*2.3.2.2. Cortical auditory evoked potentials (cAEPs)*

These potentials were initially thought to be generated in the primary auditory cortex (PAC), located deeply in the white matter of the lateral fissure of the transverse gyrus of Heschl. However, other different areas, including the second auditory cortex (SAC) and the insula are capable of eliciting cAEPs. There is considerable inter-subject and inter-hemispheric variabil‐ ity [27], and the whole structure remains to be elucidated.

cAEPs are characterized by a series of waves, which can be systematized as follows: (a) short latency waves: N13/P17/N30. These waves are typically recorded from the PAC. This com‐ plex is absent in the SAC and (b) intermediate latency waves: peak between 60 and 100 ms. These waves, which are always present in the SAC, can also be present in the PAC.

A stimulation frequency of 2.18 would be adequate. The bandwidth filter is 0.5–1500 Hz, with the notch filter off, and a minimum intensity of 70 dB (sensation level) is applied to the contralateral ear.

#### *2.3.3. Visual evoked potentials (VEPs)*

These waves exhibit the characteristics of near-field potentials generated from the primary visual cortex. In the surgery room, the most widely (and probably the only) technique used for stimulation is the application of flashes of light. A normal VEP in response to a patternreversal checkerboard is a positive midoccipital peak that occurs at a mean latency of 100 ms with three separate phases: an initial negative deflection (Nl or N75), a prominent positive deflection (P1 or P100) and a later negative deflection (N2 or N145).

Although we think that VEP has undeniable utility, there have been some questions about its efficacy. However, more recent results have demonstrated stable recordings and a strong correlation with the postoperative visual function [28,29]. Therefore, intraoperative VEP monitoring will be mandatory for surgeries harbouring a risk of visual impairment [30].

Stimulation is performed by flashing light-emitting diodes at 2.18 Hz, with 10 μs pulse width, and a bandwidth of 10–1000 Hz. We considered an increase in latency of 10% or a reduction in amplitude greater than 50% amplitude compared with baseline as alarm criteria [6].

In some cases, VEP can be directly recorded from the cortical surface. In these cases, poten‐ tials are much more stable, require fewer stimuli (in fact, a very small number of stimuli can induce the response) and are 2–3 orders of magnitude higher than the scalp recording [31].
