Biomedical Research Unit

Electrogustometry is a convenient method to examine taste acuity in clinical situations. Some basic properties of neural activity in the cerebral cortex in response to electrogustatory stimulation were revealed by measuring magnetoencephalography (MEG) signals with a whole-cortex-type system in response to varying intensities of anodal DC currents focally applied to the tongue surface in human subjects. Independent component analysis was used to eliminate stimulus artifacts in MEG signals. Electrogustatory stimulation with intensities of induced electric taste evoked responses bilaterally, mainly in the opercular-insular cortex with a mean onset latency of~350 ms, while subthreshold electrogustatory stimulation induced modest responses in the cortex. Stronger stimulation induced a tingling sensation and elicited large transient responses in both the opercular-insular and somatic sensory cortices. This is the first description of the basic properties of human MEG responses to electrogustatory stimulation.

Shales display significant seismic anisotropy that is attributed in part to preferred orientation of constituent minerals. This orientation pattern has been difficult to quantify because of the poor crystallinity and small grain size of clay minerals. A new method is introduced that uses high-energy synchrotron X-rays to obtain diffraction images in transmission geometry and applies it to an illite-rich shale. The images are analyzed with the crystallographic Rietveld method to obtain quantitative information about phase proportions, crystal structure, grain size, and preferred orientation ͑texture͒ that is the focus of the study. Textures for illite are extremely strong, with a maximum of 10 multiples of a random distribution for ͑001͒ pole figures. From the three-dimensional orientation distribution of crystallites, and single-crystal elastic properties, the intrinsic anisotropic elastic constants of the illite aggregate ͑excluding contribution from aligned micropores͒ can be calculated by appropriate medium averaging. The illitic shale displays roughly transverse isotropy with C 11 close to C 22 and more than twice as strong as C 33 . This method will lend itself to investigate complex polymineralic shales and quantify the contribution of preferred orientation to macroscopic anisotropy.

Within the context of seismic wave propagation, fractures can be described as thin layers or linear-slip interfaces. In this paper, numerical simulations of elastic wave propagation in a medium with a single fracture represented by these two models are performed by 2D finite-difference codes: a variable-grid isotropic code for the thin-layer model and a regular-grid anisotropic code for the linear-slip model. Numerical results show excellent agreement between the two models for wavefields away from the fracture; the only discrepancy between the two is the presence of a slow wave traveling primarily within the fracture fluid of the thin-layer model. The comparison of the computational cost shows that modeling of the linear-slip model is more efficient than that of the thin-layer model. This study demonstrates that the linear-slip model is an efficient and accurate modeling approach for the remote seismic characterization of fractures.

Recent studies on noninvasive recordings from human brain have shown the existence of tasteelicited activation areas in the cerebral cortex. While functional MRI (f-MRI) and positron CT (PET) are often used for these studies, the magnetoencephalogram (MEG) is the most commonly used instrument for these noninvasive measurements. One advantage of the MEG measurements is the ability to measure rapid taste-elicited time-course data. In this current study, we used brain magnetic fields to quantitate the stimulus latencies evoked by different taste stimuli that use different peripheral transduction mechanisms. Recent work has shown that taste stimuli that presumably act through different transduction processes show different MEG-measured latencies. Here, we measured the latencies due to citric acid and sucrose and compared these with the latency due to the action of the taste-modifying substance contained in miracle fruit during stimulation by citric acid. Miracle fruit has the property of changing the sour taste of acids to sweet taste. The use of this taste-modifying substance allows us to compare the latency of two very different sweet-taste-evoking substances. D

A method for determining elastic properties of rock fragments is presented. The method consists in creating two component composite specimens of a continuous Wood's metal matrix embedded with rock fragment inclusions. Six different cylindrical samples were prepared in this manner. The volumetric concentration of the fragments varies from 0.43 to 0.5. The principal idea of the method is to use an effective media theory to determine the elastic properties (K and G) of the rock fragments from acoustic measurements (resonance and ultrasonics) of the elastic properties of the Wood's metal/rock fragment composite specimen a solid Wood's metal specimen. Contrary to anticipated, the measured elastic parameters for the composite specimens lie outside the bounds of the corresponding elastic parameters for the 100% Wood's metal and 100% rock samples. These results cannot be predicted using zero-frequency effective media theories (e.g., Kuster-Toks6z method). Indeed, our data agrees with a recently published frequency-dependent, effective media theory for a strongly scattering medium 1 . It is thus evident that the commonly used frequency independent theory are not adequate to model the behavior of these composite media with volumetric concentrations above 0.4.

