waves, a process termed phase precession. Upon entering a place field, place cells will fire in bursts at a particular point in the phase of the underlying theta waves. However, as an animal progresses through the place field, the firing will happen progressively earlier in the phase. It is thought that this phenomenon increases the accuracy of the place coding, and aids in plasticity, which is required for learning. Directionality In some cases place cells show directionality, meaning they will only fire in a location when the subject is travelling in a particular direction. However, they may also be omnidirectional, meaning they fire regardless of the direction the subject. The lack of directionality in some place cells might occur particularly in impoverished environments, whereas in more complicated environments directionality is enhanced. The radial arm maze is one such environment where directionality does occur. In this environment, cells may even have multiple place fields, of which one is strongly directional, while the others are not. In virtual reality corridors, the degree of directionality in the population of place cells is particularly high. The directionality of place cells has been shown to emerge as a result of the animal's behaviour. For example, the receptive fields become skewed when rats travel a linear track in a single direction. Recent theoretical studies suggest that place cells encode a successor representation which maps the current state to the predicted successor states, and that directionality emerges from this formalism. This computational framework also provides an account for the distortion of place fields around obstacles. Sensory input Place cells were initially believed to fire in direct relation to simple sensory inputs, but studies have suggested that this may not be the case. Place fields are usually unaffected by large sensory changes, like removing a landmark from an environment, but respond to subtle changes, like a change in color or shape of an object. This suggests that place cells respond to complex stimuli rather than simple individual sensory cues. According to the functional differentation model, sensory information is processed in various cortical structures upstream of the hippocampus before actually reaching the structure, so that the information received by place cells is a compilation, a functional derivative, of different stimuli. Sensory information received by place cells can be categorized as either metric or contextual information, where metric information corresponds to where place cells should fire and contextual input corresponds