### abstract ###
When introduced into a novel environment, mammals establish in it a preferred place marked by the highest number of visits and highest cumulative time spent in it.
Examination of exploratory behavior in reference to this home base highlights important features of its organization.
It might therefore be fruitful to search for other types of marked places in mouse exploratory behavior and examine their influence on overall behavior.
Examination of path curvatures of mice exploring a large empty arena revealed the presence of circumscribed locales marked by the performance of tortuous paths full of twists and turns.
We term these places knots, and the behavior performed in them knot-scribbling.
There is typically no more than one knot per session; it has distinct boundaries and it is maintained both within and across sessions.
Knots are mostly situated in the place of introduction into the arena, here away from walls.
Knots are not characterized by the features of a home base, except for a high speed during inbound and a low speed during outbound paths.
The establishment of knots is enhanced by injecting the mouse with saline and placing it in an exposed portion of the arena, suggesting that stress and the arousal associated with it consolidate a long-term contingency between a particular locale and knot-scribbling.
In an environment devoid of proximal cues mice mark a locale associated with arousal by twisting and turning in it.
This creates a self-generated, often centrally located landmark.
The tortuosity of the path traced during the behavior implies almost concurrent multiple views of the environment.
Knot-scribbling could therefore function as a way to obtain an overview of the entire environment, allowing re-calibration of the mouse's locale map and compass directions.
The rich vestibular input generated by scribbling could improve the interpretation of the visual scene.
### introduction ###
When introduced into a novel environment rats establish in it a preferred place characterized by the highest frequency of visits, by the highest cumulative dwell time, by an upper bound on the number of stops per roundtrip performed from it, by low outbound trajectory speed and high inbound trajectory speed, with the speed relationship reversed in later stages of the session CITATION CITATION.
The existence of highly organized behavior across the whole arena in reference to this so-called home base CITATION illustrates the influence a preferred place might have on the overall organization of exploratory behavior and prompts the search for other types of preferred places that may have a similar influence on the organization of exploratory behavior.
In contrast to the situation with rats, there is an ambiguity regarding the establishment of home bases in mice: while some studies mention home bases, their establishment is mostly of a low occurrence in the forced exploration setup.
Even though mice readily establish home bases near physical objects CITATION CITATION and near nesting material CITATION, they are reported to fail to establish distinct home bases during forced exploration of a relatively featureless environment CITATION CITATION .
When mice used as a control group in another study were injected with saline and placed in the exposed portion of a large open field arena, they established in it preferred places, typically not more than one per session, which they visited sporadically, tracing in them a tortuous path full of twists, turns, and bends that looked like a knot.
We termed these places knots, and the behavior performed in them - knot-scribbling.
This knot phenomenon, which appeared in its full blown form in the saline-injected mice, was subsequently uncovered, albeit in a less striking form, also in intact mice.
In the present study we describe the full-blown knot phenomenon in saline-injected mice and only then verify the existence of knots in intact mice.
To uncover knots we utilize a basic feature of the locomotor path its curvature.
Depending on the size of the window that is used for measuring path curvature this measure discloses various features of path texture, from overall large-scale curvature to fine-grained tortuosity CITATION.
In the present study we use a window size that makes the fine-grained tortuosity of the path as conspicuous as possible.
Curvature has been measured previously only as a cumulative measure of paths without coupling a particular degree of curvature to particular topographical locations CITATION CITATION.
To uncover knots we first calculate fine-grained path curvature for each data point on the path traced by the mouse.
Then we partition the arena into small 5 5cm unit areas and summate, for each unit area, the curvature of the path included within that unit area.
The magnitude of path curvature per unit area is then color coded in a visualization of the paths, thus highlighting the unit areas that are marked by the highest curvature.
Having uncovered a high curvature locale we then design and use algorithms that define its boundaries and quantify some of its features.
Finally, we ask whether the knots we discover are also endowed with the classical home base features, and consider their function.
