it plays a role in emotion, memory and learning. the main olfactory bulb connects to the amygdala via the piriform cortex of the primary olfactory cortex and directly projects from the main olfactory bulb to specific amygdala areas. the amygdala passes olfactory information on to the hippocampus. the orbitofrontal cortex, amygdala, hippocampus, thalamus, and olfactory bulb have many interconnections directly and indirectly through the cortices of the primary olfactory cortex. these connections are indicative of the association between the olfactory bulb and higher areas of processing, specifically those related to emotion and memory. amygdala associative learning between odors and behavioral responses takes place in the amygdala. the odors serve as the reinforcers or the punishers during the associative learning process; odors that occur with positive states reinforce the behavior that resulted in the positive state while odors that occur with negative states do the opposite. odor cues are coded by neurons in the amygdala with the behavioral effect or emotion that they produce. in this way odors reflect certain emotions or physiological states. odors become associated with pleasant and unpleasant responses, and eventually the odor becomes a cue and can cause an emotional response. these odor associations contribute to emotional states such as fear. brain imaging shows amygdala activation correlated with pleasant and unpleasant odors, reflecting the association between odors and emotions. hippocampus the hippocampus aids in olfactory memory and learning as well. several olfaction-memory processes occur in the hippocampus. similar to the process in the amygdala, an odor is associated with a particular reward, i.e. the smell of food with receiving sustenance. odor in the hippocampus also contributes to the formation of episodic memory; the memories of events at a specific place or time. the time at which certain neurons fire in the hippocampus is associated by neurons with a stimulus such as an odor. presentation of the odor at a different time may cause recall of the memory, therefore odor aids in recall of episodic memories. olfactory coding in habenula in lower vertebrates (lampreys and teleost fishes), mitral cell (principal olfactory neurons) axons project exclusively to the right hemisphere of habenula in an asymmetric manner. it is reported that dorsal habenula (hb) are functional asymmetric with predominant odor responses in right hemisphere. it was also shown that hb neurons are spontaneous active even in absence of olfactory stimulation. these spontaneous active hb neurons are organized into functional clusters which were proposed to govern olfactory responses. (jetti, sk. et al. 2014, current biology) depression models further evidence of the link between the olfactory bulb and emotion and memory is shown through animal depression models. olfactory bulb removal in rats effectively causes structural changes in the amygdala and hippocampus and behavioral changes similar to that of a person with depression. researchers use rats with olfactory bulbectomies to research antidepressants. research has shown that removal of the olfactory bulb in rats leads to dendrite reorganization, disrupted cell growth in the hippocampus, and decreased neuroplasticity in the hippocampus. these hippocampal changes due to olfactory bulb removal are associated with behavioral changes characteristic of depression, demonstrating the correlation between the olfactory bulb and emotion. the hippocampus and amygdala affect odor perception. during certain physiological states such as hunger a food odor may seem more pleasant and rewarding due to the associations in the amygdala and hippocampus of the food odor stimulus with the reward of eating. orbitofrontal cortex olfactory information is sent to the primary olfactory cortex, where projections are sent to the orbitofrontal cortex. the ofc contributes to this odor-reward association as well as it assesses the value of a reward, i.e. the nutritional value of a food. the ofc receives projections from the piriform cortex, amygdala, and parahippocampal cortices. neurons in the ofc that encode food reward information activate the reward system when stimulated, associating the act of eating with reward. the ofc further projects to the anterior cingulate cortex where it plays a role in appetite. the ofc also associates odors with other stimuli, such as taste. odor perception and discrimination also involve the ofc. the spatial odor map in the glomeruli layer of the olfactory bulb may contribute to these functions. the odor map begins processing of olfactory information by spatially organizing the glomeruli. this organizing aids the olfactory cortices in its functions of perceiving and discriminating odors. adult neurogenesis the olfactory bulb is, along with both the subventricular zone and the subgranular zone of the dentate gyrus of the hippocampus, one of only three structures in the brain observed to undergo continuing neurogenesis in adult mammals. in most mammals, new neurons are born from neural stem cells in the sub-ventricular zone and migrate rostrally towards the main and accessory olfactory bulbs. within the olfactory bulb these immature neuroblasts develop into fully functional granule cell interneurons and periglomerular cell interneurons that reside in the granule cell layer and glomerular layers, respectively. the olfactory sensory neuron axons that form synapses in olfactory bulb glomeruli are also capable of regeneration following regrowth of an olfactory sensory neuron residing in the olfactory epithelium. despite dynamic turnover of sensory axons and interneurons, the projection neurons (mitral and tufted neurons) that form synapses with these axons are not structurally plastic. the function of adult neurogenesis in this region remains a matter of study. the survival of immature neurons as they enter the circuit is highly sensitive to olfactory activity and in particular associative learning tasks. this has led to the hypothesis that new neurons participate in learning processes. no definitive behavioral effect has been observed in loss-of-function experiments suggesting that the function of this process, if at all related to olfactory processing, may be subtle. clinical significance the olfactory lobe is a structure of the vertebrate forebrain involved in olfaction, or sense of smell. destruction of the olfactory bulb results in ipsilateral anosmia. other animals evolution fossil endocast of a tyrannosaurus cranial vault, showing extensive olfactory bulb (structure to the left). tyrannosaur dinosaurs, as well as carnivorous animals in general, have highly developed olfactory bulbs to seek out prey. comparing the structure of the olfactory bulb among vertebrate species, such as the leopard frog and the lab mouse, reveals that they all share the same fundamental layout (five layers containing the nuclei of three major cell types; see "anatomy" for details), despite being dissimilar in shape and size. a similar structure is shared by the analogous olfactory center in the fruit fly drosophila melanogaster, the antennal lobe. one possibility is that vertebrate olfactory bulb and insect antennal lobe structure may be similar because they contain an optimal solution to a computational problem experienced by all olfactory systems and thus may have evolved independently in different phyla – a phenomenon generally known as convergent evolution. "the increase of brain size relative to body size—encephalization—is intimately linked with human evolution. however, two genetically different evolutionary lineages, neanderthals and modern humans, have produced similarly large-brained human species. thus, understanding human brain evolution should include research into specific cerebral reorganization, possibly reflected by brain shape changes. here we exploit developmental integration between the brain and its underlying skeletal base to test hypotheses about brain evolution in homo. three-dimensional geometric morphometric analyses of endobasicranial shape reveal previously undocumented details of evolutionary changes in homo sapiens. larger olfactory bulbs, relatively wider orbitofrontal cortex, relatively increased and forward projecting temporal lobe poles appear unique to modern humans. such brain reorganization, beside physical consequences for overall skull shape, might have contributed to the evolution of h. sapiens' learning and social capacities, in which higher olfactory functions and its cognitive, neurological behavioral implications could have been hitherto underestimated factors."