Geoffrey R. Hammond

Correlates of human handedness in primary motor cortex: a review and hypothesis

A review of the research on anatomical and functional asymmetries in human primary motor cortex suggests that the area of hand representation is greater in the dominant than in the non-dominant hemisphere and that there is a greater dispersion of elementary movement representations with more profuse horizontal connections between them. The more profuse interconnections in motor cortex (M1) of the dominant hemisphere might form a neural substrate which favors the formation of experience-dependent excitatory and inhibitory interactions between elementary movement representations. Motor practice might lead to more precise spatiotemporal coordination of the activity of the elementary movement representations in M1 of the dominant than that of the non-dominant hemisphere, thus leading to more dexterous behavior of the dominant than that of the non-dominant hand.

Hemispheric differences in temporal resolution

A review of the relevant clinical and experimental literature gives the conclusion that the cerebral hemispheres differ in temporal resolution of input, with the language-dominant hemisphere showing finer acuity. This conclusion is supported by evidence from performance of patients with unilateral brain damage on tests of temporal resolution, performance of developmental dyslexics on similar tasks, and left-right sensory field differences in temporal acuity in normal human subjects. While it is unlikely that a hemispheric difference in temporal resolution is sufficient to give a complete account of lateralized functions, such attempts to show more primitive physiological differences between the hemispheres are more likely to be fruitful than attempts which differentiate the hemispheres in terms of higher-order psychological functions.

Independence of force production by digits of the human hand

The hypothesis that handedness stems from a greater ability to produce independent forces in the digits of the preferred than the non-preferred hand was investigated in 20 right-handed males who made a sustained isometric flexion of the distal phalanx of a single digit (the instructed digit). Instructed flexion forces were accompanied by non-instructed forces in all other digits. Mean non-instructed force was least when the thumb was the instructed digit, and increased progressively when the index, middle, ring, and little finger was the instructed digit. Both flexion and extension were recorded in non-instructed digits. There was no asymmetry in production of non-instructed force, and hence no evidence for greater independence of force production in the digits of the preferred than the non-preferred hand.

Excitatory and inhibitory processes in primary motor cortex during the foreperiod of a warned reaction time task are unrelated to response expectancy

Reaction time (RT) is shortened when a warning signal precedes the response signal, a finding attributed to response preparation during the foreperiod between the warning and response signals. In a previous experiment, we delivered transcranial magnetic stimulation (TMS) during the short constant foreperiod of a warned RT task and found simultaneous suppression of motor evoked potential (MEP) amplitude and reduction of short-interval intracortical inhibition (SICI) on warned trials (Sinclair and Hammond in Exp Brain Res 186:385–392, 2008). To investigate the extent to which these phenomena are associated with response preparation we measured MEP amplitude and SICI during the foreperiod of a warned RT task in which three different warning signals specified the probability (0, 0.5, or 0.83) of response signal presentation. MEP amplitude was suppressed (Experiment 1) and SICI reduced (Experiment 2) equally in all of the warned conditions relative to when TMS was delivered in the inter-trial interval (ITI) suggesting that the modulation of primary motor cortex excitability during the foreperiod does not depend on momentary response expectancy induced by the warning signal. The reduction of SICI and suppression of MEP amplitude can be explained by assuming that a warning signal induces automatic motor cortical activation which is balanced by a competing inhibition to prevent premature response. A composite measure which weighted both speed and accuracy of response was positively correlated with the MEP amplitude during both the foreperiod and the ITI, suggesting that high motor cortical excitability is associated with optimized preparatory strategies for fast and accurate response.

Differential effects of ecstasy on short-term and working memory: a meta-analysis.

Quantitative analysis of studies examining the effect of ecstasy on short-term and working memory in the verbal and visuo-spatial domain was undertaken. Thirty verbal short-term memory, 22 verbal working memory, 12 visuospatial short-term memory and 9 visuospatial working memory studies met inclusion criteria. Ecstasy users performed significantly worse in all memory domains, both in studies using drug-naïve controls and studies using polydrug controls. These results are consistent with previous meta-analytic findings that ecstasy use is associated with impaired short-term memory function. Lifetime ecstasy consumption predicted effect size in working memory but not in short-term memory. The current meta-analysis adds to the literature by showing that ecstasy use in humans is also associated with impaired working memory, and that this impairment is related to total lifetime ecstasy consumption. These findings highlight the long-term, cumulative behavioral consequences associated with ecstasy use in humans.

Temporal regularity of tapping by the left and right hands in timed and untimed finger tapping.

