This paper describes the application of computer-aided three-dimensional reconstruction to measurements of the length of the organ of Corti (mean +/- SD, 35.58 +/- 1.41 mm), scala tympani outer wall (40.81 +/- 1.97 mm), scala tympani inner wall (18.29 +/- 1.47 mm), and Rosenthal's canal center (15.98 +/- 1.33 mm) in eight adult male cochleas. The Rosenthal's canal center ranged between 1 3/4 and 2 turns, did not appear to be linearly related to the organ of Corti, and lay near the basal end of the latter. The length of the organ of Corti measured with three-dimensional reconstruction differed by 7.6% +/- 3.2% ("cutting angle difference") from that derived from traditional two-dimensional reconstruction on the plane perpendicular to the plane of section, and by 2.6% +/- 1.7% ("geometric difference") from that derived from two-dimensional reconstruction on the axial plane at right angles to the modiolar axis.

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Horrobin (2001) has proposed that phospholipid metabolism is linked to human brain growth, and that deviations in the metabolism may be linked to creativity as well as mental disorders. The present literature review leads to a framework or model which states that brain white matter growth causes the overlying cortex to expand tangentially, like a balloon, and that this expansion affects the cortex's capacity to differentiate afferent signals. The neuroanatomical description of this model is based on publications linking human white matter growth and mass to the thickness of the overlying cortex, and of some linking cortical thickness and surface area (inversely). The link between the surface area of a cortical region and its differentiation capacity is based on previous work on hemispheric differences and functional lateralisation in the human auditory cortices. The link between differentiation capacity and inappropriate responses or "loose associations" is based on publications linking perceptual deficits and abnormal cortical structure, especially abnormal laterality. Finally, perceptual deficits have been linked to aspects of schizophrenia or other "disorders".

We have studied spiral ganglion cell (SGC) survival and soma size in neonatally pharmacologically deafened kittens. They were implanted with a four-electrode array in the left cochlea at 100 to 180 or more days of age. Eight animals were chronically stimulated approximately 1000 hours over approximately 60 days with charge-balanced, biphasic current pulses; three were unstimulated controls. Using three-dimensional computer-aided reconstruction of the cochlea, the SGC position and cross-sectional area were stored. SGC position was mapped to the organ of Corti by perpendicular projections, starting from the basal end. The basal region of the cochlea was divided into three 4-mm segments. SGC survival (number per 0.1 mm of the length of the organ of Corti) and soma size for stimulated cochleae were compared statistically with implanted but unstimulated cochleae. There was no evidence of an effect of electrical stimulation on SGC survival under this protocol and with this duration. On the other hand, the cell size on the stimulated side was significantly larger than the control side in the middle segment (4 to 8 mm from the basal end). SGCs undergo a reduction in size after prolonged auditory deprivation; however, these changes may be partially moderated after chronic intracochlear electrical stimulation.

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We present histological data from 21 post-mortem, adult human cases that indicate the neocortex on the left planum temporale (secondary auditory cortex) is thinner but longer than that on the right side. The volumes of the left and right regions are approximately equal. Thus, the left planum temporale cortex is long and thin and the right short and thick. The present data fit excellently with previous studies of the volume, surface area, cytoarchitectonics, and neuronal structures of these areas. From these studies we suggest that the hemispheric differences arise from a so-called "balloon model" of cortical development. In this the cortex is extended and stretched by white matter growth. The stretching is greater on the left side, leaving greater distances between neuronal columns and more tangentially (to the pial surface) oriented dendrites on that side. This difference in fine structure can result in more independent activity of individual columns on the left, and could be an anatomical factor in the usual dominance of the left hemisphere for speech perception (Seldon, 1982, 1985).

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