Comparing Brains

Nearly all fMRI studies use multiple subjects and perform statistical analyses across data collected from multiple subjects. This practice introduces a number of practical problems that must be addressed.

A first, relatively simple step toward comparing activity across the brains of multiple subjects is to transform the representations of those brains so that they are similar in overall size and similarly oriented in space. To accomplish this, the brain images are rigidly rotated and linearly scaled into a common "box." The Talairach stereotactic coordinate system is the most widely used standard coordinate system box for comparing brains. In the full Talairach transformation, a rigid rotation and translation to a standardized orientation is followed by a piecewise linear scaling of the anterior, middle, and posterior portions of each hemisphere, independently. The standard orientation in three dimensions is determined by the line between two interhermispheric fiber bundles—the anterior commissure (AC) and the posterior commissure (PC)—and the plane between the two hemispheres. The anterior, middle, and posterior portions of each hemisphere are defined in terms of the AC-PC line: The portion ofeach hemisphere in front ofthe anterior commisure is the anterior, the portion of each hemisphere between the AC and PC points is the middle, and the portion behind the posterior commisure is the posterior.

In some software packages an abbreviated form of the Talairach transformation is performed in which the brain is scaled as a whole, without piecewise linear portions for each hemisphere. This has the advantage of being much faster and simpler to implement. Indeed, some packages compute this transformation automatically by comparing the given brain to a standard "average" brain that was generated by transforming the anatomical MR images of 305 brains to Talairach coordinates and averaging. This process eliminates the tedious and often tricky steps of finding the AC-PC line and other landmarks for each individual brain. Despite the fact that the individual anatomy of the AC-PC line is ignored and the transformation has fewer degrees of freedom than the full Talairach transformation, this automatic process yields data that are adequate for most purposes.

The more serious problems with either the simplified Talairach transformation or the full Talairach transformation are caused by the fact that real brains are not rigid transformations of one another. The simplified Talairach transformation, being strictly linear, cannot account for these differences. Even the full Talairach transformation, which is only piecewise linear with a small number of pieces, is clearly inadequate for dealing with these individual differences in brain anatomy. More powerful nonlinear approaches have therefore been developed.

There are a wide range of approaches to more general transformations of brains to facilitate data display and intersubject comparison. One approach is to permit complicated nonlinear warping procedures based on sulcal and gyral landmarks to guide computer-generated distortions of one brain into another (or to a standard). Another approach, which is also based on nonlinear transformations, is to try to match the perimeters of given brain slices between different brains. Perhaps the most widely used alternative to Talairach and these other nonlinear transformations is to "inflate" the brain as a means of removing all sulci and gyri. This inflation is often followed by cutting the inflated brain in a small number of places to permit "flattening."

The goal of inflating is to obtain a three-dimensional, smooth, nonconvoluted surface representation of cortex. The goal of flattening is to lay that surface representation on a flat plane. Given that a brain hemisphere (when inflated) looks like an ellipsoid, it is necessary to cut it in one or more places to allow it to be flattened. Qualitatively, this is similar to the need to cut a globe to get a flattened representation of the world. Quantitatively, however, for an inflated brain, the analogy is closer to a cylinder. When a globe is cut and flattened, it is necessary to create many cuts or else there will be some very large distortions. On the other hand, when a cylinder is cut, the resulting surface can be flattened with virtually no distortions. In these terms, the inflated cortex is more like a cylinder than a sphere, and the distortions are not terribly large.

Flattened representations are visually and logically very appealing. Unlike Talairach coordinates, however, they are not three dimensional and only apply to the cortical surface. Subcortical structures cannot be represented. (Talairach invented his system for subcortical structures, although it does not include the cerebellum.)

Given the good spatial resolution of fMRI and the ability to detect activations in individual subjects, some researchers eschew averaging across subjects. Their position seems to be that the right way to compare across subjects is to look at each individual's functional map (preferably in a flattened brain format to facilitate intersubject comparison). Elaborations on this approach can include warping within the flattened space and could therefore eventually include averaging in that space.

Recent developments in brain comparisons involve the use of more sophisticated algorithms to inflate the brain to a sphere but then warp the surface borders on that sphere (associated with major and almost universal sulcal/gyral landmarks) toward a common standard. This transformation permits a smoother and more effective comparison of activation sites across the brains of different subjects than do the Talairach transformations.

Conquering Fear In The 21th Century

Conquering Fear In The 21th Century

The Ultimate Guide To Overcoming Fear And Getting Breakthroughs. Fear is without doubt among the strongest and most influential emotional responses we have, and it may act as both a protective and destructive force depending upon the situation.

Get My Free Ebook


Post a comment