Poster: Light sensor Various types (photo-transistors, -resistors or -diodes) exhibit a wide range of response speeds and sensitivity to: brightness (from projector or ambient light), incident light angle, wavelength. Figure 2: Projector or ambient light brightness variations can affect trigger timing when using a fixed threshold. Figure 3: Filtering can create a sharper signal (red line), but the MEG antialiasing filter can create artefacts and smooth the onsets at typical sampling rates (right). Challenge When participants are presented with visual stimuli, the precise time of their appearance on screen often needs to be recorded together with the MEG data in order to separate it into trials, for averaging across trials and subjects, and ultimately to compare results among studies. This can be done with a photosensor exposed to the projected image, to generate triggers or markers when the brightness crosses a threshold, but the factors outlined in this section influence their timing and can lead to: missed or spurious markers, jitter in time between trials and subjects, difficulty in interpreting absolute times reported in the literature. Light from the projector is reflected and transmitted through an optical fiber cable to a phototransistor circuit outside the magnetically shielded room (Fig. 5-7), thus avoiding metal and currents inside. The circuit outputs an amplified version of the signal with small offset which can be used for precise post-processing (Fig. 3 and 8 blue), e.g. using software to detect and mark light on and off events. For immediate trial generation, a second copy of the signal is output, which has been filtered (high-pass) to reduce the rise time and remove any offset, allowing a threshold near zero, based on noise levels (Fig. 3 and 8 red). Thus triggers can be generated reliably and as early as possible. Variable amplification at both stages gives flexibility, functioning in various configurations (supine or seated), and with a wide range of projector and ambient light brightness. The large amplification factors can make the circuit sensitive to electromagnetic noise e.g. from cell phones. Shielding was added to the circuit box to reduce interference. Recommended solution Hardware design Processing software Recording MEG data continuously instead of immediately separating into trials using the light triggers allows further processing of the sensor signal for more accurate marking of events and the possibility of adjusting the onset time after collection. A MATLAB function was written to generate new markers at light on/off events following the steps on the right. By finding both on and off events, missing or incorrect events are more easily detected. Projector Frames are displayed gradually from top to bottom. Light intensity changes relatively slowly. Overall brightness decreases with time. Brightness can oscillate. When exactly is the stimulus onset time? The chosen definition should be reported, e.g.: earliest detected brightness change above noise, 50% of max amplitude (15 ms after start of frame display). Figure 1: Relatively slow brightness change of 2 LCD projectors (blue: Sanyo Xtrax PLC-XP51, red: InFocus IN5108) when the image goes from black to white (time zero), and back to black (50 ms). Computer Signals from the stimulus computer are usually delayed variably with respect to the actual display due to: video card and drivers, presentation software, programming, e.g. requesting displays at intervals not respecting the display frame rate. Figure 4: Overlay of many trials marked by stimulus computer signals, of the photosensor channel, showing timing variation of one MEG sample (left) when codes are synchronized with the projector, and one display frame (right) when they are not. Figure 5: A white square is displayed at the same time as the stimulus, on the light receiver attached to the edge of the MEG screen. Figure 6: A 45° mirror (metallised plastic film glued on plastic base) directs the light into a robust 1 mm plastic optical fiber (50 ft TOSLINK optical audio cable, connector cut off). First stage variable amplification Second stage amplification High-pass filter Figure 7: Circuit box near acquisition computer, showing the optical cable and power connections (left), amplification knob and output channel switches (top), and DC37 output cable (right) which connects to the MEG electronics console. Figure 9: Simple amplification and filtering circuit, using a single quad op- amp chip. The photosensor is glued to the optical cable connector (far right). 1.Find time points where the signal crosses a threshold based on the amplitude range. 2.Average short time windows around each occurrence to get the shape of the signal. 3.Match this average shape to the original signal, finding peaks in cross-correlation and sum of squared errors. Each well separated (according to the display frame period) peak above a certain match level is marked. 4.Average again based on the new markers to calculate or select the desired onset point, avoiding trial-level noise. Add this offset to each marker from step 3 and save them in the dataset. Practical considerations for accurately recording visual stimulus onset times with a photosensor circuit Marc Lalancette 1, Thilakshan Kanesalingam 2 1 Diagnostic Imaging, The Hospital for Sick Children, Toronto, Ontario, Canada 2 Electrical & Computer Engineering, McMaster University, Hamilton, Ontario, Canada Beta MATLAB code: