For this paper, I'd like to revisit some of the questions left unanswered in my last paper regarding the sense of smell. In that paper, I outlined some of what is now understood about how smells are identified and the initial levels of how that information is coded in the brain; for this paper I would like to trace that path (as much as possible) through to my experience of smell and then to see if my experience matches the proposed models. From this perspective, I'd like to take a last look at the "brain = behavior" equation and the notion of the "I - function" and see if I can't make some final sense of it all in a way which is not utterly dissonant with my own experiences.
Let's look at smell again, then. My last paper left off with the following conclusions regarding the olfactory system. There are between 500 and 1000 unique protein receptor genes which are expressed only in the olfactory epithelium. These receptors each respond to a unique odorant or to a unique feature on an odorant molecule (epitopes). It is suggested that there is a one - to - one relationship between a specific odorant, its protein receptor, and the sensory neuron: that is, any given sensory neuron expresses only one type of receptor and is therefore responsive to only one kind of odorant. Each type of neuron is randomly distributed across one of four zones within the olfactory epithelium. The information from this population coding is then reorganized, as these axons leave the epithelium and travel to the olfactory bulb, into a very specific, spatially organized map of activity across the several hundred kinds of receptors. The span between the 1000 types of receptor neurons, and discrimination amongst 10,000 odors, is bridged in the interpretation of the ratios and relationships of activity level across the population. The olfactory bulb was compared to an operators switchboard, and the process of odor identification was likened to determining which switchboard lights were flashing. The obvious question then becomes, what parts of the brain watch over the olfactory bulb, monitor its activity and interpret that activity? What parts of the brain assign meaning and identity to each pattern of stimulation, and then choose an appropriate response?
Some of these questions have been addressed by Walter Freeman in his investigations, and he has several useful insights into the process of preattentive perception, or the almost instantaneous recognition of the familiar. Not unexpectedly, it turns out that the analogy between the olfactory bulb and an operators switchboard is useful only in the most superficial way. It holds water in the sense that the bulb does organize its information spatially, and it transmits this reorganized information to the olfactory cortex; the olfactory cortex then synthesizes a new set of signals, and this is sent to many other parts of the brain(2). However, the difficult - to - resist tendency to compare any neural functioning to any bit of automated machinery continually does a disservice to the reality of the situation, which is that perception is often driven by internally set priorities which vary with time and experience. The observations which arose from these experiments and which give an example of these internally driven perception patterns have had a rather dramatic impact on my sense of how we learn things.
Freeman inserted 60 electrodes 0.5 mm apart in a grid-like arrangement into the surface of the olfactory bulb of entrained animals (2). These animals were trained to recognize certain odors and to behave in a specific way upon recognition; the electrodes allowed the researchers to trace EEG patterns simultaneously across the entire bulb as the animal experienced training and then recognition (2). While EEG readings of almost any animal while awake are irregular and seemingly without pattern, the recognition of something familiar corresponded with a "burst" in the EEG - waves being more orderly for a few cycles. A pattern corresponding to a particular odorant was found in the shape of the carrier wave in EEGs recorded simultaneously while the amplitudes varied, but surprisingly, the shape of the wave was not consistent with repeated recognition of the same odorant (2). The investigators did not find a pattern of neural activity that was constant for a particular known scent until they mapped carrier wave amplitude with respect to the space across the entire bulb (2). From this information they were able to devise a kind of contour map which emerged every time the animal got a whiff of something familiar, even though the shape of the waves themselves varied (2). However, if the animal was subsequently trained to a new, second smell, the map of the first one changed (2). That is to say, once a rabbit is trained to recognize odorant A, there is a specific map A of carrier wave amplitudes across the bulb which arises every time the rabbit is exposed to A. If the rabbit is subsequently trained to recognize a new smell B, then a specific map B arises which is the same every time rabbit is exposed to B, and a new map C also arises which reflects the recognition of A. The pattern of recognition unique to A has changed after the rabbit has gotten familiar with B. This is a remarkably succinct demonstration of two related points: first, that experience is clearly brought to bear not just in an animals response to stimuli but in its very perception; and secondly that some of these changes occur in the olfactory bulb, not exclusively in the "higher functioning" parts of the brain. In a very basic way, every new thing learned changes the shape of our understanding of what we already know.
Just the fact that every object of recognition has a unique pattern of electrical expression across a particular part of the brain is in itself a good basis for a first look at the neuroanatomy of memory. If we accept the observation that pattern of neural activity associated with recognition changes as new things are added to the repertoire of the familiar, then questions arise about not just memory and recognition, but also about comprehension. Freeman defines "gestalt" as "a meaning-laden perception... that is unique to each individual" (2). This term is useful both in that it allows for a changing significance over time, and in that unique past experience is inherent to it. How does the brain travel from perception to gestalt? The EEG patterns collected by FreemanŐs group begin to shed some light on how meaning is assigned to the familiar and how the brain both reflects and directs changes in that meaning. It is also an example of how changes and processes in the brain are considerably more cooperative and anatomically integrated than had been previously thought. There are two other influences on the process of recognition which Freeman postulates and which I think bear considering before we leave the olfactory bulb for other regions of the brain.
