Updating egocentric representations in human navigation

Seven experiments tested whether human navigation depends on enduring representations, or on momentary egocentric representations that are updated as one moves. Human subjects pointed to unseen targets, either while remaining oriented or after they had been disoriented by self-rotation. Disorientation reduced not only the absolute accuracy of pointing to all objects ('heading error') but also the relative accuracy of pointing to different objects ('configuration error'). A single light providing a directional cue reduced both heading and configuration errors if it was present throughout the experiment. If the light was present during learning and test but absent during the disorientation procedure, however, subjects showed low heading errors (indicating that they reoriented by the light) but high configuration errors (indicating that they failed to retrieve an accurate cognitive map of their surroundings). These findings provide evidence that object locations are represented egocentrically. Nevertheless, disorientation had little effect on the coherence of pointing to different room corners, suggesting both (a) that the disorientation effect on representations of object locations is not due to the experimental paradigm and (b) that room geometry is captured by an enduring representation. These findings cast doubt on the view that accurate navigation depends primarily on an enduring, observer-free cognitive map, for humans construct such a representation of extended surfaces but not of objects. Like insects, humans represent the egocentric distances and directions of objects and continuously update these representations as they move. The principal evolutionary advance in animal navigation may concern the number of unseen targets whose egocentric directions and distances can be represented and updated simultaneously, rather than a qualitative shift in navigation toward reliance on an allocentric map.     

Lost in virtual space: studies in human and ideal spatial navigation

The authors describe 3 human spatial navigation experiments that investigate how limitations of perception, memory, uncertainty, and decision strategy affect human spatial navigation performance. To better understand the effect of these variables on human navigation performance, the authors developed an ideal-navigator model for indoor navigation whose optimizing algorithm uses a partially observable Markov decision process. The model minimizes the number of actions (translations and rotations) required to move from an unknown starting state to a specific goal state in indoor environments that have perceptual ambiguity. The authors compared the model's performance with that of the human observer to measure human navigation efficiency. Experiment 1 investigated the effect of increasing the layout size on spatial way-finding efficiency and found that participants' efficiencies decreased as layout size increased. The authors investigated whether this reduction in navigation efficiency was due to visual perception (Experiment 2), memory, spatial updating strategy, or decision strategy (Experiment 3).     

Bilateral lesion of the cerebellar-dentate nucleus impairs egocentric sequential learning but not egocentric navigation in the rat

The involvement of the cerebellum in procedural learning is demonstrated in visuomotor-sequence tasks, as lesion of this area impedes the acquisition of new sequences. Likewise, the lateral cerebellum appears to be involved in the acquisition of new sequences, but not in the execution of learned sequences. In contrast, the dentate nucleus participates only in the execution of learned visuomotor sequences. In previous studies, disruption of the procedural elements of spatial navigation following cerebellar or dentate lesions has been reported. However, as praxic strategies (egocentric learning) are included in the procedural elements of the navigation, the participation of the cerebellar-dentate nucleus in egocentric procedural learning processes has not been evaluated. Therefore, using colchicine, bilateral lesions were made in the cerebellar-dentate nucleus of Sprague-Dawley rats, and these rats were given two tasks: egocentric-based motor sequence learning in the radial maze and egocentric navigation in the Morris water maze. The lesioned rats were unable to use the sequential information in the short term and showed delayed long-term acquisition, which was probably due to the inability to detect the sequence. No effects on the egocentric navigation task were observed. Our results indicate that the cerebellar-dentate nucleus is involved in the detection of egocentric sequential information but not in the use of this information in the navigation process. Further, they show differential involvement of the cerebellar-dentate nucleus in the execution of learned visuomotor sequences, as the dentate lesion disrupted the acquisition of new egocentric-motor-based sequences.     

Evidence of separable spatial representations in a virtual navigation task

Three experiments investigated spatial orientation in a virtual navigation task. Subjects had to adjust a homing vector indicating their end position relative to the origin of the path. It was demonstrated that sparse visual flow was sufficient for accurate path integration. Moreover, subjects were found to prefer a distinct egocentric or allocentric reference frame to solve the task. "Turners" reacted as if they had taken on the new orientation during turns of the path by mentally rotating their sagittal axis (egocentric frame). "Nonturners," by contrast, tracked the new orientation without adopting it (allocentric frame). When instructed to use their nonpreferred reference frame, both groups displayed no decline in response accuracy relative to their preferred frame; even when presented with reaction formats based on either ego or allocentric coordinates, with format unpredictable on a trial, both groups responded highly accurately. These findings support the assumption of coexisting spatial representations during navigation.     

