The open field is one of the simplest rodent behavioral tasks, used for measuring locomotor activity, anxiety, and response to a novel environment. The subject is placed into a walled arena and allowed to explore freely. General measures of activity (e.g. duration active vs. resting), as well as specific positional measures (e.g., time spent in center vs. periphery), are quantified by the investigator. The anxiety component of the test is based on rodents' aversion to open spaces and their tendency to be thigmotaxic, or to stay near the walls of an enclosure. Subjects who spent more time in the periphery of the open field are perceived to be more anxious.
Historically, quantifying behavior in the open field test was a manual process. A grid was drawn on the floor of the arena and investigators would count the number of line crosses and the entries into the center. This painstaking process was repeated for each individual subject, making it very time consuming to analyze large batches of animals.
Fortunately, today's technology makes it much easier to efficiently analyze open field behavior in a high-throughput fashion, taking advantage of automated activity tracking tools. Video-based activity tracking and photobeam-based activity tracking are the two main tools available. So what are the differences and how do you choose?
Video-based open field testing relies on a video camera mounted above the arena. Each trial is recorded and subsequently analyzed by computer-based tracking. Automatically generated variables include: distance traveled, ambulatory time, resting time, and average velocity. The investigator can optionally set up analysis zones (e.g., center vs. periphery) to segment these variables by zone occupancy and generate additional variables, such as number of zone entries and latency to enter each zone. The data can be further broken down by time bins to analyze changes in behavior over the duration of the test, for example minute by minute. Interesting visualizations can be generated, like the animal's tracked path.
For high-throughput testing of multiple subjects in parallel, segmented open fields enable simultaneous testing under a single video camera.
One of the main advantages of video-based tracking is that video data is high resolution, meaning the resulting position data is very precise and you can always go back to the raw videos for additional information if needed. For example, if you find an interesting result from the automatically generated data and you want to go back to see what else was happening at that moment, you can always review the original video. You might be interested in self-grooming behavior, sniffing, fecal boli, or other variables.
- High spatial resolution
Photobeam-based open field testing requires a chamber outfitted with infrared beam sensors. As a subject moves through the arena it interrupts a set of photobeams and its X,Y position is tracked. Photobeam systems automatically generate a similar set of variables to video-tracking systems and can often count rearing and episodes of stereotypy as well. These systems also make it simple to set up zone-based tracking, analyze data in time bins, and generate visualizations.
An additional benefit of photobeam systems is that they are typically higher-throughput. For example, a single system from San Diego Instruments can be configured to run 32 open fields simultaneously.
- Rearing and stereotypy
Whether you decide to choose the high resolution of video tracking or the high-throughput capabilities of photobeam systems, rest assured you'll be generating accurate and reliable open field data with these two technologies. Don't hesitate to contact us with any questions about your specific use case. As experts in all things behavior, we're here to help!