In defense of the sinistrals: anatomy of handedness and the safety of prenatal ultrasound.

Once again there is a flurry of interest in handedness, this time in the guise of a question: whether a demonstrated increased ‘risk’ of left-handedness among those exposed to prenatal ultrasound denotes brain damage in such babies1,2. This is an old debate in a new vernacular in the sense that left-handedness has always been viewed with suspicion as far as the integrity of the brain of its practitioners is concerned (note the negative meaning of the word ‘sinistral’), and not always on cultural or religious grounds3. The genesis of handedness itself is subject to controversy with a recent review concluding, for example, that handedness is all acquired4. An ugly fact, however, serves as nature’s sobering remark: the congenitally blind displays the same statistics of hand preference as do the sighted5,6. The fundamental problems hampering resolution of any laterality questions are as follows. First, the precise anatomy of handedness as a unique and ubiquitous human characteristic, which would also provide an insight as to its skewed distribution. Second, the fact that the use of one hand or the other in performing a task is entirely subject to a person’s volition, in turn subject to improvement by practice. Because of its technical nature the second aspect will not be examined here in detail but it is important to note that the saying ‘practice makes perfect’ has a neural basis within the motor cortex, i.e. activity-dependent synaptic modification7. The first issue, which is intimately related to cerebral lateralization of motor control, has been clarified recently with significant implications for the subject under review (the safety of prenatal ultrasound). The novel understanding is that the callosal traffic underpinning lateralities of executive functions of speech and handedness is one-directional, with the direction of callosal traffic displayed/codified as handedness. The theory is buttressed by unequivocal clinical and experimental evidence8–11. While highlighting the dichotomous nature of neural handedness (as opposed to manifest or self-declared handedness, see below), the theory provides verifiable answers as to the nature of hitherto unresolved laterality questions (some of which are to be touched upon later). It discards the notion of pathological left-handedness as the return of old superstitious beliefs in scientific garbs, unworthy of attention. The hemisphere that houses the controlling moiety of the neuronal aggregate devoted to executive functions is the major hemisphere with the hand opposite to it the dominant hand. (This contralaterality is due to the crossing of the pyramidal tracts, a grossly visible anatomical landmark, in the medulla oblongata.) Physiologically this means that the dominant hand, because of its direct connection to the major hemisphere, where all commands are planned and executed, flings into action faster than the non-dominant when called upon to perform. Those commands to be carried out by the nondominant hand require an extra step, and thus additional time, taken by the command to reach from the command center on the left to the exclusive motor apparatus for the left, located in the right hemisphere. There is thus a callosum-length distance by which the non-dominant hand is farther away from the command center than the dominant. This anatomical scheme is supported by the fact that the activity of motor cortices in bimanual actions is strikingly similar to that seen with movements of the non-dominant limb alone, as documented in emission technology9–11 as well as in electromagnetic studies