In daily activities, hand use is dominated by individuated thumb and finger movements, and by grasping. This thesis focused on the level of ???independence??? of the digits and its relationship to hand grasps, from the level of the motor units to the level of synergistic grasping forces. Four major studies were conducted in healthy adult volunteers. First, spike-triggered averages of forces produced by single motor units in flexor pollicis longus (FPL) in a grasp posture showed small but significant loading of the index, but not other fingers. This reflected a neural rather than anatomical coupling, as intramuscular stimulation produced minimal effect in any finger. Also, FPL had a surprisingly large number of low-force motor units and this may account for the thumb???s exceptional dexterity and force stability compared with the fingers. Second, independent control of extensor digitorum (ED) was more limited than flexor digitorum profundus (FDP), as more ED motor units of a ???test??? finger were recruited inadvertently by extension than by flexion of adjacent digits. Third, ???force enslavement??? in maximal voluntary tasks was greater in digit extension than flexion. The distribution of force enslavement (and deficits) matched the pattern of daily use of the digits (alone and in combination), and reveals a neural control system which preferentially lifts fingers together from an object by extension but allows an individual digit to flex to contact an object so the finger pads can engage in exploration and grasping. Finally, during grasping, irrespective of whether a digit had been lifted from the object, coherence among forces generated by the digits was similar. In addition, the coherence between finger forces was independent of any contraction of the thumb, was stable over 2 months, and required no learning. The pattern of coherence between digital grasping forces may be closely related to the level of digit independence and daily use. Overall, the grasp synergy was remarkably invariant over the various tasks and over time. In summary, this thesis demonstrates novel aspects of the properties of FPL, the lack of complete independence of the digits, and robustness in the production of flexion forces in hand grasps.

Quiescence is a non-proliferative cellular state that is critical to tissue repair and regeneration. Although often described as the G0 phase, quiescence is not a single homogeneous state. As cells remain quiescent for longer durations, they move progressively deeper and display a reduced sensitivity to growth signals. Deep quiescent cells, unlike senescent cells, can still re-enter the cell cycle under physiological conditions. Mechanisms controlling quiescence depth are poorly understood, representing a currently underappreciated layer of complexity in growth control. Here, we show that the activation threshold of a Retinoblastoma (Rb)-E2F network switch controls quiescence depth. Particularly, deeper quiescent cells feature a higher E2F-switching threshold and exhibit a delayed traverse through the restriction point (R-point). We further show that different components of the Rb-E2F network can be experimentally perturbed, following computer model predictions, to coarse-or fine-tune the E2F-switching threshold and drive cells into varying quiescence depths.

This thesis explores the role of age in second language (L2) acquisition and first language (L1) attrition. The focus is on Voice Onset Time (VOT) in the production and categorical perception of word-initial L1 and L2 stops in highly advanced L1 Spanish learners of L2 Swedish. Using as the point of departure a maturational constraints perspective and the Critical Period Hypothesis (CPH), Study I examines the impact of age of onset (AO) of L2 acquisition on the production of L2 Swedish voiceless stops. The results show that there are AO effects even in the speech of highly advanced L2 learners and that the incidence of nativelike L2 learners is considerably lower than earlier assumed. However, conclusions like these are only possible when speaking rate is accounted for, thereby highlighting the importance of speaking rate effects on VOT as a measure of nativelikeness. Like Study I, Study II reveals age effects on the same L2 learners’ categorical perceptions of L2 Swedish stops. Moreover, after combining the results with the data from Study I, the incidence of nativelike behavior drops remarkably with no late L2 learner performing within the range of native-speaker production and perception. The results suggest that L2 acquisition of phonetic/phonological aspects is especially sensitive to AO effects. It is concluded that theories on maturational constraints, including the CPH, cannot be refuted on the basis of the present data. Study III concerns the same participants’ production and perception of L1 Spanish stops. Age of reduced contact (ARC) is identified as an important predictor for L1 attrition and retention of voiceless stop production, although not of stop perception. This discrepancy is related to different activation thresholds as proposed by the Activation Threshold Hypothesis (ATH). It is further suggested that early bilinguals are more dependent on high-frequency L1 use than late bilinguals when compensating for age effects, but only in production. / <p>At the time of doctoral defence the following papers were unpublished and had a status as follows: Paper 1: Accepted. Paper 2: In press. Paper 3: Manuscript.</p> / Age of onset and ultimate attainment in second language acquisition, The Bank of Sweden Tercentenary Foundation, grant no. 1999-0383:01 / First language attrition in advanced second language speakers, Swedish Research Council, grant no. 421-2004-1975