# Somatosensory Mismatch Response in Patients with Cerebral Palsy
## 1 Introduction  
Event-related potential (ERP) is a measured brain response to a specific sensory, cognitive, or motor event. ERPs are measured by means of electroencephalography (EEG), which is a noninvasive electrophysiological method of monitoring the brain's electrical activities. Mismatch negativity (MMN) is a component of an ERP observable at specific recording sites (e.g., frontal) and latency (\(\sim\)120-200 ms) relative to the moment of stimulus presentation, as detected in the EEG signal. MMN occurs when a sequence of repetitive standard stimuli is interrupted by an odd or deviant stimulus, i.e., when the brain detects a change in a background of homogeneous events [1]. For example, if one hears a series of identical tones (i.e., standard stimuli), and then a slightly different tone (i.e., odd stimulus) is introduced, MMN reflects the brain's automatic response to this deviation. The odd stimulus must differ from the standard stimulus in at least one stimulus attribute, in frequency, duration, or intensity. MMN is associated with automatic, subconscious memory processes [1], through its role in detecting and registering deviations from expected stimuli. A sequence of frequent stimuli causes a track or a regular pattern of stimuli in the sensory memory, and any new incoming stimulus is then compared with that created memorytrack. When a stimulus breaks that regular pattern, MMN is elicited, indicating that the brain has detected a mismatch. This process is thought to be related to preattentive, automatic memory mechanisms. MMN reflects the brain's ability to compare incoming sensory information with stored memory representations, contributing to the early stages of memory formation and updating [2]. MMN has been originally discovered and studied intensively by auditory stimuli [3], but there is evidence of MMN in the visual [4; 5] and somatosensory [6; 7; 8] modalities also. In the visual domain, it involves changes in visual stimuli, such as color, shape, orientation, and emotional expression [9; 10]. In the somatosensory domain, it has been assessed most commonly using vibrotactile stimuli and is known as somatosensory MMN (sMMN). For example, different durations, different frequencies of stimulation, or stimulation of different body parts is used for standard and odd stimuli [11; 12]. The better a subject can distinguish the deviant from the standard stimuli, the larger the MMN [13]. Especially in the auditory domain, MMN has been widely employed to study speech and language development. This research includes both typical cases, such as infants exposed to one or two languages with normal development [14], and atypical cases, like children having problems with a specific language impairment [15]. Visual MMN in the context of cognitive impairment and aging can provide information about age-related changes in sensory processing and cognitive function [16; 17]. Furthermore, visual MMN seems to be sensitive enough to disclose gender differences [18; 19]. In the tactile domain, few studies reported the existence of somatosensory MMN in healthy individuals [6; 11; 20; 21]. However, to our knowledge, contrary to MMN studies in the auditory and visual modalities, much less is known about the effects of mismatch in the sMMN modalities, and related developmental studies are very sparse. The reported somatosensory ERPs showed different results depending on the stimulus properties, such as duration, spatial location, and vibrotactile frequencies. For example, [PERSON] et al. [11] used a vibratory mismatch paradigm and observed sMMN as a negative deflection at 100-200 ms, while [PERSON] et al. [22] found sMMN as a positive deflection at 100-200 ms, using a topographical mismatch paradigm. [PERSON] et al. adopted a two-point discrimination paradigm and obtained a negative potential at \(\sim\)140 ms (N140) and two positive components at around 300 and 500 ms [23]. On the other hand, using a temporal discrimination task, [PERSON] et al.