Blink reflex elicited by auditory stimulation: clinical study in newborn infants

Blink reflex can be elicited by sudden strong auditory stimulation. Using a special transducer wer recorded this reflex which appears as a microvibration of the eyelid, and named it auditory-evoked eyelid microvibration (AMV). As the reflex pathway of AMV exists in the brainstem, AMV is an easy and useful way of knowing the function of the brainstem, especially in newborn periods. AMV was studied in infants from 25 to 43 weeks in conceptional age, to establish the normal value and to analyse the change of AMV in cases of neonatal asphyxia and intracranial hemorrhage. The mean latency of AMV in full-term infants was 31.0 +/- 67. msec. After 33 weeks in conceptional age, AMV was observed constantly with almost the same latency and amplitude as adult values. AMV appeared during both waking and sleep stages, except in active sleep. Marked change in AMV was noted such as delayed latency or disappearance of the response in neonatal asphyxia or intracranial hemorrhage. Infants who continued to have these abnormalities for a certain period, died or exhibited neurological sequelae.     

Brain-stem auditory evoked responses to hypercarbia in preterm infants

To determine the effect of acute hypercarbia on brain-stem function in preterm neonates, we compared brain-stem auditory evoked responses (BAERs) during 8% CO₂ breathing to those elicited during room air breathing in 12 healthy preterm infants during the first week of life. End-tidal CO₂ (ETpCO₂), respiratory rate and depth were monitored throughout the protocol. Absolute wave latencies and interpeak intervals of the BAERs were ana;yzed from duplicate trials. During 8% CO₂ breathing, ETpCO₂, respiratory rate depth of respiration increased significantly (p < 0.05). The absolute latency of wave V was prolonged (P < 0.025). Values of absolute peak latencies I and III were unaffected by the hypercarbic state. These data demonstrate that elevations in pCO₂ which elicit ventilatory responses also effect the BAER. The specific effects on ventilatory pattern, peak V latency and interpeak interval III–V indicate brain-stem responsiveness and alterations in the more central components of the auditory pathway. These findings raise important considerations regarding the influence of hypercarbia on brain-stem function preterm infants and the clinical management of such infants with abnormalities of gas exchange.     

Hemodynamic responses to speech and music in newborn infants

We used near‐infrared spectroscopy (NIRS) to study responses to speech and music on the auditory cortices of 13 healthy full‐term newborn infants during natural sleep. The purpose of the study was to investigate the lateralization of speech and music responses at this stage of development. NIRS data was recorded from eight positions on both hemispheres simultaneously with electroencephalography, electrooculography, electrocardiography, pulse oximetry, and inclinometry. In 11 subjects, statistically significant (P < 0.02) oxygenated (HbO₂) and total hemoglobin (HbT) responses were recorded. Both stimulus types elicited significant HbO₂ and HbT responses on both hemispheres in five subjects. Six of the 11 subjects had positive HbO₂ and HbT responses to both stimulus types, whereas one subject had negative responses. Mixed positive and negative responses were observed in four neonates. On both hemispheres, speech and music responses were significantly correlated (r = 0.64; P = 0.018 on the left hemisphere (LH) and r = 0.60; P = 0.029 on the right hemisphere (RH)). On the group level, the average response to the speech stimuli was statistically significantly greater than zero in the LH, whereas responses on the RH or to the music stimuli did not differ significantly from zero. This suggests a more coherent response to speech on the LH. However, significant differences in lateralization of the responses or mean response amplitudes of the two stimulus types were not observed on the group level. Hum Brain Mapp, 2010. © 2009 Wiley‐Liss, Inc.     

