HeterochromaticFlickerSimplified demonstration of heterochromatic flicker photometry on ArduinoUpdatedSep 3, 2012bypetteri....@gmail.com Introduction Details Arduino sketch (heterochromaticFlicker.ino) Counting the "button presses" during sampling period Limiting how often subject can confirm Arduino Debug ( heterochromaticFlickerExclIncrementDecrements.ino) Python GUI (heterochromaticFlicker.py) LabVIEW VI (heterochromaticFlicker.vi) Scientific References Heterochromatic flicker photometry Commercial products Critical Flicker Fusion (CFF) measurements Commercial productsIntroductionDemonstration how to alternate between different LED channels as is needed to implement psychophysical techniques such asheterochromatic flicker photometryorcritical flicker fusionmeasurement. Arduino had been used similarly in the work ofSun et al.(Hillman Lab, Columbia University) to control LED lights in their open-sourceSPLASSHframework for multispectralin vivoimage acquisition. SeeHeterochromaticFlicker#Scientific_Referencesfor further information.

Demonstration of heterochromatic flicker technique using our proposed Arduino-based system and LabVIEWDetailsThe basic idea for the heterochromatic flicker photometry implementation is that there aretwoLEDs of different color (heterochromatic) which are switched on in counter-phase.1. LED 1 is ON for first 500 ms of 1000 ms while LED 2 is OFF during this period2. LED 1 is OFF for the second 500 ms of 1000 ms while the LED 2 is ON during this periodThis is considered to flicker at a frequency of 2 Hz maybe counter-intuitively to people with engineering background that might take this scheme to have to rectangular signals having a frequency of 1 Hz to be phase-shifted 180 degrees in respect to each other.In our implementation, the subject of the experiment is faced with a flickering annulus (ring of light) and is asked to adjust the intensity of only the other LED, until theperceptionof flicker is eliminated or minimized. The other LED is set at fixed level by the experimenter.The subject responses are read using digital inputs of which 2 are needed for 1-axis joystick indicating subject's desire to increase or decrease light intensity, and 1 digital input is needed to read the "confirm"button that the subject is instructed to press when he perceives no flickering. Thus, the joystick and the button are connected to a 5V power supply and ground via pulldown resistors and when pressed the Arduino detects that digital pin to be HIGH.We don't want to change the light intensity during the 500ms/500ms flicker period, thus the digital inputs are buffered during the whole second cycle and decision of whether subject asked is done based on these buffered samples. Thus, a sampling rate of 1 Hz for digital input in theory would be sufficient if subject pressed long enough the buttons and joystick.In our application, Arduino need to return the intensity values (duty cycles in other words) of the both LEDs when the subject presses the confirm button (which can be combined with some other data if needed in the Python GUI and written to disk as a text file for example usingttylog)Arduino sketch (heterochromaticFlicker.ino)Compared to other examples (ControlPWMandCueLightTask), additional conditional rules need to be introduced to make sure that the system behaves as we like it to behave:Counting the "button presses" during sampling periodWe can count how many times the subject presses the button during the 1 sec sampling period. The resolution of counting depends on the digital sampling rate, in practice we get around 20 samples per second with current settings. We can then set a threshold of for counts that can be considered as a "real press", in our case only one was sufficient but in some other applications this check could be useful.For tutorial on debouncing the digital input (filter noise), see the tutorial ofLadyAdaor our other exampleCueLightTask.The variables are initialized as following: // i.e. the number of each channel pressed during // the 1 second period (2xflickerPeriod) int count_joystickUP = 0; int count_joystickDown = 0; int count_buttonPressed = 0; // if pressed or not int boolean_joystickUP = LOW; int boolean_joystickDown = LOW; int boolean_buttonPressed = LOW;The basicfor-loop in The Arduino sketch for the flickering scheme looks like the following: for (int i = 1; i = pressThreshold) { boolean_joystickUP = HIGH; // ledOut_ch1++; } // Evaluate whether the buttons are really pressed if (count_joystickDown >= pressThreshold) { boolean_joystickDown = HIGH; // ledOut_ch1--; }Now the user control of the LED is hard-wired only the 'ch1', but could be modified to be switch-controlled from the Python GUI for example. This was omitted from the example from the sake of simplicity.Python GUI (heterochromaticFlicker.py)The basic implementation of Python GUI is simple with only two sliders for adjusting the intensity of the flickering lights: def on_changed(self, widget): val = widget.get_value() name = widget.get_name() if name == "Channel1": self.ser.write("b" + chr(int(val))) elif name == "Channel2": self.ser.write("g" + chr(int(val))) else: print "ERROR: Invalid widget name, in on_changed function"The case conditional to identify which LED channel want to be modified is implemented similarly as for other examples (ControlPWMandCueLightTask)Screen capture of the heterochromatic flicker Python GUI.LabVIEW VI (heterochromaticFlicker.vi)As shown inControlPWM, LabVIEW can be used to control the LEDs using Arudino.Demonstration of how to use LabVIEW with Arduino is done using theLabVIEW Interface for Arduino(LIFA) package provided byNational Instruments(NI, the company behind LabVIEW and vast selection of data acquisition instruments). The example is modified from the example "Simple LED" provided by NI.You need to upload the Arduino sketchLVIFA_base.pdein order to make Arduino responsive to commands sent from LabVIEW:Screen capture of LVIFA_base.pde with its accompanying files.The LabVIEW Virtual Instrument block diagram for the heterochromatic flicker looks like that without user inputs (to increment/decrement LED intensity):

