In human and non-human animals, conceptual knowledge is partially organized according to low-dimensional geometries that rely on brain structures and computations involved in spatial representations. Recently, two separate lines of research have investigated cognitive maps, that are associated with the hippocampal formation and are similar to world-centered representations of the environment, and image spaces, that are associated with the parietal cortex and are similar to self-centered spatial relationships. We review evidence supporting cognitive maps and image spaces, and we propose a hippocampal-parietal network that can account for the organization and retrieval of knowledge across multiple reference frames. We also suggest that cognitive maps and image spaces may be two manifestations of a more general propensity of the mind to create low-dimensional internal models.
Organizing Knowledge in Low-Dimensional SpaceEvery second, our brains process an amazing amount of information, perceive a dynamic and complex sensory environment, and spontaneously generate countless thoughts. Making sense of this vast amount of data must rely on some structure and organizational principles [1,2]. Determining exactly what these organizational principles are has proved to be a formidable challenge [3][4][5]. However, convergent evidence from neural, cognitive, and information sciences is pointing toward a fascinating hypothesis: that the human brain may organize knowledge into low-dimensional spaces (see Glossary) that we can easily navigate, explore, and manipulate as we, for example, navigate a familiar environment, explore a picture in a frame, or manipulate an object in our hands [1,2,6,7]. In other words, the neural machinery that evolved to map objects and structure events in the physical world may have been recycled to map and structure knowledge within our minds [6].Although this idea, broadly taken, has a venerable tradition [8][9][10][11], research in cognitive science and neuroscience has only recently provided solid empirical ground for this view. We review here evidence suggesting that the neurocognitive structures and algorithms that are recruited to represent and navigate space are also recruited to represent and navigate (nonspatial) conceptual knowledge. In particular, we focus on, and contrast, world-centered cognitive maps (that are usually associated with the hippocampal formation) and self-centered image spaces (usually associated with the parietal cortex). We then attempt to integrate cognitive maps and image spaces with the mechanisms of a hippocampal-parietal network, inspired by current models of spatial navigation and based on complementary reference frames (allocentric and egocentric). Finally, we discuss the role of low-dimensional conceptual spaces in cognition.
World-Centered Cognitive Maps and the Hippocampal FormationA long history of neuropsychological studies with amnesic patients [12,13] and more recent neuroimaging experiments [14] have shown that the hippocampal formation (i.e., the hippoca...