in animals. vep stimuli the diffuse-light flash stimulus is rarely used nowadays due to the high variability within and across subjects. however, it is beneficial to use this type of stimulus when testing infants, animals or individuals with poor visual acuity. the checkerboard and grating patterns use light and dark squares and stripes, respectively. these squares and stripes are equal in size and are presented, one image at a time, via a computer screen. vep electrode placement electrode placement is extremely important to elicit a good vep response free of artifact. in a typical (one channel) setup, one electrode is placed 2.5 cm above the inion and a reference electrode is placed at fz. for a more detailed response, two additional electrodes can be placed 2.5 cm to the right and left of oz. vep waves normal visual evoked potential the vep nomenclature is determined by using capital letters stating whether the peak is positive (p) or negative (n) followed by a number which indicates the average peak latency for that particular wave. for example, p100 is a wave with a positive peak at approximately 100 ms following stimulus onset. the average amplitude for vep waves usually falls between 5 and 20 microvolts. normal values are depending on used stimulation hardware (flash stimulus vs. cathode ray tube or liquid crystal display, checkerboard field size, etc.). types of vep some specific veps are: monocular pattern reversal (most common) sweep visual evoked potential binocular visual evoked potential chromatic visual evoked potential hemi-field visual evoked potential flash visual evoked potential led goggle visual evoked potential motion visual evoked potential multifocal visual evoked potential multi-channel visual evoked potential multi-frequency visual evoked potential stereo-elicited visual evoked potential steady state visually evoked potential auditory evoked potential auditory evoked potentials (aep) can be used to trace the signal generated by a sound through the ascending auditory pathway. the evoked potential is generated in the cochlea, goes through the cochlear nerve, through the cochlear nucleus, superior olivary complex, lateral lemniscus, to the inferior colliculus in the midbrain, on to the medial geniculate body, and finally to the cortex. auditory evoked potentials (aeps) are a subclass of event-related potentials (erps). erps are brain responses that are time-locked to some "event", such as a sensory stimulus, a mental event (such as recognition of a target stimulus), or the omission of a stimulus. for aeps, the "event" is a sound. aeps (and erps) are very small electrical voltage potentials originating from the brain recorded from the scalp in response to an auditory stimulus, such as different tones, speech sounds, etc. brainstem auditory evoked potentials are small aeps that are recorded in response to an auditory stimulus from electrodes placed on the scalp. aeps serve for assessment of the functioning of the auditory system and neuroplasticity. they can be used to diagnose learning disabilities in children, aiding in the development of tailored educational programs for those with hearing and or cognition problems. somatosensory evoked potential normal somatosensory evoked potential (tibial nerve) somatosensory evoked potentials (sseps) are ep recorded from the brain or spinal cord when stimulating peripheral nerve repeatedly. sseps are used in neuromonitoring to assess the function of a patient's spinal cord during surgery. they are recorded by stimulating peripheral nerves, most commonly the tibial nerve, median nerve or ulnar nerve, typically with an electrical stimulus. the response is then recorded from the patient's scalp. although stimuli such as touch, vibration, and pain can be used for ssep, electrical stimuli are most common because of ease and reliability. ssep can be used for prognosis in patients with severe traumatic head injury. because ssep with latency less than 50 ms is relatively independent of consciousness, if used early in comatose patient, it can predict outcome reliably and efficiently. for example, comatose patients with no responses bilaterally has 95% chance of not recovering from coma. but care should be taken analyzing the result. for example, increased sedation and other cns injuries such as the spinal cord can affect sep. because of the low amplitude of the signal once it reaches the patient's scalp and the relatively high amount of electrical noise caused by background eeg, scalp muscle emg or electrical devices in the room, the signal must be averaged. the use of averaging improves the signal-to-noise ratio. typically, in the operating room, over 100 and up to 1,000 averages must be used to adequately resolve the evoked potential. the two most looked at aspects of an ssep are the amplitude and latency of the peaks. the most predominant peaks have been studied and named in labs. each peak is given a letter and a number in its name. for example, n20 refers to a negative peak (n) at 20ms. this peak is recorded from the cortex when the median nerve is stimulated. it most likely corresponds to the signal reaching the somatosensory cortex. when used in intraoperative monitoring, the latency and amplitude of the peak relative to the patient's post-intubation baseline is a crucial piece of information. dramatic increases in latency or decreases in amplitude are indicators of neurological dysfunction. during surgery, the large amounts of anesthetic gases used can affect the amplitude and latencies of sseps. any of the halogenated agents or nitrous oxide will increase latencies and decrease amplitudes of responses, sometimes to the point where a response can no longer be detected. for this reason, an anesthetic utilizing less halogenated agent and more intravenous hypnotic and narcotic is typically used. clinical uses sep findings do not by themselves lead to a specific diagnosis, and