To examine the effects of multiple, aligned fractures in rock, we have developed a two-dimensional elastic finite difference code for fractured media. Fractures are incorporated into the model explicitly as displacement-discontinuity boundary conditions. The wavefield is computed using a 4th-order staggered grid scheme. Simulations were performed for a broadband explosion point source (center frequency 374 Hz) located at the center of the model. The model consisted of 90 horizontal fractures spaced approximately 1/8 of a wavelength apart. The normal and shear fracture stiffnesses were selected such that the normal incidence transmission coefficient is 0.6. The simulations show strong scattering attenuation of the P-wave in the vertical direction (± to the fractures) and channeling of guided waves in the horizontal direction (ll to the fractures). The same code was also used to model wave propagation in an anisotropic medium with equivalent effective moduli for the 90-fracture system. Significant differences between the amplitudes, velocities, and frequency content of the waves in the explicit and equivalent medium fracture models were observed. These differences result from frequency-dependent time delays and filtering across each fracture and channeling along fractures that are not included in the zero-frequency effective medium description. These effects are especially interesting because they illustrate that the dynamic properties of fractured rock include significant amplitude anisotropy that may prove useful in the characterization of fractured rock.

The contributions of frictional slip on the nonlinear, hysteretic deformation of sandstone in the reversible regime (i.e., prior to the onset of permanent deformation) for uniaxial strain compression are investigated through an analysis of a Hertz±Mindlin face-centered cubic sphere pack model and laboratory stress±strain tests on Berea sandstone. The analysis demonstrates that the dynamic 1 moduli are path-independent functions of the strain. The analysis also reveals that for uniaxial strain consolidation it is possible to decompose the volumetric strain into a path-independent contribution from nonlinear grain contact deformation and a path-dependent contribution from frictional (slip) compaction. Laboratory stress±strain measurements on Berea sandstone support these ®ndings and, in addition, reveal that frictional compaction accounts for a signi®cant portion of the volumetric strain of Berea sandstone. 7

Elastic wave conversions across a fracture in rock subjected to shear stress are observed during laboratory ultrasonic transmission tests. Compressional waves (P-waves) and shear waves (S-waves) normally incident on a sheared fracture are partially converted to waves with particle motions that are not present in the incident waves. The amplitudes of the converted waves increase signi®cantly with increasing shear stress applied to the fracture. The particle motion of the converted wave changes its phase by 1808 when the direction of the shear is reversed. These wave behaviors can be explained by adding the coupling (dilational) components in the fracture stiness matrix of the displacement±discontinuity boundary conditions for a non-welded interface. The shear-induced wave conversion presented in this paper has a wide range of potential applications for monitoring and determining the magnitude of shear stress acting on fractures in geologic and engineered materials. 7

This study demonstrates that joint-parallel compressive stresses were integral to the development of joint sets on Vancouver Island, British Columbia. The mapped study area contains paleostress indicators in the form of calcareous concretions, which have multiple, internal fractures in precisely the same orientation as one of the surrounding joint sets in the sandstone matrix. Field and laboratory tests indicate that the stiffest concretions are the most likely to be fractured; however, results from numerical simulations using measured rock properties preclude an origin for the concretion fractures from either a far-field uniaxial tensile or compressive stress. Fracturing is only found to be possible if the concretions possessed a lower Poisson's ratio than the sandstone at the time of fracturing. In the latter case, a far-field uniaxial compressive stress may have generated tensile effective stresses in the vicinity of high modulus concretions, seeding the field site with an initial population of concretion fractures and joints. Given the close spacing of some joints, their extension cannot be satisfactorily explained without invoking grain-scale compression-driven tensile fracturing mechanisms. q

Poroelastic analysis has traditionally focused on the relationship between dry or drained constants which are assumed known and the saturated or undrained constants which are assumed unknown. However, there are many applications in this field of study for which the main measurements can only be made on the saturated/undrained system, and then it is uncertain what the effects of the fluids were on the system, since the drained constants remain a mystery. The work presented here shows how to deduce drained constants from undrained constants for anisotropic systems having symmetries ranging from isotropic to orthotropic. Laboratory ultrasound data are then inverted for the drained constants in three granular packings: one of glass beads, and two others for distinct types of more or less angular sand grain packings. Experiments were performed under uniaxial stress, which resulted in hexagonal (transversely isotropic) symmetry of the poroelastic response. One important conclusion from the general analysis is that the drained constants are uniquely related to the undrained constants, assuming that porosity, grain bulk modulus, and pore fluid bulk modulus are already known. Since the resulting system of equations for all the drained constants is linear, measurement error in undrained constants also propagates linearly into the computed drained constants.

