Linear and nonlinear temporal interaction components of mid-latency auditory evoked potentials obtained with maximum length sequence stimulation.

A maximum length sequence (MLS) is a quasi-random sequence of clicks and silences that enables simultaneous recording of linear components and nonlinear temporal interaction components (NLTICs). NLTICs are produced when the stimulation rate is fast enough such that several stimuli occur within the memory length of the system. The present study was designed to characterise the NLTICs of auditory mid-latency responses (MLR). Forty normally hearing subjects (19-45-year-old) were tested at MLS rates between 20 and 120 clicks/s. Linear components could be identified at all rates. The NLTICs of the MLS-MLR were identified in only a few subjects. This suggests two possibilities: (1) there may not be strong nonlinear temporal interactions within the MLR generators; (2) the memory length of the MLR is much shorter than expected from the linear component rates. If so, NLTICs should be obtained at higher rates of stimulation.

The choice of distracting task can affect the quality of auditory evoked potentials recorded for clinical assessment.

Auditory evoked potential (AEP) recordings often require subjects to ignore the stimuli and stay awake. In the present experiment, early (ABR), middle (MLR), and late latency (LLR) AEPs were recorded to compare the effect of five different distracting tasks: (1) doing nothing eyes open, (2) reading, (3) watching a movie, (4) solving a three-digit sum, and (5) doing nothing eyes closed (or counting the stimuli for LLR). Results showed that neither the amplitudes nor the latencies of the ABR, MLR, or LLR were affected by task. However, the amount of pre-stimulus activity (noise) or amplitude rejection was significantly and differently affected by the distracting task. For the ABR, the math task was the noisiest but, for the MLR, the amount of noise was greater when watching a movie. As for the LLR, reading and watching a movie yielded the lowest percentage of rejected traces. In conclusion, the choice of distracting task depends on the AEP being measured and should be chosen to improve the quality of the AEP traces and thus reduce recording time.

Linear and nonlinear changes in the auditory brainstem response of aging humans.

OBJECTIVE: This experiment was designed to characterize the changes in linear and nonlinear temporal interactions in the aging auditory brainstem of humans using maximum length sequence (MLS) stimulation. METHODS: The MLS technique uses a quasi-random sequence of clicks and silences to determine the linear (linear averaging of single responses) and nonlinear (interactions between pairs or triplets of responses) temporal interactions in the auditory brainstem response (ABR). A group of 30 normal hearing females aged between 11 and 61years were tested. They were divided in three groups: young (<24years); middle (24-41years) and old (>41years). RESULTS: The linear component showed a decrease in wave 5 amplitude with age which could also be related to increased thresholds for higher frequencies. The nonlinear component of the MLS-ABR showed significant differences between middle and old groups for wave 1 and 5 latencies and inter-wave interval (1-5). CONCLUSIONS: The results suggest that the linear and nonlinear components of the MLS-ABR could be more sensitive to changes occurring in the auditory system before any functionally significant changes in hearing status. SIGNIFICANCE: The nonlinear components of the auditory brainstem could be used as an investigative tool to assess early changes in the aging auditory brainstem in young middle-aged women.

Selective attention increases the temporal precision of the auditory N100 event-related potential.

Selective attention increases the amplitude of the averaged N100 event-related potential (ERP). This increase may result from more neurons responding to the stimulus or from the same number of neurons better synchronised with the stimulus, or both. We investigated the synchronization mechanism using a new response latency jitter measurement algorithm that performed well for all the signal-to-noise ratios obtained in the experiment. We found that the significantly increased N100 amplitude is accounted for by a significantly decreased latency jitter variance for the attended stimuli.

The amplitude modulation of the quadriceps H-reflex in relation to the knee joint action during walking.

