Doing something for the sheer joy and playfulness of it – just because it is fun and feels good in and of itself – is a key impetus behind many of our creative and life projects.

But what, really, is this elusively powerful driver of our playful activity?  How does the urge to play arise? What’s happening differently in our minds-brains-bodies when our urge to play is burning bright and strong, compared to when it’s gone, or has diminished to a mere dull flicker?

How might we study play and begin to piece together parts of these deep puzzles?  Although there are many places we could look, a team of ingenious behavioral neuroscience researchers recently rigged together a new way to let us peer into brains and bodies at play, of interacting creatures small, and large.  But before we take a closer look at their animal study, and their findings, we first need to take a small detour, into the surprisingly complex playworld of hide-and-seek.

Let’s play hide-and-seek

Playing hide-and-seek is complicated.  To sometimes assume the role of the one who is hiding, but at other times to take on the role of the seeker, draws on a surprisingly large and complex array of cognitive, social, motivational, and physical skills.

For example, if a child is playing the role of the hider she must remain quiet and hidden even while the seeker closely approaches her or passes nearby her hiding place, inhibiting any urges to move, burst into giggles, or otherwise reveal her hidden presence.  When playing the role of the seeker, the child must wait and fully and loudly count out the required time, keeping her eyes closed or her back turned, and not peeking while her playmates steathily find and slide into their hiding places.  Other flexible perspective-taking abilities are also needed.  For example, the hiding child needs to know that just because she can see the seeker, it does not necessarily mean that the seeker can also see her.

Some of this complexity is revealed by the age at which young children first begin to fully succeed at the game.  A laboratory-based study conducted by researchers from Canada and Italy found that only a few 3-year-olds could successfully play hide-and-seek, but children who were a little older, including most 4-year-olds and nearly all 5-year-olds, were adept at the game.  The mistakes the youngest children made were often ones of not alternating the role of hider and seeker (for example, telling the experimenter to hide, but then also themselves hiding) or not really “hiding” (as in telling the experimenter where he or she was going to hide, not trying to hide from view, or not remaining physically hidden, or not remaining quiet).  The children’s skill at playing hide-and-seek was strongly positively correlated with another ability related to understanding another person’s perspective and knowledge – that of keeping a secret.

Given this complexity – and the clear challenges the game poses to young human children – could other creatures really learn to play hide-and-seek?  And, if they could, might this provide some insights into the deep motivational and rewarding origins of play?

Small creatures with big play urges

Behavioral neuroscience researchers have known for some time that young adolescent male laboratory rats are eager and enthusiastic playmates.  They jump into lots of rough-and-tumble play with their adolescent peers, and joyfully engage in all sorts of hand-and-finger chasing and tickling exploits with their human handlers.  But what are the neural underpinnings of the drive to play in these small young creatures?  And could such play urges extend to more complex and rule-based games requiring them to take on different roles at different times, such as those in hide-and-seek?

To begin to examine the neural correlates of these small furry creatures’ big motivation to play researchers at the Bernstein Center for Computational Neuroscience and Humboldt University in Berlin devised a novel two-player rat-and-human hide-and-seek game.

Placed around a large nearly 100-square-foot (5 x 6m²) dimly lit lab room were several smaller hiding places (two transparent and two opaque boxes), three large human-size “cardboard” hiding places, and a shoebox-sized “start box” with a remote cable-controlled opening mechanism.

At three weeks of age, each animal individually began a five-to-ten-day familiarization with the room and with the experimenter – starting with lots of gentle handling, touching, and carrying, and later more vigorous and energetic tickling and hand games.  Next, the rats were successively first trained to either hide or to seek.  Crucially, throughout the training the animals only ever received “social play rewards” – touching, and hand games and playful interactions with the experimenter. No food or other tangible rewards were offered.

