D

search time and platform crossings are assessed to characterize a subject's search behavior during the probe trial. The quadrant search measure is obtained by conceptually dividing the pool into four equal quadrants and then measuring the amount of time that the subject spends searching in each quadrant. The platform crossing measure is the number of times a subject crosses the exact place where the platform had been located during training. For comparison, the number of times a subject crosses the equivalent location in the other quadrants is determined. Animals that have learned the (now presumed) location of the underwater platform spend significantly more time searching in the trained quadrant than in each of the other three quadrants (Figure 8B). They also cross the place where the platform had been located during training significantly more often than the corresponding place in the other quadrants (Figure 8C). Thus, animals that have learned the location of the platform selectively search in the correct quadrant.

Modern automated monitoring systems also allow the acquisition of additional control data during the probe trials. For example, one can monitor the swimming behavior of each animal during the probe trials to determine general mobility. Thus, one can assess total path length and swim velocity for individual mice (see Figure 9).

It is generally held that data acquired during the probe trial is the best indicator of the animals using a spatially biased search strategy to locate the platform during training and performance. Data acquired during training (i.e., measurement of escape latencies) may be less informative and in some instances can be dissociated from the performance during the probe trail.

To control for motivational factors and perceptual and motor abilities, animals are tested in the visible platform version of the Morris water maze task (Figure 8D). In this variant of the task, the escape platform is clearly indicated by placement of a visible

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