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  • Homework 2

    The evoked responses were calculated over 600 ms time window, with 100 ms pre-stimulus and 500 ms post-stimulus. Evoked responses for triggers 2, 3, and 16 are visualized in Figures \ref{fig_topo2}, \ref{fig_topo3} and \ref{fig_topo23}. Since all sensors were planar gradiometers, the signal is strongest over the areas of the corresponding brain activity. The following Matlab script was used to calculate and visualize the pre-recorded data set:


    The highest response for trigger 2 is located over the visual cortex so the stimulus is most likely visual. The two peaks are also found from reference articles (Shigeto 1998) and (Herrmann 2000). The response is greater on the left side, which means the stimulus was most likely in the right visual hemifield as can be seen from Fig. \ref{fig_vishemi}

    Trigger 3 seems to correspond to an audio stimulus. Highest activation is over auditory cortex and the responses have clear peak at 100 ms, which is typical for certain audio stimuli (Roberts 2000). Based on the measurements in (Roberts 2000), the frequency of the tone is most likely over 600 Hz. Since there is activation on the left and right sides of the cortex, the stimulus is most probably heard by both ears.

    The last stimulus type is most likely median nerve stimulation and more precisely left median nerve stimulation since the activation is seen over the right somatosensory cortex. The response in Fig. \ref{fig_trig23} is very similar as the one in (Huttunen 2006) Fig. 1.

    Distribution of Work

    • Manu wrote some of the Matlab script and contributed to the writing of the homework.

    • Antti M. assisted in writing the Matlab script, searched three of the papers, and initialized the report-writing platform.

    • Antti R. found one of the papers, wrote the basic structure of the homework, and compiled the LaTex file.

    • Tuomo was reassigned to the group when the assignment was already completed, although read through the assignment and checked through the neurobiological interpretation and found it to be valid

    \label{fig_topo2} Sensor-level responses for trigger 2