Function Of Stimulus Rate Research Articles

Predicting stimulus‐rate sensitivity of human somatosensory fMRI signals with MEG

With increasing stimulus rate (SR), cortical EEG and MEG responses typically decrease in amplitude whereas BOLD fMRI signals increase. To address this discrepancy, we predicted BOLD responses with squared MEG waveforms using a recently proposed energy-density model. Tactile stimuli were delivered to finger tips at SRs of 1, 4, or 10 Hz in successive 25-s blocks, and brain signals were detected from area 3b of the primary somatosensory cortex of nine healthy adults using a 306-channel whole-scalp neuromagnetometer and a 3-T fMRI magnet. The main MEG deflections decreased in amplitude as a function of SR, whereas the BOLD signals increased from 1- to 4-Hz SR, with no further change at 10 Hz. MEG energy densities, obtained over the whole stimulus train and convolved with different hemodynamic response functions, predicted both the shape and amplitude of the BOLD signals well, and incorporation of nonlinear terms into the model did not offer any further advantage. Thus, squared MEG waveforms obtained over the entire stimulus train provided an appropriate estimate of area 3b neuronal activity associated with the BOLD signal.

An investigation into the relationship between input–output nonlinearities and rate-induced nonlinearities of click-evoked otoacoustic emissions recorded using maximum length sequences

The maximum length sequence (MLS) technique allows otoacoustic emissions (OAEs) to be recorded using clicks presented at very high presentation rates. It has previously been found that increasing the click presentation rate leads to increasing suppression (termed “rate-suppression”) of the MLS evoked OAE (Hine, J.E., Thornton, A.R.D., 1997. Transient evoked otoacoustic emissions recorded using maximum length sequences as a function of stimulus rate and level. Ear Hear. 18, 121–128). It has been suggested that the source of rate-suppression arises from the same nonlinear processes that give rise to the well-known nonlinear growth of OAEs. Based on this assumption, a simple model of rate-suppression (Kapadia, S., Lutman, M.E., 2001. Static input–output nonlinearity as the source of nonlinear effects in maximum length sequence click-evoked OAEs. Br. J. Audiol. 35, 103–112) predicts that both input–output (I/O) nonlinearity and rate-suppression can be unified by characterising the stimulus in terms of its acoustic power which, at high rates, is proportional to the click presentation rate. The objective of this study was to test this simple model by recording MLS OAEs from a group of normally hearing adults over a range of stimulus rates from 40 to 5000 clicks/s, and of stimulus levels from 45 to 70 dB peSPL. The results are broadly in agreement with the predictions from the model, though there appears to be some tendency for the model to slightly overestimate the degree of rate-suppression for a given degree of I/O nonlinearity. It is also suggested that the model may break down more significantly in the presence of spontaneous OAEs.

Developmental changes in refractoriness of the cortical auditory evoked potential

Objective This study examined morphological changes in the cortical auditory evoked potential (CAEP) waveform as a function of varying stimulation rate. Stimuli were presented in a paradigm which indirectly assesses the refractory properties of the underlying neuronal generators. Methods CAEPs were recorded in 50 normal-hearing children (3–12 years) and 10 young adults (24–26 years). A speech sound was presented in a stimulus train with sequentially decreasing inter-stimulus intervals (ISIs) of 2000, 1000, 560, and 360 ms. Latencies and amplitudes of the P1, N1, and P2 components at the Cz electrode were examined as a function of stimulus rate and age. Results Results revealed significant changes in the CAEP as a function of age and stimulation rate. At younger ages the N1–P2 component was elicited only at the slowest stimulation rates, and was more clearly apparent at successively faster stimulation rates as age increased. Conclusions We have described a stimulus paradigm that allows examination of the development of refractoriness by highlighting the interaction between age and rate on CAEP morphology. Significance Complex maturational patterns of CAEP components are best understood when the effects of both age and stimulus rate on the CAEP waveform are considered.

Deciphering the neural code: neuronal discharge variability is preferentially controlled by the temporal distribution of afferent impulses

Neuronal discharge variability has typically been studied as a function of stimulus rate in the context of a Poisson point process. Here we report that novel discharge behavior is elicited in a model cerebellar Purkinje cell when the timing of afferent impulses follows a pulsed Gaussian stimulation paradigm. We show that under these circumstances effective convergence superseded frequency of activation as the independent variable controlling cell discharge. This provides strong evidence that firing rate alone is an insufficient foundation for the neural code.

