GRAND ROUNDS: Topographical Heading Disorientation - A Case Study

Cholinergic modulation of spatial learning, memory and navigation

Spatial learning, including encoding and retrieval of spatial memories as well as holding spatial information in working memory generally serving navigation under a broad range of circumstances, relies on a network of structures. While central to this network are medial temporal lobe structures with a widely appreciated crucial function of the hippocampus, neocortical areas such as the posterior parietal cortex and the retrosplenial cortex also play essential roles. Since the hippocampus receives its main subcortical input from the medial septum of the basal forebrain (BF) cholinergic system, it is not surprising that the potential role of the septo-hippocampal pathway in spatial navigation has been investigated in many studies. Much less is known of the involvement in spatial cognition of the parallel projection system linking the posterior BF with neocortical areas. Here we review the current state of the art of the division of labour within this complex 'navigation system', with special focus on how subcortical cholinergic inputs may regulate various aspects of spatial learning, memory and navigation.

From objects to landmarks: the function of visual location information in spatial navigation

Landmarks play an important role in guiding navigational behavior. A host of studies in the last 15 years has demonstrated that environmental objects can act as landmarks for navigation in different ways. In this review, we propose a parsimonious four-part taxonomy for conceptualizing object location information during navigation. We begin by outlining object properties that appear to be important for a landmark to attain salience. We then systematically examine the different functions of objects as navigational landmarks based on previous behavioral and neuroanatomical findings in rodents and humans. Evidence is presented showing that single environmental objects can function as navigational beacons, or act as associative or orientation cues. In addition, we argue that extended surfaces or boundaries can act as landmarks by providing a frame of reference for encoding spatial information. The present review provides a concise taxonomy of the use of visual objects as landmarks in navigation and should serve as a useful reference for future research into landmark-based spatial navigation.

Medial temporal lobe projections to the retrosplenial cortex of the macaque monkey

The projections to the retrosplenial cortex (areas 29 and 30) from the hippocampal formation, the entorhinal cortex, perirhinal cortex, and amygdala were examined in two species of macaque monkey by tracking the anterograde transport of amino acids. Hippocampal projections arose from the subiculum and presubiculum to terminate principally in area 29. Label was found in layer I and layer III(IV), the former seemingly reflecting both fibers of passage and termination. While the rostral subiculum mainly projects to the ventral retrosplenial cortex, mid and caudal levels of the subiculum have denser projections to both the caudal and dorsal retrosplenial cortex. Appreciable projections to dorsal area 30 [layer III(IV)] were only seen following an extensive injection involving both the caudal subiculum and presubiculum. This same case provided the only example of a light projection from the hippocampal formation to posterior cingulate area 23 (layer III). Anterograde label from the entorhinal cortex injections was typically concentrated in layer I of 29a–c, though the very caudal entorhinal cortex appeared to provide more widespread retrosplenial projections. In this study, neither the amygdala nor the perirhinal cortex were found to have appreciable projections to the retrosplenial cortex, although injections in either medial temporal region revealed efferent fibers that pass very close or even within this cortical area. Finally, light projections to area 30V, which is adjacent to the calcarine sulcus, were seen in those cases with rostral subiculum and entorhinal injections. The results reveal a particular affinity between the hippocampal formation and the retrosplenial cortex, and so distinguish areas 29 and 30 from area 23 within the posterior cingulate region. The findings also suggest further functional differences within retrosplenial subregions as area 29 received the large majority of efferents from the subiculum. © 2012 Wiley Periodicals, Inc.

Impaired head direction cell representation in the anterodorsal thalamus after lesions of the retrosplenial cortex

The retrosplenial cortex (RSP), a brain region frequently linked to processes of spatial navigation, contains neurons that discharge as a function of a rat's head direction (HD). HD cells have been identified throughout the limbic system including the anterodorsal thalamus (ADN) and postsubiculum (PoS), both of which are reciprocally connected to the RSP. The functional relationship between HD cells in the RSP and those found in other limbic regions is presently unknown, but given the intimate connectivity between the RSP and regions such as the ADN and PoS, and the reported loss of spatial orientation in rodents and humans with RSP damage, it is likely that the RSP plays an important role in processing the limbic HD signal. To test this hypothesis, we produced neurotoxic or electrolytic lesions of the RSP and recorded HD cells in the ADN of female Long-Evans rats. HD cells remained present in the ADN after RSP lesions, but the stability of their preferred firing directions was significantly reduced even in the presence of a salient visual landmark. Subsequent tests revealed that lesions of the RSP moderately impaired landmark control over the cells' preferred firing directions, but spared the cells directional stability when animals were required to update their orientation using self-movement cues. Together, these results suggest that the RSP plays a prominent role in processing landmark information for accurate HD cell orientation and may explain the poor directional sense in humans that follows damage to the RSP.

What does the retrosplenial cortex do?

The past decade has seen a transformation in research on the retrosplenial cortex (RSC). This cortical area has emerged as a key member of a core network of brain regions that underpins a range of cognitive functions, including episodic memory, navigation, imagination and planning for the future. It is now also evident that the RSC is consistently compromised in the most common neurological disorders that impair memory. Here we review advances on multiple fronts, most notably in neuroanatomy, animal studies and neuroimaging, that have highlighted the importance of the RSC for cognition, and consider why specifying its precise functions remains problematic.

Parahippocampal and retrosplenial contributions to human spatial navigation

Spatial navigation is a core cognitive ability in humans and animals. Neuroimaging studies have identified two functionally defined brain regions that activate during navigational tasks and also during passive viewing of navigationally relevant stimuli such as environmental scenes: the parahippocampal place area (PPA) and the retrosplenial complex (RSC). Recent findings indicate that the PPA and RSC have distinct and complementary roles in spatial navigation, with the PPA more concerned with representation of the local visual scene and RSC more concerned with situating the scene within the broader spatial environment. These findings are a first step towards understanding the separate components of the cortical network that mediates spatial navigation in humans.