In “seek” trials, the experimenter closed the lid of the start box, and hid at one of the larger hiding places.  In “hide” trials, the experimenter left the start box open, and the experimenter assumed a very still posture next to the start box, and began counting out loud. When the experimenter hid, the sounds made while she moved to her chosen hiding place were masked with white noise. There were also multiple decoy “cables” to each of the possible hiding places so the animals could not simply follow the cable that provided the hidden experimenter remote control for opening of the start box.

Of the six animals initially trained by one experimenter, all six learned to seek, and five learned both to hide and to successfully switch between the hiding and seeking roles.  Of four additional animals, trained by a different experimenter in the same setting, all four learned to seek, hide, and also to switch roles.

But, you may ask:  What did the animals actually learn?  Were they really playing the game?  Did the animals actually enjoy the game?  Were they actually playing just for the fun and joy of it?

Play, laughing at play, and let’s play more please!

Many aspects of the animals’ activity suggested that they had really learned something about “hiding and seeking” and had developed some appropriate game-playing strategies.  For example, when the experimenter hid in random (non-predictable) locations from one trial to the next, the rats took longer to find the experimenter than when the experimenter consistently hid in one location across a series of five trials.

In such consistent trials, the animals searched for the experimenter increasingly quickly and directly – making a beeline toward where she was hidden with scarcely a pause – showing that they remembered where the experimenter had hidden on the previous several trials.  Also, when they were the hiders (but not as seekers), the animals showed a clear and significant preference for the opaque and cardboard boxes over the transparent “see-through” boxes.

In the wild, rats are most active during the night and so most of their play will occur in darkness.  Rather than a visual cue to signal that they want to play, such as a puppy’s “play bow” or a monkey’s “open mouth,” adolescent rats of the type the researchers studied (the Long-Evans hooded strain) give a variety of different vocal chirps or calls. These calls are ultrasonic vocalizations at a frequency of close to 50 kHz, and are emitted during social play with peers, and during other positive affective states, such as when they are being “tickled” by human handlers.

Such “calls to play” or play signals are especially frequent in juvenile or adolescent rats. The chirping calls, and their specific timing – such as anticipatory calls given just before launching a playful nape attack or chase – seem to help maintain a playful mood or motivation, and to promote cooperative play.

A close look at the vocalizations of the rats during the researchers’ hide-and-seek sessions showed that, for both seek and hide trials, the animals’ calls (all of which were inaudible to the experimenter but visible on the Audacity recordings) sharply increased at those times when the rats were enthusiastically darting away from the start box.  There were also many such calls during the tickling and finger-chase-play interactions with the experimenter but fewer when the animals were quietly choosing where to “hide” and also during their hiding time.

The timing and patterns of the animals’ chirping calls suggested that the animals were indeed enjoying the hide-and-seek game.

And – like young toddlers who often exclaim “do it again, do it again!” or “more, more!” when they love the playful motions or sounds that their parents or adults are making – so the adolescent rats often seemed to want to prolong the hiding portion of the game, darting away from the experimenter to a new “hiding place” even when they’d clearly been found out in their hiding place. These and other behavioral indicators, such as their quick and lively search, and springy “joy jumps,” all converged in an impression that this was all good fun.

Motivational and reward systems in the brain

What, then, was happening in the brains of these small creatures as they enthusiastically played this complex socially-interactive rule-based game?  To find out, the researchers focused their attention on a region at the front of the brain – the medial prefrontal cortex – known to be involved in social play and reward-based play motivation in rats.  After the animals had learned the hide-and-seek game, the experimenters implanted electrodes (tetrodes) in the medial prefrontal cortex of five of the anesthetized animals.  Then, after they’d recovered from the surgery and were again happily playing hide-and-seek, the researchers tracked the patterns and changes in the firing activity of individual neurons as the rats now took on the role of the seeker, and then that of the hider.

The electrode recordings revealed that the patterns of brain cell firing differed markedly depending on specific timepoints and the animals’ particular role in the game.  Firing of neurons increased strongly in nearly 30% of the 177 neurons the researchers were able to record from at the timepoint when the start box lid was closed – the environmental signal that the rat was, on the next trial, to be in the “seeker” role.