Electrical stimulation of the auditory nerve: direct current measurement in vivo.

Neural prostheses use charge recovery mechanisms to ensure the electrical stimulus is charge balanced. Nucleus cochlear implants short all stimulating electrodes between pulses in order to achieve charge balance, resulting in a small residual direct current (DC). In the present study we sought to characterize the variation of this residual DC with different charge recovery mechanisms, stimulation modes, and stimulation parameters, and by modeling, to gain insight into the underlying mechanisms. In an acute study with anaesthetised guinea pigs, DC was measured in four platinum intracochlear electrodes stimulated using a Nucleus C124M cochlear implant at moderate to high pulse rates (1200-14,500 pulses/s) and stimulus intensities (0.2-1.75 mA at 26-200 microseconds/phase). Both monopolar and bipolar stimulation modes were used, and the effects of shorting or combining a capacitor with shorting for charge recovery were investigated. Residual DC increased as a function of stimulus rate, stimulus intensity, and pulse width. DC was lower for monopolar than bipolar stimulation, and lower still with capacitively coupled monopolar stimulation. Our model suggests that residual DC is a consequence of Faradaic reactions which allow charge to leak through the electrode tissue interface. Such reactions and charge leakage are still present when capacitors are used to achieve charge recovery, but anodic and cathodic reactions are balanced in such a way that the net charge leakage is zero.

Reduction in excitability of the auditory nerve following acute electrical stimulation at high stimulus rates: III. Capacitive versus non-capacitive coupling of the stimulating electrodes

Safe electrical stimulation of neural tissue is typically achieved using charge-balanced biphasic current pulses, which are designed to minimize the generation of direct current (DC) and the production of harmful electrochemical products. However, due to the kinetics of the charge injection process, neural stimulators must also use capacitive coupling or electrode shorting techniques, to ensure DC levels are minimal. Previous studies have reported a reduction in excitability of the auditory nerve following acute simulation at high rates and intensities. Elevated levels of DC were reported in these studies despite using charge-balanced biphasic pulses and electrode shorting. The present study was designed to investigate the extent to which DC contributed to these stimulus induced reductions in auditory nerve excitability. Adult guinea pigs were bilaterally implanted and unilaterally stimulated for two hours using charge-balanced biphasic current pulses and stimulus rates of 200, 400 or 1000 pulses/s (pps) at a stimulus intensity well above clinical levels (0.34 μC/phase). DC levels were controlled using either electrode shorting, or electrode shorting with capacitive coupling. Electrically evoked auditory brainstem responses (EABRs) were recorded before and periodically following the acute stimulation. It was found that the extent of reduction in the EABR amplitude was a function of stimulus rate. While there was little change in the EABR following stimulation at 200 pps, significant post-stimulus reductions in the EABR amplitude were observed at stimulus rates of 400 and 1000 pps during the three hour post-stimulus monitoring period. Stimulation using capacitively coupled electrodes, which eliminated all DCs, showed reductions in EABR amplitudes similar to those observed following stimulation using electrode shorting alone. While there was no significant difference in the extent of reduction in EABR amplitudes for capacitive coupling versus electrode shorting at stimulus rates of 200 pps ( P>0.05) and 400 pps ( P>0.05), there was a significant difference at 1000 pps ( P<0.001). The present findings indicate that the major component of the stimulus induced reductions observed in auditory nerve excitability appear to be associated with stimulus induced neuronal activity, although elevated levels of DC (>2.5 μA) can also contribute to these changes. However, although statistically significant, the effects of DC are very small compared to the effects of high rate, high intensity stimulation per se.

Transient Evoked Otoacoustic Emissions Recorded Using Maximum Length Sequences as a Function of Stimulus Rate and Level

