**2.2. Free-running techniques**

*xx xy xz x y yx yy yz z zx zy zz*

*J*

*J*

at the patient–electrode interface.

210 Neurooncology - Newer Developments

or electrocardiography (EKG).

*<sup>x</sup> <sup>J</sup>*

sss

sss

ç ÷ é ù ¶ æ ö ç ÷ ç ÷ ê úç ÷ ¶Æ ç ÷ = -ê úç ÷ ¶ ç ÷ ê úç ÷ è ø ê ú ë ûç ÷ ¶Æ

sss

This tensorial equation can be represented for every spatial dimension *x, y, z* as follows:

; ,, *i ix iy iz J i j ki xyz xyz*

Hence, for the same voltage source, the current obtained depends on the conductivity of the different structures. Consequently, structures with higher resistivity, such as the skull and the skin, will only allow a lower current [14]. Similarly, if we recall that higher frequency oscillations of the cortex imply smaller synchronized regions, we can understand why frequencies above the beta band (13–30 Hz) are extremely difficult to record from the scalp. The necessity to record very small currents is the main reason why a low impedance is needed

In general, two types of recordings can be distinguished in neurophysiology [15]: near-field and far-field potentials. These concepts are completely different from the same words used in electromagnetic theory. The generators of near-fieldpotentials are locatedin the cerebral cortex with limited spreading on the scalp. That is, we assume that the neurons responsible for the potential are in the immediate proximity of the region in which this potential is observed. However, far-field potentials originate from the deepest structures (white matter, basal ganglia

We can divide the neural response recorded by electrodes into three types according to the type of stimuli that induces the response in the neural tissue [13]: (i) electrically induced responses. We apply a controlled stimulation to activate different structures. Among these, we have all the types of evoked potentials or the response of a muscle after electrical stimula‐ tion of its innervating nerve (stimulated electromyography or sEMG); (ii) neural response by involuntary stimulation. Those responses appearing in the neural tissue after surgical aggression, e.g., mechanical compression or torsion, ischemia or heating induced by electro‐ coagulation, must be included in this group. A typical example of this is the neurotonic discharge of a muscle (free electromyography or fEMG) induced by stretching or overheat‐ ing of a cranial nerve (CN); (iii) physiological response of the NS. These are spontaneously induced responses that are intrinsically generated by the neural activity, either as a physio‐ logical or pathological expression of the activity, and can include electrocorticography (ECoG)

or brainstem nuclei), and their distribution throughout the scalp is more extensive.

¶Æ ¶Æ ¶Æ =- - - = ¶¶¶ r r r r

sss

*y*

(4)

(5)

æ ö ¶Æ

*z*

ç ÷ è ø ¶ The recordings obtained using these techniques can be acquired continuously and do not require a command from the neurophysiologist to be released.
