Frank Musiek, Ph.D.
This commentary is one that is consistent with the mission of the Pathways column. It
brings attention to topics that perhaps have been underplayed in the audiology
community. This Pathways article will focus on an auditory evoked potential (AEP) that
should be considered more than it has been – especially for use in preschool-aged
children – the P1 response.
The P1 is part of late or cortical AEP complex. It precedes the N1 and P2 responses in
time and has been overshadowed by these 2 more investigated responses. Anyone that
has recorded cortical AEPs with the aim of obtaining the N1 and P2 responses also has
likely recorded a P1 response (figure 1). In adults it occurs from about 50 to 80 msec
depending on the type and intensity of the stimulus and recording technique. However,
the P1 has a notable maturational course. Newborns demonstrate a P1 (often of poor
morphology) that occurs around 300 msec which decreases in latency and improves in
waveform morphology rather markedly up to about 2 years of age where latencies
approach 100 msec. Amplitudes also grow in the first 2 years of life from approximately
2 microvolts to often greater than 5 microvolts. Complete maturation of the P1 may not
occur however until well into the teenage years, though obviously slowed markedly post
2-3 years of age. As the P1 matures post early childhood the amplitude actually
decreases! Adult latencies settle in and around 60 msec but amplitudes decrease and
the waveform in young adults is smaller than in young children! (see Campbell, Cardon,
Sharma, 2011). This is indeed an unusual occurrence but highlights the possibilities for
use of the P1 in the toddler and younger population. It should be mentioned that in the
preschool years the P1 changes its morphology along with the N1, P2 complex whereas
the N1, P2 becomes more dominant and the P1 less dominant with increasing age.
Similar to other AEPs, the P1 latency can be a biomarker for maturation of the central
auditory system (see Eggermont and Ponton, 2003). It likely reflects increased synaptic
activity and myelination of the central auditory system over time. The P1 generator is
considered by most to be the auditory cortex with possible contributions from the
thalamo-cortical connections to auditory cortex. Since it is generated by the auditory
cortex, the P1 as with other middle and late evoked potentials is affected by sleep state.
However, it has been demonstrated that it can be consistently obtained with awake
young children.
Anu Sharma and colleagues have nicely shown how the P1 can be utilized in measuring
the positive influences of early versus later cochlear implantation (Sharma, Dorman,
Spahr, 2002). In their work they have established latency norms for preschool children
to show the influence of cochlear implants on auditory development. Of course, the use
of P1 with cochlear implants is critically important but Sharma’s work also shows the
ability to establish consistent, normative data for infants and young children for an AEP
that is generated by the auditory cortex. The P1 has already been shown to be useful
for the auditory evaluation of children with sensorineural hearing loss and auditory neuropathy (see Campbell et al. 2011). Furthermore, the P1 clinical utility seems to
have capability to use with additional clinical populations.
The advantage of the P1 over the more commonly used ABR in evaluating young
children/newborns is that it reflects function of more rostral structures in the central
auditory system. This is not to say that the P1 should be used instead of the ABR but
rather as a supplement to it. The ability to record the P1 in young children as well as its
known generator sites could open up many other clinical populations to its use. Perhaps
among the best candidates are those children with various central auditory deficits. The
P1 measure could be utilized for children with various neurological disorders where the
central auditory system may have been compromised. One of the auditory disorders for
which the P1 would seem to be applicable and also sorely needed is Landau Kleffner
syndrome (Pearl, Carrazana, Holmes, 2001). Landau Kleffner syndrome is essentially a
seizure disorder – often subtle and difficult to diagnose, that can and often does affect
the central auditory system. These children, often toddler age, show regression in
speech and language and auditory awareness. Since this disorder affects primarily the
cortex, ABR testing usually results in normal results – sometimes leading to missing an
auditory diagnosis. Since these children cannot be tested behaviorally with central
auditory procedures, the need for and objective test such as auditory evoked potentials,
that can provide some insight to the integrity of the central auditory system, is
paramount. In these kinds of clinical instances where the need is great it seems the P1
could be of considerable help. It is time to give the P1 a chance!
References
- Campbell, J., Cardon, G. and Sharma, A. (2011) Clinical Application of the P1 Cortical
Auditory Evoked Potential Biomarker in Children with Sensorineural Hearing Loss and
Auditory Neuropathy Spectrum Disorder - Eggermont JJ, Ponton CW. Auditory-evoked potential studies of cortical maturation in
normal hearing and implanted children: correlations with changes in structure and
speech perception. Acta Otolaryngol. 2003;123:249–252. - Pearl PL, Carrazana EJ, Holmes GL (2001). "The Landau–Kleffner Syndrome".
Epilepsy Curr. 1 (2): 39–45. - Sharma A, Dorman MF, Spahr AJ. Rapid development of cortical auditory evoked
potentials after early cochlear implantation. Neuroreport. 2002b;13:1365–1368.