The influence of intentional and incidental learning on acquiring spatial knowledge during navigation

In order to study the influence of intentional and incidental learning conditions on route learning, young adults walked a route through a university building. Half of the participants focused their attention on the route (intentional learning condition), while the other half did not (incidental learning condition). Five tests of spatial knowledge were employed: a route-length-estimation, landmark recognition, landmark ordering, map-drawing and navigation task. The intentional group performed better than the incidental group on the map-drawing and navigation task. No difference between the intentional and incidental group was found on the landmark-recognition and landmark-ordering task. Moreover, the intentional group overestimated the walking distance, while the incidental group underestimated it. These results suggest that route knowledge (landmark recognition and landmark ordering) requires less effortful processing than survey knowledge (developing a map-like representation and actual navigation).     

A single unstable visual cue impairs spatial learning in a water maze

The role of a prominent visual cue (a light) in spatial learning was examined. Rats were trained to locate a hidden platform in a 4-arm radial water maze. For some rats the light predicted platform location. For other rats the light moved from trial to trial and did not predict platform location. While the predictive light did not facilitate performance above that seen in rats trained without a light, the moving light significantly impaired performance. In a second experiment, the temporal parameters of the moving light were manipulated. For one group the light was visible for the entire trial. In a second group the light was visible only before the rats reached the platform and in a third group the light was visible only after the rats reached the platform. The moving light impaired performance only when it was presented for the entire trial. When rats were subsequently retrained with a new platform location and a moving light in the familiar room, the light did not impair performance in any of the groups. We suggest that the prominent visual cue served as a directional cue, that competed with other directional cues, in the unfamiliar environment and interfered with performance.     

Cue combination in human spatial navigation

This project investigated the ways in which visual cues and bodily cues from self-motion are combined in spatial navigation. Participants completed a homing task in an immersive virtual environment. In Experiments 1A and 1B, the reliability of visual cues and self-motion cues was manipulated independently and within-participants. Results showed that participants weighted visual cues and self-motion cues based on their relative reliability and integrated these two cue types optimally or near-optimally according to Bayesian principles under most conditions. In Experiment 2, the stability of visual cues was manipulated across trials. Results indicated that cue instability affected cue weights indirectly by influencing cue reliability. Experiment 3 was designed to mislead participants about cue reliability by providing distorted feedback on the accuracy of their performance. Participants received feedback that their performance with visual cues was better and that their performance with self-motion cues was worse than it actually was or received the inverse feedback. Positive feedback on the accuracy of performance with a given cue improved the relative precision of performance with that cue. Bayesian principles still held for the most part. Experiment 4 examined the relations among the variability of performance, rated confidence in performance, cue weights, and spatial abilities. Participants took part in the homing task over two days and rated confidence in their performance after every trial. Cue relative confidence and cue relative reliability had unique contributions to observed cue weights. The variability of performance was less stable than rated confidence over time. Participants with higher mental rotation scores performed relatively better with self-motion cues than visual cues. Across all four experiments, consistent correlations were found between observed weights assigned to cues and relative reliability of cues, demonstrating that the cue-weighting process followed Bayesian principles. Results also pointed to the important role of subjective evaluation of performance in the cue-weighting process and led to a new conceptualization of cue reliability in human spatial navigation.     

