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First published online June 11, 2007
Journal of Experimental Biology 210, 2128-2136 (2007)
Published by The Company of Biologists 2007
doi: 10.1242/jeb.002634

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The co-activation of snapshot memories in wood ants

Paul Graham, Virginie Durier and Thomas Collett^*

Sussex Centre for Neuroscience, School of Life Sciences, University of Sussex, Brighton, BN1 9QG, UK

View larger version (9K): [in this window] [in a new window]   Fig. 1. Experimental arrangements. (A–D) The positions of food sources (F1, F2), the start and local landmarks are shown for the different experimental configurations. Landmarks were black cylinders approximately 45 cm in height. (A,B) The direct paths to F1 and F2 from the start are 45° apart. The configuration in B was used for two experiments: restricted and unrestricted training. The grey line and circle represent the barrier and metal ring used to prevent access to F1 during `restricted training' to F2. (C,D) The direct paths to F1 and F2 from the start are 90° apart.

View larger version (29K): [in this window] [in a new window]   Fig. 2. Gradual change in trajectories with 45° separation between F1 and F2. (A) The trajectories of ants (N=6) after the feeding site was switched to F2. Paths are grouped by run number. The grey area is the triangle with vertices S, F1 and F2. (B) The initial direction of each path was categorised as F1, F2, intermediate or other. The first three categories are 22.5° wide as shown in the legend. The `other' category represents trajectories that fall outside this range. Stacked bar charts show the change in proportions of each category with run number (N=6; n=32, 60, 46, 37). N, number of ants; n, number of trajectories. This convention is followed throughout. (C) Distribution of headings for all post-switch trajectories (N=6; n=175). (D) Scatter plot showing the change in length of the initial segment plotted against number of trials after the switch. The length of this initial segment was defined by the distance between the start and the location of the first obvious turn determined by eye. Independent observers agreed closely on the presence and location of the first obvious turn in a trajectory. Bars show means and 95% confidence intervals for bins of 10 runs, as in B. The ratio of trajectories containing distinct turns was 100%, 92%, 88% and 56%, respectively, for the four bins.

View larger version (19K): [in this window] [in a new window]   Fig. 3. Paths after reaching F1. Paths after the switch are shown from the point where ants have reached within 20 cm of F1 until the ants reach F2 or the end of the recording period. Trajectories are grouped according to the number of trials after the switch. (A) Experimental configuration as in Fig. 1B. Trajectories are from six ants and the proportion of trajectories that reached within 20 cm of F1 were 58% (runs 1–10), 17% (runs 11–20) and 3% (runs 20+). (B) Experimental configuration as in Fig. 1A. Trajectories are from 20 ants and the proportion of trajectories that reached within 20 cm of F1 were 81% (runs 1–10), 32% (runs 11–20) and 13% (runs 20+).

View larger version (25K): [in this window] [in a new window]   Fig. 4. Change in trajectories of ants with restricted training. (A) The trajectories of ants (N=30) during the final training run (left) and in probe trials after the food position has been switched to F2. Paths are grouped by initial heading (F1, intermediate and F2, respectively). A barrier (Fig. 1B) prevented access to the white region of the arena during post-switch training runs. (B) As in Fig. 2B, trajectories are categorised by their initial heading and grouped by run number. The groups contain 29, 40, 41 and 53 trajectories, respectively. (C) Distribution of initial headings for all probe trials after the switch (N=30; n=153).

View larger version (35K): [in this window] [in a new window]   Fig. 5. Paths of ants with restricted training, after reaching F1 or F2. (A) Probe tests grouped according to the number of training trials after the switch to F2. Paths are shown from the point where ants reached within 20 cm from F1. Trajectories are from 30 ants and the proportions of trajectories that reached within 20 cm of F1 were 68% (runs 1–10), 47% (runs 11–20) and 40% (runs 20+). (B) As above, for test runs that reached within 20 cm of F2. The proportions of trajectories that reached within 20 cm of F2 were 34%, 86% and 81% for the three groups.

View larger version (34K): [in this window] [in a new window]   Fig. 6. Trajectory change with 90° separation between F1 and F2. Trajectories at the end of training to F1 and at different periods after the switch to F2 are shown for two groups of ants for which the direct path to F2 was rotated by 90° from the direct path to F1. (A) Experimental arrangement as in Fig. 1C. (B) Experimental arrangement as in Fig. 1D.

View larger version (23K): [in this window] [in a new window]   Fig. 7. Time course of changes in the direction of the initial segment. (A,B) The change of initial heading with experience is shown for individual ants after a 45° switch (A) or a 90° switch (B) between F1 and F2. The trace for each ant is a smoothed version of the raw headings using a median filter (window size, three runs). In A and B the upper and lower shaded areas represent the ranges over which headings are categorised as F1 and F2, respectively.

View larger version (5K): [in this window] [in a new window]   Fig. 8. Paths simulated by averaging weighted vectors. Insets show sets of simulated trajectories for food sites 45° apart (top left) and 90° apart (bottom right). The food was 100 cm from the start. Within each cluster, trajectories are shown for different weights of F2 (0 to 1 with increments of 0.2). The normalised headings of these trajectories after 80 cm are plotted. A normalised heading of 0 is directly toward F1 and a normalised heading of 1 is directly toward F2.

View larger version (17K): [in this window] [in a new window]   Fig. 9. The summation of feature attractors. (A) Hypothetical attractor in which retinal error is converted to a yaw force of the ant. (B) At three different goal locations (F, Fnear and Ffar) the stored retinal position of a local landmark, or, equivalently, equilibrium point of an associated attractor, will vary. (C,D) Weighted averages of attractors. If two attractors from snapshots taken close together (e.g. F and Fnear) are averaged across retinal position (C) then the resulting combined attractor has a single equilibrium point that moves as the relative weightings change (solid circles). If the two attractors come from snapshots taken far apart (F and Ffar) then the resulting combined attractor (D) has two stable equilibrium points (at F and Ffar) that do not move as the weighting is changed. The zero crossings between the two equilibrium points (dashed portion of the attractor curve) are unstable as turns are away rather than towards the zero point. (E,F) Simulated routes to two feeders using spatially limited attractors of different weights. Further explanation is given in the text.