Solubilization of manganese and trace metals in soils affected by acid mine runoff.

Manganese solubility has become a primary concern in the soils and water supplies in the Alamosa River basin, Colorado due to both crop toxicity problems and concentrations that exceed water quality standards. Some of the land in this region has received inputs of acid and trace metals as a result of irrigation with water affected by acid mine drainage and naturally occurring acid mineral seeps. The release of Mn, Zn, Ni, and Cu following saturation with water was studied in four soils from the Alamosa River basin. Redox potentials decreased to values adequate for dissolution of Mn oxides within 24 h following saturation. Soluble Mn concentrations were increased to levels exceeding water quality standards within 84 h. Soluble concentrations of Zn and Ni correlated positively with Mn following reduction for all four soils studied. The correlation between Cu and Mn was significant for only one of the soils studied. The soluble concentrations of Zn and Ni were greater than predicted based on the content of each of these metals in the Mn oxide fraction only. Increases in total electrolyte concentration during reduction indicate that this may be the result of displacement of exchangeable metals by Mn following reductive dissolution of Mn oxides.

Time-dependence of SI RA neuron response to cutaneous flutter stimulation.

Spike discharge activity of RA-type SI cortical neurons was recorded extracellularly in anesthetized monkeys and cats. Multiple applications (trials) of 10-50 Hz sinusoidal vertical skin displacement stimulation ("flutter") were delivered to the receptive field (RF). Analysis revealed large and systematic temporal trends not only in SI RA neuron responsivity (measured as spikes/s and as spikes/stimulus cycle), but also in entrainment, and in phase angle of the entrained responses. In contrast to SI RA neurons, the response of RA skin afferents to comparable conditions of skin flutter stimulation exhibited little or no dynamics. The occurrence and form of the SI RA neuron response dynamics that accompany skin flutter stimulation are shown to depend on factors such as stimulus frequency and the locus of the recording site in the global cortical response pattern. Comparison of recordings obtained in near-radial vs tangential microelectrode penetrations further reveals that the SI RA neuron response dynamics that occur during skin flutter stimulation are relatively consistent within, but heterogeneous across column-sized regions. The observed SI RA neuron response dynamics are suggested to account, in part, for the improved capacity to discriminate stimulus frequency after an exposure ("adaptation") to skin flutter stimulation (Goble and Hollins, J Acoust Soc Am 96: 771-780, 1994). Parallels with recent proposals about the contributions to visual perception of short-term primary sensory cortical neuron dynamics and synchrony in multineuron spike activity patterns are identified and discussed.

Cortical responses to single mechanoreceptive afferent microstimulation revealed with fMRI.

The technique of intraneural microneurography/microstimulation has been used extensively to study contributions of single, physiologically characterized mechanoreceptive afferents (MRAs) to properties of somatosensory experience in awake human subjects. Its power as a tool for sensory neurophysiology can be greatly enhanced, however, by combining it with functional neuroimaging techniques that permit simultaneous measurement of the associated CNS responses. Here we report its successful adaptation to the environment of a high-field MR scanner. Eight median-nerve MRAs were isolated and characterized in three subjects and microstimulated in conjunction with fMRI at 3.0 T. Hemodynamic responses were observed in every case, and these responses were robust, focal, and physiologically orderly. The combination of fMRI with microstimulation will enable more detailed studies of the representation of the body surface in human somatosensory cortex and further studies of the relationship of that organization to short-term plasticity in the human SI cortical response to natural tactile stimuli. It can also be used to study many additional topics in sensory neurophysiology, such as CNS responses to additional classes of afferents and the effects of stimulus patterning and unimodal/crossmodal attentional manipulations. Finally, it presents unique opportunities to investigate the basic physiology of the BOLD effect and to compare the operating characteristics of fMRI and EEG as human functional neuroimaging modalities in an unusually specific and well-characterized neurophysiological setting.

Frequency-dependent response of SI RA-class neurons to vibrotactile stimulation of the receptive field.

