A Functional Architecture of Optic Flow in the Inferior Parietal Lobule of the Behaving Monkey

The representation of navigational optic flow across the inferior parietal lobule was assessed using optical imaging of intrinsic signals in behaving monkeys. The exposed cortex, corresponding to the dorsal-most portion of areas 7a and dorsal prelunate (DP), was imaged in two hemispheres of two rhesus monkeys. The monkeys actively attended to changes in motion stimuli while fixating. Radial expansion and contraction, and rotation clockwise and counter-clockwise optic flow stimuli were presented concentric to the fixation point at two angles of gaze to assess the interrelationship between the eye position and optic flow signal. The cortical response depended upon the type of flow and was modulated by eye position. The optic flow selectivity was embedded in a patchy architecture within the gain field architecture. All four optic flow stimuli tested were represented in areas 7a and DP. The location of the patches varied across days. However the spatial periodicity of the patches remained constant across days at ∼950 and 1100 µm for the two animals examined. These optical recordings agree with previous electrophysiological studies of area 7a, and provide new evidence for flow selectivity in DP and a fine scale description of its cortical topography. That the functional architectures for optic flow can change over time was unexpected. These and earlier results also from inferior parietal lobule support the inclusion of both static and dynamic functional architectures that define association cortical areas and ultimately support complex cognitive function

Axonal Varicosity Density as an Index of Local Neuronal Interactions

Diffuse transmission is an important non-synaptic communication mode in the cerebral neocortex, in which neurotransmitters released from en passant varicosities interact with surrounding cells. In a previous study we have shown that the cholinergic axonal segments which were in the microproximity with dopaminergic fibers possessed a greater density of en passant varicosities compared to more distant segments, suggesting an activity-dependent level of en passant varicosities in the axonal zone of interaction. To further evaluate this plastic relationship, the density of cholinergic varicosities was quantified on fiber segments within the microproximity of activated or non-activated pyramidal cells of the prefrontal cortex (mPFC). Repetitive 14 days patterned visual stimulation paired with an electrical stimulation of the cholinergic fibers projecting to the mPFC from the HDB was performed to induce persistent axonal plastic changes. The c-Fos early gene immunoreactivity was used as a neuronal activity marker of layer V pyramidal cells, labelled with anti-glutamate transporter EAAC1. Cholinergic fibers were labeled with anti-ChAT (choline acetyltransferase) immunostaining. The density of ChAT+ varicosities on and the length of fiber segments within the 3 µm microproximity of c-Fos positive/negative pyramidal cells were evaluated on confocal images. More than 50% of the pyramidal cells in the mPFC were c-Fos immunoreactive. Density of ChAT+ varicosities was significantly increased within 3 µm vicinity of activated pyramidal cells (0.50±0.01 per µm of ChAT+ fiber length) compared to non-activated cells in this group (0.34±0.001; p≤0.05) or control rats (0.32±0.02; p≤0.05). Different types of stimulation (visual, HDB or visual/HDB) induced similar increase of the density of ChAT+ varicosities within microproximity of activated pyramidal cells. This study demonstrated at the subcellular level an activity-dependent enrichment of ChAT+ varicosities in the axonal zone of interaction with other neuronal elements

Improvement of the temperature characteristic of 1.3 μm Gainasp laser diodes with GaInAsP/InP short-period superlattice barriers

We report on the characteristics of tensile-strained 1.3 μm InGaAsP multi-quantum well lasers with InP/GaInAsP short-period superlattices (SPSLs) in barriers and waveguide. Growth was carded out by all solid source atomic layer molecular beam epitaxy (ALMBE) without growth interruptions. Infinite length threshold current densities are as low as 176 A/cm2 per quantum well for as cleaved broad area lasers. The values for the characteristic temperature T0 are as high as 90 K for cavity lengths of 1200 μm. The improvement in To is attributed to the increased effective barrier height by the short-period superlattices.Peer Reviewe

Low temperature InP/Si technology: from Si substrate preparation to epitaxial growth

3 páginas, 2 figuras.InP layers have been grown on Si(001) substrates by using a low temperature process, both for the Si surface preparation (400°C<T Si<550°C) and for the growth process itself (Tg <350°C) using solid source atomic layer molecular beam epitaxy. Strain-free InP on Si layers, with an etch pit density of ~1-2×107 cm-2, showing an excellent morphology and good optical quality have been obtained using a buffer layer involving strain layer superlattices (SLS) of elastically dissimilar materials. This result implies an actual advancement towards monolithic integration of III-V devices to conventional CMOS-Si circuits.The authors wish to thank the CEC for their support under ESPRIT III project 6854 'BLES' (buffer layer engineering in semiconductors).Peer reviewe

Electrical tuning and switching response of defect mode in 3D photonic crystal infiltrated with nematic liquid crystal defect layer

Vytauto Didžiojo universitetasŠvietimo akademij

Observation of dislocation generation in highly strained quantum well lasers during operation

5 páginas, 8 figuras.For the first time, we have obtained the real-time observation, by x-ray topography, of the laser degradation due to the formation of misfit dislocations. We drive the laser diode under a high-energy, high-flux synchrotron micro-x-ray beam. We simultaneously obtain both the laser characteristic curve and the structural characteristics during operation, i.e. the presence of misfit dislocations, laser diode curvature and strain status. From the strain we measure the temperature increase in the epilayer due to the injection current. In other lasers device degradation occurred without the formation of misfit dislocations, but by mass transport, and in other lasers no structural degradation was observed.Peer reviewe

Laser nanosources based on planar photonic crystals as new platforms for nanophotonic devices

Two-dimensional photonic crystal lasers have been fabricated on III–V semiconductor slabs. Tuning of the spontaneous emission in micro and nanocavities has been achieved by accurate control of the slab thickness. Different structures, some of them of new application to photonic crystal lasers, have been fabricated like the Suzuki-phase or the coupled-cavity ring-like resonators. Laser emission has been obtained by pulsed optical pumping. Optical characterization of the lasing modes have been performed showing one or more laser peaks centred around 1.55 ?m. Far field characterization of the emission pattern has been realized showing different patterns depending on the geometrical shape of the structures. These kinds of devices may be used as efficient nanolaser sources for optical communications or optical sensor

Laser nanosources based on planar photonic crystals as new platforms for nanophotonic devices

Two-dimensional photonic crystal lasers have been fabricated on III–V semiconductor slabs. Tuning of the spontaneous emission in micro and nanocavities has been achieved by accurate control of the slab thickness. Different structures, some of them of new application to photonic crystal lasers, have been fabricated like the Suzuki-phase or the coupled-cavity ring-like resonators. Laser emission has been obtained by pulsed optical pumping. Optical characterization of the lasing modes have been performed showing one or more laser peaks centred around 1.55 μm. Far field characterization of the emission pattern has been realized showing different patterns depending on the geometrical shape of the structures. These kinds of devices may be used as efficient nanolaser sources for optical communications or optical sensors