*2.2.2.2. ECoG*

ECoG is used to record the bioelectrical activity directly from the cortical surface. Several types of tumours that are located in the cortex can induce epilepsy or irritative activity, which are defined by the presence of a spike or sharp waves and its combinations. Hence, it is very relevant to assess the presence of these activities. In this sense, ECoG can discriminate between different functionalregions in the cortex, namely [9,19] (i) the spiking area, where the irritative activity can be observed; (ii) the lesional area, where abnormal slowing or loss of activity is observed; and (iii) the non-pathological area, which is defined by the absence of the abovementioned activities. The identification of these regions helps the surgeon to select the cortical region through which to approach the tumour [6,20].

As we have stated previously, it is very important to monitor the presence of epileptic activity during electrical stimulation of the cerebral cortex.

The settings for ECoG should consist of a bandwidth filter of 0.5–100 Hz, with the notch on, a gain of 750–1500 μV/div and a time base of 15–30 mm/s. As stated previously, some type of mathematical analysis can be helpful for the assessment of ECoG [5,6].

## **2.3. Evoked potentials**

The signals recorded using these techniques have the common feature that a previous stimulation must be elicited. Some of these, due to a very small amplitude, must be aver‐ aged. Before each technique is discussed in detail, we shall briefly explain why an average is needed [21,22]. Averaging is an extraordinarily powerful tool to separate the signal from its noisy environment.

A neurophysiological measurement (*m*) consists of the signal to be acquired (*s*) and the noise (*n*) [23]. Consider *m*(*t*)=*s*(*t*) + *n*(*t*). In many cases, the noise amplitude is greater than the signal (*n* > *s*). Therefore, to reduce the noise and enhance the signal we perform several rounds of stimulation (say *M*), such that, for each *k*th stimulus, the expression of the measure will be as follows:

$$m\_k\left(t\right) = s\_k\left(t\right) + n\_k\left(t\right); k = 1, 2, \dots, M\tag{6}$$

We can see that the average of *<sup>M</sup>* leads to *<sup>m</sup>*(*t*), *s*(*t*) and *<sup>n</sup>*(*t*), denoted by, *m*¯, *<sup>s</sup>*¯, *<sup>n</sup>*¯, and can be written as follows:

$$\overline{m} = \frac{1}{M} \sum\_{i=1}^{M} m\_i \left( t \right) = \frac{1}{M} \sum\_{i=1}^{M} \mathbf{s}\_i \left( t \right) + \frac{1}{M} \sum\_{i=1}^{M} n\_i \left( t \right) = \overline{s} + \overline{n} \tag{7}$$

This expression embodies the justification of the method. However, to be truly useful, it is necessary that two conditions are met: (i) there is no causal relationship between the signal and the noise and (ii) the noise varies randomly from one stimulus to the next. The mean for any variable that varies randomly is *n*¯ =0; therefore, the greater the stimulus number, then the greater will be the similarity between *<sup>m</sup>*¯ and *<sup>s</sup>*¯. Furthermore, it is easy to prove (see Van Drongelen), but omitted here, the signal estimated from the measurement, improves by a factor of 1 *M* .

#### *2.3.1. Somato-sensory evoked potentials (SSEPs)*

good recording. We can use the International System 10–20 [16] to position the electrodes, but

EEG can be used to monitor the state of the cerebral cortex, and the main indications in IONM are blood flow alterations and epileptic activity. Bioelectrical activity directly depends on blood flow, and a reduction of this variable will be observed as a slowing of the brain activity denoted by the appearance of theta/delta activity [17]. Epileptic activity can appear after a perfusion alteration or as a consequence of an insult to the cortex (mechanical, chemical or electrical). Considering that high-voltage electrical stimulation is common during IONM, EEG should be used in all patients with an increased risk of epileptic seizure. Similarly, we must be aware that general anaesthetics can increase the likelihood of seizures [18]. Customarily,

The bandwidth filter should be at least 0.5–30 Hz, with the notch on. For this type of record‐ ing, higher frequencies are uncommon in the presence of anaesthesia. The gain must be set

ECoG is used to record the bioelectrical activity directly from the cortical surface. Several types of tumours that are located in the cortex can induce epilepsy or irritative activity, which are defined by the presence of a spike or sharp waves and its combinations. Hence, it is very relevant to assess the presence of these activities. In this sense, ECoG can discriminate between different functionalregions in the cortex, namely [9,19] (i) the spiking area, where the irritative activity can be observed; (ii) the lesional area, where abnormal slowing or loss of activity is observed; and (iii) the non-pathological area, which is defined by the absence of the abovementioned activities. The identification of these regions helps the surgeon to select the cortical

As we have stated previously, it is very important to monitor the presence of epileptic activity

The settings for ECoG should consist of a bandwidth filter of 0.5–100 Hz, with the notch on, a gain of 750–1500 μV/div and a time base of 15–30 mm/s. As stated previously, some type of

The signals recorded using these techniques have the common feature that a previous stimulation must be elicited. Some of these, due to a very small amplitude, must be aver‐ aged. Before each technique is discussed in detail, we shall briefly explain why an average is needed [21,22]. Averaging is an extraordinarily powerful tool to separate the signal from its

between 7 and 15 μV/div and the time base at approximately 15–30 mm/s.

it is more common to use a reduced version of this system.

some degree of quantification should be useful.

region through which to approach the tumour [6,20].

during electrical stimulation of the cerebral cortex.

mathematical analysis can be helpful for the assessment of ECoG [5,6].

*2.2.2.2. ECoG*

212 Neurooncology - Newer Developments

**2.3. Evoked potentials**

noisy environment.

These are potentials that are generated at several points of the somato-sensory pathway.

Electrical stimulation is performed along the path of a peripheral nerve. In the upper limb, the median nerve or ulnar nerve (at wrist or elbow) is commonly used. In the lower limb, the posterior tibial nerve is commonly used. In general terms, nerve stimulation is achieved in the region closest to the cathode (i.e., negative electrode), where it produces a cationic output current [24] that depolarizes the membrane. To avoid anodic block, it is very important to place the anode distally and the cathode proximally.

Stimulation can be performed through the use of auto-adherence electrodes or subdermal needles [23].

It is customary to name the recorded waves (potentials) according to two criteria: polarity; the downward deflection of the wave is considered to be positive (P) and upward deflection negative (N) and latency; the time (measured in milliseconds) during which the potential appears with greater frequency.

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.