Three types of experiment were carried out on anesthetized monkeys and cats. In the first, spike discharge activity of rapidly adapting (RA) SI neurons was recorded extracellularly during the application of different frequencies of vibrotactile stimulation to the receptive field (RF). The second used the same stimulus conditions to study the response of RA-I (RA) cutaneous mechanoreceptive afferents. The third used optical intrinsic signal (OIS) imaging and extracellular neurophysiological recording methods together, in the same sessions, to evaluate the relationship between the SI optical and RA neuron spike train responses to low- vs high-frequency stimulation of the same skin site. RA afferent entrainment was high at all frequencies of stimulation. In contrast, SI RA neuron entrainment was much lower on average, and was strongly frequency-dependent, declining in near-linear fashion from 6 to 200 Hz. Even at 200 Hz, however, unambiguous frequency-following responses were present in the spike train activity of som

Stability of rapidly adapting afferent entrainment vs responsivity.

Spike discharge activity was recorded from low-threshold, rapidly adapting, skin mechanoreceptive afferents (RA afferents) dissected from the median (forelimb) or tibial (hindlimb) nerves in anesthetized monkeys and cats. The spike activity was evoked by delivery of controlled sinusoidal vertical skin displacement ("flutter") stimuli to the receptive field (RF). The stimuli (15-30 Hz; 30-400 microm peak-to-peak amplitude; duration 0.8-15 s) were superimposed on a static skin indentation (0.5-1.0 mm) which was either maintained continuously throughout the run or applied trial-by-trial. The neural activity and the analog signal of the position of the stimulator probe were digitized at 10 kHz resolution and stored for off-line analysis. The main goal was to determine whether changes in the RA afferent response to skin flutter stimulation may be responsible for the enhanced capacity to discriminate stimulus frequency that accompanies a relatively brief (approximately 1 min) pre-exposure to such stimulation in humans. To this end, the spike train data were evaluated using methods that enabled independent measurement of entrainment and responsivity. Responsivity (response intensity) was measured as the average number of spikes/stimulus cycle, while entrainment (the degree to which evoked spike train activity is phase-locked to the stimulus) was quantitatively assessed using statistical techniques developed for the analysis of "circular" (directional) data, supplemented by methods based on the calculation of power spectra from point process data. The methods are demonstrated to enable quantification of RA afferent entrainment over a range of stimulus durations and amplitudes substantially greater than reported in previous studies. While RA afferent responsivity was found to decline to a minor extent (10-20%) both across and within stimulus trials, entrainment remained consistently high and stable, and exhibited no temporal trends or dependence on any other measured factor. The average phase angle of the entrained RA afferent response also remained stable both within and across trials, showing only a tendency to increase slightly during the initial 100-500 ms after stimulus onset. The results imply that the improved capacity to discriminate stimulus frequency that develops in response to an exposure to cutaneous flutter stimulation is not attributable to a change in RA afferent entrainment per se.

fMRI of the responses to vibratory stimulation of digit tips.

Three studies were carried out to assess the applicability of fMRI at 3.0 T to analysis of vibrotaction in humans. A novel piezoelectric device provided clean sinusoidal stimulation at 80 Hz, which was initially applied in separate runs within a scanning session to digits 2 and 5 of the left hand in eight subjects, using a birdcage RF (volume) coil. Significant clusters of activation were found in the primary somatosensory cortex (SI), the secondary somatosensory cortex (SII), subcentral gyrus, the precentral gyrus, posterior insula, posterior parietal regions (area 5), and the posterior cingulate. Digit separation in SI was possible in all subjects and the activation sites reflected the known lateral position of the representation of digit 2 relative to that of digit 5. A second study carried out in six additional subjects using a surface coil, replicated the main contralateral activation patterns detected in study one and further improved the discrimination of the digits in SI. Significant digit separation was also found in SII and in the posterior insula. A third study to investigate the frequency dependence of the response focused on the effect of an increase in vibrotactile frequency from 30 to 80 Hz, with both frequencies applied to digit 2 during the same scanning session in four new subjects. A significant increase in the number of pixels activated within both SII and the posterior insula was found, while the number of pixels activated in SI declined. No significant change in signal intensity with frequencies was found in any of the activated areas.

