Analysis of the marble burying response: marbles serve to measure digging rather than evoke burying.

Marble burying has been suggested as a model of obsessive-compulsive disorder (OCD) based on the fact that specific serotonin reuptake inhibitors selectively influenced this response. Studying the behaviour we observed that mice also exhibited intense digging activity in the absence of glass marbles. This digging activity showed no habituation either between sessions or within a session, and it could be inhibited by psychotropic drugs in a manner similar to the marble burying response. On the basis of the results a methodological and a theoretical conclusion can be drawn. First, it is concluded that glass marbles themselves do not necessarily provide a fear-provoking stimulus but they serve rather as a convenient means of measuring the intensity of digging activity. Alternative ways of measuring the intensity of digging gave high correlations with marble burying. Second, the behaviour observed is not burying (the marbles) but digging/burrowing (the bedding material), which is elicited by the presence of a "diggable" ground. The putative compulsive nature of this behaviour may add support to the hypothesis that marble burying may be a model of OCD.