*2.3.1.3. Cortical SSEP*

Different points will be used throughout the path that reflect the activity of various nerve structures [15]. Recording is performed by placing and properly fixing subdermal electrodes (other types, such as cork-screws or discs, can clearly also be used) according to the 10–20 IS.

**•** Erb's point. The active electrode is located on the midclavicularline, 2 cm above the clavicle. The reference electrode can be placed several centimetres away. A wave called N9 is recorded. It represents the near-field generated by the afferent arrival of CNAP in the

**•** Cervical. The active electrode is located on the second spinose apophysis (C2) or fifth (C5) cervical vertebra, with the reference electrode in the Fz. At this location, we can identify several potentials as follows: (a) N11. This potential is likely a presynaptic travelling wave arising near the root entry zone of C6 and C7 and action potentials ascending in the dorsal columns. It is also known as the dorsal column volley. (b) N13. This potential is generated by the synapse of neurons spanning from dorsal columns onto the nucleus cuneatus. (c) P/ N14. This is a distant potential generated by the caudal part of the medial lemniscus at the location of the brainstem. Latency and morphology can vary between individuals (i.e., they

**•** Scalp. The active electrode is placed 5 cm in the lateral direction and 2 cm caudally in relation to the vertex (C4′/C3′ for right/left). The reference electrode is located on Fz. Typically, several potentials are registered as follows: (a) N18. Reflecting the postsynaptic potential activity of the ventrocaudal nucleus of the thalamus. (b) N20. This is a near-field potential that is generated by postsynaptic potentials at the hand cortical region. (c) P25 may be an average of the independent posterior frontal and parietal generators. (d) The N35 peak is attributed to the sense of pain and temperature (conveyed by small myelinated fibres).

**•** Lumbar point. The reference electrode is located in the spinous process of L1 with the

**•** Scalp. The active electrode is located in the midline, 2 cm caudal to the vertex (Cz′), with the reference electrode located at Fz. The waves are called P37 and originate from the cortical area of the foot. A potential can also be registered in the fronto-central region with a latency of 38 ms (N38). Because the orientation of the dipole inside the longitudinal fissure is variable, P38 is sometimes maximal over the ipsilateral scalp (paradoxical localization). The most common parameters employed for stimulation are trains between 4.18 and 7.1 Hz, with a 200–300 μs pulse width and an amplitude that is double the threshold of the motor

reference electrode on the iliac crest. It is called LP (lumbar point) or N22.

can be positive or negative) and may have one or more phases.

*2.3.1.1. Upper limb SSEP*

214 Neurooncology - Newer Developments

brachial plexus.

*2.3.1.2. Lower limb SSEP*

**•** Popliteal fossa. Labelled N8.

**•** Cervical. It is known as N30.

During surgery of the cortex, it is very common to use phase reversal of SSEP to identify the transition between motor and somato-sensory areas, which usually occurs at the central sulcus (CS).

Thalamo-cortical projections from the ventrocaudal nucleus synapse in layerIV of the primary somato-sensory area. However, the rostral part of area 3 is located in the anterior wall of the central sulcus, and thus the current sources generated by these afferents can be modelled by a dipole oriented parieto-frontally rather than in a normal position relative to the surface.

We always use cortical SSEP when we need to identify the primary somato-sensory region and/or the motor area (Brodmann area 4). It is very important to keep in mind that in the case of tumours, a significant distortion of the anatomy can be observed [25,26].

The somato-sensory region corresponding to the forearm can be easily identified by the greater amplitude of the complex N1/P1/N2 waves [25].

The recording will have a bandwidth of 10–1500 Hz, with the notch off, a gain of 10–30 μV/div and a base time of 5–10 ms/div for the upper and the lower limbs, respectively. A 20-elec‐ trode grid placed at the lateral region of the fronto-parietal transition can be very helpful. By contrast, for the lower limb SSEP, a 4-8-electrode strip is placed at the medial region. The reference electrode should be placed as far as possible, i.e., at the contralateral earlobe, whereas the ground electrode should be as close as possible, e.g., at the ipsilateral earlobe [13,20].

#### *2.3.2. Auditory evoked potentials (AEPs)*

Brainstem AEPs (BAEP) are generated by different relay nuclei along the auditory pathway. They are far-field potentials that originate from very deep structures.
