To determine whether head-dipping could be validated as a measure of exploration a modified hole-board was developed with four holes in the floor, under which novel objects could be placed. Two criteria for considering head-dipping as a measure of exploration were proposed: firstly that it should reflect novel aspects of the environment; secondly, that exposure to the hole-board should result in information storage. That head-dipping reflected novelty was indicated by the longer duration of head-dips on initial exposure if objects were present, and also on a second exposure when objects were introduced for the first time. Information storage was indicated by habituation on re-exposure to the hole-board. A significant positive correlation between head-dipping in the "four" and "sixteen" hole-boards was obtained for rats, but not for mice. This provided some indirect evidence that rat head-dipping in the "sixteen hole-board" also reflects exploration. (+)Amphetamine and alcohol were tested in the modified hole-board, and (+)amphetamine decreased and alcohol increased the frequency and duration of head-dips.

Two aspects of the reliability of the hole-board apparatus were investigated-the similarity between scores of different samples of the same population on their first exposure to the apparatus, and the test-retest reliability. Rats and mice were given a 5-min exposure to the hole-board and then retested for 5 min after 1, 2 or 8 days. Male rats and mice showed good initial exposure reliability, whereas the female mouse groups differed significantly. All animals showed a positive test-retest correlation (range 0.31-0.78), but a homogeneous group (e.g. all animals habituating) produced higher correlations (range 0.60-0.99). Comparison of scores on the two 5-min exposures showed that not all groups showed significant habituation, but the animals exposed to the hole-board for two 10-min periods showed both significant habituation and test-retest reliability.