Epilogue
How and where in the brain are all the bits assembled to form a complete image?
Nobody knows! But there are some theories concerning this binding problem.
It is clear that we have a modular brain. The brain is not centrally organized. Instead different parts of the brain not only control different abilities they operate independently. It is a very complex structure well described by Koch and Laurant (1999):
"Advances in neuroscience have revealed the staggering complexity of even "simple" nervous systems. This reflected in their function, their evolutionary history, their structure and the coding schema they use to represent information. These four viewpoints need all play a role in any future science of "brain complexity".
It is important to emphasize the stark differences between brains and computers. Individual transistors are homogeneous and non-adaptive. The interconnectivity of transistor gates is very low. In the central processing unit of any microprocessor, one gate is connected, on average, to two or three others. This pales in comparison to interneuronal convergence and divergence ratios, often in the tens of thousands. The standard von Neumann computer architecture enforces a strict separation between memory and computation. Software and hardware, which can be easily separated in a computer, are completely interwoven in brains - a neuron's biophysical makeup is intrinsically linked to the computations (for instance to detect temporal coincidence). Furthermore, brains wire themselves up during development as well as during adult life, by modifying, updating, replacing connections, and even in some circuits by generating new neurons. While brains do indeed perform something akin to information processing, they differ profoundly from any exiting computer in the scale of their intrinsic structural and dynamic complexity"
A special issue the journal Visual Cognition (1999) was dedicated to the understanding towards the visual integration. Some authors defend the idea that integration primarily on low level processed based on neural circuits whereas others present evidence that integration cannot occur without attention. The first theory is based on the synchronization of oscillatory activity of neurons. (Wolf Singer and his group is working on this theory of oscillations in the 35-75 -Hertz range)
There is also presented in this issue a new approach to the problem. That is the use of pharmacology to investigate the neuro-chemical of the binding processes. One scientist (Giersch) has shown that lorazepam (a benzodiaspine) impairs grouping of local contour elements into a global configuration.
It appears that the GABAergic system plays an important role. It is involved in lateral inhibition between cells.
The second theory deals with the relationship between attentional processes and grouping. Separate elements can be rapidly integrated in parallel across the visual field whereas integration of different features requires attention.
Vidyasagar (1999) pretends to have found indications that would solve the binding problem. Attentional spotlight originates in the dorsal stream and helps serially searching the field for conjunction of the relevant target feature in the temporal stream.
Anyhow in the next century hopefully will tell us more about the binding problem and also to know more about the question what is awareness or consciousness?
Logothesis ends his article in Scientific American (November 1999):
"The picture of the brain that starts to emerge from these studies is a system whose processes create states of consciousness in response not only to sensory inputs but also to internal signals representing expectations on past experiences. In principle, scientists should be able to trace the networks that support these interactions. The task s huge, but our success identifying neurons that reflect consciousness is a good start".
Figure 1: The great mystery. Our visible world is split up in four parts and processed in our brain in separate channels. Apparently we see only one non- divided world. Where is the assembly -hall? How it is functioning to bring all the pieces together so the split-up world becomes one 'reality'?