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Results for “HANDEDNESS”

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At least 919 records · Page 51Linked to original sources
PubMed1979

On the physical origin of biological handedness.

In the racemic conglomerate crystallization of over 1,000 samples of D,L-sodium-ammonium tartrate the effect of 32P beta irradiation on the weight, optical activity, and crystallite size was measured. Both weight and optical activity showed a statistical dependence on the intensity of beta irradiation. The crystallite size is also affected by the presence of 32P. Asymmetric crystals are suggested to have been potential mediators between asymmetric parity violating forces and molecular asymmetry so that stereo-selective prebiotic chemical reactions involving crystals need not be considered 'chance' processes. No measurable difference in the energy content of optical isomers was found. An upper limit for the direct contribution of weak interactions to electromagnetic ones has been calculated. The mechanism of stereoselective crystal seeding by beta particles is discussed.

Crystallization↗
PubMed1995

Effect of a chiral impulse on the weak interaction induced handedness in a prebiotic medium.

Previously we have carried out simulation of the Weak Neutral Current and symmetry breaking transition bifurcation process addressing some of the issues raised by critics of the approach. We now include the effects of a chiral impulse on the transition and show that under certain cases this could greatly alter the transition time. Examples of a chiral impulse could be a nearby Supernova and an impulse of beta-emitters. We briefly discuss the possibility of these processes in the early solar system.

Biophysical Phenomena↗
PubMed1995

Genetic vs. environmental contributions to human handedness: insights gained from studying individuals with unilateral hand injuries.

Four hundred eleven individuals who had suffered temporary unilateral hand injuries were assessed on five hand preference behaviors after their recovery. Measures of shifts to contralateral hand use experienced during and after the recovery period were taken. The best predictor of hand use shifts during the injury recovery period was whether the injury was to the preferred or nonpreferred hand. The incidence of postinjury contralateral shifts was low but was greater than that observed over a simulated injury period in a control group of 384 individuals who had not experienced a hand injury.

Adolescent↗
PubMed1996

The degree of lateralization of paw usage (handedness) in the mouse is defined by three major phenotypes.

Lateralization of paw usage in the laboratory mouse may be a useful model system in which to assess the genetic and developmental cause of asymmetry of hand usage. With a set number of paw reaches from a centrally placed food tube, individual mice from an inbred strain will exhibit a reliable number of left and right paw reaches. For a single inbred strain, there are approximately equal numbers of left-pawed and right-pawed mice, but strain differences have been reported in the degree of lateralization of paw preference. We reported a preliminary strain survey in which the strains appeared to fall into two groups of highly lateralized and weakly lateralized paw preference (Biddle et al., 1993). We review here our expanded survey of genetically different strains and stocks of the laboratory mouse, including different species and subspecies. The major genetic trait is the degree of lateralization of paw preference and the strain differences appear to fall into three major classes of highly lateralized, weakly lateralized, and ambilateral preference. The trait exhibits both additivity and dominance in preliminary reciprocal crosses, depending on which strain pairs are used. The wide difference between strains that have highly lateralized and ambilateral paw preference suggests specific genetic tools that could be used to begin a genetic dissection of the causes of this trait. Preliminary assessment of the size of the corpus callosum in three strains with significantly different degrees of lateralization suggests that genetically determined deficiencies and absence of this structure are not the direct cause of the strain differences in the trait of degree of lateralization. In the expanded survey, some strains appear to exhibit a directional deviation from equal numbers of mice with left and right paw usage. Therefore, direction of paw usage may not be a genetically neutral trait, but replicate assessments and genetic tests are needed to confirm this.

Animals↗
PubMed2004

Human handedness: is there a difference in the independence of the digits on the preferred and non-preferred hands?

In non-human primates, comparative studies show that greater dexterity is associated with greater independent control over the digits. Studies of humans, however, show no difference in the degree of independent control of single digits on the more dexterous preferred hand and its less dexterous partner. We wondered whether there might be a difference in the degree of independent control on the two hands during performance of functionally relevant tasks. Many object manipulation tasks require the ability to produce and control forces with more than one finger at the same time. We hypothesized that asymmetrical independence, with greater independence on the preferred than the non-preferred hand, would be evident with a task that requires the simultaneous production of force in two digits. We examined digit individuation when subjects produced flexion forces with a single digit in isolation, and simultaneous flexion forces in all ten combinations of two digits. Consistent with previous studies, we found no difference between the individuation of the digits on the preferred and non-preferred hands during force production with single digits in isolation. Similarly, no asymmetry was present when forces were produced by two digits. However, separation of two-digit forces into thumb-to-digit opposition forces and non-opposition forces showed that although there was no difference in individuation between the two hands for non-opposition forces, the digits on the preferred hand were more independent than those on the non-preferred hand for opposition forces. We suggest that this independence asymmetry is not itself the underlying cause of the dexterity differences between the hands, but rather arises from a difference in the capacity for use-dependent reorganization of the motor cortical circuits controlling each hand, and that this difference might underlie the dexterity differences between the hands.