EEG evidence of stimulus-directed response dynamics in human somatosensory cortex.

Dense multichannel recordings of scalp electroencephalogram (EEG) were obtained in the vicinity of primary somatosensory cortex, time-locked to repetitive vibrotactile stimulation of sites on the right index finger of a single human subject. Frequency-domain analysis of cross-trial averages revealed prominent 'driving' responses in the EEG at the frequency of stimulation, which under specific stimulus conditions displayed pronounced changes in amplitude and topographic organization over brief (4 s) durations of stimulus exposure. The changes were systematic and physiologically coherent, evolving toward driving-response topographies observed in the same subject in conjunction with periodic microstimulation of single mechanoreceptive afferents whose receptive fields occupied corresponding positions on the digit. This dynamic process was orderly and reproducible, and could be controlled by manipulating factors such as the amplitude, frequency, and temporal spacing of the stimuli. The results are tentatively interpreted in light of a previously proposed neurophysiological model of stimulus-driven response plasticity in mammalian somatosensory cortex.

Time-frequency analysis of vibrotactile driving responses by matching pursuit.

A new method of time-frequency analysis, based on the Matching Pursuit (MP) algorithm, was used to extract and quantify EEG 'driving' or frequency-following responses produced in human primary somatosensory cortex (SI) by 33 Hz vibrotactile stimulation of the right index fingertip in a single subject. EEG signals were recorded from a 5 x 5 array of electrodes centered over the left hand area, time-locked to repeated presentations of four vibratory stimulus amplitudes. The MP algorithm was used to decompose the edited and and filtered EEG signals into waveforms selected from a large and redundant dictionary. Statistical discrimination of the vibratory stimulus amplitudes was then readily achieved in terms of trial-by-trial measures of response amplitude constructed in automated fashion from the calculated MP parameters. The results were orderly and physiologically coherent, and potentially open the way to correlation of psychophysical magnitude estimates with measures of neurophysiological response on a trial-by-trial basis. The approach developed here appears well suited to detection and characterisation of time dependent or transient target signals embedded in a noisy background.

A general statistical framework for frequency-domain analysis of EEG topographic structure.

A wide variety of rhythmic electrophysiological phenomena--including driven, induced, and endogenous activities of cortical neuronal masses--lend themselves naturally to analysis using frequency--domain techniques applied to multichannel recordings that discretely sample the overall spatial pattern of the rhythmic activity. For such cases, a large but so far poorly utilized body of statistical theory supports a third major approach to topographic analysis, complementing the more familiar mapping and source-recovery techniques. These methods, many of which have only recently become computationally feasible, collectively provide general solutions to the problem of detecting and characterizing systematic differences that arise--not only in the spatial distribution of the activity, but also in its frequency-dependent between-channel covariance structure--as a function of multiple experimental conditions presented in conformity with any of the conventional experimental designs. This application-oriented tutorial review provides a comprehensive outline of these resources, including: (1) real multivariate analysis of single-channel spectral measures (and measures of between-channel relationships such as coherence and phase), (2) complex multivariate analysis based on multichannel Fourier transforms, and (3) complex multivariate analysis based on multichannel parametric models. Special emphasis is placed on the potential of the multichannel autoregressive model to support EEG (and MEG) studies of perceptual and cognitive processes.

Periodic microstimulation of single mechanoreceptive afferents produces frequency-following responses in human EEG.