The recently developed technique of recording transient evoked otoacoustic emissions (TEOAEs) using clicks presented according to maximum length sequences (MLSs) enables very high stimulation rates to be used. The aim of this study was to provide normative data on the relationship between TEOAEs recorded conventionally (at 40 clicks/sec) and those recorded using the MLS technique (at 11 maximum rates between 100 and 5000 clicks/sec) to establish a baseline for future clinical studies. TEOAEs were recorded at 12 rates from 12 normally hearing adult ears at click levels decreasing in 5 dB steps from 68 dB peSPL. The morphology of the waveforms and the pattern of the input/output functions with latency were similar for conventional and MLS TEOAEs. The only major difference between TEOAEs recorded at the different rates was in their absolute amplitude. As the click rate was increased from 40 clicks/sec there was a reduction in amplitude that reached a near asymptote at approximately 1500 clicks/sec. When expressed as a percentage reduction in amplitude compared with that recorded at 40 clicks/sec, this MLS "rate effect" was independent of stimulus level over all but the lowest test level (38 dB peSPL SPL). Over a wide range of amplitudes of conventionally recorded TEOAEs (21 to 450 microPa for the 9 to 13 msec section of the otoacoustic emission), the mechanism involved in the MLS rate effect seems to act in a way that reduces the amplitude by an almost constant proportion, whatever its original size.

A framework for assessing the relative efficiency of stimulus sequences in evoked response measurements

A general matrix-based framework for characterizing the performance of cross-correlation techniques for recovering the response to an arbitrary stimulus sequence is presented. In general an infinite number of recovery sequences can be identified for any given stimulus sequence. The recovery sequence that minimizes the noise in the recovered response is derived and the effect of the recovery operation on the noise is analyzed. This general framework is employed to develop analytic expressions for the relative efficiencies of conventional signal averaging and binary MLS based methods for the most commonly used recovery operation and the one that optimizes SNR assuming the background noise is uncorrelated. The results depend on the ratio of response length to MLS minimum pulse interval and the amplitude loss as a function of stimulus rate. Examples based on measured amplitude loss functions for ABR recordings are employed to evaluate the relative efficiencies of MLS techniques. When the noise is uncorrelated conventional signal averaging is from two to five times as efficient as MLS techniques. The relative advantage of averaging is shown to decrease when the noise is dominated by low-frequency components.

Are frontal and parietal somatosensory evoked potentials functionally dissociated by changing stimulus rate?

The cortical somatosensory evoked potentials are known to be sensitive to relatively small changes in the stimulus repetition rate of the afferent nerve. However, conflicting reports exist as to whether frontally and parietally recorded potentials at a given latency show differential behaviors as a function of stimulus rate. Because such dissociations of frontal and parietal potentials can have significant implications for the SEP generation mechanisms, the present study was undertaken to further describe in detail these effects on frontal, central and parietal waveforms after median nerve stimulation. Increasing stimulus repetition rate from 1 Hz to 5 Hz had the following effects: (i) in 9 of 16 subjects, the frontal P20 diminished while parietal N20 clearly remained unaltered, (ii) the central P22 was reduced in all subjects, (iii) frontal N30 and parietal P27 were attenuated in all subjects, the average magnitude of the reductions being nearly equal for these deflections. The results support the view that changing stimulus rate can functionally dissociate frontal and parietal activity around 20 ms, indicating that several partially independent neural populations can contribute to the frontal P20. The results did not lend support for functional dissociation of frontal N30 from parietal P27.

The development of stimulus following in the cochlear nerve and inferior colliculus of the mouse

The decrement of evoked response amplitudes during the presentation of repetitive clicks was examined quantitatively at the level of the eighth nerve and inferior colliculus in mice aged 13–60 days postnatal. The amplitudes of both these potentials were found to decline during the course of stimulation, this being much more severe at the onset of hearing than in adults. Furthermore the following response at the level of the cochlear nerve was adult-like by day 18, while the response at the level of the inferior colliculus continued to improve through day 24. Recordings in the inferior colliculus were consistently obtained in two different regions along the frequency axis. The regions that responded best to a lower range of frequencies (e.g. 3–9 kHz) showed a more rapid and severe decrement in the evoked response to repetitive stimulation than those regions responding best to a higher range of frequencies (e.g. 8–17 kHz). This was found to be the case for repetitive click stimuli and repetitive tone bursts. Single unit responses in the inferior colliculus were consistent with this differential decline as a function of stimulus rate seen along the frequency axis.

Stimulus rate determines regional brain blood flow in striate cortex.

Intravenous bolus administration of oxygen 15-labeled water and positron emission tomography were used to measure changes in brain blood flow induced by two modes of photic stimuli over a wide range of repetition rates. These stimuli (patterned-flash and reversing checkerboard) were chosen in order to determine whether stimulus luminance or stimulus frequency was responsible for previously observed increases in blood flow in the striate cortex during photic stimulation. The response curves of blood flow change as a function of stimulus rate were nearly identical for both stimuli. These results suggest that elementary stimulus variables, such as repetition rate, can have a major effect on local cerebral responses, as measured with positron emission tomography and other radiotracer methods.

