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Mammalian mechanism of time-place learning clarified

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02.06.2008

We are all aware of the places you can better avoid in the dead of night. And we all know that the freshest, tastiest food can be bought at the open-air market early in the morning.

In the animal kingdom, too, it is of the utmost importance to circumvent certain places at certain times (due to predators) and to visit others at other times (due to the availability of food). Until recently, the exact way in which mammals established this link between time and place remained a mystery.

However, researchers at the University of Groningen have finally unravelled the secret. Their findings will be published in Current Biology on 3 June 2008.

The researchers began by observing how mice in the lab could be taught to connect time and place – the so-called ‘time-place learning’. Prof. Menno Gerkema, the last author of the article, explains that an attempt was made to emulate natural circumstances as much as possible. ‘Animals always have to weigh up the situation when gathering food. To them, food is never free. A mouse can always be seized by predators. We tried to incorporate that risk in our experiment.’

Time-place learning
The researchers used a construction with three passages through which the mice could run. At the end of each passage, the mice could find food behind a platform. Depending on the time of day, however, an electric shock was applied through the platform. It soon became evident that the mice were perfectly capable of finding those places where food was freely available at certain times and of avoiding certain places where food could only be obtained at the cost of incurring an unpleasant shock. This finding was extraordinary in itself because, up to the present, no method of studying time-place learning in mammals had been constructed.

Biological clock
But what kind of mechanism do the mice use in this time-place learning? The researchers suspected that the mice made use of their biological clock, just as birds and bees do in time-place learning. To test this hypothesis, use was made of genetically modified mice that lack certain genes (called ‘Cry1’ and ‘Cry2’) so that they had to work without a biological clock. These mice turned out to be incapable of obtaining food in the passages at the appropriate moments. With this finding, the researchers have finally been able to ascertain that mammals make use of the biological clock in time-place learning.

Humans
People also learn to connect time and place. ‘A subconscious link between time and place is established in a great number of learning processes. If, for example, you cannot recall something, it helps to revisit the spot where the idea that you want to remember originated. The forming of associations between place and time helps us structure our memory.’ Gerkema surmises that time-place learning occurs in much the same way in humans as it does in mice, because both have roughly the same biological clock system.

Alzheimer’s disease
The researchers, Menno Gerkema and Eddy van der Zee, now wish to investigate how time-place learning changes as people grow older. Gerkema: ‘In humans, you see that the ability to connect time and place declines as one grows older. Some patients with Alzheimer’s disease completely lose their notion of time at a certain moment. This is a dramatic development, in view of the fact that they do not recognize the difference between day and night. That is why round-the-clock care is necessary and the patient usually has to be hospitalized.’ Insight into time-place learning may help in postponing this moment as long as possible.

Eelco Salverda | Source: alphagalileo
Further information: www.rug.nl
images.cell.com/images/Edimages/CurrentBiology/May29/6467main.pdf

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