the dorsal and ventral hippocampus, consequently altering fear conditioning in rats. historically, the earliest widely held hypothesis was that the hippocampus is involved in olfaction. this idea was cast into doubt by a series of anatomical studies that did not find any direct projections to the hippocampus from the olfactory bulb. however, later work did confirm that the olfactory bulb does project into the ventral part of the lateral entorhinal cortex, and field ca1 in the ventral hippocampus sends axons to the main olfactory bulb, the anterior olfactory nucleus, and to the primary olfactory cortex. there continues to be some interest in hippocampal olfactory responses, in particular, the role of the hippocampus in memory for odors, but few specialists today believe that olfaction is its primary function. function theories of hippocampal functions over the years, three main ideas of hippocampal function have dominated the literature: response inhibition, episodic memory, and spatial cognition. the behavioral inhibition theory (caricatured by john o'keefe and lynn nadel as "slam on the brakes!") was very popular up to the 1960s. it derived much of its justification from two observations: first, that animals with hippocampal damage tend to be hyperactive; second, that animals with hippocampal damage often have difficulty learning to inhibit responses that they have previously been taught, especially if the response requires remaining quiet as in a passive avoidance test. british psychologist jeffrey gray developed this line of thought into a full-fledged theory of the role of the hippocampus in anxiety. the inhibition theory is currently the least popular of the three. the second major line of thought relates the hippocampus to memory. although it had historical precursors, this idea derived its main impetus from a famous report by american neurosurgeon william beecher scoville and british-canadian neuropsychologist brenda milner describing the results of surgical destruction of the hippocampi when trying to relieve epileptic seizures in an american man henry molaison, known until his death in 2008 as "patient h.m." the unexpected outcome of the surgery was severe anterograde and partial retrograde amnesia; molaison was unable to form new episodic memories after his surgery and could not remember any events that occurred just before his surgery, but he did retain memories of events that occurred many years earlier extending back into his childhood. this case attracted such widespread professional interest that molaison became the most intensively studied subject in medical history. in the ensuing years, other patients with similar levels of hippocampal damage and amnesia (caused by accident or disease) have also been studied, and thousands of experiments have studied the physiology of activity-driven changes in synaptic connections in the hippocampus. there is now universal agreement that the hippocampi play some sort of important role in memory; however, the precise nature of this role remains widely debated. a recent theory proposed – without questioning its role in spatial cognition – that the hippocampus encodes new episodic memories by associating representations in the newborn granule cells of the dentate gyrus and arranging those representations sequentially in the ca3 by relying on the phase precession generated in the entorhinal cortex rats and cognitive maps the third important theory of hippocampal function relates the hippocampus to space. the spatial theory was originally championed by o'keefe and nadel, who were influenced by american psychologist e.c. tolman's theories about "cognitive maps" in humans and animals. o'keefe and his student dostrovsky in 1971 discovered neurons in the rat hippocampus that appeared to them to show activity related to the rat's location within its environment. despite skepticism from other investigators, o'keefe and his co-workers, especially lynn nadel, continued to investigate this question, in a line of work that eventually led to their very influential 1978 book the hippocampus as a cognitive map. there is now almost universal agreement that hippocampal function plays an important role in spatial coding, but the details are widely debated. later research has focused on trying to bridge the disconnect between the two main views of hippocampal function as being split between memory and spatial cognition. in some studies, these areas have been expanded to the point of near convergence. in an attempt to reconcile the two disparate views, it is suggested that a broader view of the hippocampal function is taken and seen to have a role that encompasses both the organisation of experience (mental mapping, as per tolman's original concept in 1948) and the directional behaviour seen as being involved in all areas of cognition, so that the function of the hippocampus can be viewed as a broader system that incorporates both the memory and the spatial perspectives in its role that involves the use of a wide scope of cognitive maps. this relates to the purposive behaviorism born of tolman's original goal of identifying the complex cognitive mechanisms and purposes that guided behaviour. it has also been proposed that the spiking activity of hippocampal neurons is associated spatially, and it was suggested that the mechanisms of memory and planning both evolved from mechanisms of navigation and that their neuronal algorithms were basically the same. many studies have made use of neuroimaging techniques such as functional magnetic resonance imaging (fmri), and a functional role in approach-avoidance conflict has been noted. the anterior hippocampus is seen to be involved in decision-making under approach-avoidance conflict processing. it is suggested that the memory, spatial cognition, and conflict processing functions may be seen as working together and not mutually exclusive. role in memory