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, 17 (2), 93-102

Visual Cues for the Retrieval of Landmark Memories by Navigating Wood Ants

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Visual Cues for the Retrieval of Landmark Memories by Navigating Wood Ants

Robert A Harris et al. Curr Biol.

Abstract

Background: Even on short routes, ants can be guided by multiple visual memories. We investigate here the cues controlling memory retrieval as wood ants approach a one- or two-edged landmark to collect sucrose at a point along its base. In such tasks, ants store the desired retinal position of landmark edges at several points along their route. They guide subsequent trips by retrieving the appropriate memory and moving to bring the edges in the scene toward the stored positions.

Results: The apparent width of the landmark turns out to be a powerful cue for retrieving the desired retinal position of a landmark edge. Two other potential cues, the landmark's apparent height and the distance that the ant walks, have little effect on memory retrieval. A simple model encapsulates these conclusions and reproduces the ants' routes in several conditions. According to this model, the ant stores a look-up table. Each entry contains the apparent width of the landmark and the desired retinal position of vertical edges. The currently perceived width provides an index for retrieving the associated stored edge positions. The model accounts for the population behavior of ants and the idiosyncratic training routes of individual ants.

Discussion: Our results imply binding between the edge of a shape and its width and, further, imply that assessing the width of a shape does not depend on the presence of any particular local feature, such as a landmark edge. This property makes the ant's retrieval and guidance system relatively robust to edge occlusions.

Figures

Figure 1
Figure 1
Experimental Arena and Landmarks Ants were trained with either (A) a two-edged landmark or (B) a one-edged gradient landmark. Ants were placed in a starting pot at the center of the arena. They then walked to a sucrose reward placed at the landmark's base. (C) Ratio of black to white pixels across the three experimental gradients. The slopes of the steep and plateau gradients were identical, with the plateau offset by 40 cm of solid black. The physical width of each stimulus was 120 cm. Further details are in the Experimental Procedures.
Figure 2
Figure 2
Routes of Ants during Their Approach to Training Landmarks (A–D) The left of each panel shows, for a given training condition, the mean route. Here and in the other figures, the error bars give standard errors. The columns of histograms in each panel show, for successive 15 cm bins during the approach, the retinal position of the left edge of the stimulus (left) and the ants' fixation point on the landmark (right). The histograms were calculated from every recorded frame within the 15 cm bin. In this and the following figures, N is the number of recorded ants, and n is the number of recorded paths. (A) Food at left edge of a shallow gradient (N = 22, n = 295). (B) Food 10 cm inset from left edge of a shallow gradient (N = 44, n = 838). (C) Food placed at the left edge of a 40-cm-wide wall (N = 28, n = 175). (D) Food 10 cm inset from the left edge of the wall (N = 71, n = 243).
Figure 3
Figure 3
Tests for Multiple Memories and Different Retrieval Cues (A) Simulated routes generated when the left edge of a gradient is held in a single retinal position over the whole route. The fixed positions (7°–56°) correspond to single snapshots stored on the direct route at 80, 40, 19, and 7 cm from the goal. (B–F) Mean routes (solid lines) and simulated routes (dashed lines) for different landmark heights and distances. Ants were trained to an 82-cm-high left gradient placed 80 cm from the start and were subsequently tested with either an 82- or a 40-cm-high gradient placed at 30 cm from the starting location. (B) Mean route with training landmark (N = 34, n = 223). (C) Tests with the landmark 30 cm from start. The mean route combines data from tests with 40- and 82-cm-high gradients (N = 44, n = 59). The simulated route supposes that ants set their initial direction with the left edge placed 7° to the left of their midline. (D) Mean path, as in (C). Simulated path with distance from start controlling the retrieval of desired edge positions. (E and F) Mean path from tests with 82-cm-high ([E] N = 35, n = 116) or 40-cm-high ([F], N = 9, n = 43) gradient at 30 cm from the start. Paths are simulated with apparent height controlling the retrieval of desired edge positions.
Figure 4
Figure 4
Ants Trained with the Shallow Gradient (A) Mean routes of ants trained to find food 10 cm from the left edge of the shallow gradient and tested with a narrower (steep) or wider (plateau) gradient (shallow, N = 16, n = 308; steep, N = 14, n = 70; plateau, N = 14, n = 87). (B) Mean routes of ants trained to find food at the left edge of the shallow gradient and tested with steep and plateau gradients (shallow, N = 10, n = 209; steep, N = 9, n = 52; plateau, N = 9, n = 34). (C–E) Different numbers of memories were used for simulating routes to the three gradients. Desired edge positions are retrieved according to the apparent width of the landmark.
Figure 5
Figure 5
Individual Routes of Ants Trained with the Shallow Gradient (A and B) Individual, mean, and simulated paths of two ants. (A) Routes similar to the population mean (shallow, n = 35; steep, n = 8; plateau, n = 10) and (B) routes when training was consistently biased to the right of the population mean (shallow, n = 30; steep, n = 6; plateau, n = 8). Simulated paths are generated from a look-up table of memories based on the individual's mean training route.
Figure 6
Figure 6
Ants Trained to Two-Edged Landmarks (A–F) Mean and simulated routes after training to a 40-cm-wide two-edged landmark. (A and B) Ants trained to food 10 cm in from the left edge. (A) Tests with left-edge gradients (steep, N = 16, n = 70; shallow, N = 16, n = 65; plateau, N = 16, n = 82). (B) Tests with right-edge gradients (steep, N = 21, n = 60; shallow, N = 15, n = 62; plateau, N = 24, n = 66). (C and D) Ants trained to food at the left edge and tested with right-edge gradients. (C) Mean routes of the population (training, N = 16, n = 143; steep, N = 17, n = 64; shallow, N = 15, n = 58). (D) Mean routes of an idiosyncratic ant (training, n = 14; steep, n = 6, shallow n = 5), with routes to right-edge gradients simulated from three snapshots. (E) Experimental and simulated paths of ants trained to a 40-cm-wide landmark and tested with an 80-cm-wide landmark (40 cm, N = 23, n = 262; 80 cm, N = 22, n = 98). (F) Mean and simulated routes of two idiosyncratic individuals (40 cm, n = 11 and 8; 80 cm, n = 5 and 4).

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