Visual landmarks facilitate rodent spatial navigation in virtual reality environments

Because many different sensory modalities contribute to spatial learning in rodents, it has been difficult to determine whether spatial navigation can be guided solely by visual cues. Rodents moving within physical environments with visual cues engage a variety of nonvisual sensory systems that cannot be easily inhibited without lesioning brain areas. Virtual reality offers a unique approach to ask whether visual landmark cues alone are sufficient to improve performance in a spatial task. We found that mice could learn to navigate between two water reward locations along a virtual bidirectional linear track using a spherical treadmill. Mice exposed to a virtual environment with vivid visual cues rendered on a single monitor increased their performance over a 3-d training regimen. Training significantly increased the percentage of time avatars controlled by the mice spent near reward locations in probe trials without water rewards. Neither improvement during training or spatial learning for reward locations occurred with mice operating a virtual environment without vivid landmarks or with mice deprived of all visual feedback. Mice operating the vivid environment developed stereotyped avatar turning behaviors when alternating between reward zones that were positively correlated with their performance on the probe trial. These results suggest that mice are able to learn to navigate to specific locations using only visual cues presented within a virtual environment rendered on a single computer monitor.     

High-fat diets impair spatial learning in the radial-arm maze in mice

It has been suggested that hyperglycemia and insulin resistance triggered by energy-dense diets can account for hippocampal damage and deficits of cognitive behaviour. We wonder if the impairment of learning and memory processes detected in diet-induced obese (DIO) mice is linked to diet composition itself. With this purpose we have evaluated learning performance in mice undergoing a short-term high-fat (HF) treatment, which leads to a pre-obese state characterized by increased adiposity without significant changes of glucose and insulin plasma levels. C57BL/6J mice were fed either a HF (45 kcal% from fat) or control diet (10 kcal% from fat) during 8 weeks. Learning performance was evaluated by using the four-arm baited version of the eight-arm radial maze test (RAM). Mice were trained to learn the RAM protocol and then memory was tested at different time-points. Time spent to consume food placed in baited arms and errors committed to find them were measured in all sessions. DIO mice significantly spent more time in learning the task and made a greater number of errors than controls. Moreover, retention tests revealed that both working and total memory errors were also more numerous in DIO mice. The current results show that short-term DIO impairs spatial learning and suggest that impairment of hippocampal learning elicited by HF diets might be perceptible before metabolic alterations linked to obesity develop.     

Does active learning benefit spatial memory during navigation with restricted peripheral field?

Spatial learning of real-world environments is impaired with severely restricted peripheral field of view (FOV). In prior research, the effects of restricted FOV on spatial learning have been studied using passive learning paradigms – learners walk along pre-defined paths and are told the location of targets to be remembered. Our research has shown that mobility demands and environmental complexity may contribute to impaired spatial learning with restricted FOV through attentional mechanisms. Here, we examine the role of active navigation, both in locomotion and in target search. First, we compared effects of active versus passive locomotion (walking with a physical guide versus being pushed in a wheelchair) on a task of pointing to remembered targets in participants with simulated 10° FOV. We found similar performance between active and passive locomotion conditions in both simpler (Experiment 1) and more complex (Experiment 2) spatial learning tasks. Experiment 3 required active search for named targets to remember while navigating, using both a mild and a severe FOV restriction. We observed no difference in pointing accuracy between the two FOV restrictions but an increase in attentional demands with severely restricted FOV. Experiment 4 compared active and passive search with severe FOV restriction, within subjects. We found no difference in pointing accuracy, but observed an increase in cognitive load in active versus passive search. Taken together, in the context of navigating with restricted FOV, neither locomotion method nor level of active search affected spatial learning. However, the greater cognitive demands could have counteracted the potential advantage of the active learning conditions.

     

Influence of spatial environment on maze learning in an African mole-rat

In subterranean species where excavation is energetically expensive, efficient spatial navigation is vital to reducing the costs of locating important resources such as food and mates. While spatial navigational ability is positively correlated with sociality in subterranean mammals, we have a less clear understanding of the role of habitat complexity on navigational ability. We tested spatial navigational ability and memory in 12-18-month captive Natal mole-rats (Cryptomys hottentotus natalensis) maintained in a simple environment with no environmental enrichment and newly captured wild individuals from natural, complex burrow systems. In maze trials, mole-rats captured freshly from the wild made significantly fewer navigational errors, were more likely to successfully navigate the maze, travelled shorter distances and as a consequence, completed the maze in less time. Male mole-rats from both experimental treatments were more likely to complete the maze than females. Memory retention of the maze was tested on day two, seven, 30 and 60, respectively. The results were variable, although both groups showed a significant memory retention 60?days after testing. Our results highlight the potential importance of the environment (microhabitat complexity) on spatial cognitive performance in mole-rats.