Distinct error-correcting and incidental learning of location relative to landmarks and boundaries

Abstract

Associative reinforcement provides a powerful explanation of learned behavior. However, an unproven but long-held conjecture holds that spatial learning can occur incidentally rather than by reinforcement. Using a carefully controlled virtual-reality object-location memory task, we formally demonstrate that locations are concurrently learned relative to both local landmarks and local boundaries but that landmark-learning obeys associative reinforcement (showing "overshadowing" and "blocking" or "learned irrelevance"), whereas boundary-learning is incidental, showing neither overshadowing nor blocking nor learned irrelevance. Crucially, both types of learning occur at similar rates and do not reflect differences in levels of performance, cue salience, or instructions. These distinct types of learning likely reflect the distinct neural systems implicated in processing of landmarks and boundaries: the striatum and hippocampus, respectively [Doeller CF, King JA, Burgess N (2008) Proc Natl Acad Sci USA 105:5915-5920]. In turn, our results suggest the use of fundamentally different learning rules by these two systems, potentially explaining their differential roles in procedural and declarative memory more generally. Our results suggest a privileged role for surface geometry in determining spatial context and support the idea of a "geometric module," albeit for location rather than orientation. Finally, the demonstration that reinforcement learning applies selectively to formally equivalent aspects of task-performance supports broader consideration of two-system models in analyses of learning and decision making.

Fig. 1.

Virtual reality task. (A) Trial structure (after initial collection of objects). Participants replace the cued object after a short delay phase and received feedback during learning trials (object appears in correct location immediately after the response and is collected) but not testing trials. (B) Virtual arena from the participant's perspective (i, replace phase; ii, feedback phase; different viewpoints) showing the intramaze landmark (traffic cone), the boundary (circular wall), the extramaze orientation cues (mountains, which were projected at infinity), and one object (vase). ITI, intertrial interval.

Fig. 2.

Overshadowing experiment. The landmark is overshadowed by the boundary but not vice versa. (A) Four different groups (columns, 12 participants per group) learned four object-locations with either one of (simple learning) or both of the landmark (L) and boundary (B) present (compound learning; Upper) and were tested with either landmark or boundary alone (Lower). (B) Boundary overshadows landmark (i.e., the presence of the boundary during learning reduces learning to the landmark in group LB1 compared with group L) but not vice versa (i.e., the presence of the landmark during learning does not reduce learning to the boundary in group LB2 compared with group B). Bars indicate mean distance between response and correct location during test phase in virtual meters, ± SEM; *, P < 0.05; ns, not significant. +, landmark; circle, boundary; dots, object locations.

Fig. 3.

Blocking experiments. Landmark learning obeys associative reinforcement; boundary learning does not. (A) Three experiments investigate blocking between two local landmarks (Experiment LL), between a local landmark and a boundary (Experiment LB), and between opposing sections of a boundary (Experiment BB). In each experiment, 16 participants performed initial learning of eight object-locations over eight blocks (three example blocks shown) with different spatial configurations of the two cues: landmarks L1 (light orange) and L2 (dark orange) in Experiment LL (Left); landmark L (orange) and boundary B (green) in Experiment LB (Center); or opposite sections B1 (light green) and B2 (dark green) of an enclosing boundary in Experiment BB (Right; note here that the boundary radius changed from block to block). Four object locations were paired with one cue (L1, L, or B1), and four were paired with the other cue (L2, B, or B2), indicated by little dots in the same colors as the associated cues. For example, when landmarks L1 and L2 were moved relative to each other, the four L1-associated objects (light orange dots) kept a fixed bearing to landmark L1 (light orange plus sign) but not to landmark L2 (dark orange plus sign) and vice versa for L2-associated objects (dark orange dots). Subsequently, participants performed compound learning, where both cues were fixed, allowing associations to be learned to the previously unpaired cue. (B) Performance was tested with either cue alone. Learning to the landmark was blocked by prior learning to either another landmark (see Test L1 and Test L2 in Experiment LL) or to the boundary (see Test L in Experiment LB). Learning to the boundary was unaffected by prior learning to the landmark (see Test B in Experiment LB) or to the opposite section of the boundary (see Test B1 and Test B2 in Experiment BB). **, P < 0.001; ns, not significant. Orange +, landmark; green circle/half circle, boundary; dots, object locations (orange is associated with the landmark; green is associated with the boundary).