N100

Types

There are three subtypes of adult auditory N100.

• N100b or vertex N100, peaking at 100 ms.
• T-complex N100a, largest at temporal electrodes at 75 ms
• T-complex N100c, follows N100a and peaks at about 130 ms. The two T-complex N100 evoked potentials are created by auditory association cortices in the superior temporal gyri.

Elicitation

The N100 is often known as the "auditory N100" because it is elicited by perception of auditory stimuli. Specifically, it has been found to be sensitive to things such as the predictability of an auditory stimulus, and special features of speech sounds such as voice onset time.

During sleep

It occurs during both REM and NREM stages of sleep though its time is slightly delayed.{{Cite journal

Stimulus repetition

The N100 depends upon unpredictability of stimulus: it is weaker when stimuli are repetitive, and stronger when they are random. When subjects are allowed to control stimuli, using a switch, the N100 may decrease. This effect has been linked to intelligence, as the N100 attenuation for self-controlled stimuli occurs the most strongly (i.e., the N100 shrinks the most) in individuals who are also evaluated as having high intelligence. Indeed, researchers have found that in those with Down syndrome "the amplitude of the self-evoked response actually exceeded that of the machine-evoked potential". Being warned about an upcoming stimulus also reduces its N100.{{Cite journal

The amplitude of N100 shows refractoriness upon repetition of a stimulus; in other words, it decreases at first upon repeated presentations of the stimulus, but after a short period of silence it returns to its previous level. Paradoxically, at short repetition the second N100 is enhanced both for sound{{Cite journal

With paired clicks, the second N100 is reduced due to sensory gating.{{Cite journal

Voice onset time

The difference between many consonants is their voice onset time (VOT), the interval between consonant release (onset) and the start of rhythmic vocal cord vibrations in the vowel. The voiced stop consonants /b/, /d/ and /g/ have a short VOT, and unvoiced stop consonants /p/, /t/ and /k/ long VOTs. The N100 plays a role in recognizing the difference and categorizing these sounds: speech stimuli with a short 0 to +30 ms voice onset time evoke a single N100 response but those with a longer (+30 ms and longer) evoked two N100 peaks and these are linked to the consonant release and vocal cord vibration onset.{{Cite journal | doi-access = free

Top-down influences

Traditionally, 50 to 150 ms evoked potentials were considered too short to be influenced by top-down influences from the prefrontal cortex. However, it is now known that sensory input is processed by the occipital cortex by 56 ms and this is communicated to the dorsolateral frontal cortex where it arrives by 80 ms.{{Cite journal

Another top-down influence upon N100 has been suggested to be efference copies from a person's intended movements so that the stimulation that results from them are not processed.{{Cite journal

Development in children

The N100 is a slow-developing evoked potential. From one to four years of age, a positive evoked potential, P100, is the predominant peak.{{Cite journal

The various types of N100 mature at different times. Their maturation also varies with the side of the brain: N100a in the left hemisphere is mature before three years of age but this does not happen in the right hemisphere until seven or eight years of age.

Clinical use

The N100 may be used to test for abnormalities in the auditory system where verbal or behavioral responses cannot be used,{{Cite journal | doi-access = free

High density mapping of the location of the generators of M100 is being researched as a means of presurgical neuromapping needed for neurosurgery.{{Cite journal

Many cognitive or other mental impairments are associated with changes in the N100 response, including the following:

• There is some evidence that the N100 is affected in those with dyslexia and specific language impairment.
• The sensory gating effect upon N100 with paired clicks is reduced in those with schizophrenia.
• In individuals with tinnitus, those with smaller N100 are less distressed than those with larger amplitudes.{{Cite journal | doi-access = free
• Migraine is associated with an increase rather than decrease in N100 amplitude with repetition of the high-intensity stimulation.{{Cite journal
• Headache sufferers also have more reactive N100 to somatosensory input than nonsufferers{{Cite journal

The N100 is 10 to 20% larger than normal when the auditory stimulus is synchronized with the diastolic phase of the cardiac blood pressure pulse.{{Cite journal

Relationship to mismatch negativity

The Mismatch negativity (MMN) is an evoked potential that occurs at roughly the same time as N100 in response to rare auditory events. It differs from the N100 in that:

• They are generated in different locations.{{Cite journal
• The MMN occurs too late to be an N100.{{Cite journal
• The MMN, unlike N100, may be elicited by stimulus omissions (i.e., not hearing a stimulus when you expect to hear one).{{Cite journal

Though this suggests that they are separate processes, arguments have been made that this is not necessarily so and that they are created by the "relative activation of multiple cortical areas contributing to both of these 'components'".{{Cite journal

History

Pauline A. Davis at Harvard University first recorded the wave peak now identified with N100. The present use of the N1 to describe this peak originates in 1966 and N100 later in the mid 1970s.{{Cite journal

Due to magnetoencephalography, research is increasingly done upon M100, the magnetic counterpart of the electroencephalographic N100. Unlike electrical fields which face the high resistance of the skull and generate secondary or volume currents, magnetic fields which are orthogonal to them have a homogeneous permeability through the skull. This enables the location of sources generating fields that are tangent to the head surface with an accuracy of a few millimeters. New techniques, such as event-related beam-forming with magnetoencephalography, allow sufficiently accurate location of M100 sources to be clinically useful for preparing surgery upon the brain.

References

References

1. Shaul S. (2007). Evoked response potentials (ERPs) in the study of dyslexia: A review. pp. 51–91. In (Breznitz Z. Editor) Brain Research in Language. Springer {{ISBN. 978-0-387-74979-2
2. Davis PA. (1939). Effects of acoustic stimuli on the waking human brain. J Neurophysiol 2: 494–499 [http://jn.physiology.org/cgi/content/citation/2/6/494 abstract]
3. (1966). "Acoustic relations of the human vertex potential". The Journal of the Acoustical Society of America.
4. Hämäläinen M, Hari R, Ilmoniemi RJ, Knuutila J. (1993). Magnetoencephalography-theory, instrumentation, and applications to noninvasive studies of the working human brain. Reviews of modern Physics. 65: 413–497. {{OCLC. 197237696