Previously the modulation of the quadriceps H-reflex has only been investigated in the initial part of the gait cycle, and it was suggested that the quadriceps H-reflex modulates with relative high reflex gain at heel contact and decreases during the subsequent part of stance (Dietz et al. 1990b). The objectives of the present study was to elaborate on the previous results by increasing the measurement resolution around heel contact and include additional measures in order to relate the H-reflex modulation to the mechanical function of the knee extensors throughout the gait cycle. EMG profiles were measured in quadriceps and the antagonistic hamstring muscles simultaneously with the knee joint kinematics in ten subjects during treadmill walking at preferred speed. H-reflex excitability was measured in vastus lateralis (VL) and rectus femoris (RF) at 11 selected positions during the gait cycle. The resulting excitability curves showed a significant modulation of the quadriceps H-reflex during the gait cycle. The H-reflex amplitude increases shortly after heel contact and reflex inhibition is present in the remaining part of stance and most of the swing phase. The modulation of the quadriceps H-reflex during walking does not follow the classical pattern of reciprocal inhibition between antagonistic muscles. It is suggested that at least during the stance phase the modulation of the quadriceps H-reflex is controlled by presynaptic inhibition. The present results confirm the idea that the excitability of the quadriceps H-reflex is controlled to comply with the different mechanical demands on the muscle during the gait cycle in humans.

Timing of cortical excitability changes during the reaction time of movements superimposed on tonic motor activity.

Seated subjects were instructed to react to an auditory cue by simultaneously contracting the tibialis anterior (TA) muscle of each ankle isometrically. Focal transcranial magnetic stimulation of the leg area of the motor cortex (MCx) was used to determine the time course of changes in motor-evoked potential amplitude (MEP) during the reaction time (RT). In one condition the voluntary contraction was superimposed on tonic EMG activity maintained at 10% of maximal voluntary contraction. In the other condition the voluntary contraction was made starting from rest. MEPs in the TA contralateral to the stimulation coil were evoked at various times during the RT in each condition. These were compared to the control MEPs evoked during tonic voluntary activity or with the subject at rest. The RT was measured trial by trial from the EMG activity of the TA ipsilateral to the magnetic stimulus, taking into account the nearly constant time difference between the two sides. The MEPs became far greater than control MEPs during the RT (mean = 332%, SD = 44 %, of control MEPs, P < 0.001) without any measurable change in the background level of EMG activity. The onset of this facilitation occurred on average 12.80 ms (SD = 7.55 ms) before the RT. There was no difference in the onset of facilitation between the two conditions. Because MEPs were facilitated without a change in the background EMG activity, it is concluded that this facilitation is specifically due to an increase of MCx excitability just before voluntary muscle activation. This conclusion is further reinforced by the observation that MEPs evoked by near-threshold anodal stimuli to the MCx were not facilitated during the RT, in contrast to those evoked by near-threshold transcranial magnetic stimulation. However, several observations in the present and previous studies indicate that MEP amplitude may be more sensitive to alpha-motoneuron activity than to motor cortical neuron activity, an idea that has important methodological implications.

Neural mechanisms involved in the functional linking of motor cortical points.

We sought to understand the basic neural processes involved in the functional linking of motor cortical points. We asked which of the two basic neural mechanisms, excitation or inhibition, is required to functionally link motor cortical points. In the ketamine-anaesthetized cat, a microstimulation electrode was positioned at a point (control point) that was identified by the following three characteristics of the EMG responses: the muscle(s) activated at threshold, any additional muscles recruited by supra-threshold stimulation, and their relative latency. A second distinct point (test point) producing activation of a muscle at a different joint was then identified. At this test cortical point the GABA(A) receptor antagonist bicuculline was ejected iontophoretically, while stimulating the control point near threshold. A combined response was elicited consisting of the response normally elicited at the control point plus that elicited at the test point. Thus, an artificial muscle synergy was produced following disinhibition of the test point. This was never the case when glutamate was ejected at the test point, even when supra-threshold stimuli were used at the control point. Therefore, simply increasing the excitability of a cortical point was not sufficient to release the muscle(s) represented at that point into a muscle synergy. Kynurenate, a broadly acting excitatory amino acid receptor antagonist, ejected at the bicuculline point reversed the effect of bicuculline. This shows that the release phenomenon was mediated synaptically and was not due to spread of the stimulating current. We suggest that release from inhibition may be one of the neural mechanisms involved in functionally linking motor cortical points. This functional linking may be part of the ensemble of motor cortical mechanisms involved in recruitment of muscle synergies.