The temporal characteristics of repetitive finger tapping by the left and right hands were examined in two experiments. In the first experiment, interresponse intervals (IRIs) were recorded while right-handed male subjects tapped in synchrony with an auditory timing pulse (the synchronization phase) and then attempted to maintain the same tapping rate without the timing pulses (the continuation phase). The left and right hands performed separately, at four different rates (interpulse intervals of 250, 500, 750, and 1500 ms). There was no asymmetry of the asynchronies of the timing pulses and the associated responses in the synchronization phase or of the IRIs in either phase, but there was an asymmetry of chronization phase or of the IRIs in either phase, but there was an asymmetry in the temporal dispersion of the responses in both phases. in the second experiment, right-handed males tapped separately with each hand at three different speeds: as quickly as possible, at a fast but steady rate, and at a slow rhythmical rate. The speed asymmetry present when tapping as quickly as possible (with the preferred hand tapping more quickly ) was reduced when tapping at the fast steady rate and was absent when tapping at the slow rhythmical rate. The temporal dispersion of the IRIs produced by the nonpreferred hand was greater than the temporal dispersion of those produced by the preferred hand in all speed conditions. These results show smaller temporal dispersion of tapping by the preferred hand in right-handed males under different conditions, including submaximal speeds at which both hands respond at the same rate. This suggests that the motor system controlling the preferred hand in right-handers had more precise timing of response output than that controlling the nonpreferred hand.

Parameters affecting gap detection in the rat.

The present research used a startle amplitude reduction paradigm to investigate the ability of the rat’s auditory system to track rapidly changing acoustic transients. Specifically examined was the ability of brief gaps in otherwise continuous noise to reduce the amplitude of a subsequently elicited acoustic startle reflex. The duration of the gap, time between gap offsetand startle elicitation (the interstimulus interval or ISI), and rise-fall characteristics of the gap were systematically varied. Consistent with previous research, gaps reliably reduced startle amplitude. Gaps 2 msec long were reliably detected, and a 50-msec ISI resulted in the greatest amplitude reduction. Gaps presented at short ISIs produced amplitude reduction that followed a different time course than did gaps presented at longer ISIs. These results may reflect differences in the length oftime available for the processing of the stimulus and may involve two different processes.

Modulation of long-interval intracortical inhibition and the silent period by voluntary contraction

Transcranial magnetic stimulation was used to examine the effect of voluntary contraction on the magnitude of long-interval intracortical inhibition (LICI) and the duration of the silent period in intrinsic hand muscles. The magnitude of LICI acting on the first dorsal interosseus (FDI) measured with a paired-pulse protocol with an inter-pulse interval of 100 ms decreased with increasing tonic level of voluntary abduction force generated by the index finger. LICI in abductor pollicis brevis (APB) decreased from the condition in which the index finger was at rest to the conditions in which it was abducted, whereas LICI in abductor digiti minimi (ADM) was unaffected by the level of index finger abduction. During voluntary abduction of the index finger, the magnitude of LICI was least in FDI, intermediate in ADM, and greatest in APB, suggesting that it may be a mechanism by which tonic activation of hand muscles is fractionated. The magnitude of LICI increased with conditioning stimulus intensity, but intensity did not interact with abduction force. The duration of the silent period (SP) in FDI decreased with the level of voluntary index finger abduction and increased with eliciting stimulus intensity. Within-subject correlations showed that the effects of voluntary drive on SP duration and motor-evoked potential amplitude did not covary, implying an indirect effect of voluntary drive on SP duration. It is proposed that whereas voluntary drive directly reduces the magnitude of slow-acting inhibition acting on the active movement representations and near neighbors, sensory feedback from the contracting muscle acts to limit its time course.

Differential generalization of habituation across contexts as a function of stimulus significance

Orienting to one of two levels of stimulus significance (a distress squeal or a simulated mimic squeal) in female hooded rats was measured by suppression of ongoing drinking. Subsequent generalization tests with the same stimulus presented in different contexts showed that generalization of habituation across contexts was a function of stimulus significance: Habituation to the distress squeal was restricted to the context in which it first had been presented, whereas habituation to the mimic squeal generalized across the different contexts.

Inhibition of the rapid movement of optically detectable axonal particles by colchicine and vinblastine

The rapid saltatory motion of intra-axonal particles detected by dark-field microscopy in myelinated axons isolated from sciatic nerves of adult Xenopus laevis was inhibited by colchicine or vinblastine at a concentration of larger than or equal to 0.1 mM. Both the predominant somatopetal transport and the somatofugal transport of these round particles were inhibited. The reduction in numbers of moving particles was apparent first in the juxtanodal portions of the isolated axons within about 1 h. No particles could be detected moving by 3-5 h after application of the colchicine or vinblastine. During the phase of partial inhibition, those particles that were still progressing along the axon did so at apparently normal velocities while they were in motion, but remained stationary increasingly frequently and for progressively longer periods. Colchicine or vinblastine at a concentration of less than or equal to 10 micronM caused no observable inhibition within 4 h of application. Colchicine at a concentration of larger than or equal to 10 mM caused local accumulation of round particles, and vinblastine at a concentration of larger than or equal to 2.5 mM caused fragmentation of rod-shaped organelles, believed to be mitochondria. Electron microscopy of nerve fibers treated with 5 mM colchicine showed a progressive loss of microtubules from the axoplasm, such that approximately 70% of the microtubules had disappeared after 4h.