Analyses of the patterns of firing showed that some clusters of neurons were mostly active during the seeking phase of the game.  Other groups of neurons were most active during hiding.  Still other clusters of neurons were especially active during the brief periods of intense experimenter-rat interaction (touching and hand games) that ended each seeking or hiding trial.

The play-to-play hypothesis

Despite the central role that play and play-like activities have in our own lives and those of other animals, probing and fully charting out the complex social, cognitive, motivational, and neurobiological bases of such activitites in many animals has been challenging.  There has been extensive research on some sorts of play in a few species – for example, play fighting has been much studied in adolescent rats, but many other types of play, such as object play, have been less often studied.

The initial findings reported by the Berlin-based researchers – using a creatively ingenious playful format that gives small creatures the opportunity to themselves make choices about where and how they will hide, or where and how they will seek out a hidden playmate – open new opportunities and challenges for researchers of play.  Their findings suggest that by using experimental procedures that give other animals more room for choice, and more room for play, we might be able to learn much more about what “drives play” and the motivation of playing just to play.  Hide-and-seek anyone?

To think about

• The particular strain of rats used in the hide-and-seek study (Long-Evans) has, in other research, been characterized as typically “bold” or exploratory rather than shy and reticent (e.g., they quickly raise themselves up on their hind legs to look about them, and move more quickly into the center of an exposed field).  Would comparatively “shy” rats also learn the hide-and-seek game and learn to quickly switch their roles between hiding and seeking?
• In this study, adolescent playful rats learned to play hide-and-seek with the human experimenter. Would older rats also successfully learn the two different roles required during hiding and seeking? Could they enjoy the game as much as the younger animals?  Or could the game be changed in ways that would increase their playfulness and playful enjoyment?
• We often learn from watching others.  How much could younger or older animals learn by observing other animals in the game?
• Why do we so often focus on extrinsic rewards in thinking about what moves us, rather than also intrinsic rewards, such as our desire for play?
• A recent report on “the power of play” in the clinical journal Pediatrics affirmed, “Play is not frivolous; it is brain building” (p. 5).  Much of the evidence for substantial brain changes related to the opportunities for play has been found using juvenile rats.  For example, rats that were denied the opportunity to play as pups (kept in sparse cages without any toys) were less adept problem-solvers later and showed markedly impaired (immature) medial prefrontal cortex development.  Why do we tend to downplay the many social and health-promoting roles of play, not only for children but also for youth and adults of all ages?  What playful counter-moves can we let loose, hoist, or heave against such heavy anti-play sentiment?

References

Bell, H. C., McCaffrey, D. R., Forgie, M. L., Kolb, B., & Pellis, S. M. (2009). The role of the medial prefrontal cortex in the play fighting of rats.  Behavioral Neuroscience, 123, 1158–1168.

Panksepp, J., & Burgdorf, J. (2003). “Laughing” rats and the evolutionary antecedents of human joy?  Physiology & Behavior, 79, 533–547.

Pellis, S. M., Pellis, V. C., Pelletier, A., & Leca J-B. (2019). Is play a behavior system, and, if so, what kind?  Behavioural Processes, 160, 1–9.

Peskin, J., & Ardino, V. (2003). Representing the mental world in children’s social behavior: Playing hide-and-seek and keeping a secret.  Social Development, 12, 496–512.

Reinhold, A. S., Sanguinetti-Scheck, J. I., Hartmann, K., & Brecht, M. (2019). Behavioral and neural correlates of hide-and-seek in rats. Science, 365(Sept. 13), 1180–1183.

Weiss, A., & Neuringer, A. (2012).  Reinforced variability enhances object exploration in shy and bold rats.  Physiology & Behavior, 107, 451–457.

Yogman, M., Garner, A., Hutchinson, J., Hirsh-Pasek, K., & Golinkoff, R. M. (2018).  The power of play: A pediatric role in enhancing development in young children. Pediatrics, 142, 1–16.