Biomedical Engineering Reference
In-Depth Information
bellar white matter of young children with autism, relative to control subjects [17].
However, this same group found that the cerebellar gray matter volume in autistic
young children did not differ from control subjects, but this difference was noticed
in older patients. Additionally, a functional MRI (fMRI) study has examined the
cerebellum, and has found that abnormalities of cerebellar development in autism
may have different implications for motor and attentional functioning [37].
Recently, a computer image analysis study, based on the analysis of the con-
figuration of minicolumns (“
the basic functional unit of the brain that organizes
neurons in cortical space
[38, 39]”), provided evidence of diminished width for
pyramidal cell arrays in the neocortex of autistic patients [40]. This study showed
that diminution was greatest within the peripheral neuropil space of the mini-
column. A second study using the Gray Level Index (GLI) corroborated these
findings [41]. This study showed diagnosis-dependent effects in distance between
adjacent local maxima in the GLI. The tighter packing of cortical modules suggests
increased cellular density. This increase in the total number of minicolumns in
autism requires a scale increase (roughly a 3/2 power law) in white mater to main-
tain modular interconnectivity [42]. This additional white matter takes the form
of short-range connections that make up the bulk of intracortical connections [42].
More recent analysis of intra and inter-cluster distances using Delaunay triangula-
tion indicates that reported increased cellular density in autism is the result of an
increased number of minicolumns rather than an increase in the total number of
cells per minicolumn [43]. Furthermore, the pyramidal neurons that populate the
cell minicolumns in autism appear smaller and have reduced nucleolar size [43].
Nucleolar size in pyramidal cells reflects metabolic demand. Large nucleoli
are seen in correspondingly large pyramidal cells. These neurons provide for long
projections, e.g., cortical efferent axons to spinal cord motor neurons. On the
other hand, small nucleoli are seen primarily in smaller pyramidal cells. These
neurons are characterized by short intracortical projections. The findings in autism
suggest a metabolic bias that makes short corticocortical connections efficient at
the expense of longer association and commisural connections. Anatomically the
results can be seen in structural MRI studies as an increase in the outer radiate
compartment of white matter [44]. This compartment is made up of late myelinat-
ing short association fibers. Contrariwise, five out of eight structural MRI studies
of the corpus callosum have reported reduction in its total area and/or one of its
subdivisions [45]. The findings explain why autistics excel in performance in tasks
that require information processing within a given brain area (e.g., visual discrim-
ination) but otherwise perform poorly in those that require inter-areal integration
(e.g., joint attention, language).
Brainstem
Like some of the other brain regions, inconsistencies exist between patho-
logical findings related to the brainstem in autism. In a study by Bauman et al.
