5 Study 2 - Error-Related Potentials
5.3 ErrP offline analysis
computed to split the raw EEG data recorded at different sensor locations into statistically in-dependent signals. This is a widely used technique to reduce typical EEG artifacts like muscle activity or eye movements and blinks. It was shown by [Delorme et al., 2007] that ICA can be used to efficiently reduce those mentioned artifacts without significantly distorting the un-derlying EEG data which effectively raises the signal-to-noise ratio (SNR). The analysis of the raw 14-channel EEG data resulted in 14 independent components, with two of them expos-ing a very high amplitude and typical topographic localization towards the frontal part of the skull (see Figure 5.2). Since these components strongly correlate with artifactual eye blinks and movements they were rejected from the data by setting their corresponding weights in the unmixing matrix to zero. A backprojection resulted in a cleaned up dataset which was used for throughout the rest of the analysis. This preprocessing step was applied for both low- and high task difficulty conditions. Further, all epochs were bandpass filtered with cor-ner frequencies of 3 and 20Hz.
5.3.2 Analysis of error vs. correct trials
The epochs of each experimental setting (high- and low task difficulty) were analyzed with respect to known ErrP characteristics like the Ne and Pe component, the spatial location of their maximum amplitudes, trial-to-trial variability and cross subject variability. For the low task difficulty condition it turned out to be fairly difficult to reproduce the results mentioned in [Ferrez, 2008]. Morphologically a strong negative peak around 250-350ms after the onset of an erroneous feedback stimulus was expected. For a correct feedback it was unclear what brain response was to be expected. Considering the the similarity to the given task to focus on the target letter, a correct feedback (i.e. the target letter) might elicit a P300 as well. To test whether there exists a significant statistical difference between the mean amplitudes of the correct and error condition a t-test was computed for all samples along the time axis. Data of all subjects were used as input and thus the results were expected to reveal time inter-vals and channel locations common to all subjects with significant difference between both conditions. For the experiment with low task difficulty, a rather weak statistical difference between erroneous and correct epochs could be detected. The significance values for each timestep and channel are drawn in the matrix of figure 5.3(B). The plots below and on the right side of the color coded matrix visualize the row and columns means of this matrix, i.e.
the mean statistical significance per channel for each timestep. All colored cells in this matrix correspond tosp ace×t i mepairs that passed a two-tailed paired t-test at a 0.01 significance level. Therefore, all these cells show at what time for each channel a significant difference in mean amplitudes between erroneous and correct trials exist. For the low task difficulty experiment(B)the most significant areas are found primarily in parietal and occipital loca-tions with a latency of 250-300ms and 400-500ms after stimulus onset. Areas with weaker statistical significance are also found towards frontal and central sites with maximas occur-ring around Fz and Cz. These areas are activated at 350-400ms and 500ms after a stimulus has been presented. When taking the mean significance values along the channel axis into
5.3 ErrP offline analysis
O1 O2 Pz Cz P3 P4 C3 C4 Fz F3 F4 AFz Fp2 Fp1
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O1 O2 Pz Cz P3 P4 C3 C4 Fz F3 F4 AFz Fp2 Fp1
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Figure 5.3. The plots show color coded significant differences between erroneous and correct trials. Brighter values correspond to higher significance. The side plots show the cross-sectional average along their respective axis. Differences between erroneous and correct trials in the high task difficulty condition (A) are much more pronounced than in the low task difficulty condition (B). For (B), most significant areas are around occipital and central areas while (A) shows highest significance over frontal areas.