Deafness alters auditory nerve fibre responses to cochlear implant stimulation

Here we characterized the relationship between duration of sensorineural hearing loss and the response of the auditory nerve to electrical stimulus rate. Electrophysiological recordings were made from undeafened guinea pigs and those ototoxically deafened for either 5 weeks or 6 months. Auditory neuron survival decreased significantly with the duration of deafness. Extracellular recordings were made from auditory nerve fibres responding to biphasic, charge-balanced current pulses delivered at rates of 20 and 200 pulses/s via a monopolar scala tympani stimulating electrode. The response to 20 pulses/s electrical stimulation of the deafened cochlea exhibited a decrease in spike latency, unaltered temporal jitter and unaltered dynamic range (of nerve firing rate against stimulus current), and a reduction in threshold after 6 months of deafness. The response to a 200-pulse/s stimulus was similar except that the dynamic range was greater than with 20 pulses/s and was also greater in deafened animals than in undeafened animals. Deafness and pulse rate are related; in deaf animals spike recovery appears to be complete between successive stimulus pulses at a low rate (20 pulses/s), but incomplete between pulses at a moderate pulse rate (200 pulses/s). These results suggest that changes in the function of individual auditory nerve fibres after deafness may affect clinical responses during high-rate stimulation such as that used in contemporary speech processing strategies, but not during lower rate stimulation such as that used to record evoked potentials.     

Cortical somatosensory evoked potentials to posterior tibial nerve stimulation in newborn infants

Successful cortical recordings of somatosensory-evoked potentials (SEPs) to posterior tibial nerve (PTN) stimulation were obtained in 21 (87.5%) for P1 and 22 (91.7%) for N1 of 24 infants who were followed up for at least 3 years and had a normal outcome. There were linear decreases with increasing post menstrual age in both P1 and N1 peak latency. Of the four cases with diplegia later, three showed definite abnormalities, no responses and delayed latency in PTN SEPs respectively, however, the other case showed normal responses. Of the three cases with mental retardation, two showed relatively long latency and borderline responses respectively, and the other case showed normal responses. As the pathway of PTN SEPs traverses the periventricular area of the brain likely to be affected by ischemic lesions in premature infants, abnormalities in the responses might indicate a later motor disorder.     

Hemodynamic response to visual stimulation in newborn infants using functional near-infrared spectroscopy

Brain activity is associated with physiological changes, which alter the optical properties of tissue. These changes can be detected by near-infrared spectroscopy (NIRS). Aim of the study was to determine changes in cerebral oxygenation in response to stimulation in the visual cortex in newborn infants during spontaneous sleep in the first days of life. We used an in-house developed multichannel NIRS imaging instrument, the MCP-II, to measure changes in concentration of oxyhemoglobin (O(2)Hb) and deoxyhemoglobin (HHb) in specific brain areas. In 10 out of 15 subjects, a significant increase in O(2)Hb and/or a significant decrease in HHb were found in one or more channels over the occipital cortex. During stimulation, O(2)Hb increased by a mean of 0.98 mumol/l, HHb decreased by a mean 0.17 mumol/l, and total-Hb increased by a mean of 0.81 mumol/l. The hemodynamic response to visual stimulation in the occipital cortex in newborn infants is similar to adults. The increase in O(2)Hb and the simultaneous decrease in HHb during stimulation suggest an increase in cerebral blood flow (CBF) that overcompensates for the increased oxygen consumption (CMRO(2)) in the activated cortical area.     

Ipsilateral and contralateral recordings of auditory brainstem responses to monaural stimulation

The developmental changes of the ipsilateral and contralateral auditory brainstem responses (ABRs) were studied in 105 normal infants and children. In both ipsilateral and contralateral recordings, the peak and interpeak latencies shortened with increasing age, while the amplitudes of wave V had a tendency to become higher. Contralateral ABR amplitudes were always smaller than those of ipsilateral ABRs. In the contralateral recording, wave I was absent and the contralateral wave II and wave III complex began to separate after birth (25%); separation percentage reached 80–100% at 7 months of age. Our results suggest that the contralateral recording of ABRs is a useful measure of developmental changes in infant auditory pathways.     

