Thesis - Instituto de Telecomunicações

4.2. ELECTRODERMAL ACTIVITY 69To extract information from an electrodermal recording, two computational steps areperformed, namely event search and parameter extraction. The first step, event search, isperformed by manual annotation or by identification of the peaks and valleys of the EDAsignal.4.2.1 Modeling DifficultiesThe search for better EDA models is related to two effects typically encountered withinelectrodermal event detection and quantification: the overlapping of events and the occurrenceof low amplitude events. These problems can lead to distorted estimated values forthe events, and, on some occasions, a pair of events can be detected as a single one. Bothproblems affect event search and parameter extraction steps. We describe in more detailthese modeling problems in the following paragraphs.Event Overlapping The most difficult EDA modeling problem is the quantification ofoverlapping events [94]. When an SCR event to be quantified occurs immediately afteranother SCR event, the computed values are masked. The amplitude of the second eventappears to be smaller than it would be in the case of a single event. The overlapping occursmore frequently in the decay zone of the preceding event (an example is depicted in figure4.10), but in some cases the delay between successive activations is lower than the rise timeof the first event, and the overlapping occurs in the rise zone (see figure 4.12). In bothcases, the parameters cannot be correctly extracted via the normal procedures.Low Amplitude Event In some situations an EDA event occurs with low amplitudecompared to the decreasing exponential of a previous SCR event or in the rise zone of ahigher amplitude event. In this case the detection techniques based on visual inspection or inthe search for first derivative zeros will fail. These events are normally masked by a strongerone and fail to be accounted for even when having the same amplitude of other easilydetectable SCR events. In figure 4.11 a recorded signal, that presents the superimpositionof a low amplitude event in a decreasing zone of a previous EDA response, is presented.