive cylinder (in this case gray, which is used for half of the rats in each group the other half have a black cylinder). The criterion (five correct choices in succession) is obtained after 2 days of training. Thirteen days after acquisition, the rats are tested for what has been retained of the learning principle (gray or black positive cylinder). 

Several studies have tested visual function after exposure to carbon monoxide. To minimize variability, McFarland et al. (1944) studied brightness discrimination in a small group of well-trained subjects. The subjects reported decreases in visual sensitivity at approximately 4.5% COHb. Another study reported no adverse effects on visual discrimination or depth perception in subjects with 8% or 12% COHb (Ramsey 1973). Luria and McKay (1979) reported no decrement in night vision with COHb of 9%. Davies et al. (1981) reported that at 7% COHb there was no effect on visual function. The studies described above used various visual paradigms, which could account for the differences in results. 

Delayed alternation is another possible training protocol for the T/Y-maze, in which animals are rewarded for choosing any goal box in trial one. They are then returned to the start box and released after an inter-trial interval. In trial two, they have to enter the arm not visited in trial one (non-match) and are rewarded. Typically, animals acquire a criterion of 80% correct responses after a short training period. When tested in the presence of A THC, there was a significant drop in performance coupled with a reduction in monoamine turnover in their prefrontal cortex (Jentsch et al. 1997). Animals treated with a similar dose (5 mg/kg) A THC, however, were not impaired in brightness discrimination (Jentsch et al. 1996) or visual discrimination of forms procedures (Mishima et al. 2001) administered in the same apparatus. 

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