Wednesday, November 14, 2012

Social Spatial Working Memory in Rats

    This week's blog post is going to focus on an interesting research experiment conducted by Mike Brown (not the ex-Lakers coach) who studied social spatial working memory in rats.  In laymen's term, he was studying to see if rats would remember the locations that other rats had visited and foraged, so that they would not revisit those locations after food sources had been exhausted.  As a quick introduction, it is important to remember that many animals, including rats, are social creatures that forage in groups.  Foraging in groups provides numerous costs and benefits. On the one hand, animals that forage together are usually more successful in finding new sources of food, utilizing those sources, and protecting each other from predators.  On the other hand, these animals may be competing with each other for the same resources, thus limiting the total quantity of food that they may have been able to secure had they gone foraging alone.  Think of humans, for example, if we are set to roam freely in the forest with a rifle and hunting equipment, I would imagine most people to travel in packs and search for food together.  One the one hand, they may be able to combine their navigating abilities and resources in order to search the forest, most efficiently, for food.  On the other hand, if they hunt a deer, then they would have to share their prize between the group.  No one will go hungry, but everyone will receive a smaller portion (it's like a Democracy).  Another interesting idea about foraging strategies is that of the producer-scrounger model.  This model makes the claim that animals can be one of two types: producers, who find and procure food, and scroungers, who find other animals that have procured food.  The model predicts that animals distribute themselves between these two behaviors, depending on the availability of food in an environment and the number of competitors around.  Connecting this to last week's post... success in foraging for food also depends on social learning through observation and imitation.  If one species is better able to discover a way of foraging for food and teach their friends and relatives, then they may have more success and eventually fitness in an environment.  This also goes to show that long-term memory is at work in food foraging.  Michael Brown and his team were interested in working-memory as a participant in foraging.
    The basis for this research comes from a previous experiment done with a pole box maze.  In this maze, pairs of rats search for sucrose pellets hidden on top of vertical poles.  The finding in this study was that during the early trials, rats preferred to visit the poles that the other rat had already visited, or was recently foraging in.  In later trials, however, the rats learned that no food was found during a revisit to those locations, so they began to visit new locations.  This suggests that rats are learning and that memory is at play, but critics of these results suggested that some form of local enhancement was at play.  Reviewing from last week, local enhancement in this situation would mean that the mere presence of a rat next to a pole would draw the attention of the other rat to that location and cause it to start exploring and foraging by that pole.  The presence of the other rat, therefore, and not memory, may be controlling behavior.
     The first experiment conducted by Mike Brown and his team involves a radial-arm maze.  This maze has a central hub and usually eight arms sticking out of it, each approximately 1-m long.  A small amount of food, such as sucrose pellets, is placed at the end of each arm.  The arms are constructed of PVC tubing, and they are wide enough for the rats to pass each other as they travel through the arms.  In a preliminary training, pairs of rats were exposed to the maze and trained to obtain pellets from the end of the maze arms.  If both rats entered a maze arm in one trial, then food was taken away the next day (less food was available for them in the central hub as a form of punishment), but if both members of a pair obtained pellets from the food cups at the end of the arm, then pellets were placed again in the food cup the following day.  After this training, a free-choice test was conducted.  The choice of a maze arm was defined as a rat having all four paws in the maze arm.  A pair of rats were placed in the central hub and were allowed to make choices for six minutes.  Then, finally a forced-choice test was conducted.  Before each trial, four maze arms were randomly chosen to be closed off by putting plastic inserts into the maze arms.  The rats were then placed in the central arena and were allowed to enter the other four maze arms (all of which had food pellets in them), until all four accessible maze arms had been chosen by at least one rat.  Then, the rats were lifted and the maze arms were rotated... this was done to make sure that the next choices rats made were not done because of the odor cues left behind by rats.  Rotating the arms was supposed to eliminate the possibility of odor as an explanation.  The rats were then placed back in the central arena and were allowed to make choices again until all four locations, that were not visited prior to rotation, had been visited by at least one rat.  This specifies that the location must be visited, even if a previously visited maze arm was in that location.