organic diseases cannot necessarily be excluded with normal sep findings. findings must be interpreted in the context of the patient’s clinical presentation. evaluating the peripheral responses with seps could contribute to the diagnosis of peripheral nerve damage. furthermore, seps could be abnormal in different pathologies such as multiple sclerosis (ms), hereditary spinocerebellar degenerations, hereditary spastic paraplegia, aids and vitamin b12 or vitamin e deficiency. in patients with ms, evoked potential findings often complement findings on mri. in the acute stage after a traumatic spinal injury or brain trauma, the absence of sep responses do not correlate with prognosis. however, an early return to normal or preserved cortical responses in the subacute stage correlate with a positive outcome. seps can be helpful to evaluate subcortical and cortical function in comatose patients and are less sensitive to sedative drugs than eeg. sep´s and baep´s together are the best tools to assist in the confirmation of brain death in comatose patients clinical consideration in children as in the adult, sep findings in combination with the clinical assessment and eeg findings can contribute to the determination of prognosis in comatose children. in high risk newborns, tracking sep findings over time can be helpful for outcome prognostication. several neurodegenerative disorders have abnormal findings in spinal and cortical sep components. moreover, compressive lesions on the spine (e.g. arnold-chiari malformation or mucopolysaccharidosis) are associated with abnormal seps, which may precede abnormalities on mri. laser evoked potential conventional sseps monitor the functioning of the part of the somatosensory system involved in sensations such as touch and vibration. the part of the somatosensory system that transmits pain and temperature signals is monitored using laser evoked potentials (lep). leps are evoked by applying finely focused, rapidly rising heat to bare skin using a laser. in the central nervous system they can detect damage to the spinothalamic tract, lateral brain stem, and fibers carrying pain and temperature signals from the thalamus to the cortex. in the peripheral nervous system pain and heat signals are carried along thin (c and a delta) fibers to the spinal cord, and leps can be used to determine whether a neuropathy is located in these small fibers as opposed to larger (touch, vibration) fibers. motor evoked potentials motor evoked potentials (mep) are recorded from muscles following direct stimulation of exposed motor cortex, or transcranial stimulation of motor cortex, either magnetic or electrical. transcranial magnetic mep (tcmmep) potentially offer clinical diagnostic applications. transcranial electrical mep (tcemep) has been in widespread use for several years for intraoperative monitoring of pyramidal tract functional integrity. during the 1990s, there were attempts to monitor "motor evoked potentials", including "neurogenic motor evoked potentials" recorded from peripheral nerves, following direct electrical stimulation of the spinal cord. it has become clear that these "motor" potentials were almost entirely elicited by antidromic stimulation of sensory tracts—even when the recording was from muscles (antidromic sensory tract stimulation triggers myogenic responses through synapses at the root entry level). tcmep, whether electrical or magnetic, is the most practical way to ensure pure motor responses, since stimulation of sensory cortex cannot result in descending impulses beyond the first synapse (synapses cannot be backfired). tms-induced meps have been used in many experiments in cognitive neuroscience. because mep amplitude is correlated with motor excitability, they offer a quantitative way to test the role of various types of intervention on the motor system (pharmacological, behavioral, lesion, etc.). tms-induced meps may thus serve as an index of covert motor preparation or facilitation, e.g., induced by the mirror neuron system when seeing someone's else actions. in addition, meps are used as a reference to adjust the intensity of stimulation that needs to be delivered by tms when targeting cortical regions whose response might not be as easily measurable, e.g., in the context of tms-based therapy. intraoperative monitoring somatosensory evoked potentials provide monitoring for the dorsal columns of the spinal cord. sensory evoked potentials may also be used during surgeries which place brain structures at risk. they are effectively used to determine cortical ischemia during carotid endarterectomy surgeries and for mapping the sensory areas of the brain during brain surgery. electrical stimulation of the scalp can produce an electric current within the brain that activates the motor pathways of the pyramidal tracts. this technique is known as transcranial electrical motor potential (tcmep) monitoring. this technique effectively evaluates the motor pathways in the central nervous system during surgeries which place these structures at risk. these motor pathways, including the lateral corticospinal tract, are located in the lateral and ventral funiculi of the spinal cord. since the ventral and dorsal spinal cord have separate blood supply with very limited collateral flow, an anterior cord syndrome (paralysis or paresis with some preserved sensory function) is a possible surgical sequela, so it is important to have monitoring specific to the motor tracts as well as dorsal column monitoring. transcranial magnetic stimulation versus electrical stimulation is generally regarded as unsuitable for intraoperative monitoring because it is more sensitive to anesthesia. electrical stimulation is too painful for clinical use in awake patients. the two modalities are thus complementary, electrical stimulation being the choice for intraoperative monitoring, and magnetic for clinical applications.