eye tracking with media lab (case 19711)

how do i run an asl eye tracker with media lab?

OK - the goal here is to make your DirectRT / MediaLab computer communicate properly with your EyeTracker system. Since there are a plethora of different systems out there, my instructions here will be fairly general. However, they should get you pointed in the right direction. If you have any questions, feel free to reply to this message, or send me an email at 'sales AT empirisoft.com'

- - -

Most folks who do eye tracking actually have two computers set up. One runs MediaLab or DirectRT and the other runs the eyetracking software. It's probably possible to run both programs on the same computer, but it may cause more problems than it's worth.

We're also assuming that you want to show a picture or question on the MediaLab computer and then begin to capture data on the eyetracker system.

So all we need to do is tell the eyetracker system to start and stop recording at the appropriate times.

I'm guessing that your general experiment looks something like this:

1. Show a picture or question
2. Send a 'start' signal through the parallel port
3. Wait for your subject to respond or react
4. Send a 'stop' signal to tell the eyetracker to end data collection
5. Rinse and repeat

Everything OK so far?

Your eyetracker system probably comes with an external hardware box, which contains connections for your camera, the computer, etc. There should also be a port which can send and receive TTL logic signals. On the systems I'm most familiar with, this port has 25 pins and is D-shaped, similar to an old-style parallel printer port.

You can check the documentation that comes with your eyetracker system for more details.

Fortunately, both DirectRT and MediaLab can communicate with the outside world through your computer's parallel port. This port contains, among other things, 8 wires which can be toggled on and off.

If one of these wires is connected between the MediaLab computer and the Eyetracker computer, we can send stop and go signals back and forth.

Check your documentation for best results, but I *think* you can go to the campus bookstore and buy a regular 25 pin parallel cable (check the computer section) and be in business.

[I]Don't[/I] start connecting random cables between your eyetracker system and your MediaLab computer without first checking with the proper user's manuals. We're [B]NOT[/B] responsible any explosion which may occur.

Then, you'll need to program MediaLab or DirectRT to send out a TTL signal at the appropriate time.

For MediaLab, see page 85 in the manual:

[QUOTE]<value:location:duration>

For example, say you present an image item for 5 seconds and at its onset you want to send a signal of 255 to port 888 for 10ms. To present it for 5 seconds, you would use parameter value of (d5). To add the TTL signal at its onset, you would add: (d5,<255:888:10>)
[/QUOTE]

For DirectRT, see page 33 of the manual.

* * *

In any case, the parallel port is 8 bits wide, which means that it can have 256 possible values assigned to it.

The bits are assigned like this:

D7 D6 D5 D4 D3 D2 D1 D0
which correspond to decimal values
128 64 32 16 87 4 2 1

If you want to activate only the DO line, you'd send the decimal value '1' to the parallel port.

00000001

If you want to activate D1, you'd send decimal '2'.

00000010

D4 corresponds to decimal 16

00010000

and D7 is 128

10000000.

You can also add bits together. 7 looks like this

00000111.

Etc.

Remember, there 10 types of people in the world - those who understand binary and those who don't. :)

Then, you just need to program your eyetracker software to start and stop recording when the proper TTL signals are received.

So to follow the template given above, your experiment file could look something like this:

1. Show a picture or question
2. Send a 'start' signal through the parallel port. Program the eyetracker software to accept a 'start' signal on wire D0. This wire will normally 'live' at 0 volts, but will pulse to +5V for 10 mS before returning to zero.

[B] (d5,<1:888:10>) [/B]

3. Wait for your subject to respond or react
4. Send a 'stop' signal to tell the eyetracker to end data collection. Program the eyetracker to accept a stop signal on wire D1.

[B] (d5,<2:888:10>)[/B]

5. Rinse and repeat



Does this help at all?

- John

It turns out that at least one eyetracker system is set up to automatically start and pause recording when Pin 1 on the parallel port is toggled between 1 and 0.

Pin 1 on the parallel port is the 'strobe' bit, according to

[url]http://www.beyondlogic.org/spp/parallel.htm[/url]

The strobe bit is Bit 0 of the parallel printer's control register, which is found at the base address + 2. That is, if your printer is found at decimal 888, the control register is at 890.

