# Neurofeedback Effects on EEG Connectivity among Children with Reading Disorders: I. Coherence
### Neurofeedback  
Neurofeedback (NFB) is the most employed term to refer to the biofeedback of brain activity, and it is generally intended to normalize brain activity. There are two modalities: biofeedback based on EEG/MEG and biofeedback based on functional magnetic resonance imaging at rest. In this paper, the term NFB is used to refer to EEG-based biofeedback. NFB involves a type of learning called \"operant conditioning,\" where a given behavior (for example, an EEG event) receives a reward signal (positive or negative) as a consequence; as a result of pairing behavior with a reward, the probability of the occurrence of this EEG event increases.  
Several NFB protocols have been used to treat RD. In children, protocols have been based on abnormal EEG characteristics described for RD populations: [PERSON] et al. [43] and [PERSON] et al. [44; 45; 46] trained power values, and [PERSON] et al. [47], [PERSON] and [PERSON] [48], and [PERSON] et al. [49] trained power and coherence values. In some of these studies, the design of the NFB protocol also considered some neuropsychological aspects associated with reading, for example, its left lateralization. In this sense, the protocol that reinforces the reduction in the theta/alpha ratio has a limitation. However, there is a mitigating factor to this limitation, since the regions involved in reading and the connections between them may be different in children with RD and not specific to the left hemisphere.  
There is consensus that children with learning disabilities have more theta activity (absolute and relative power) and a lower amount of alpha relative power (RP) when compared to typically developed children of the same age; however, this does not occur in specific regions. Furthermore, in neurotypical children [50] and young adults [51], a certain amount of resting alpha seems to be necessary for the correct performance of a mental task in the regions involved in that specific task. It should also be considered that [PERSON] and [PERSON] [24] reported that the theta/alpha ratio could be an electroencephalographic marker of brain maturity and that in many studies, excessive theta/alpha ratios have been found in children with learning disorders [23; 24; 52; 53]. Considering all this evidence, an NFB treatment that positively reinforces a reduction in the theta/alpha ratio at the lead, which presents the most abnormal theta/alpha ratio, was proposed, with the aim of normalizing the ratio and improving cognitive performance.  
On the other hand, the theta/beta ratio has repeatedly been used to characterize EEG profiles in clinical populations [24]. A higher theta/beta ratio is characteristic of ADHD, and it has been proposed that an excess of the theta/beta ratio in frontal-central leads could be considered a biomarker for top-down control of attention [54]. [PERSON] and [PERSON] [24] also reported that there is no difference between theta/alpha and theta/beta ratios in the eyes-open condition, but it is important to consider that having the eyes closed facilitates alpha production. A protocol based on the theta/beta ratio was not considered in this or previous studies because a consensus regarding the beta deficit has not been reached in studies comparing children with learning disorders and neurotypical control children. On the other hand, in all NFB studies on learning disorders carried out by the [PERSON] group, children with ADHD were excluded.