The first of these is hypothetical. Freeman suggests the existence of a group of neurons called the nerve cell assembly, which are stimulated by other neurons during learning. The behavior of these cells is dictated by Hebbs Rule, that "synapses between neurons that fire together become stronger, as long as the synchronous firing is accompanied by a reward" (2). These neurons effectively connect the sensory input neurons to each other by receiving input from several of them at once; when simultaneous excitation among them occurs in connection with a consequence, the Hebbian synapses of the nerve cell assembly are selectively strengthened (2). That strengthening is reflected in increased sensitivity in the post-synaptic cell; in this case, the synapses between the connected neurons which are excited by input (2). The consequence of such a system is that it would help coordinate the "burst" which occurs with recognition, and that this would then help the input associated with a familiar scent stand out against less important incoming information. These assemblies thus strengthened would be a physical "repository for past associations" (2), reflecting not just recognition of a smell but the experiences which went along with it, assuming that the experiences were significant in some way.
The second influence which modulates our sensitivity to familiar scents is not hypothetical, but would complement the activity of the postulated nerve cell assemblies. This is the "priming" of the olfactory bulb, which has the overall effect of increasing sensitivity (2). One way this is accomplished is through system-wide arousal, like hunger or thirst, sexual attraction, or fear; these states are directed by other parts of the brain through both axonal connections to the bulb and the release of modulatory chemicals (2). Another way that priming is accomplished is through input itself. Freeman's lab work has shown that a neuron's output can increase with repeated stimulation, so that its response is essentially "louder" the second time it is stimulated, and even more so the third time, until some maximum threshold is reached (2). The Hebbian nerve cell assemblies, embedded in the primed olfactory bulb, magnify and amplify the excitatory signals of a familiar smell, so that "the input rapidly ignites an explosion of collective activity" (2).
These are all key examples of how the act of perceiving is driven as much by internal state and past experience as it is by the object perceived; it is as subjective as it is objective. And, at least in part, the sensory system is capable of modulating itself and its recognition and its response - it is not just reporting, it is interpreting as well.
The synchronized activity of the neurons in the olfactory bulb upon recognizing an odor has the effect of putting the message in capital letters and then putting several exclamation points on the end. The synchronized signals are additive, whereas the unsynchronized signals reflecting irregular everyday life essentially cancel each other out (2). This is comparable to the effect of several people in a room. If you walk into a room of twenty people who are all talking amongst themselves, you hear a murmur of voices in which individual sentences or stories are hard to distinguish. If, however, those twenty people are all singing "Happy Birthday", then you get the message loud and clear. Such are the effects of synchrony. Thus, a synchronized signal has priority: it insures that the olfactory cortex gets the message. This works because of the extensive branching that links the two areas, so that each bulb neuron synapses with several thousand cortical cells, and each cortical cell receives input from several thousand bulbar cells (2). Presumably, the cortex has a system in place for generating its own collective bursts, which one would expect to be qualitatively different from those of the bulb (2). If this is so, it would indeed act as a filter, screening out the ambient noise of unfamiliar smells and transmitting only the news of something already known. This collective burst would then be transmitted to other parts of the brain, a message with its edges smoothed out and its salient features highlighted. Primarily, these messages seem to go to the entorhinal complex and the limbic system, "where signals are combined with those from other sensory systems" (2).
The system of cross-wiring between bulb and cortex has another important consequence: it allows any one cell in the cortex to "view" the activity of several thousand bulbar cells. Thus, in the one direction, the message from the outside world is refined, while in the other direction, the window through which one part of the brain views the world is enlarged. The scope of its vision is wider (4).
Which leads us, finally, to a model of the relationship between memory and smell, and to an idea of how gestalts might be formed. It relies rather heavily on this progression through layers, in which every step is a kind of distillation of the message: bulk is lost, but clarity and concentration is gained. Thus, if there is are ways for the cells of the limbic system to build associative relationships in the same way that the olfactory bulb can, and they are receiving messages from all five senses, and the associations are reinforced by experience, then we've described an organization which links "multimedia" perspectives, and assigns meaning to them. It provides a possible basis for understanding why, for example, a smell which reminds us of something often accompanies a heightened sense of awareness: the smell is one part of a cascade, and it has alerted our bodies to look out for the other, corroborating parts. Perhaps the act of remembering is just that: a way for our senses to look around for something that used to be there, and to take stock of what changes have occurred. The fact that the limbic system is involved in both memory and emotions certainly provides some clue as to why the act of remembrance can be so emotionally laden.
Another satisfying aspect of this admittedly enormously incomplete model is that it accounts for changing perspective. Memories donŐt go away as much as they morph over time, with new experiences, shifting to accommodate new patterns but never completely disappearing.
Although the experiments described were done with animals in a specific, rewarded learning system, it is not hard to imagine similar patterns guiding more complex experiences. For the purposes of this paper, it is not quite so important to know precisely how such processes differ in a non-controlled environment, or for that matter in a nervous system which has a "more developed" anatomy, or "sense of self". What is important is that one can begin to imagine how "brain = behavior", or how a physical system can give rise to such a rich and complex set of experiences as that of being human. Rather than being reductive or overly simplistic, the model that begins to emerge is one that intrigues, and then finally convinces you that such a thing is possible.
Text:
(1) Delcomyn, Fred, Foundations of Neurobiology, W.H. Freeman and Co., New York, New York, 1998.
(2) Freeman, Walter, "The Physiology of Perception", Sci. Am., Feb., 1991.
(4) Mishkin, M. and Appenzeller, T. "The Anatomy of Memory", Scientific American, June 1987.
World Wide Web:
(6) "The Memory of Smells", from The Mystery of Smell, Howard Hughes Medical Institute.
(7) "Cortical Columns, Modules, and Hebbian Cell Assemblies" reprinted from , by William Calvin, The Handbook of Brain Theory and Neural Networks, MIT Press, 1995.
(8) "Debunking the Digital Brain", by Christof Koch.
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