Welcome back to the Maze Engineers blog! Today we’re going to talk about how basic object recognition is tested in mice/rats. Object recognition is a complex process that requires multiple brain regions. In monkeys and humans, research often focuses on visual object recognition and how the visual system segregates and groups different objects, or how specialized regions work to process information regarding faces. Other research uses the novel object recognition test as a means to study learning and memory and often employs match-to-sample (or non-match-to-sample) tasks. In these tasks, a person or animal is asked to indicate whether an object they are looking at matches (or does not match) one that they saw earlier. Training animals (usually mice) on these complicated tasks can take quite a bit of time and usually requires food or water deprivation to motivate the animals.
An alternative approach often used when studying object recognition in mice is the novel object recognition test which takes advantage of the natural tendency of mice to seek out and investigate new things. The procedure is fairly simple.
- Put a mouse in a cage for 10 min with two identical objects. This is the sample object.
- Put the mouse back in its home cage for an hour.
- Put it back in the cage after swapping one sample object for a new (novel) object.
- Determine how much time the animal spends interacting with the new object vs. the sample object
(Fig. 1). A video record of the test is used to quantify interaction. This can be the number of directed contacts (purposefully touching the object with its mouth, nose, or paws) or times within the object’s area (nose pointing to object and within 2 cm).
Normally, mice will spend more time interacting with the novel object, indicating that they can detect differences between the two objects, that they stored the features of the sample object in memory, and that they could retrieve that memory in the future.
The primary advantages of the novel object recognition test are:
- No training required
- Can be done in one session
- Does not require aversive stimuli or food/water restriction
- Is easy to do, not expensive, and can be replicated easily in any lab
Thus, this task can be considered a non-match-to-sample task that is a quick and powerful way to test object recognition after lesions, drug administration, or natural aging.
For the novel object recognition test to work properly and give meaningful results, several things need to be considered. First, several variables need to be counterbalanced across mice, including the identities of the objects (sample or novel) and their locations (left or right). Second, objects should be comparable in complexity because this influences the amount interaction (high complexity = more interaction). Third, care must be taken to avoid objects that move (like marbles) or boxes that are made out of material that encourages chewing.
If the number of interactions is low, it could be because:
- Animals are stressed. Pre-exposure to the test environment before testing is important.
- Animals are distracted. Reducing the size of the test box and using a box with opaque walls might help.
- Object is too small or not interesting enough. Or too big and scary. Use objects with multiple sensory features that are a moderate size.