Readout of percepts
Given the synthesis of a wave packet corresponding to a percept, the next question is, how is it conveyed to those parts of the brain that will respond with it? The AM patterns are observed in dendritic potentials, but they are conveyed by action potentials of the many neurons from the bulb and widely disseminated over the olfactory cortex. While the commonality of the carrier of the population output is limited to a large fraction of the total macroscopic activity manifested in the EEGs, it consists of a very small fraction of the microscopic neuronal activity in the bulb, perhaps only one part in 1,000 to 10,000. How in the next stage, the olfactory cortex, is the population output extracted by the receiving neurons under the conditions of an extremely low signal to noise ratio?
One explanation is that because the output of each bulbar neuron diverges widely over the cortex (Figure 3), each cortical neuron sums input from neurons scattered widely over the bulb, thereby effecting a spatial integration over the bulbar population. The only activity that survives this integration is that which has a common instantaneous frequency, provided further that the phase dispersion of the carrier is under a quarter of a cycle of the dominant frequency of oscillation. All other activity averages toward zero. But the common activity is the carrier. It is the cooperative output of the population that creates and transports the perceptual information, and the measurements of the phase gradient of the carrier show that its spread is under the specified limit to avoid degradation by phase dispersion.
In a word, divergence in the bulbar output pathway "launders" the bulbar message, and it can only do so if the carrier oscillates (Freeman, 1991). The divergence will do this optimally if the undesired components are not spatially coherent, that is, at different frequencies at different points in the transmitting cortex. This answer explains the utility of the divergence in the output pathway of the bulb and also of the oscillatory carrier. However, this answer cannot account for the utility of the chaotic carrier, because a limit cycle carrier or a noise carrier would serve as perhaps well if it were spatially coherent. Then why is brain activity chaotic? This fundamental question remains unanswered, but there are some directions in which to search for answers. One suggestion is that chaos is inevitable in so large and complex a system as the simplest brain with its many interconnections. Engineers and computer scientists already find undesired chaos emergent in their more advanced systems. If this is the source and significance of brain chaos, perhaps the study of brain dynamics can show how to function in chaos.
Yet the evidence suggests that brain chaos is the result of design by evolution and is not an epiphenomenon of hypercomplexity. Brains are intrinsically unstable. Even when animals are at rest and immobile their brains ceaselessly generate temporally unpatterned activity reflected in their EEGs. The chaotic olfactory EEG is not, as is commonly said, "a noise like the roar of a crowd at a ball game", because the EEG is not the smoothed and degraded sum of single cell activities. It is a deterministic output like a complex modern symphony that is orchestrated by the system. It is not a product of the bulb or cortex alone but of these and other parts of the olfactory system in feedback interaction. If they are surgically separated from each other, the activity vanishes (Freeman, 1975).
One role for this mechanism might be to provide unpatterned activity as a form of exercise for neurons, which must keep active or die. In contrast to this view the "grandmother cell" hypothesis requires that neurons wait in silence through days or years for their trigger stimuli to appear and then fire faultlessly on feature presentation. This is biologically unrealistic. But a more important postulated role of chaos for brain function is predicated on the fact that chaotic systems not only destroy information; they create it. The essence of a perceptual act may consist in the neurodynamics which creates a neural activity pattern from the raw materials of stimulus, experience and expectancy. The greatest achievement by an act of perception is the immediate creation of the meaning for the subject of some relevant information from a continuous environmental inflow that is complex beyond all measure.
Man-made information processing systems face this same infinite rate of information flow, and they handle the task by use of filters (Figure 16). The filters define what is "signal" and remove everything else as "noise". In all cases it is the engineer or observer who decides what is signal and what is noise, either by direct construction of the filter or by use of a reference standard with which to compute an error, so as to shape a filter through a "teaching" procedure as in back propagation. Such an agent in a brain would be a homunculus and is not admissible. Brains do not allow direct intervention by outside observers to determine what is feature or signal and what is to be ground or noise. Brains act to organize themselves and to shape the environment and the relations of their sensors to the environment, as in sniffing or moving the eyes. The raw act of perception by hypothesis may be replacement of an infinite environmental inflow with a finite dimensioned chaotic activity pattern. The fractal dimension may be determined within the system and may vary according to prevailing circumstances and constraints. The requisite perceptual meaning may be freshly minted under the stimulus conditions and the state of the brain holding at the instant of the state transition of the system away from the basal chaotic attractor into a wing.