This video demonstrates how to enable high resolution gray scale displays and perform accurate response time measurements on conventional computers. This is achieved using two affordable devices, a video switcher and an RT box, which are connected to the computer. Next, the display is calibrated to ensure image intensities are accurate.
A sign wave grading with and without increased gray scale resolution is used to demonstrate the improved gray scale resolution. Simulated button responses from a light signal are used to demonstrate the accuracy of the response time measurements. The main advantage of the video switcher over existing methods like b plus plus and data pex is that it enables researchers to generate monochromatic displays with high grade level resolution using conventional computer hardware.
It can also provide trigger signals to synchronize visual displays with physiological recordings or response time measurements. The RT box enables measurements of response times with high accuracy. The advantage of the RT box over a computer keyboard is that it uses a microprocessor and high resolution clock to record the identities and timing of button events.
So the measured response times are not effective by potential timing uncertainties or biases associated with data transmission and processing in the host computer. It also greatly simplifies the design of user programs. Before installing the video switcher, turn off the computer and monitor, then disconnect the monitor from the computer graphics card.
Connect the video switcher directly to the computer graphics card using the provided VGA cable. If the computer graphics card doesn't have a VGA port, but has an integrated digital visual interface, or DVII port instead, then A-D-V-I-V-G-A adapter is required. Secure the connection with the small screws on the connector.
Connect the monitor's VGA cable to the output port of the video switcher, and secure the connections using the provided power adapter. Connect the video switcher to a power source. Now turn on the computer and the monitor.
The switch button on the video switcher allows the user to switch back and forth into display modes in the color mode. The video switcher does not alter the video signal from the computer graphics card. In the gray scale mode, the display becomes monochromatic.
In this mode, pixel luminance is determined mostly by the video signal from the blue channel of the graphics card only slightly by the red channel and not at all by the green channel. The ratio of the contributions of the blue and red channels to the output of the video switcher is called the blue to red ratio or BR ratio, and is provided by the manufacturer. For most monitors, the relationship between pixel gray level and pixel luminance is a power function.
The exponent of the power function is normally called gamma. To determine the gamma of a display, one can use a photometer to measure the luminance of a large uniform square at different pixel gray levels and fit a power function to the result. A more sensitive method to determine gamma is to compare a spatiotemporal mixture of equal quantities of zero and maximum pixel levels in one viewing area of the display.
Then compare the luminance of the pixel mixture with an adjacent area of homogeneous gray level. First, match the gray level value for half of the maximum luminance. Next, a new mixture of pixels with luminance intensities of just determined half level and maximum level is made.
This is then matched to the gray that corresponds to three quarters luminance. This procedure is repeated until seven. Values from one eighth to seven eighths have been determined.
Gamma can be determined by fitting a power function to the results. Now set the background luminance and contrast using the gamma and corresponding minimum and maximum luminance values. The effect of increased gray scale resolution can be shown by comparing two sine wave gradings.
The grading on the left uses the full capacity of the video switcher. The grading on the right is the result of an eight bit display without using the video switcher. Begin by connecting the RT box to the USB port of a computer with the provided cable download and install the driver.
So the computer system can recognize the RT box using stimulus software that can record accurate stimulus onset times. The RT box will convert between timestamps and the response time is measured as the difference between the button press time and stimulus onset. When reliable stimulus onset times are unavailable.
Provide the RT box trigger signals that are synchronized with the stimulus onset via a light sensor, an audio response, or the TTL signal from the video switcher or similar device to verify the accuracy of the RT box. The response times of a photo diode to light flashes in the center of the screen are measured. The stimulus onset time is returned by the screen flip function of psych toolbox.
The time difference between the light signal and the onset of stimulus is expected to be half of a video refresh rate or 5.88 milliseconds for an 85 hertz CRT. The difference observed is used to correct the nominal onset of stimulus, and the variability of the measurements reflects the precision of the RT box. This data shows that the RT box functions with very high precision.
After watching this video, you should have a good understanding of how to enable high grade level display, unconventional computer hardware using the video switcher, and how easily and accurately measure response times using the RT box.
