The mystery of smell
It might be usefull to take a quick look at the use that mother nature saw for emotional reactions and an emotional based motivation in the more advanced species. The more obvious advantage is the possibility of "long distance reactions": If you can se a treathening situation as just that - threatening - you can react accordingly, even if you are at safe distance in time or space. But I will suggest that the really significant advantage is the general ability to proces stimuli in the kind of virtual reality commonly refered to as abstract thinking. Abstract here refers to something outside any sense-modality, so that any visual representation would be abstract in relation to sound perception, i.e. you hear a well known voice and "see a face" in your mind, even if you are just on the phone.
What has all this to do with smell? Well, even if it is not totaly evident I tend to think that there must be a very strong connection between the function of smell and the emotional reactions that we have. Or rather that the latter is a updated version of the first.
Lets go back in time to trias, the era when dinosaurs largely ruled the world and where the first mamals evolved. Jeholodens were one of the early mamals, a long-tailed, quadrupedal mammal, about 5 inches long. An insectivore, it had relatively advanced, grasping hands, but a primitive pelvis and hind limbs. It may have had large eyes, and may have been nocturnal (most active at night) which is significant.
Being a mamal meant internal regulation of temperature instead of the reptiles and dinosaurs primitive reliance on the ambient temperature. It meant that they could be active at night when it was cold and when the hungry thecodonts would be "sleeping", or rather inactive from hypothermia.
This meant that the world was largely theirs in the dark hours, but it introduced the slight problem of navigating in complete darkness. In order to overcome this limitation early mamals develloped an acute sense of smell. The natural selection for variations more suited to take advantage of the cold dark night would enshure that a the early mamals got the needed equipment. This makes it very likely that the early devellopment of the mamalian brain had nothing to do with more intelligence as such, but was a function of the evolving olfactory bulp.
There is however one slight glitch in this theory: The sense of smell isnt really very usefull. With visual information you can do a lot of analysis: the angular difference from each eye will give you a clue to distance and the visual brain will actually give you a 3-dimentional derivate from two different 2-dimentional representations in each eye. Similarly with hearing: The outer ears are designed to distort the sound according to the direction its coming from and with added phase analysis between the sounds in the two ears you will get a sense of room with at least two dimensions represented clearly.
The trouble with smell is that you cant really do anything with it. Light moves in straight lines and the eye will produce a perfect array representing the distribution in space of the souces of the light. The molecules that are detected as smell just hangs or drifts in the air. The one thing you can do with smell is recognition. You can recognise a specific smell as that particular smell. End of story.
This begs the question: If the start of mamalian brain-devellopment was due to increased olfactory capabillites, then what are those extra capabillities used for? Nature dont waste energy, if there was a larger olfactory bulp develloped, the there must have been a vital use for it.
It has been suggested that the olfactory bulp became the site for imaginations: It is deep dark night and some early mamal get wind of a suiting prey. What it needs to do is a bit of hunting. Had there been light it would have had an easy translation between the visual information and the adequate hunting behaviour. The visuals would be processed in 3D and make a nice source for spatial navigation of the legs and teeth. If you now replace the visual stimulation with smell you loose the link between stimuli and behavior, a smell is not suggestive of direction. It would seem that what the brain actually did in lack of visual was to enhance the stimuli of smells with vivid imaginations of the scenario. If the hearing was good it could support the virtual visual and give a bit of direction. But most importantly the stimuli would be placed in a "space" where coordination of bodymovements would take place. The smelled objects in this imagined space would have to have a pretty strong motivating effect as well and this fits very well with the very strong instinctive reaktions that we can have to certain smells. If you have learned to loathe a specific smell by getting sick after eating say asparges, this loathe will be almost impossible to overcome, even if you know that youn didnt get sick from the asparges but from a virus you contracted.
So to summerize: Recognition of a specific smell as coming from a prey animal would lead to strong arrousal and keen motivation, it would conjure up a vivid "visualisation" of the prey in 3 dimensions, guided also by sounds, whatever visual perception was possible, and the proprioceptive feedback from the moving legs. Any specific significant smell would be recognised based on experience and trigger a huge motivated response.
This would all be a curiocity in natural history if it wasnt for one significant aspect: Given that this worked so well for smell, why not try the same trick based on other senses? Why not take any significant sign - be it visual or audital - and make it trigger an appropriate (based on your personal experience) response?
Any such general ability would come pretty close to what we understand as our emotional reactions and I think that this gives a great set of clues to what normal emotional function is like and how to deal with it.
Now there is a lot of speculation in this theory - a lot of things could turn out not to be the case in reality - but that is not so important. The central point is to get a sense of the dynamic involved in interpretation of significant signs in the inviroment and the ways that imagined space plays a role in the proces of alligning present stimuli with past experience. The way that experience based imaginations will serve as a guiding matrix for any significant sense input.