Adult↗
PubMed2004

Perceived reachability: the roles of handedness and hemifield.

The impact of reaching experience on the ability to predict one's own reaching range was investigated. Left- and right-handed participants made verbal estimates about the reachability of a target object for both arms in ipsilateral and contralateral frontal space. There was a significant overestimation bias in both groups and for both hands. The overestimation bias increased with the target object's eccentricity in contralateral space. The implications of these findings for models of motor control and motor imagery are discussed.

Adult↗
PubMed2006

Asymmetries of long-latency intracortical inhibition in motor cortex and handedness.

Paired-pulse transcranial magnetic stimulation was used in three experiments to measure the properties of long-latency intracortical inhibition (LICI) acting on the relaxed first dorsal interosseus muscle of the left and right hand in right-handed volunteers. The experiments show that LICI is asymmetrical: it emerges more rapidly and is greater in the dominant than non-dominant hand shortly after activation of the LICI circuits, and is greater with low-intensity conditioning stimulus intensities in the dominant than non-dominant hand. These findings suggest that asymmetrical function of long-latency inhibitory circuits in motor cortex might contribute to the asymmetrical dexterity between the hands, possibly through their inhibitory control of the circuits responsible for short-latency inhibition.

Adolescent↗
PubMed1999

Human handedness and asymmetry of the motor cortical silent period.

We performed transcranial magnetic stimulation of the motor cortex in 22 left-handed and 25 right-handed subjects during active contraction of a small hand muscle. Motor evoked potentials had the same latency, amplitude and threshold on both sides of the body, whilst the silent period duration was shorter in the dominant hand. Silent periods elicited by nerve and brainstem stimulation were the same in both hands. Since the latter part of the cortical silent period is due mainly to withdrawal of corticospinal input to spinal motoneurones, we speculate that the results are compatible with the suggestion that tonic contractions of the non-dominant hand are associated with a greater involvement of the corticospinal tract than those of the dominant hand. It also seems likely that there is an asymmetry in the excitability of cortical inhibitory mechanisms with those responsible for the cortical silent period being less excitable in the dominant motor cortex.

Adult↗
PubMed2001

Asymmetry of motor cortex excitability during a simple motor task: relationships with handedness and manual performance.

Transcranial magnetic stimulation (TMS) was used to assess the relative contribution of the corticospinal (CS) pathway in activating the first dorsal interosseous (FDI) muscle in each hand of 16 right- (RH) and 16 left-handed (LH) subjects with varied degrees of hand preference. It was hypothesised that asymmetry in corticospinal activation of the two hands may be related to hand preference and interlimb differences in manual performance. Subjects performed isometric index finger abduction at force levels of 0.5 N, 1 N and 2 N while TMS was applied at resting threshold intensity (T), 0.9T, or 0.8T. The amount of contraction-induced facilitation of the muscle evoked potential (MEP) was used as an estimate of corticospinal involvement in the task. Patterns of MEP facilitation in each hand were compared with measures of manual performance (finger tapping speed, Purdue pegboard, maximal FDI strength). Threshold TMS intensities for an MEP in FDI at rest were similar in LH and RH subjects, and did not vary between hands. Facilitation of the MEP with voluntary activation was larger overall on the left side (P<0.05), but the asymmetry was dependent on the degree of lateralisation of hand preference. For subjects with consistent hand preference (either LH or RH), MEP facilitation in active FDI was larger for the left hand. For non-consistent RH subjects, contraction-induced MEP facilitation was larger in the right FDI muscle than the left. Asymmetry of MEP facilitation was not correlated with differences between hands in finger tapping speed or performance in the pegboard task, but was associated with relative differences in FDI strength. MEP facilitation tended to be larger in the stronger FDI muscle of the pair. We conclude that corticospinal involvement in the command for index finger abduction is generally greater when the left hand is used, although in RH subjects the asymmetry is influenced by the degree of lateralisation of hand preference. The corticospinal asymmetry is not related to speed or dexterity of finger movements, but the association with muscle strength suggests that it may be influenced in part by preferential use of one hand for tasks which strengthen the FDI muscle.

Adult↗