Red berries of a tropical plant called miracle fruit, Richadella dulcifica, reduce the sour and aversive taste of acids and add sweet and palatable taste. To elucidate the brain mechanism of this unique action of miracle fruit, we recorded taste-elicited magnetic fields of the human cerebral cortex. The initial taste responses were localized in the fronto-parietal opercular/insular cortex reported as the primary taste area. The mean latency of the response to citric acid after chewing miracle fruit was essentially the same as that for sucrose and was 250-300 ms longer than that for citric acid. Since it is known that stimulation with acids after the action of miracle fruit induces both sweetness and sourness responses in the primate taste nerves, the present results suggest that the sourness component of citric acid is greatly diminished at the level of subcortical relays, and mostly sweetness information reaches the cortical primary taste area. We propose the idea that the qualitative aspect of taste is processed in the primary taste area and the affective aspect is represented by the pattern of activation among the different cortical areas.

When two tones are presented in a short time interval, the response to the second tone is suppressed. This phenomenon is referred to as forward suppression. To address the effect of the masker laterality on forward suppression, magnetoencephalographic responses were investigated for eight subjects with normal hearing when the preceding maskers were presented ipsilaterally, contralaterally, and binaurally. We employed three masker intensity conditions: the ipsilateral-strong, left-right-balanced, and contralateral-strong conditions. Regarding the responses to the maskers without signal, the N1m amplitude evoked by the left and binaural maskers was significantly larger than that evoked by the right masker for the left-strong and left-right-balanced conditions. No significant difference was observed for the right-strong condition. Regarding the subsequent N1m amplitudes, they were attenuated by the presence of the left, binaural, and right maskers for all conditions. For the left-and right-strong conditions, the subsequent N1m amplitude in the presence of the left masker was smaller than those of the binaural and right maskers. No difference was observed between the binaural and right masker presentation. For left-right-balanced condition, the subsequent N1m amplitude decreased in the presence of the right, binaural, and left maskers in that order. If the preceding activity reflected the ability to suppress the subsequent activity, the forward suppression by the left masker would be superior to that by the right masker for the left-strong and left-right-balanced conditions. Furthermore, the forward suppression by the binaural masker would be expected to be superior to that by the left masker owing to additional afferent activity from the right ear. Thus, the current results suggest that the forward suppression by ipsilateral maskers is superior to that by contralateral maskers although both maskers evoked the N1m amplitudes to the same degree. Additional masker at the contralateral ear can attenuate the forward suppression by the ipsilateral masker.

Many studies involving the application of geophysical methods in the field of gas hydrates have focused on determining rock physics relationships for hydrate-bearing sediments, with the goal being to delineate the boundaries of gas hydrate accumulations and to estimate the quantities of gas hydrate that such accumulations contain using remote sensing techniques.

When two tones are presented in a short interval of time, the presentation of the preceding tone (masker) suppresses the response evoked by the subsequent tone (signal). To address the processing in forward suppression, we applied 2- and 4-kHz maskers, followed by a 1-kHz signal at varying signal delays (0 to 320 ms) and measured the signal-evoked N1m. A two-way analysis of variance revealed a statistically significant effect for signal delay in both masker presentation conditions. The N1m peak amplitude at the signal delay of 320 ms was significantly larger than those of 10, 20, 40, and 80 ms (p < 0.05). No significant enhancement for the very short signal delay was observed. The results suggest that the enhancement of N1m peak amplitude for short signal delay conditions is maximized when the frequency of the masker is identical to that of the signal.

A two-dimensional elastic finite difference code was developed to examine the characteristic of seismic waves propagation in a fractured rock containing multiple, aligned fractures. The displacement-discontinuity boundary conditions were used to model the fractures explicitly. The effect of fracture spacing on the wavefiled generated by an explosion source was examined by changing the spacing from 0.6 to 0.15 of a wavelength. A wavefiled was computed for a transversely anisotropic (TI) medium with elastic moduli equivalent to the effective static moduli of the most densely fractured system. The results showed significant differences between the amplitudes, velocities, and frequency content of the waves in the explicit and equivalent medium fracture models. These differences result from frequency-dependent time delays and filtering across each fracture and channeling along fractures that are not included in the zero-frequency effective medium description. These effects lead to an unusually strong velocity and amplitude anisotropy which cannot explained by the TI medium approximation. The characterization may prove useful in characterizing fractures in reservoir rock.

This paper examines the characteristics of channel wave propagation in a waveguide formed by two parallel fractures using two-dimensional elastic finite difference simulation and mode analysis. Velocity dispersion of the fracture channel waves computed from f-k analysis of the synthetic waveforms shows good agreement with the mode solution. The dispersion characteristics of the fracture channel waves are compared to those for classic Rayleigh channel waves in a low velocity layer with properties selected so that the S-wave acoustic impedance of the layer matches that of the fracture waveguide at the central frequency of the source. Significantly different velocity-frequency behavior for the various modes was observed for the two types of waveguides. In addition, the fracture waveguide shows high-frequency thickness resonances in the vicinity of the source.