Testing strategies in behavioral teratology: III. Microanalysis of behavior.

After having been tested in a standard behavioral teratology test battery, as reported in Part I of this study, a random sample of male offspring of rat dams exposed to methylmercury starting two weeks prior to pairing until weaning were further tested in a wheel-shaped activity monitor and in an operant conditioning paradigm (spatial alternation discrete trial acquisition). In the activity monitor a non-significant increase in locomotor activity, a significant interaction between locomotion and radial activity monitor compartments and a significant increase in stereotyped locomotion was observed. While the preliminary training phases of operant conditioning showed only weak effects due to methylmercury, strong and highly significant increases of the number of unresponded trials and of the response latency, as well as of the session to session variation of these measures were detected during the spatial alternation schedule. A microanalysis of this observation revealed that the number of unresponded trials and its instability were affected in essence due to an increased number of response pauses, and to a lesser degree to an increased length of these pauses. In addition, short attention spans (number of trials responded between two pauses) were about four times more frequent in treated animals. The distributions of both response latency and duration were shifted significantly to longer values. These observations are interpreted as an indication of a reduced attention and behavioral stability, induced by the schedule challenge. This interpretation is compared with the signs of minimal brain dysfunction in school-children.