A PC-based visual stimulus generator

An inexpensive video stimulator for Visual Evoked Potential (VEP) and Pattern ElectroRetinoGraphy (PERG) measurements was developed using standard PC components (motherboard, CPU, RAM, VGA-card). The program is running under DOS (PC-DOS or MSDOS). DOS is available from different sites (e.g., www.bootdisk.com, or www.freedos.org). The current version of the software was tested with MSDOS 6.2.

Most of the parameters, e.g., stimulus type, stimulation timing, screen resolution (320x240 – 1600x1200), fixation point size and color, are programmable. The vertical refresh rate is adjustable (~60-150Hz depending on the Monitor) using the VBEHZ utility: http://www.informatik.fh-muenchen.de/~ifw98223/vbehz.htm 

The current version of the software supports luminance, pattern reversal and pattern onset/offset stimulation. 
Checkerboard, vertical and horizontal bar, windmill and dartboard  and sinusoidal grating patterns are available. Red-green random-dot stereogram (requires red-green spectacles) and texture segregation modes are also supported. 

The printer port of the PC is used as synchronization output. Synchronization occurs during the vertical retrace.

In pattern offset mode a 1x1 pixel checkerboard stimulus pattern is generated, the screen luminance remains constant during pattern onset/offset stimulation. Calibration of the mean luminance in the pattern-offset phase is not necessary.

The program was developed for CRT displays. Don't use LCD monitors. The frame rate of the LCDs is determined by the internal circuitry. Images sent to the LCD monitor are stored in the internal RAM of the LCD-controller and displayed asynchronously. 

The current version of the software is available here: stimul.zip (only the executable and documentation file).

An obsolete version of the video stimulus generatior using palette manipulations for is available under the GNU General Public License is also available stimul_old.zip (including the source code). Before using the program please read the copyright and license conditions: http://www.gnu.org/licenses/gpl.html

Installation: unzip stimul.zip or stimul_old.zip to a folder (e.g., to C:\STIMUL), and install program according to information in readme.txt.

The easyest way to generate visual stimuli is to use a Windows-based (95-XP) PC for remote control the the visual stimulus generator.


  1. Install the "Vision Explorer" software on a Windows-based computer.
  2. Connect the COM1 port of the video stimulus generator to a free COM port of the Windows PC (Com1 - 4 are supported) with a null-modem cable.
  3. Start "Vision Explorer"
  4. Setup video stimulator parameters using the menu -> Settings -> Hardware commands.
  5. Configure Video Stimulator parameters (Com-port number, viewing distance (mm), Frame rate (should also be adjusted using the VBEHZ program on the video stimulus generator), screen width (mm) and default pattern size. Store setting using the OK-Button.

Video stimulus parameter can be programmed using the Menu -> Stimulation -> Video -> Stimuls dialog.

The color, size and shape of the fixation object be changed using the Menu -> Stimulation -> Video -> Fixation Object dialog.