The Atlas · Chapter 2

The P1–N1–P2 Complex

The obligatory cortical response is a sequence of peaks that unfold over the first quarter-second after a sound. Learning to name them, and to know which one matters when, is the foundation of everything that follows.

Reading the waveform

By convention the components are named by polarity and order — P for a vertex-positive peak, N for a vertex-negative one — and numbered in sequence. The obligatory complex is P1, N1, P2 and, especially in children, a later N2. The peaks are also referred to by their typical adult latency: P1 (P50), N1 (N100), P2 (P200), N2 (N250).

The obligatory cortical response. P1, N1, P2 and N2 unfold across the first 50–300 ms after a sound — far later and far larger than the brainstem response. In the awake adult the N1–P2 swing dominates and is the feature tracked for objective audiometry. Vertex-positive plotted upward; latencies are typical adult values and vary with stimulus and state. Schematic — not to scale.

These latencies dwarf those of the brainstem response — N1 alone arrives ten times later than the entire ABR — and so do the amplitudes. The cortical response is measured in microvolts rather than fractions of one, which is why it needs far fewer stimulus repetitions to record [19].

The components one by one

P1(~50–100 ms) is modest in adults but is the dominant, most robust peak in young children — so much so that it becomes the workhorse of paediatric work, where its latency tracks central maturation [14].

N1(~100 ms) is the largest deflection in the awake adult and the most studied cortical component, with multiple generators around auditory cortex [4]. It is exquisitely sensitive to stimulus onset, to the interval between stimuli, and to arousal — it fades in drowsiness and is unreliable in infants.

P2 (~180–200 ms) follows N1. In practice the N1–P2 amplitude — the peak-to-trough swing — is the single measure most often tracked, because it is large, repeatable and grows with stimulus level [5]. N2(~200–300 ms) is more prominent in children and adds little to routine adult testing.

What changes the waveform

The components are not fixed. As stimulus level rises, N1–P2 amplitude grows and latency shortens — the relationship that lets the response estimate threshold. Age reshapes the whole complex: the infant response is dominated by a broad positivity, N1 emerges only in later childhood, and latencies shorten as the cortex matures[15]. Faster stimulus rates and the listener drifting toward sleep both shrink the response, which is why timing and state are controlled so carefully during recording[6].

The acoustic change complex

The same onset machinery responds not only to a sound beginning but to a change withinan ongoing sound. A shift in frequency, intensity or spectrum partway through a continuous stimulus evokes a second N1–P2 — the acoustic change complex (ACC) [8]. It is elicited by both spectral and intensity changes [9].

The acoustic change complex. A change within an ongoing sound — of pitch, level or spectrum — evokes a second onset-like N1–P2 response, time-locked to the change. Because it only appears if the change was registered, the ACC is an objective index of suprathreshold discrimination and of aided audibility. Schematic — not to scale.

The ACC is powerful because it is an objective index of discrimination, not just detection: the change response only appears if the auditory system registered the change. That makes it a natural tool for suprathreshold testing and for confirming that a hearing aid or implant delivers audible, distinguishable speech cues[10].

Carry this away.P1, N1, P2 (and N2) name the obligatory peaks by polarity and order. N1–P2 amplitude is the adult workhorse; P1 latency is the paediatric one. A change inside an ongoing sound re-triggers the response as the acoustic change complex — the bridge from detection to discrimination.