Screen capture of the block diagram of LabVIEW heterochromatic flicker photometry.The corresponding front panel GUI looks as following:

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Heterochromatic Flicker Electroretinograms Reflecting Luminance and Cone Opponent Activity in Glaucoma Patients.Investigative Ophthalmology & Visual Science(July 21, 2011).http://dx.doi.org/10.1167/iovs.11-7538. OBrien, Kevin, Bill Smollon, Bill Wooten, and Billy Hammond. Determining Heterochromatic Flicker Photometry Frequency for Macular Pigment Optical Densitometry by Critical Flicker Fusion Frequency.Journal of Vision11, no. 15 (December 27, 2011): 5555.http://dx.doi.org/10.1167/11.15.55. van den Berg, Thomas J.T.P., Luuk Franssen, Bastiaan Kruijt, and Joris E. Coppens. Psychophysics, Reliability, and Norm Values for Temporal Contrast Sensitivity Implemented on the Two Alternative Forced Choice C-Quant Device.Journal of Biomedical Optics16, no. 8 (2011): 085004.http://dx.doi.org/10.1117/1.3613922. Brown, TimothyM., Sei-ichi Tsujimura, AnnetteE. Allen, Jonathan Wynne, Robert Bedford, Graham Vickery, Anthony Vugler, and RobertJ. Lucas. Melanopsin-Based Brightness Discrimination in Mice and Humans. Current Biology 22, no. 12 (June 19, 2012): 11341141.http://dx.doi.org/10.1016/j.cub.2012.04.039.Commercial products Oculus C-Quantstraylight meter, which was further modified byvan den Berg et al., 2011with custom Matlab code. PerimetersfromOptosforflicker perimetryapplications for example. Macuscope, The MacuScope is a "heterochromatic flicker photometer" - the gold standard for measuring macular protective pigment density (MPPD). SeeYouTube videoto see the Macuscope in action. MPS 9000 Macular Pigment Screener, commercially available? Evaluated in the following papers e.g.:Bartlett et al., 2010,Loughman et al., 2011andMurray et al., 2011Critical Flicker Fusion (CFF) measurements Porter, T. C. Contributions to the Study of Flicker. Paper II.Proceedings of the Royal Society of London70 (January 1, 1902): 313329. Ives, Herbert E. 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Determining Heterochromatic Flicker Photometry Frequency for Macular Pigment Optical Densitometry by Critical Flicker Fusion Frequency.Journal of Vision11, no. 15 (December 27, 2011): 5555.http://dx.doi.org/10.1167/11.15.55. Bowles, Kristen E, and Timothy W Kraft. ERG Critical Flicker Frequency Assessment in Humans.Advances in Experimental Medicine and Biology723 (2012): 503509.http://dx.doi.org/10.1007/978-1-4614-0631-0_63.Commercial products FLIM Flicker/Fusion Frequencydevice bySCHUHFRIED GmbH [www.lafayettelifesciences.com/product_detail.asp?ItemID=36 Flicker Fusion System Model 12021] fromLafayette Instrument