A larger hippocampus is associated with longer-lasting spatial memory

Abstract

Volumetric studies in a range of animals (London taxi-drivers, polygynous male voles, nest-parasitic female cowbirds, and a number of food-storing birds) have shown that the size of the hippocampus, a brain region essential to learning and memory, is correlated with tasks involving an extra demand for spatial learning and memory. In this paper, we report the quantitative advantage that food storers gain from such an enlargement. Coal tits (Parus ater) a food-storing species, performed better than great tits (Parus major), a nonstoring species, on a task that assessed memory persistence but not on a task that assessed memory resolution or on one that tested memory capacity. These results show that the advantage to the food-storing species associated with an enlarged hippocampus is one of memory persistence.

Figure 1

Predicted outcomes if an enlarged hippocampus is associated with different memory capacity, resolution, or persistence. The storers are represented by the unbroken lines, the nonstorers by the dashed lines. Heavy lines represent images that are near to each other and thin lines represent images that are far apart. (A) If the species differ in memory capacity, performance should be similar when there is little to remember, but diverge as memory load increases. The pair of lines for each group are the predictions for the way in which performance levels should change with increasing numbers of sample images when the choice images differ in their proximity (see also C). (B) If the species differ in spatial resolution, performance should be similar when the images are far apart, but differ when they are close together. (C) If the species differ in memory persistence, the differences should be apparent from the smallest memory load and remain the same as memory load increases, regardless of proximity.

Figure 2

A schematic of the experimental design showing examples of the variation in the number of sample images and in the distance at which choices were presented. From one to four white squares were displayed in the sample phase. In the Choice phase of the Near condition, one image (and only one, regardless of the number of sample images) was placed in one of the positions immediately adjacent to the target. These positions are marked here by stippled outlines, which were not shown to the birds. The birds were presented only with the white squares. In the Choice phase of the Far condition, there was no sample in the positions immediately adjacent to the target. The combination of one to four samples with Near and Far conditions resulted in a total of eight different conditions.

Figure 3

Final retention intervals (seconds) reached by the birds at the end of the experiment. Data are means and SE. The storers are represented by the filled circles, the nonstorers by the open circles. Heavy lines represent images that are near to each other and thin lines represent images that are far apart.

Figure 4

Final distances between choice images achieved at each of four retention intervals in experiment 2. The storers are represented by the filled circles, the nonstorers by the open circles.