James G. Donnett

Knowing Where Things Are: Parahippocampal Involvement in Encoding Object Locations in Virtual Large-Scale Space

The involvement of the medial temporal-lobe region in allocentric mapping of the environment has been observed in human lesion and functional imaging work. Cognitive models of environmental learning ascribe a key role to salient landmarks in representing large-scale space. In the present experiments we examined the neural substrates of the topographical memory acquisition process when environmental landmarks were more specifically identifiable. Using positron emission tomography (PET), we measured regional cerebral blood flow changes while normal subjects explored and learned in a virtual reality environment. One experiment involved an environment containing salient objects and textures that could be used to discriminate different rooms. Another experiment involved a plain empty environment in which rooms were distinguishable only by their shape. Learning in both cases activated a network of bilateral occipital, medial parietal, and occipito-temporal regions. The presence of salient objects and textures in an environment additionally resulted in increased activity in the right parahippocampal gyrus. This region was not activated during exploration of the empty environment. These findings suggest that encoding of salient objects into a representation of large-scale space is a critical factor in instigating parahippocampal involvement in topographical memory formation in humans and accords with previous studies implicating parahippocampal areas in the encoding of object location.

Directional control of hippocampal place fields

Abstract Pyramidal cells in the rat hippocampus fire whenever the animal is in a particular place, suggesting that the hippocampus maintains a representation of the environment. Receptive fields of place cells (place fields) are largely determined by the distance of the rat from environmental walls. Because these walls are sometimes distinguishable only by their orientation with respect to the outside room, it has been hypothesised that a polarising directional input enables the cells to locate their fields off–centre in an otherwise symmetrical environment. We tested this hypothesis by gaining control of the rat’s internal directional sense, independently of other cues, to see whether manipulating this sense could, by itself, produce a corresponding alteration in place field orientation. Place cells were recorded while rats foraged in a rectangular box, in the absence or presence of external room cues. With room cues masked, slow rotation of the rat and the box together caused the fields to rotate accordingly. Rotating the recording box alone by 180° rarely caused corresponding field rotation, while rotating the rat alone 180° outside the environment and then replacing it in the recording box almost always resulted in a corresponding rotation of the fields. This shows that place field orientation can be controlled by controlling the internal direction-sense of the rat, and it opens the door to psychophysical exploration of the sensory basis of the direction sense. When room cues were present, distal visual cues predominated over internal cues in establishing place field orientation.

Using a mobile robot to test a model of the rat hippocampus

A model of how internal and external sensory information contribute to the firing of place cells in the rat hippocampus, and of how these cells contribute to the rats spatial behavior, is tested on a miniature mobile robot. The experiments show that crude visual, odometric and short-range proximity information provided by sensors on the robot is sufficient to enable the formation of a robust spatial code within rectangular environments. They further show that the model of navigation can accurately return the robot to an unmarked goal location. Since the rats perceptual systems are probably similarly crude, these results support our intuition that the model of hippocampal function is reasonable in not demanding too much of its inputs. The combined robotic and neuronal simulation can be used to make predictions regarding both electrophysiological and behavioral experiments. Finally, the model is applied to the neural basis of navigation in humans.

The Representation of Space and the Hippocampus in Rats, Robots and Humans

Abstract Experimental evidence suggests that the hippocampus represents locations within an allocentric representation of space. The environmental inputs that underlie the rat’s representation of its own location within an environment (in the firing of place cells) are the distances to walls, and different walls are identified by their allocentric direction from the rat. We propose that the locations of goals in an environment is stored downstream of the place cells, in the subiculum. In addition to firing rate coding, place cells may use phase coding relative to the theta rhythm of the EEG. In some circumstances path integration may be used, in addition to environmental information, as an input to the hippocampal system. A detailed computational model of the hippocampus successfully guides the navigation of a mobile robot. The model’s behaviour is compared to electrophysiological and behavioural data in rats, and implications for the role of the hippocampus in primates are explored.

Neural Recording Using Digital Telemetry

Digital telemetry (DT) offers a method of collecting the electrical signals produced by neural activity and transmitting them wirelessly to a receiver/decoder for analysis and storage. The wirelessness means that activity can be recorded from a subject that is behaving relatively normally, which opens up a number of research and therapeutic opportunities - for example, in the study of spatial encoding, or in pre-seizure activity in an epileptic subject. In this chapter we first review the history of neural recording and describe the classic analog method of data processing, outlining the technical problems that need to be solved in collecting and transmitting tiny electrical signals within a noisy environment. We then outline digital signal processing together with the basic principles of telemetry, describing how DT solves these problems in a way that preserves signal fidelity while allowing subjects to move around in an unconstrained way. We finish by describing several situations in which DT is enabling advances to occur both in the laboratory and in the clinic.