account, it can be observed that the channels with highest statistical difference are the oc-cipital followed by parietal and central areas. In general, highest significances were expected to occur at frontal parts of the head where the amplitude of error-related potentials is usu-ally high and ErrPs were not expected to occur in correct trials. On the other hand, during correct trials which show the expected feedback which was also focused at the beginning of the speller matrix presentation, a P300 component was likely to occur due to the similarity to the prior speller task. Therefore additional significant differences around central to pari-etal/occipital sites could be expected. From the values shown in(B)it is rather difficult to prove a high correlation between the hypothesis and the actual measurement. Frontal parts did pass the t-test but most only slightly above significance level. Since the t-test only mea-sures statistical significant deviations from the mean it does not answer the question how stable the onset latency of possible error-related components are To measure the degree of latency uniformity, i.e. the degree of time jitter from trial to trial, the inter-trial coherency (ITC)1can be computed which is solely based on phase angles of the signal and independent of the signal amplitude. A frequency 5Hz was used in the analysis since the associated ITC values were maximal compared to the remaining frequency band. Especially for the time in-tervals of interest around 200-400ms [Ferrez, 2008, Dal Seno et al., 2009] the phase coherency ranges from 0.15 to almost 0. The blue shaded area of these plots depict the 0.01 significance level. Since the ITC values of the erroneous trials never exceed this level it could be confirmed that there was no significantly latency-stable ERP component within the erroneous trial
con-1sometimes referred to asphase-locking factor
dition. For the correct trial condition a slightly significant consistency could be detected at the Fz site at 200-250ms after stimulus onset. The result was equal for the lateralized sites around Fz which in turn coincides with the t-test seen in figure 5.3(B). It should be noted however, that due to the low number of erroneous trials in this setting the statistical evalu-ation should be taken with a grain of salt. It is possible that minor differences could not be recognized due to the low number of samples.
Exactly the same experiment has also been conducted with more stringent time constraints by lowering presentation intervals and thereby increasing the task difficulty. Afterwards the same analysis as explained above has been conducted. The results for this high task difficulty experiment are visualized in 5.3(A)and figure 5.4 (bottom). Here the statistical difference between error and correct conditions are much more explicit with prominent peaks at laten-cies of 200-250ms (prefrontal to central), 300ms (frontal to occipital), 400ms (frontal), 500ms (frontal to occipital) and 600ms (frontal to occipital) after stimulus onset. For the erroneous condition at Fz location, ITC values do by far exceed the 0.01 significance level from 200ms onwards with a much larger spectral power increase after stimulus onset compared to the low task difficulty experiment. Even without statistical analysis it can be observed in the single-trial visualization (Figure 5.4 bottom) that a amplitude modulation with similar latencies in response to an erroneous stimulus is present. This signal is composed of 2 main peaks at 300ms (negative deflection) and 400ms (positive deflection) and 2 smaller peaks at 200ms and 500ms. This is exactly what was to be expected for ErrPs which occur at 200-300ms at fronto-central locations. This also coincides with differences found in the t-test results shown in figure 5.3(A). The analysis of the correct trial condition revealed that no time-stable 5Hz components are present for this condition at a 0.01 significance level whereas the erroneous condition shows highly significant phase correlations for the 5Hz frequency. Compared to the low task difficulty experiment, no significant difference in spectral power increase could be measured for the correct trial condition as both groups only show around 1dB increase over baseline level.
5.3 ErrP offline analysis
−7.2
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Figure 5.4. Single-trial visualization of frontal erroneous (left) and correct (right) trial ERPs for low task difficulty (top)andhigh task difficulty (bottom)experiments. The colored plot shows a 10-epoch smoothed visualization of single trial amplitudes. In addition, an event-related spectral perturbation (ERSP) and inter-trial coherence (ITC) plots are shown below. The blue shaded areas denote the 0.01 significance level which has to be exceeded in order to be regarded as a significantly stable time interval with respect to the stimulus onset.
User wants to grasp objectA
BCI predicts objectB
ErrP elicited
ErrP recognized (True Positive)
ErrP not recognized (False Negative)
Flash Stimulus/Object
Predict most likely object
BCI predicts objectA
No ErrP elicited
ErrP recognized (False Positive) ErrP not recognized
(True Negative)
Grasp Object
False recognition Correct recognition
IntentRecognitionPhase (Multiple Trials)
Error CorrectionPhase (Single Trial)
Execution Phase
(1) (3)
(4) (2)
Figure 5.5. Schematic view of an error correction method using error-related potentials. Green paths corre-spond to increased or unchanged performance, whereas red paths denote performance loss.