Topological data analyses are widely used for describing and conceptualizing large volumes of neurobiological data, e.g., for quantifying spiking outputs of large neuronal ensembles and thus understanding the functions of the corresponding networks. Below we discuss an approach in which convergent topological analyses produce insights into how information may be processed in mammalian hippocampus-a brain part that plays a key role in learning and memory. The resulting functional model provides a unifying framework for integrating spiking data at different timescales and following the course of spatial learning at different levels of spatiotemporal granularity. This approach allows accounting for contributions from various physiological phenomena into spatial cognition-the neuronal spiking statistics, the effects of spiking synchronization by different brain waves, the roles played by synaptic efficacies and so forth. In particular, it is possible to demonstrate that networks with plastic and transient synaptic architectures can encode stable cognitive maps, revealing the characteristic timescales of memory processing.

A central question for any model of visual word identification is the representation of the position at which letters are encoded (e.g., calm vs. clam). In this article, we examine whether the orthographic-specific characteristics of a writing system-namely, Thai-shape the process of letter position coding. Thai is an alphabetic script that lacks interword spaces and has an orthographic order that does not necessarily correspond to the phonological order for initial vowels. This implies that the initial letter position coding in Thai needs to be flexible enough that readers can successfully encode the letter positions of words. To compare letter position coding in Thai to that in English, we conducted an experiment that paralleled Experiment 3 in Gomez, Ratcliff, and Perea (Psychological Review, 115, 577-600, 2008), including 23 conditions (single-letter replacements, letter transpositions, letter migrations, and a corresponding control). We obtained fits from Gomez et al.\'s overlap model, which is a model that has been shown to account for letter position coding in the Roman alphabet across this variety of letter manipulations. The overlap model was found to successfully fit the Thai data. Our results revealed that the position encoding was better for the first letter than for the rest of the positions in both languages; however, in English the position uncertainty grows as a function of letter order quite abruptly, whereas in Thai it grows gradually. Thus, the orthographic-specific characteristics of the Thai writing system do play a role in shaping the process of letter position coding.

The hippocampus is believed to have evolved to support allocentric spatial representations of environments as well as the details of personal episodes that occur within them, whereas other brain structures are believed to support complementary egocentric spatial representations. Studies of patients with adult-onset lesions lend support to these distinctions for newly encountered places but suggest that with time and/or experience, schematic aspects of environments can exist independent of the hippocampus. Less clear is the quality of spatial memories acquired in individuals with impaired episodic memory in the context of a hippocampal system that did not develop normally. Here we describe a detailed investigation of the integrity of spatial representations of environments navigated repeatedly over many years in the rare case of H.C., a person with congenital absence of the mammillary bodies and abnormal hippocampal and fornix development. H.C. and controls who had extensive experience navigating the residential and downtown areas known to H.C. were tested on mental navigation tasks that assess the identity, location, and spatial relations among landmarks, and the ability to represent routes. H.C. was able to represent distances and directions between familiar landmarks and provide accurate, though inefficient, route descriptions. However, difficulties producing detailed spatial features on maps and accurately ordering more than two landmarks that are in close proximity to one another along a route suggest a spatial representation that includes only coarse, schematic information that lacks coherence and that cannot be used flexibly. This pattern of performance is considered in the context of other areas of preservation and impairment exhibited by H.C. and suggests that the allocentric-egocentric dichotomy with respect to hippocampal and extended hippocampal system function may need to be reconsidered.

Statistical regularities in our environment enhance perception and modulate the allocation of spatial attention. Surprisingly little is known about how learning-induced changes in spatial attention transfer across tasks. In this study, we investigated whether a spatial attentional bias learned in one task transfers to another. Most of the experiments began with a training phase in which a search target was more likely to be located in one quadrant of the screen than in the other quadrants. An attentional bias toward the high-probability quadrant developed during training (probability cuing). In a subsequent, testing phase, the target\'s location distribution became random. In addition, the training and testing phases were based on different tasks. Probability cuing did not transfer between visual search and a foraging-like task. However, it did transfer between various types of visual search tasks that differed in stimuli and difficulty. These data suggest that different visual search tasks share a common and transferrable learned attentional bias. However, this bias is not shared by high-level, decision-making tasks such as foraging.

This paper describes the rehabilitation process of a patient with severe topographical disorientation. The study demonstrates the sustained effects of a tailor-made, meticulous rehabilitation programme based on the gradual development of compensatory strategies. The patient (RB) had a memory impairment specific to environmental landmarks. He was able to recognise objects in his environment, but was unable to identify any salient object as a landmark and was also unable to derive any directional information from a chosen landmark. As such, his topographical disorientation syndrome was complex in that he had elements of both landmark agnosia and a heading disorientation, as described by Aguirre and D\'Esposito (1999). Because of this dual damage to the exocentric framework, the tools and methods used in RB\'s rehabilitation programme were all based on his intact egocentric frame of reference. Remarkable training effects were found for routes he used frequently. After years of training he could walk these routes without the aid of the written information he had used previously, which can be interpreted as a form of implicit learning. In the 12 years we followed this patient some transfer occurred, as the patient was ultimately able to identify his own landmarks. However, RB remains dependent on other people to construct new routes for him on the basis of these landmarks.