Neural and hemodynamic responses to optogenetic and sensory stimulation in the rat somatosensory cortex

Introducing optogenetics into neurovascular research can provide novel insights into the cell-specific control of the hemodynamic response. To generalize findings from molecular approaches, it is crucial to determine whether light-activated circuits have the same effect on the vasculature as sensory-activated ones. For that purpose, rats expressing channelrhodopsin (ChR2) specific to excitatory glutamatergic neurons were used to measure neural activity, blood flow, hemoglobin-based optical intrinsic signal, and blood oxygenation level-dependent (BOLD) functional magnetic resonance imaging (fMRI) during optogenetic and sensory stimulation. The magnitude of the evoked hemodynamic responses was monotonically correlated with optogenetic stimulus strength. The BOLD hemodynamic response function was consistent for optogenetic and sensory stimuli. The relationship between electrical activities and hemodynamic responses was comparable for optogenetic and sensory stimuli, and better explained by the local field potential (LFP) than the firing rate. The LFP was well correlated with cerebral blood flow, moderately with cerebral blood volume, and less with deoxyhemoglobin (dHb) level. The presynaptic firing rate had little impact on evoking vascular response. Contribution of the postsynaptic LFP to the blood flow response induced by optogenetic stimulus was further confirmed by the application of glutamate receptor antagonists. Overall, neurovascular coupling during optogenetic control of glutamatergic neurons largely conforms to that of a sensory stimulus.     

Inconsistency of auditory middle latency and steady-state responses in infants

Auditory middle latency and steady-state responses (MLR/SSRs) were recorded in normal infants (aged 3 weeks to 28 months) and adults. SSR amplitudes were maximum using stimulus presentation rates near 40 Hz in adults. By contrast, the infant data showed no consistent amplitude maximum across the rates tested (9-59 Hz). With the exception of the brain-stem response wave V to MLR Na deflection, MLR components in infants' responses to 10.85 Hz clicks did not show any consistent pattern. To investigate the hypothesis that the 40 Hz SSR is derived from overlapping of the 10 Hz MLR components, 43.4 Hz SSRs were synthesized from the responses recorded at 10.85 Hz and compared with those recorded at 43.4 Hz. The predictive accuracy of the synthesized 43.4 Hz SSRs was significantly better in adults than in infants. The results of these studies indicate the presence of large age-related differences in the auditory MLR and SSR, and in the relationship between the two responses.     

Visual evoked responses to light emitting diode (LED) photostimulation in newborn infants

The use of a Xenon discharge stroboscope to elicit VERs in a neonatal intensive care unit has several disadvantages. To overcome these a photostimulator has been developed which consists of an array of 25 red light emitting diodes (LEDs). The LEDs are driven by pulses of fixed length and current and the output intensity is controlled by the pulse repetition rate. Such a photostimulator is compact and can be hand-held inside a cot or incubator. Using this method VERs have been recorded in healthy full-term and pre-term infants which are comparable in wave form and latency to those elicited by a stroboscope.     

Brainstem auditory evoked responses in infants and children with AIDS.

Brainstem auditory evoked responses were measured in 16 infants and children with acquired immunodeficiency syndrome (AIDS) and in 9 normal infants and children. Two stimulation rates were used: a conventional rate of 10 Hz and a high rate of 50 Hz. Latencies of waves III, IV, and V on the left were significantly longer in the AIDS group when a stimulation rate of 10 Hz was used. With a higher stimulation rate of 50 Hz, significant differences between the two groups occurred in the latencies of waves I, III, and V bilaterally, but there were no significant differences in the interpeak latencies. A measure of the differential effect of the increasing stimulus rate on the two groups was significant for wave I latency and for I-III and I-V interpeak latencies on the left, revealing that increasing stimulation rate prolongs these measures more in the AIDS group. Increased brainstem auditory evoked response stimulation rate may unmask abnormalities in infants and children with AIDS that are not observed when the lower stimulation rate is used.     

Tomographic mapping of human cerebral metabolism: subcortical responses to auditory and visual stimulation

We measured cerebral glucose metabolism with positron computed tomography during audio-visual stimulation in 42 studies of 21 subjects. Metabolic activations and stimulus-induced asymmetries were examined in subcortical structures (thalamus, caudate, lenticular nuclei). Bilateral activations of the thalamus occurred with verbal stimuli. The head of the left caudate was activated when subjects used visual imagery as a strategy to identify sequences of tones. These two types of stimuli produced dominant (left) hemisphere cortical activations in this same group of subjects. Clinical evidence has implicated the participation of subcortical (thalamus and basal ganglia) structures in the processing of language and auditory information. The present results demonstrate this functional role directly in normal subjects.