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INVESTIGATION OF SPECIFIC LEARNING DIFFICULTIES BASED ON THE INFORMATION FLOW IN MULTICHANNEL EEG SIGNALS

The main purpose of this paper is to study the information flow in multi-channel Electroencephalogram (EEG) recordings of δ, θ, α, β rhythms in the case of Specific Learning Difficulties (SLD) and to compare the findings with healthy subjects. The Directed Transfer Function (DTF) was used in order to determine the directional flow in the frequency domain between any given pair of channels. EEG recordings corresponding to 19 healthy subjects and 38 SLD subjects, were used to estimate the directional influences. The Bootstrap technique was used to compare the mean values of the influences extracted from the two investigated groups and to determine the confidence interval of the influences for each group. It has been found that the mean DTF of SLD subjects was significantly lower than that of healthy controls, for the δ and θ frequency bands. These findings seem to suggest that SLDs are associated with limitations in the speed of information processing, concerning the orienting a coordinated response (θ band) and/or the signal detection-evaluation and decision making (δ band). Introduction The term Specific Learning Difficulties (SLD) has been used to describe individuals having reading related difficulties despite normal intelligence, adequate training and satisfactory educational opportunities. Approximately 10-15% of the population have SLD. The supposed pathogenetic mechanisms underlying SLD are referred to genetic, neurobiological and cognitive variables. Despite extensive research in this field, the nature of what constitute the core deficits in SLD has remained poorly understood and heavily debated. In general, the functional significance of varying brain activity can be seen in the vicinity of the underlying neural circuits. For instance, it is assumed that alpha band activity reflects an increased excitability level of neurons in certain cortical areas, which may be related to an enhanced information transfer in thalamocortical circuits and is strongly correlated with working memory as well as with long-term memory. Beta bursts being related to cortico-cortical interactions, shift the system to an attention state that consequently allows for gamma synchronization and perception. Theta EEG activity being associated with cortico-hippocampal interactions, is interpreted as being correlated with ‘orienting’ a coordinated response indicating alertness, arousal. Delta EEG activity, being observed in the sleeping brain and in epilepsy, is associated with the signal detection-evaluation and decision making and is linked to the cortical-subcorical interactions [1]. It is believed that the way different regions of brain communicate to each other can give useful information about learning difficulties [2]. The Directed Transfer Function (DTF) [3][4] has been introduced as an estimator of the intensity of activity flow between brain structures, depending on the frequency correlation of any given pair of channels in a multivariate dataset. For the analysis of relations between brain structures, a multichannel model is more appropriate as it takes into account all signals simultaneously and not pair-wise. This overcomes the general disadvantage of pair-wise analysis, which in certain cases can lead to incorrect conclusions especially when some channels are fed from common signal sources [5]. Additionally, the multichannel model creates a common base with regard to the whole system providing an absolute scale for comparison of quantitative results. The DTF function is also sensitive to the time delay of signals and it expresses both direct and indirect causal influences between structures. The DTF has been applied recently in order to examine the differences between the pattern of activity flow in patients before and after the evolution of an epileptic seizure [6]. Moreover, the main centers from which EEG activity is spreading during sleep and wakefulness and differences in coherence patterns between sleep stages have been investigated by means of the DTF [7]. The purpose of the present paper was to study the EEG patterns of information flow in two groups of children, healthy subjects and SLD subjects, and to determine the possible frequency bands and the EEG channels for which the information flow patterns are significantly different between the two groups. The whole analysis of information flow has been focused on EEG rhythms δ, θ, α, β. Methods The Directed Transfer Function Let be the k-channel EEG measurements at time point t as shown in Figure 1. (t)] k , X t (t),X [X X(t) ... = ), ( 2 1 Figure 1. k-channel EEG signal The signal matrix X(t) can be described by the Multivariate Autoregressive (MVAR) process [8] as