Mechanisms of microtunneling in rock substrates: distinguishing endolithic biosignatures from abiotic microtunnels.

Rock-dwelling, endolithic micro-organisms can create tubular microcavities (TMCs) by the dissolution of rock substrates. Microtunnels can also conceivably be formed by abiotic processes, and collectively, these structures are here termed tubular microcavities. A textural record of life in subseafloor environments is provided by biological TMCs, and it is imperative to distinguish these from abiological tunnels. To this end, the morphologies and petrographic context of tunnels formed by chemical solution, physical abrasion, and biological processes are here described. Biological TMCs in volcanic glass are restricted to sites that were connected to early fluid circulation. Their shapes, distribution, and the absence of intersections exclude an origin by chemical dissolution of pre-existing heterogeneities such as, radiation damage trails, gas-escape structures, or fluid inclusion trails. Rather their characteristics are best explained by microbial dissolution, involving perhaps, cellular extensions that provide a mechanism of localizing and directing microtunnel formation as observed in terrestrial soils. Biological TMCs are contrasted with ambient inclusion trails (AITs) found in cherts and authigenic minerals. These differ in exhibiting longitudinal striae, a constant diameter, and polygonal cross-section, sometimes with terminal inclusions. The origin(s) of AITs remain unclear but they are hypothesized to form by migration of crystalline or organic inclusions in sealed substrates, in contrast to biotic TMCs that form in open systems. We present diagnostic morphological and petrographic criteria for distinguishing these different types of TMCs. Moreover, we argue that AIT-type processes are not viable in volcanic glass because of the absence of crystalline millstones, localized chemical solution agents, and elevated fluid pressures, necessary to drive this process.

Growth of synthetic stromatolites and wrinkle structures in the absence of microbes - implications for the early fossil record.

Stromatolites and wrinkle structures are often taken to be an important indicator for early life. While both may be shaped by microbial mat growth, this can be open to doubt, so that the contribution of abiotic processes in their construction always needs to be established (Grotzinger & Knoll, 1999). We here report laboratory spray deposition experiments that can generate stromatolites and wrinkle structures in the absence of microbes. These minicolumnar and sometimes branched stromatolites are produced artificially by the aggregation of a synthetic colloid in a turbulent flow regime. They self-organize at the relatively low particle concentrations found in the outer parts of a spray beam. This contrasts with adjacent stratiform deposits that are produced by high rates of colloid deposition and relatively low sediment viscosities found in the centre of a spray beam. These stratiform laminae become subsequently wrinkled during hardening of the colloid. These results support numerical models that together suggest that physicochemical processes are capable of generating laminated sedimentary structures without the direct participation of biology. Geological environments where comparable abiogenic stromatolites and wrinkle structures may be found include: splash-zone silica sinters, desert varnish crusts and early Archean cherts formed from silica gel precursors.

Neurovascular coupling investigated with two-dimensional optical imaging spectroscopy in rat whisker barrel cortex.

Optical imaging slit spectroscopy is a powerful method for estimating quantitative changes in cerebral haemodynamics, such as deoxyhaemoglobin, oxyhaemoglobin and blood volume (Hbr, HbO2 and Hbt, respectively). Its disadvantage is that there is a large loss of spatial data as one image dimension is used to encode spectral wavelength information. Single wavelength optical imaging, on the other hand, produces high-resolution spatiotemporal maps of brain activity, but yields only indirect measures of Hbr, HbO2 and Hbt. In this study we perform two-dimensional optical imaging spectroscopy (2D-OIS) in rat barrel cortex during contralateral whisker stimulation to obtain two-dimensional maps over time of Hbr, HbO2 and Hbt. The 2D-OIS was performed by illuminating the cortex with four wavelengths of light (575, 559, 495 and 587 nm), which were presented sequentially at a high frame rate (32 Hz). The contralateral whisker pad was stimulated using two different durations: 1 and 16 s (5 Hz, 1.2 mA). Control experiments used a hypercapnic (5% CO2) challenge to manipulate baseline blood flow and volume in the absence of corresponding neural activation. The 2D-OIS method allowed separation of artery, vein and parenchyma regions. The magnitude of the haemodynamic response elicited varied considerably between different vascular compartments; the largest responses in Hbt were in the arteries and the smallest in the veins. Phase lags in the HbO2 response between arteries and veins suggest that a process of upstream signalling maybe responsible for dilating the arteries. There was also a consistent increase in Hbr from arterial regions after whisker stimulation.

Blind signal separation from optical imaging recordings with extended spatial decorrelation.

Optical imaging is the video recording of two-dimensional patterns of changes in light reflectance from cortical tissue evoked by stimulation. We derived a method, extended spatial decorrelation (ESD), that uses second-order statistics in space for separating the intrinsic signals into the stimulus related components and the nonspecific variations. The performance of ESD on model data is compared to independent component analysis algorithms using statistics of fourth and higher order. Robustness against sensor noise is scored. When applied to optical images, ESD separates the stimulus specific signal well from biological noise and artifacts.

Cortical computation of stereo disparity.

Our ability to see the world in depth is a major accomplishment of the brain. Previous models of how positionally disparate cues to the two eyes are binocularly matched limit possible matches by invoking uniqueness and continuity constraints. These approaches cannot explain data wherein uniqueness fails and changes in contrast alter depth percepts, or where surface discontinuities cause surfaces to be seen in depth, although they are registered by only one eye (da Vinci stereopsis). A new stereopsis model explains these depth percepts by proposing how cortical complex cells binocularly filter their inputs and how monocular and binocular complex cells compete to determine the winning depth signals.

Effect of changes in heart rate on pressure half time in normally functioning mitral valve prostheses.

To test the validity of a relation between the pressure half time and the diastolic time interval, previously shown in a pulse duplication system, eight patients with prosthetic mitral valves and permanent pacemaker systems were studied. Recordings were made from the apex by continuous wave or pulsed Doppler echocardiography at heart rates between 75 and 150 beats/min. The pressure half time was found to be closely correlated with the diastolic time interval although there was individual variation and in three prostheses the pressure half time attained a plateau when the diastolic time interval was more than 300 ms. It is likely that the orifice area is the main controller of pressure half time where there is stenosis of the prosthesis, but that other factors such as ventricular or atrial compliance and the diastolic time interval may modify or obscure the effect of orifice area in normally functioning prosthetic valves.