Structural asymmetries in the human forebrain and the forebrain of non-human primates and rats

Possible asymmetries of the following structures were studied: volumes of total human hemispheres, cortex and white matter volumes in post-mortem- (unknown handedness) and living brains (male right-handers); volumes of the rat primary visual cortex, its mon- and binocular subfields, its layer iv and the density of myelinated fibres in layer iv; transmitter receptor densities (NMDA, AMPA, kainate and GABAA receptors) in sensorimotor regions of the rat cortex; volume of the motor cortex and the 3D-extent of the central sulcus in the post-mortem- (unknown handedness) and living human brain (male right-handers); petalia of the hemispheres in human (male right- and left-handers) and chimpanzee brains. Histological, MRI and receptor autoradiographic techniques were used. With the notable exceptions of the transmitter receptors and the total primary visual cortex in rats and the hemispheres in chimpanzees, which do not show any significant directional asymmetry, all other parameters studied are asymmetrically distributed between the right- and left hemispheres. The regional distribution pattern and the degree of asymmetry of frontal and occipital petalia in living human brains differ between right- and left-handers.

Rise and fall of modular orthodoxy

The premise of cortical modularity is based on strong dissociations caused by focal lesions. These dissociations are rare, and their explanatory power and theoretical importance are vastly overrated. The effects of brain lesions must be considered in their totality, rather than in idiosyncratic selectivity. These effects are more consistent with a continuous, graded functional neocortical geometry, than with a modular neocortex. Distinction must be drawn between strong intrinsic modularity, and weak emergent modularity. Strong intrinsic modularity is more characteristic of the thalamus than of the cortex. The advent of neocortex may have represented an evolutionary escape from strong modularity as the dominant principle of neural organization, and a shift toward a more interactive principle of neural organization dominated by emergent properties. The latter may take the form of weak modularity, reflective of cognitive skill routinization. The extent of weak, emergent modularization may be asymmetric, more pronounced in the left hemisphere, while the right hemisphere is essentially amodular.