Cost of force development as a function of stimulus rate in rat soleus muscle.

The energy cost (determined myothermically) and mean stress (force per cross-sectional area) development of soleus muscles from adult rats were determined. Muscles were stimulated at various frequencies from 0.25 to 100 Hz over a 4-s interval, and the resulting stress-time integral was averaged over this period to yield a total heat vs. mean stress relation. This relation was characterized by three zones, i.e., 1) a linear zone corresponding to unfused twitches, 2) an intermediate zone corresponding to partial fusion during the relaxation phase only, and 3) a second linear zone corresponding to partially and fully fused tetani. The energy cost of mean stress development in the twitch zone was about four times higher than that of the fusion zone. The results are discussed in terms of the firing frequencies at which motor unit recruitment occurs in mammalian slow-twitch muscles.

The Rate Of Handling Information

Several tests were given to determine the rate of transmission of information through the human channel when the information was encoded in one to five flashing lights and S's output was pressing corresponding keys. All possible stimulus bulbs were equally probable and any number could be lighted simultaneously. (1) Increasing the stimulus rate increased the information transmission rate only up to a point where S was making a few errors. (2) Increasing the number of possible light bulbs from one through five more than tripled the maximum transmission rate. (3) Reaction time was not a function of stimulus rate. (4) The reaction time increased with the number of possible light bulbs in the stimulus but not linearly. (5) Maximum transmission occurred at a rate very close numerically to the reciprocal of the reaction time. (6) The self-pacing test resulted in as high transmission as the forced rate tests.

Auditory habituation at primary cortex as a function of stimulus rate

Rapid developing habituation was examined as a function of stimulus rate. The evoked responses (ER) recorded from primary auditory cortex were summated in awake cats. By using mean values for an n-series of clicks, it was possible to obtain values that were independent of each other. Changes in the rate of stimulation produced varying degrees of decreased ER; the faster the rate of delivery, the more marked the decrease in the ER. Three sections of the ER were measured; the early, positive wave (ER p), the late slow wave (ER N), and maximum deflection (ER PN). Both ER p and ER PN showed a decrease that as a function of the rate of stimulation, whereas ER N showed a decrease which was not correlated with the rate of stimulation. The rate of the decrease was found to be the same for the different rates of delivery. The regularity of stimulation was found not to be important. The period of spontaneous recovery is very short; approximately 1 min. Because this develops so rapidly and the spontaneous recovery is so short, we refer to this phenomena as short-term habituation.

Acoustic habituation in cats as a function of stimulus rate and the role of temporal conditioning of the middle ear muscles

1. 1. Evoked auditory responses at the medial geniculate body (MG), inferior colliculus (IC) and cochlear nucleus (CN) were recorded for 1 h from chronically implanted electrodes in cats during the presentation of two types of repetitive click stimulation — regular and random. The regular click stimuli, with equal inter-stimulus intervals, were given at rates of 1/sec, 1 5 sec and 1 10 sec. The random click stimuli, with unequal inter-stimulus intervals, had mean rates of 1/sec and 1 10 sec and minimum inter-stimulus intervals of 250 msec and 2 sec respectively. 2. 2. Acoustic habituation was observed at all auditory nuclei studied, with least habituation occurring at the CN and the greatest habituation at the MG. 3. 3. The theory of “temporal conditioning” of middle ear muscles was not confirmed. Random stimulation elicited response changes similar to those observed during regular stimulation. 4. 4. Action of middle ear muscles in acoustic habituation appears negligible since cats with inactivated middle ear muscles yielded similar results to those observed in normal cats in regard to a 1/sec stimulus.

RESPONSE OF FAST MUSCLE FIBRES TO SERIES OF IMPULSES.

ABSTRACT The peak power at the beginning of an isotonic twitch is increased if it is initiated while a previous twitch is still in progress ; the effect is cumulative over the first few twitches of a series. The time taken to drop from peak power to zero power is not affected. The power cycle in an isometric tetanus is considered and a possible method of predicting isometric tension, as a function of stimulus rate, from isotonic measurements, is explained. The steady component of tension is found to vary approximately with the logarithm of the stimulus rate in various fast muscles of the frog and dogfish. The problem of incorporating the results of the present and previous papers in an empirical muscle model is reconsidered.