Enabling High Grayscale Resolution Displays and Accurate Response Time Measurements on Conventional Computers

Summary

Conventional computer hardware can not generate visual stimuli with sufficiently high grayscale resolution and measure response times with sufficient accuracy. We describe how to use the VideoSwitcher to produce high-resolution monochromatic displays, and the RTbox to measure response times with high accuracy on conventional computer hardware.

Abstract

Display systems based on conventional computer graphics cards are capable of generating images with 8-bit gray level resolution. However, most experiments in vision research require displays with more than 12 bits of luminance resolution. Several solutions are available. Bit++ ¹ and DataPixx ² use the Digital Visual Interface (DVI) output from graphics cards and high resolution (14 or 16-bit) digital-to-analog converters to drive analog display devices. The VideoSwitcher ³ described here combines analog video signals from the red and blue channels of graphics cards with different weights using a passive resister network ⁴ and an active circuit to deliver identical video signals to the three channels of color monitors. The method provides an inexpensive way to enable high-resolution monochromatic displays using conventional graphics cards and analog monitors. It can also provide trigger signals that can be used to mark stimulus onsets, making it easy to synchronize visual displays with physiological recordings or response time measurements.

Although computer keyboards and mice are frequently used in measuring response times (RT), the accuracy of these measurements is quite low. The RTbox is a specialized hardware and software solution for accurate RT measurements. Connected to the host computer through a USB connection, the driver of the RTbox is compatible with all conventional operating systems. It uses a microprocessor and high-resolution clock to record the identities and timing of button events, which are buffered until the host computer retrieves them. The recorded button events are not affected by potential timing uncertainties or biases associated with data transmission and processing in the host computer. The asynchronous storage greatly simplifies the design of user programs. Several methods are available to synchronize the clocks of the RTbox and the host computer. The RTbox can also receive external triggers and be used to measure RT with respect to external events.

Both VideoSwitcher and RTbox are available for users to purchase. The relevant information and many demonstration programs can be found at http://lobes.usc.edu/.

Protocol

1. VideoSwitcher

1.1. Connect VideoSwitcher to a computer

The VideoSwitcher (Figure 1) receives analog video (VGA) inputs and is used to drive cathode ray tube (CRT) color monitors. Before using the VideoSwitcher, make sure the computer graphics card has either a VGA port (Figure 2A) or a Digital Visual Interface (DVI-I) port (Figure 2B) that transmits both digital and analog video signals. A DVI-to-VGA adaptor (Figure 2C) is necessary to connect the DVI-I port to the VideoSwitcher.

Turn off the computer and monitor and disconnect the monitor from the computer graphics card.

Discussion

The video switcher provides a compact, inexpensive, and easy-to-use method to generate high luminance resolution displays using conventional computer graphics cards and analog color monitors. It is connected between the analog outputs of the graphics card and the monitor. It can also switch between monochromatic and chromatic display modes, providing additional convenience. Its trigger channel allows researchers to synchronize other equipment to visual displays.

The RTbox is a compact, inexpen.......