THE DYNAMICS OF LONG-TERM EXPLORATION IN THE RAT - PART I - A PHASE-PLANE ANALYSIS OF THE RELATIONSHIP BETWEEN LOCATION AND VELOCITY

Rat exploratory behavior consists of regular excursions into the envir onment from a preferred place termed a home base. A phase plane repres entation of excursions reveals a geometrical pattern that changes duri ng exploration in both shape and size. We first show that with time an d repeated exposures to the same large environment there is a gradual increase in the length of excursions; each rat has its own characteris tic length of excursions; but all rats share a similar rate of excursi on growth. As in our experimental setup the rats perform increasingly longer paths from one location, while locomoting back and forth along the walls of the arena, exposure is more extensive at the proximal par t of the route, and less at the distal part. We consequently show that the rat's velocity pattern changes concurrently with the increase in excursion length, and in correlation with the level of exposure (famil iarity) to places. The primitive velocity pattern consists of slow pro gression while moving away from base and fast progression while return ing to it. During exposure the asymmetry in velocity is inverted. The inversion spreads across successive excursions from the home base outw ards. The rate of spread of this inversion is higher than the rate of increase in excursion length, and is similar across rats. Because it s preads more rapidly than the increase in excursion length, the global shape of the excursion trajectory changes. The dynamics of excursion s hape share similar properties with the dynamics of excursion length. B oth might reflect the same intrinsic constraints on the amount of nove lty that a rat can handle per excursion.