Sights and Sounds, part two: Sorting Through Stimuli

This post will focus primarily on visual stimuli and the method used to deliver them to the projector in our imaging facility. In an ideal world, you would be able to take everything I lay out here and translate it directly to your own facility. Alas no two snowflakes are alike. But by the end of this post you should have a better understanding of how a system like this is established. Afterwards I’ll use the same method to set up the auditory stimuli as well. The actual projector and its various components will be covered in a follow-up post.

As we explore visual stimuli I want you to keep our minimalistic design philosophy in mind. “Keep it simple, stupid” was pounded into my head by my engineering professors early on and I still find myself repeating it whenever a new problem arises.

Since I’m a visual learner, I’ll be using a series of charts to illustrate the process as I go through it. This also helps keep track of cables when it comes time to actually make all the physical connections. The coloring scheme used in the charts is simply to distinguish between the various components. In general I like to keep the inputs one color palette and outputs another.

Setting up the Video Switch

First begin by writing down every possible input you’re going to need. If you are just starting out this can be a difficult task. Without any prior experience I’d say it’s almost impossible! Luckily I have years of experience and user feedback at my disposal, so feel free to use my center’s setup as a template. I’m not saying it’s perfect but we do pride ourselves on providing as many options as possible, including room to add to the system later on if needed. The last thing we want to do is limit the research being done due to an inadequate facility.

Figure 1 - video switch inputs 

As you can see we have six distinct inputs:

• Stimulus PC - for working with Presentation and E-Prime
• Stimulus Mac – for working with Matlab and the Psychtoolbox add-on
• Eyetracking PC – for working with our Avotec eye tracker and Biopac physiological recording system
• Laptop – for those that wish to bring in their own computer
• EyeLink PC – for using our EyeLink eye tracking system
• Old Stim Mac – our old stimulus Mac. Temporarily left in place due to a lingering study. This will be removed once the study is completed

Now look at the input type for the different sources. I know what you’re thinking. How convenient that they are all the same, right? Absolutely! Keep it simple, stupid. Our projector only accepts VGA inputs so the decision of which signal type to use has already been made for us. Most computers come standard with a VGA port and there are several after-market adapters for Macs, so it’s an easy solution to go with even though it has limitations. (See Notes 1 and 2.)

Next up we add in the different outputs needed, which is again based on previous experience and requests made by our user base:

Figure 2 - video switch inputs and outputs

Our four outputs are:

•  Projector – Avotec silent vision model SV-6011 LCD projection system
• Display monitor – In the control room. Useful so that researchers can see the same image that is being projected to their subject inside the magnet
• Stimulus input monitor – In case an extra monitor is needed in the control room
• Eyetracking goggles – For tracking eye movement via goggles that mount on the head coil [not currently being utilized because we already have two separate eye tracking units that are better and easier to use]

And just like that--the hard part is done! What we need now is a video switch that can handle the job. We went with this switch by Kramer for our facility. For the sake of future proofing, a matrix switch with room to add more inputs and outputs later on is absolutely essential. If (more like WHEN!) more displays are required we’ll have the space and capacity to incorporate them into the existing setup without any disruptions.

Figure 3 - video switch

After purchasing and connecting the VGA cables, the final step is arguably the most important one. Color code every cable! I use colored electrical tape to distinguish between the various inputs and outputs (red for stim PC cables, yellow for stim Mac, etc.) and it makes my life incredibly easier when it comes time to trace back a cable. This also helps users distinguish between them and makes troubleshooting easier. Obviously it doesn't matter what colors or coding convention you use, just make sure to document it properly. I've attached a picture of the available laptop connections as an example.

Figure 4 - Laptop VGA, RCA and USB cables

Setting up the Audio Switch

Since it uses the same basic principle and we’re pretty much experts by now, I’m going to go ahead and quickly run through the auditory setup. Following the same procedure gives us this diagram:

Figure 5 - audio switch inputs and outputs

It has even less components than the video setup:

• Stimulus PC – audio for the stem PC
• Stimulus Mac – audio for the stem Mac
•  Laptop – audio for any computer our users wish to bring in
• Open connection – usually free, but currently being used by our old stem Mac
• Siemens Audio – communication system that comes standard with a 3T Trio
• Control speakers – a set of speakers setup in the control room. So researchers can hear what the subject inside the magnet is hearing
• 32 Ch. ear buds – a smaller profile audio system that is used with our 32 Ch. head coil. Expect a post on these in the future as well

The audio switch has less inputs simply because less are required and our users have not requested tapping in audio from other source yet. The audio switch we went with is this one by Burst Electronics. It has stereo RCA connectors in the back, so you’ll need to purchase a few adapters to get everything to play nicely.  Make sure to future proof just like with the video switch and buy more than what you currently need.

Figure 6 - audio switch

The Finished Product

Once the equipment is mounted on a rack and all cables have been connected, your setup might look a little something like this:

Figure 7 - equipment rank with video and audio switch

There are components pictured that I have not mentioned, but for now I will only point out what is specific to this post:

• Stimulus Mac - a Mac Mini encased in a rack mount
• Audio switch
• Video switch
• 32 Ch. ear bud system - three separate components mounted on top of each other
• Old stimulus Mac 
• Stimulus PC

Not exactly the prettiest, but you should get the idea of what it all looks like when it comes together. By all means, feel free to arrange your physical setup however is most convenient and appropriate for your space. The racks and desks pictured work just fine for our facility. FYI although it's not labeled on the photo, the projector is located on the bottom of the rack. Next is a picture of the stimulus Mac, stimulus PC and display monitors:

Figure 8 - Mac, PC and display monitor

And here’s what we end up with behind the scenes:

Figure 9 - video and audio switch cable webs with color coded tape

I considered not including the last picture since it tends to confuse and stress people out, but this will be unavoidable once you start connecting everything up. I know it’s not pretty, but there is a method to the madness! Just know you are going to have many cables running through your facility. At times it may seem overwhelming and no matter how you bundle them up and try to make them manageable they’ll still be a pain to go through. Just remember to color code everything, maintain a detailed diagram or list of the connections, and take plenty of reference photos. I’ve spent countless hours tracing connection issues, unplugging incorrect cables, and other nonsense that could have been  avoided if I had kept detailed documentation from the very beginning. Proper documentation can be the difference between a quick solution and wasting an entire day. Trust me. I learned this the hard way so that you don’t have to!

Notes:

1. Each computer must also output a video signal to a primary monitor inside the control room, otherwise it would be extremely difficult to set up the experiment. Imagine having to look inside the magnet at the projector screen each time. Any two-port VGA splitter will work, or even an entirely different type of signal (DVI, HDMI or DisplayPort) if that’s easier for you to work with. In my experience however, computers can get a bit finicky when outputting multiple signal types.
2. VGA signals are analog; where as the other types I mentioned are digital. This can lead to image degradation if the resolution is set too high. Our projector accepts signals with a maximum resolution of 1024 x 768 and a refresh rate of 60 Hz. We’ve had our share of headaches with these limitations, but thankfully nothing that prevented a study from continuing. I will be discussing some of this in greater detail in a future post centering around the projector and mirror systems used along with it.