Dense multichannel recordings of scalp electroencephalogram were obtained in the vicinity of primary somatosensory cortex, time-locked to repetitive train microstimulation of single, physiologically characterized skin mechanoreceptive afferents in the median nerve of a single human subject. Frequency-domain analysis of cross-trial averages for fast-adapting type one and slowly adapting type one afferents revealed prominent, topographically organized "driving" responses in the electroencephalogram at the frequency of stimulation, which vanished under various statistical and experimental control conditions. The responses also exhibited systematic declines in amplitude both across and within trials, and orderly changes in scalp topography as a function of the location of afferents' receptive fields on the hand. The observed response properties are tentatively explained in terms of characteristics of the pattern of afferent drive impressed on the cortex by microstimulation.

Frequency-domain measurement of vibrotactile driving responses in first-order afferent populations.

Surface recordings made at the wrist during moderate vibrotactile stimulation of a digit display rhythmic activity at the frequency of the driving stimulus. This activity is abolished by local anesthesia of the stimulated digit and by substitution of the corresponding digit of the opposite hand with the recording geometry and the load on the stimulator unchanged. Several additional features of the response are similarly incompatible with an artifactual origin in properties of the stimulus motion or the associated electromagnetic field, but consistent with previous neurophysiological observations. The frequency-domain analysis extends readily to the single-trial level, making the technique potentially useful for a variety of basic research and clinical purposes.

Multichannel PC-based data-acquisition system for high-resolution EEG.

This paper describes a complete, high-performance data-acquisition system for high-resolution EEG. The system includes preamplified EEG electrodes, rigid helmets, differential instrumentation and software-controlled gain amplifiers, digitizing and control circuitry, and optical coupling to a 486 PC. Miniaturized preamplifiers mounted on individual electrodes reduce coupling to external electromagnetic fields and minimize signal distortion caused by increased and/or imbalanced electrode impedances. Electrodes are applied quickly and with minimal skin preparation, yet with high packing densities and repositioning consistent to about 1-3 mm in repeated applications. Extensive testing under realistic conditions demonstrates that the system provides superior electrical performance compared to currently available commercial systems.

Evoked potentials as indices of adaptation in the somatosensory system in humans: a review and prospectus.

Population-level behavior of large neural aggregates can be efficiently monitored by corresponding population-level indices such as somatosensory evoked potentials (SEPs). The literature reviewed clearly indicates that SEPs undergo systematic and often marked changes under conditions of repetitive stimulation. Similar results have been reported for several mammalian species and with a diversity of stimulation, recording and analysis protocols. The most characteristic finding is a loss of SEP component amplitude as a function of decreasing time between stimulus presentations. The effects become larger and appear at longer ISIs at higher levels of the somatosensory pathway, are more readily evoked by stimulus trains than by stimulus pairs and are most pronounced for response components generated in the upper cortical layers. These findings are consistent with a recently proposed neurophysiological model of short-term plasticity in somatosensory cortex, which incorporates detailed and current information on cortical microcircuitry, receptor and neurotransmitter characteristics, topographical organization and dynamic response to repetitive sensory drive. Recommendations are provided for further research, emphasizing the potential of frequency-domain analysis methods in conjunction with mechanical vibrotactile stimuli as a vehicle for more detailed testing of the proposed neurophysiological model and for closer integration with psychophysical studies of vibrotactile adaptation.

Assessment of oviduct potency in the cow.

A test of oviduct patency was developed, based on instillation of dye to the uterine horn through a two-way catheter. In 23 cows, two cases of bilateral and five cases of unilateral blockage were detected. Of 69 cow genital tracts collected from an abattoir, nine of 138 oviducts (7 per cent) did not permit the passage of dye. Fourteen of the remaining oviducts were found to blocked using a suspension of 80 micrometer pollen grains. This suggested that some animals showing patency to the dye test in vivo may in fact have abnormalities of the oviduct sufficient to prevent the passage of the ovum. In five cows, superovulation and embryo recovery were carried out as an investigation of oviduct patency and function. Although in one of the five cows, subsequently found to have a blocked oviduct, no embryos were recovered, ambiguous results were obtained in the other animals. The unpredictable response to superovulation may limit the application of this latter technique in investigation of oviduct patency and function.