Topography of brain and phrenology of mind

Sticking electrodes just anywhere into the brain matter isn't going to produce great results. Targeting specific functionally relevant areas of the brain is essential to achieving the desired effect, preferably with previous experiments in animals.

Currently the number of possible targets for ESB are determined by two things: knowledge of the functional anatomy, and the accuracy of electrode insertion. Lets look at each of these more closely to see what is involved, and what the future holds.

Functional anatomy

Anatomists have mapped out brain structures using every available means of differentiating tissues from each other. At the grossest level the lobes, gyri and fibre tracts can be separated. Under a microscope further differentiation is possible, noting where cell phenotypes change, or using stains to mark otherwise identical cells.
Further one can deduce pathways by looking for expression of characteristic molecules, for instance enzymes involved in catacholamine synthesis. With the dawn of transgenics it is also becoming possible to find individual genes within groups of cells, and to track the expression of these genes. One can use existing, naturally occuring genes as markers, or introduce one's own, such as green-flourescent protein.

Using a variety of techniques, including electrical stimulation, ablation, transgenics and neuro-imaging one can extrapolate the functional aspects of discrete anatomical regions. The first line of testing is usually quite gross, making changes to large areas and checking for effects on behaviour. When studying specific processes like Parkinson's or addiction one can use experimental models to give a benchmark against which interventions in the brain can be measured.

Existing knowedge of pharmacology can lend insight for electrical stimulation. Major behaviouarlly relevant pathways in the brain have been deduced; such as the dopaminergic seeking pathways connecting ventral tegmental area, nucleus accumbens, hypothalamus and neo-cortex. With this foundation one can immediately begin working with such pathways, stimulating them at each point and observing behavoiural effects.

Ultimately to uncover the functional relevance of brain structures requires intense and continuous refinement, using every technique available, in a host of different experimental paradigms. As is becoming clear, brain areas thought to be exclusively for, say, vision also have other functions when tested in a sensitive enough manner.
[Rushworth et al, 2002]

As work into these areas becomes ever more refined, and new techniques are used to understand the functional relevance of anatomy, more and more possible targets for ESB will become available.

Today the target for stimulation is determined using many techniques, each refining the target zone for electrode insertion. MRI scans are used to acquire images of the patient's brain, which is then referred to a stereotactic atlas. Using the patient's images with the map one can generate co-ordinates in proportion to landmarks, such as the ventricular dimensions or anterior-posterior commissural distance. These coordinates can then be used when the patient's head is mounted in place, allowing surgery accurate to around 1mm.

Macro-stimulation can be applied during surgery to test for the desired behavioral effects. This behavoiural reference will allow fine tuning of the eletrode's placement within the designated region. Some electrode implants are being developed which automatically adjust and seek out the ideal point of stimulation.

Microelectrodes can also be introduced to record from the area, testing how stimulation is spreading to surrounding tissues. This is often desirable when a checking the site with a behavoural reference isn't possible. However adding further elctrodes is associated with greater risk of infection, longer operating times and more intrusive damage to the brain. Thus it is best kept as a backup fro when the other techniques are unable to do it alone.
[Rodriguez-Oroz et al,2004]