The degree of drive and arousal during feelings of human curiosity are proportional to the degree of conceptual conflict between the internal representations and external evidence. As such, curiosity (or the drive to pursue information, spatial or otherwise) emerges from evidence demonstrating incomplete understanding of something that is already partially understood. Curiosity is therefore facilitated by prior understanding.
Since spatial exploration in rodents is also likely motivated by the inherent rewarding nature of novelty and a desire to complete an understanding of the environment, the motivation for rodents to explore novel, safe space is conceptually similar to human curiosity and may be governed by homologous molecular and cellular processes. Therefore, rodents may exert a higher drive to explore a novel change to an already familiar environment, when compared to their drive to explore an environment that is entirely novel.
The new frontier task (NFT) capitalises on these concepts by providing spatial novelty to the already familiar homecage and by allowing mice to explore four platforms, or ‘frontiers’, placed along the cardinal axes. These frontiers are made accessible to the mice with small ladders that protrude partly into the homecage (see Fig. 1A). In the NFT, the homecage provides immediate partial understanding of the paradigm's spatial aspects.
To distinguish between different motivations to explore, the homecage during the NFT can be either dimly or brightly illuminated. When the homecage is dimly illuminated, exploration is presumed to be novelty driven and may serve as a proxy for human curiosity. In contrast, when the homecage is brightly illuminated, performance in the NFT is presumed to assess fear-driven exploration, since bright lighting is a well-accepted fearful stimulus for small nocturnal prey, like the mouse. In both cases, the frontiers remain dimly lit. Therefore, experimental manipulations that selectively affect the degree of exploration in the dim or bright version of the task may be proposed to selectively affect novelty-driven or fear-driven exploration, respectively. Exploration of the frontiers that could be driven by other factors, such as hunger, thirst, the search for a mate, or other socio-sexual factors, is minimised by ensuring experimental subjects have free access to food and water prior to the task and, during the task, do not come in contact with scents of other mice.
The NFT has been used to examine exploratory drive in group-housed animals following genetic manipulation. Adult dentate gyrus-restricted overexpression or global point mutation of neuronal calcium sensor 1 (Ncs1) both demonstrate phenotypic differences exclusively when the homecage of the NFT is illuminated with dim light. Whereas Ncs1 overexpression leads to a dramatic increase in exploration when the homecage is dimly illuminated, a point mutation that destabilises the NCS-1 protein causes a selective decrease in exploration. This work, along with support from others, is a prime basis for the notion that NCS-1 plays a key role in the drive to explore novel, safe space.
The NFT has also been used to demonstrate selectivity within the amotivation pathophenotype associated with mice following psychosocial stress. Like mice with destabilised NCS-1, mice subjected to chronic social defeat exhibit a profound impairment in exploration of the frontiers when the homecage is dimly illuminated. This finding highlights the particular susceptibility for novelty-driven, as opposed to fear-driven, exploration to disruption following psychosocial stress.
Since the chronic social defeat paradigm necessitates single housing and control animals exhibited a similar level of exploration in the task compared with other studies, it is clear that the NFT is suitable for use with mice across the housing population sizes typically employed in behavioural research. However, the degree to which the cage population affects exploration in the NFT has not been examined directly, and it is unclear how closely balanced separate groups need to be in order to avoid population size emerging as a potential confound.