Overview
The olfactory tubercle differs in location and relative size between humans, non-humane primates, rodents, birds and other animals. In most cases, the olfactory tubercle is identified as a round bulge along the basal forebrain anterior to the optic chiasm and posterior to the olfactory peduncle (Wesson & Wilson 2010). In humans and non-human primates, visual identification of the olfactory tubercle is not easy because the basal forebrain bulge is small in these animals (Milhouse & Heimer 1984). With regards to functional anatomy, the olfactory tubercle can be considered to be a part of three larger networks. First it is considered to be part of the basal forebrain, the nucleus accumbens, and the amygdaloid nuclei because of its location along the rostral ventral region of the brain; that is the front bottom part. Second it considered to be part of the olfactory cortex because it receives direct input from the olfactory bulb. Third, it is also considered to be part of the ventral striatum based on anatomy, neurochemical and embryology data.
One of the most striking features of the olfactory tubercle are the closely packed crescent shaped cell clusters which mostly located in layer III and sometimes in layer II. These cells clusters, called the islands of calleja, are innervated by dopaminergic projections from the nucleus accumbens and the substantia nigra suggesting the role that the olfactory tubercle plays in the reward system.
The olfactory tubercle is a multi-sensory processing center due to the number of innervations going to and from other brain regions such as the amygdala, thalamus, hypothalamus, hippocampus, brain stem, auditory and visual sensory fibers, and a number of structures in the reward–arousal system as well as the olfactory cortex. Due to its many innervations from other brain regions, the olfactory tubercle is involved in merging information across the senses, such as olfactory—audition and olfactory—visual integrations, possibly in a behaviorally relevant manner. Thus damage to the olfactory tubercle is likely to affect the functionality of all these area of the brain. Examples of such disruption include changes in normal odor guided behavior, and impairments in modulating state and motivational behavior (Wesson & Wilson 2011) which are common in psychiatric disorders such as schizophrenia (Rupp et al. 2005), dementia (Murphy, Nordin & Jinich 1999) and depression (Negoias et al. 2010).
The olfactory tubercle has been shown to play a large role in behavior. Unilateral lesions in the olfactory tubercle have been shown to alter attention, social and sensory responsiveness and even locomotor behavior (Koob 1978). Bilateral lesions have been shown to reduce copulatory behavior in male rats. The olfactory tubercle has also been shown to be especially involved in reward and addictive behaviors. Rats have been shown to administer cocaine into the olfactory tubercle more than the nucleus accumbens and ventral pallidum, other reward centers in the brain (Ikemoto 2003). In fact they will administer cocaine into the olfactory tubercle at about 200 times per hour and even till death.
Functional contributions of the olfactory tubercle to olfaction are currently unclear; however there is evidence of a perceptual role that it may play. Work from Zelano et al. (2007) suggest that the olfactory tubercle may be crucial in sorting out the sources of olfactory information. This suggests that it may also play a role in odor guided behavior. Thus it may link perception of odor with action through its connections with attention, reward and motivation systems of the basal forebrain (Wesson & Wilson 2011). Functional imaging data from this same group also shows that the olfactory tubercle is highly activated during tasks that engage attention, thus playing a large role in arousal related systems.
As mentioned earlier, because the olfactory tubercle is a component of the ventral striatum, it is heavily interconnected with several affective, reward, and motivation related centers of the brain. It also sits at the interface between the olfactory sensory input and state dependent behavioral modulatory circuits, that is the area that modulates behavior during certain physiological and mental states. Thus the olfactory tubercle may also play an important role in the mediation of odor approach and odor avoidance behavior, probably in a state dependent manner (Gervais 1979).
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