A computer controlled analysis of freezing behaviour

The conditioned freezing response in rats has been much used both by psycho logists and neuroscientists to investigate the behavioural effects of brain lesions and of changes in motivational state. The primary advantage of the freezing response is that it can be used without motivational manipulation s such as food or water deprivation. Previously, freezing has been measured by a human observer either from video recordings or during the test sessio ns themselves. But these methods of data collection have potential disadvan tages. In the present paper we describe a new, computer controlled, automat ed procedure for assessing conditioned freezing. Each conditioning chamber contains a mini-video camera. Behaviour is analysed on-line by means of a p rogramme which compares every two adjacent seconds of video tape to generat e a screen representing the percentage difference between them. A differenc e of < 0.05% (50 pixels) is classified as a freezing response. Experiments are described in which we measure conditioned freezing and its development over time, in response to contextual cues and to a discrete tone which had been paired with foot shock. We demonstrate our apparatus and methods of da ta analysis to be sensitive to: number of tone-shock pairings, rat strain a nd tone pre-exposure. (C) 1998 Elsevier Science B.V. All rights reserved.