The surface ablation threshold fluence of fused silica and two porcine cornea layers, the epithelium and the stroma, is characterized as a function of the laser pulse duration in the range of 100 fs-5 ps for a wavelength of 800 nm (Ti:sapphire laser system). The plateaulike region observed between 100 fs and 1 ps for the corneal layers indicates that for use in laser surgery, laser pulse durations chosen within this range should be practically equivalent. Our model predicts that the ablation threshold will decrease rapidly for pulse durations in the low end of the femtosecond regime.

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We recently suggested that movement-related inter-joint muscle synergies are recruited by selected excitation and selected release from inhibition of cortical points. Here we asked whether a similar cortical mechanism operates in the functional linking of antagonistic muscles. To this end experiments were done on ketamine-anesthetized cats. Intracortical microstimulation (ICMS) and intramuscular electromyographic recordings were used to find and characterize wrist, elbow and shoulder antagonistic motor cortical points. Simultaneous ICMS applied at two cortical points, each evoking activity in one of a pair of antagonistic muscles, produced co-contraction of antagonistic muscle pairs. However, we found an obvious asymmetry in the strength of reciprocal inhibition; it was always significantly stronger on physiological extensors than flexors. Following intravenous injection of a single bolus of strychnine, a cortical point at which only a physiological flexor was previously activated also elicited simultaneous activation of its antagonist. This demonstrates that antagonistic corticospinal neurons are closely grouped, or intermingled. To test whether releasing a cortical point from inhibition allows it to be functionally linked with an antagonistic cortical point, one of three GABA(A) receptor antagonists, bicuculline, gabazine or picrotoxin, was injected iontophoretically at one cortical point while stimulation was applied to an antagonistic cortical point. This coupling always resulted in co-contraction of the represented antagonistic muscles. Thus, antagonistic motor cortical points are linked by excitatory intracortical connections held in check by local GABAergic inhibition, with reciprocal inhibition occurring at the spinal level. Importantly, the asymmetry of cortically mediated reciprocal inhibition would appear significantly to bias muscle maps obtained by ICMS in favor of physiological flexors.

Ultrashort laser pulses are very promising tools for performing accurate dissection in the eye, especially in the corneal stroma. The development of eye femtosurgery requires basic knowledge about laser-tissue interaction. One of the most significant parameters is the ablation threshold, the minimal laser energy per unit surface required for ablation. We present here measurements of the femtosecond laser ablation threshold as a function of the pulse duration for two cornea layers (epithelium and stroma) using optical damage diagnosis. Experiments have been realized with the INRS Ti:Sapphire laser (60 fs-5000 fs, 800 nm, 10 Hz). Our experimental results are fitted with a model for laser-matter interaction in order to determine some intrinsic physical parameters.Grant identification: CIHR, FQRNT, FRSQ Research in Vision Network, NSERC.

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