Hi, I'm Ally Newton. I'm Dr.Sharon Hunter. I'm Dr.Randy Ross.
We're part of the Jerome Kern Labs and the developmental research program at the University of Colorado Denver School of Medicine. We're gonna be demonstrating infant P 50 gating a form of auditory sensory gating. Attentional deficits are common in a number of psychiatric disorders, including schizophrenia, bipolar mood disorder, autism, and attention deficit hyperactivity disorder, or A DHD.
There has been an increasing interest in the neurodevelopmental components of these attentional deficits. Neurodevelopmental meaning that while the deficits become clinically prominent in childhood or adulthood, the deficits are the results of problems in brain development that begin very early in life, in infancy or even prenatally. Despite this growing interest, there are a few methods for measuring intention and infancy, particularly methods that can be used in the first few days or weeks after birth.
P 50 sensory gating is one method that can be used in very young infants. Attention has several components. We'll concentrate here on one of the earliest components.
The brain is designed to filter out or gait repetitive non informative sensory information. The sensory gating can be measured using auditory evoke potentials. A novel auditory stimulus elicits a classic ELECTROENCEPHALOGRAPHIC or EEG record waves are designated as either positive P or negative N, and by the latency they occur after the stimulus for a typical adult response.
The early response components include N 40, P 50, and N 100 waves. Latencies in children are longer, but the waves are still termed N 40 P 50 and N 100 to maintain consistency with adult based literature. As soon as the second presentation of a stimulus, the brain inhibitory systems are active and the evoked response decreases.
We focus on the P 50 wave as occurring very early in sensory processing without conscious thought. We'll demonstrate later how to quantify this, but in general, a reduction in response to the second sound relative to the first is good sensory gating and an important first step in attention. Failure to inhibit response to the second sound represents poor sensory gating and is associated with difficulties and attention.
Because the participant is not asked to do anything, this task is ideally suited for use with young infants. There is one additional important issue. This automatic preattentive inhibitory process is ineffective during periods of heightened arousal.
Placing electrodes on a baby is enough to arouse and prevent accurate assessment of sensory gating. To counter this, babies need to be allowed to enter active sleep, the infant version of rapid eye movement or REM sleep prior to recording P 50. Sensory gating Silver, silver chloride electrodes filled with conductive pace are attached to the child with adhesive medical tape.
Electroencephalogram, EEG, and auditory evoke potentials are recorded from the vertex of the scalp at site zz. For aid in sleep staging, bipolar electrogram or EOG is recorded from electrodes directly superior and lateral to either the left or right eye. Submental, electromyogram or EMG is also recorded.
Signals are amplified 5, 000 times and filtered between 0.05 and a hundred hertz. EOG signals are amplified a thousand times and filtered between one and 200 hertz and EMG signals are amplified 10, 000 times and filtered between one and 2200 hertz. Sampling rate is set at a thousand hertz.
The data is converted to ACI format. Further analysis utilizes MATLAB software available from MathWorks and NA Massachusetts USA once the child is determined to be asleep and the electrode impedances are below 10 kilos paired clicks 50 milliseconds in duration and 500 milliseconds apart are presented through two speakers positioned on either side of the head. Inter stimulus interval between click pairs is consistent at 10 seconds.
Volume is adjusted so that each click yields an 85 decibel. Sound pressure level at the year recording is continued as long as the infant remains asleep, generally yielding between 30 and 90 minutes of continuous data. Sleep periods of greater than 60 minutes are generally necessary to identify sufficient periods of active sleep.
For analysis, The Best data is obtained while The baby is asleep. The goal is at least 15 minutes of uninterrupted active sleep. In order to obtain this amount, we generally record for an hour to an hour and a half.
There are many things that can be done to maximize the chances that the infant participant will stay asleep. This includes scheduling a visit at the infant's regular nap time, using a quiet room away from other activities, darkening the room, having a crib, baby swing and rocking chair available for whichever the baby prefers. A sheet is laid down for the baby's head at the head of the swing or Crip.
It is important to have everything ready before the baby arrives, including clean electrodes, conductive, EEG paste recording apparatus, such as a neuro scan made by com medics, neuro scan in Charlotte, North Carolina. Finally, and perhaps most importantly, you will need to schedule more time than you think you'll need. Babies are barometers for stress, and if the experimenter feels pressured, then the baby will feel stressed and not sleep as well.