     The results of this first experiment showed that rats made a higher-than-expected proportion of visits to locations not previously visited by their foraging partner, and they made a lower-than-expected number of visits to locations that had been previously visited by the other rat.  This is the tendency that we would expect of rats to avoid revisiting locations, because it is quite likely that food would be depleted in that location.  This experiment goes to show you that the choices made by one rat in a radial maze affect the choies made later in the trial by a second rat.  There may be some local enhancement at play here, however, as there was a tendency for rats to choose to visit the location that had most recently been visited by another rat.  In the free-choice trials, the other rat may still have been around near the maze arm that it had most recently visited, and so its presence there could have served as a trigger for the other rat to be interested and want to come check out the area.  To control for this, and to control for the possibility of odor cues, they conduct the forced tests, which showed that rats were making choices based on memory, since it was impossible for them to make choices based on the presence of another rat (the rats were lifted from the arena, the maze was rotated and the rats were placed back into the arena to forage).  In the forced choice tests, the rats were also more likely to visit locations that had previously not been visited before.
    The researchers expanded upon this study with a second experiment with a larger sample size.  They conducted a free-choice trial once again, as well as a forced-choice trial, but this time only allowing one of the two rats to make choices.  In the forced-choice trial, one rat was allowed to make choices in the maze arms, while the other rat had to watch from a central observation hub.  Then, the rat that had been watching was allowed to make choices, in the absence of the other rat.  The free-choice test produced very similar results to experiment 1, even with the larger sample size (26 rats).  In the observation test trials, since the observing rat was allowed to forage alone after watching, it was not affected by any position effects of a rat nearby.  After observing and then foraging, these observing rats still chose to visit those locations that had previously not been visited by the first rat.  This shows working memory at play.  As rats are observing, they are learning and remembering where they should visit next to find food, and which places they should avoid because food must be depleted from there.
    Finally, a third experiment was done to fill in some holes in the results and test new research parameters.  The limitation to experiment 2 was that the maze was not rotated, so there is the possibility that the results may have been due to odor cues left behind by the previous rat that had visited.  Experiment 3 involved rotating the maze between the first rat's run and the observing rat's run so that avoidance of a previously visited arm of the maze would be due to memory and not due to any odor cues.  The main thing manipulated in experiment 3 was the same condition or different condition groups.  For the same condition groups, after a stimulus rat went around and visited maze arms, the rat was then removed, and food was added to those arms that had been visited.  This makes the study unusual, because normally you would not expect there to be food where a rat had previously visited.  For the different condition groups, after a stimulus rat made its rounds through the maze arms, the maze arms that had not been visited were supplied with food.  After food was placed in the same or different maze arms, the observer rats were allowed to roam.  For observational training I, each pair of rats was randomly assigned to be in the same or different condition. Then, the rats were exposed to a free-choice trial, where they could visit any of the eight maze arms, all of which had food in them. Finally, the same rats were put in Observational Testing II, the rats were all put in the different condition.  The same group defied the normal expectation, whereas the different group was expected to have no food where rats had previously visited.  The results showed that there was no difference between where rats went in Observational Training I.  When the rats finally came to the Observational Training II, evidence of social learning was apparent.  Rats that had been in the same condition in observational training I stayed around baseline levels, but rats that were in the different condition visited maze arms that were different from where the stimulus rats had visited.  This shows that working memory was taking place and that rats had learned to avoid those locations that were previously visited by rats.  Also, since the maze was rotated in this case, any odor cues were ruled out as a cause.
    In conclusion, social spatial working memory apparently plays a big role in rats foraging for food.  These experiments go to show you that it is not necessarily odor cues that are causing a rat to avoid a certain area where another rat has visited, but it is actually learning and working memory that causes a rat not to revisit.  The results confirm initial predictions that rats will avoid those locations that other rats have previously visited, because it is more than likely that the first rat to attend an arm of the maze will eat the food that is available there.  If we think about this in real life, we can imagine competition in a parking garage.  If there are a very limited number of spots available, and you see someone make a left at the end of the row, you are more likely to turn the opposite way from that car, rather than follow behind it.  This is because you expect that person to take the last available spot in an area, and it will be a waste of time for you to follow that car and compete for a limited number of spaces.  It is more logical for you to search in an area that has not been visited.  The same principal is at work with these rats.  They are making a mental map of where the initial rat has visited and they are trying to avoid those locations.  It is adaptively significant for rats to exhibit this behavior, because it leads to more food procured and greater reproductive success. This definitely shows that rats are more complex and intelligent than initially thought.

Brown, M.F., Farley, R. F., & Lorek, E. J. (2007). Remembrance of places you passed: Social spatial      working memory in rats. Journal of Experimental Psychology: Animal Behavior Processes, 33(3), 213-224. doi:

1 comment:

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