Whilst not strictly a behavioural task, this is a procedure that is widely used to detect neuronal changes that represent familiarity discrimination - the ability to discriminate between novel or familiar stimuli. Novel and familiar images are displayed to a rat or mouse such that each image is viewed by only one eye. This means that the visual information is processed in only one hemisphere of the brain (the opposite one to the eye that saw the image). Thus novel and familiar objects/images can be shown to the same animal at the same time, allowing for very robust measurement of the neuronal decrement seen in familiarity discrimination, without interference from different animals being in different behavioural states, such as different states of arousal, movement or attention. Neuronal activation in each hemisphere is measured by the expression of the immediate early gene, c-fos. Increased c-fos expression correlates with the viewing of individual novel images in the rat perirhinal cortex (
Wan et al 1999) whilst novel
arrangements of multiple familiar images resulted in increased activation of hippocampal neurons (
Wan et al 1999). Such increases in c-fos expression also correlate with spatial behavioural tasks in the hippocampus (
Vann et al 2000) and with novel sounds in the auditory association cortex (
Wan et al 2001).
With recognition neurons, we can tell if a given visual element is something we have seen before and is familiar to us or is something new and so needs to be taken notice of. This can be seen by putting something new into your pet mouse's cage or giving a baby a new toy. They will spend much more time investigating the new (novel) item than something that they are very familiar with. This is essentially a 'spontaneous object recognition task', where the time taken in investigating a novel object is compared to that taken over a familiar object. Under normal conditions, more time will be spent investigating the novel object. This is known as familiarity discrimination, and is a common behaviour exhibited by all mammals. It has been shown to be dependent only on the presence of a functioning perirhinal cortex
(Barker et al 2007)
Familiarity discrimination can be used as a task to probe the relative importance of molecular and network mechanisms in memory formation and retrieval. An animal is allowed to explore two identical objects. The animal is then removed and, after a defined period of time, allowed to once again explore two objects, one of which is identical to the ones it's seen before and the other which is completely novel. As with a pet mouse, the animal will spend more time exploring the novel object than the familiar one. The first period of exploration is the 'acquisition' phase, where a memory of the object is formed, while the second period of exploration is the retrieval phase, where the familiar object is recognised and remembered.
This task probes spatial recognition memory. Two identical familiar objects are placed in the arena and the animal allowed to explore. One of those objects is then moved to a different location and the animal allowed to explore once more. Under normal conditions, the animal will explore the object that has been moved more than the one that is left in place. This is known to be dependent on the hippcampus.
This task does not probe whether an object is novel or familiar but rather whether the context of it's position is novel or familiar. Four different familiar objects are placed in an arena and that animal is alowed to explore them. The position of two objects is then swapped, and the animal allowed to re-explore. In normal circumstances, the animal will spend longer exporing the new positions of the swapped objects than those that have been left alone. This task relies on the interactions of multiple brain regions: perirhinal cortex, hippocampus and medial prefrontal cortex
(Barker et al 2007,
Warburton and Brown, 2010)