So using the IOTester utlity found with every installation of MediaLab, you can send a test signal and see if it's working.

The proper MediaLab command would be

(d5,<1:890:5000>)

For a five second low-to-high pulse on the strobe line.

For a sample MediaLab input file, see:

[URL]http://www.empirisoft.com/support/showthread.php?t=152[/URL]

[I][edited from support email][/I]
I have a question about the timestamp function of MediaLab. We are trying to synchronize eye tracking data with video data. In order to do this, I need to know exactly when MediaLab is making the stamp. It says it does so at the onset of the stimulus; however, I'm not sure if this means during the "purple screen", where it is opening the file, or when the video actually starts playing. The latter would be ideal.

Whether or not you using a MediaLab "timestamp" or a MediaLab TTL signal, the signal will be sent at the [I]onset[/I] of the trial (as opposed to when anything happens [I]during[/I] the trial like a video). In contrast, with DirectRT, a TTL signal can be sent in near perfect synch with the onset of any stimulus [I]within[/I] a trial.

UPDATE: The friendly folks at Applied Science Laboratories were kind enough to share some additional information on this topic.

The material I've posted above is correct with the following qualifications:

1. You cannot use a 25 pin cable from Radio Shack. Use the cable supplied by ASL.

2. ASL Record bit is parallel (LPT) port bit 7 (not bit 1). Recording will be ON as long as this bit is high, and be paused during any time that the bit is low. If you only turn on bit 7, this is decimal 128, hex 80, binary 1000 0000. It won't matter whether other bits are on or not, they can be used for other things if desired. Turning all the bits on (decimal 255, hex FF, binary 1111 1111), as described in the second web link, will certainly start "auto record", but only bit 7 is actually needed.

3. If desired, it is also possible to use parallel port bit 6 to open and close data files. If this option is selected, a file named with the current date and time will open when bit 6 goes high, and will remain open as long as bit 6 stays high. The file will close when bit 6 goes low. Remember that a file can start recording only if the file is opened. If the auto file open option is used turn on bit 6 to open a file; turn on bits 6 and 7 to start recording; turn of bit 7 but leave bit 6 on to stop recording without closing the file; turn off bit 6 to
close the file.

Hi,

I've another question: is for sure that directRT send TTL signal by LPT port assigned like this: D7 D6 D5 D4 D3 D2 D1 D0 (I suppose this are pins from 1 to 8)? Should I ask someone to test this with an osciloscope?

I've to connect PC's LPT output with a D-15 port from a powerlab recording equipment, so I have to build up a connector to do this and I don't want to be "responsible [for] any explosion which may occur".

thanks a lot

[QUOTE=natxo;12033]Hi,

I've another question: is for sure that directRT send TTL signal by LPT port assigned like this: D7 D6 D5 D4 D3 D2 D1 D0 (I suppose this are pins from 1 to 8)? Should I ask someone to test this with an osciloscope?

I've to connect PC's LPT output with a D-15 port from a powerlab recording equipment, so I have to build up a connector to do this and I don't want to be "responsible [for] any explosion which may occur".

thanks a lot[/QUOTE]

There's a pinout of the parallel port here:

[url]http://www.beyondlogic.org/spp/parallel.htm[/url]

An oscilloscope (or even just a voltmeter) is always a helpful tool to have.

What I understand is that pins 2 to 9 from the LPT1 connector are the ones which send data, I'm ok? Are this the pins which DirectRT uses to send information? Or this depends on which PC it is running?

I need to build a connector so I need to know which 8 pins I should select

thanks again

[quote=natxo;12197]What I understand is that pins 2 to 9 from the LPT1 connector are the ones which send data, I'm ok? Are this the pins which DirectRT uses to send information? Or this depends on which PC it is running?

I need to build a connector so I need to know which 8 pins I should select

thanks again[/quote]

Those 8 pins will be common to any PC containing a parallel port.

If you only need a single trigger line, I'd select 'D0' for convenience.

Then, by sending the value '1' or '0' to the PC's parallel port, you'll be able to control the state of that particular pin, either high or low.

How do you find out the port number to use to communicate with the eyetracker?

Thanks in advance!

Nevermind, found the answer.
Thanks!

Has anyone done this with eye tribe's eye tracker?
Thanks!