When the baby arrives, be sure it is fed, burped, and it's diaper is clean before you start swaddle. If the baby prefers wipe the head with an electrode prep pad to minimize impedance, conductive, EEG paste and cloth tape are Used to hold the electrodes in place. An electrode is Placed on the forehead or behind the ears for the ground, which acts to reduce the impact of environmental noise.
An electrode is put on the right or left mastoid for a reference electrode. The reference can also be placed on the nose as long as it lies on the same line of the face as the mastoid. A tape measure is sometimes used to measure the halfway point on the head for the CZ electrode Placement.
An electrode is placed at cz. For the EEG Two are placed to record EOG or eye movements, one on the bone above the eyebrow and one on the Bone to the right of the eye, one is put on the chin for an EMG to measure muscle tone, we check impedance levels Using neuros scan. The reading should ideally be dark blue or black indicating low impedance.
Sometimes it's beneficial to wait for the pace to warm up on the baby's head, as that will create a lower impedance reading. It may also be necessary to press the electrodes down to try and get the desired impedance Levels. The room is normally darkened at this Time, but for filming purposes, we will keep it lit.
Comments about the process are recorded concurrently, such as if eyes are open or closed, any body movement, if the electrodes are moved or displaced by the baby's movement, or to get a better reading, any extraneous noise and possible times of active sleep. If heart rate is present in tracing, move the EMG electrode to a bony or part of the chin. If the reading is noisy or messy, it may be caused by electronic interference from something in the room such as a cell phone.
The electrode not seated securely, in which case the electrode can be pushed down to make more secure, or the wires are touching another electrical source, which can be avoided by taping the wires down. When the baby wakes up, an adhesive remover facilitates the removal of the tape and paste. Baby wipes are used to clean excess paste off the baby's head.
The family is then thanked and the recording moves to offline analysis. Here's a Portion of a tracing from a typical infant. The first step is to identify periods of active sleep and periods of quiet sleep.
In particular, we're looking for a period of active sleep lasting longer than 15 minutes. Active sleep will have large eye movements, active brain waves, which are characterized by their high frequency and low voltage, similar in appearance to when the child is awake. There will also be low muscle tone indicated by a flattened EMG and notes gathered during the recording will indicate the baby's eyes are closed and that the baby has irregular breathing.
After recording filters are applied to the tracings, including ones that remove waves that occur at about a frequency of 60 hertz. 60 hertz waves are generated by most US electronics. We also use a filter that takes out frequencies below one hertz and these are generated by just irrelevant drifts in the energy field.
This helps us distinguish brainwaves from those waves that are generated by the environment. Zooming in on a three second epic. You can see where the clicks occurred.
However, identifying specific evoked potential waves is difficult from a single two second epic. We thus the epics together, the more epics the clearer each component becomes. Here with 75 epics averaged together, you can see the waves we are interested in measuring.
The first large positive wave after the stimulus is presented is termed P one or P 50. It's followed by a second wave that is negative going called the N 100 Note. In babies, these occur later than seen than is seen in adults.
The P 50 for this baby actually occurred approximately 75 milliseconds after the stimulus versus the 50 milliseconds seen in adults. We measured the amplitude of the P 50 from the preceding trough to the peak after measuring response to the first and second sounds. The second amplitude is divided by the first.
To identify a gating ratio, the latency is measured at the highest point of the peak. Here's an example of a baby who has a ratio close to one that is who failed a gate and baby who has a ratio closer to zero, reflecting a well-functioning inhibitory process, which allows normal sensory gating. A distinct cutoff for what percentage is considered normal gating has not been established.
We use a 0.5 cutoff while keeping in mind that any percentage of gating is better than none. P 50 sensory gating is an effective method for measuring an early preattentive component of attention. The method is usable even in very young infants as young as a few hours old.
This method may provide insight into early development. It may also be useful as a mechanism to assess novel interventions aimed at preventing some attentional difficulties long before they become clinically relevant. We at the Jerome Kern Laboratories hoped his presentation was helpful at providing basic instruction for assessing P 50 sensory gating in young infants.
