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Neurochemical regulation of auditory information ... - Helda - Helsinki.fi

eview, see Kujala et al. 2007b). It is suggested that the MMN represents a sensory memory

trace formation process related to the evaluation of presented stimuli. The MMN could

provide information about the amount of neuronal resources participating in automatic

(involuntary) change-detection attentional processes (Haenschel et al. 2005). Interestingly,

MMN can be recorded in neonates (Ceponiene et al. 2002), premature newborns (Cheour

et al. 1998) and even in foetuses. Hence, Draganova et al. (2007) showed that at the

28-39 week gestational age group, the discriminative MMN-like responses to frequency

change could be detected as early as 28 weeks. As a result, the MMN was considered the

ontologically first cognitive component presented in the human brain (for review, see

Naatanen et al. 2007).

According to prevailing MMN theory, it is assumed that there exist specific populations

of change-detection neurons that produce MMN response (Naatanen et al. 1978; Tiitinen

et al. 1994). Supporting this view, brain regions responsible for MMN generation were

identified. The number of studies using EEG, MEG, functional MRI (fMRI) (Schall et

al. 2003), positron-emission tomography (PET) (Muller et al. 2002) and even optical

imaging (Tse et al. 2006) showed that MMN is generated in supratemporal cortices.

However, additional parietal components are presented as well. Based on scalp current

density maps, additional confirmations of the existence of a frontal MMN generator

were presented (Deouell et al. 1998; Giard et al. 1990; Rinne et al. 2000; Yago et al.

2001a). MMN studies with intracranial recordings are of particular interest. Hence,

a recent study by Rosburg et al. (2007), performed with patients during epilepsy

surgery, showed MMN responses in the rhinal cortex. It seems that the pre-conscious

discrimination of stimulus change in the auditory cortex elicits the supratemporal

MMN and this in turn initiates a sequence of brain events that are associated with

involuntary shifting of attention, orientation, and conscious detection of this change

(Naatanen et al. 1992). This initiation of involuntary attention shifting is supposed

to be reflected in the frontal MMN subcomponent (Giard et al. 1990; Naatanen et al.

1992). Rinne et al. (2000) showed that the “centre of gravity” of the MMN source

current distribution shifts from the temporal to the frontal cortex as a function of time,

supporting the theory (Naatanen and Alho 1995) that the frontal MMN subcomponent

is generated slightly after the supratemporal subcomponent. Other observations from

human-lesion (Alain et al. 1998; Alho et al. 1994) and imaging studies (Dittmann-

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