A VR2 schedule means that roughly half the trials will be rewarded (two trials for every one that is rewarded). This would give an equal number of observations for both hand signals. As long as we didn’t simply alternate, which would make it completely predictable for the dogs, then they should stay highly motivated.

That evening, I tried VR2 on Callie.

As usual, the rustling of the hot dog bag called her to the kitchen.

“Wanna do some training?” I said in my high-pitched doggie voice.

Callie cocked her head and tore off into the living room. When I got there, she was already in the tube with her head in the chin rest. To warm up, we went through several trials as usual. Left hand up, hold it for ten seconds, and then reward. When she seemed settled in, I flipped the two-hand signal that had previously meant peas. This time, after ten seconds, instead of giving her a pea, I just touched her forehead. She thought a pea was coming and tried to lick my hand. With nothing there, she looked puzzled.

I pointed to the chin rest and said, “Touch.”

Callie quickly placed her head down. To make sure she wasn’t confused, I immediately showed her the reward hand signal, and rather than wait ten seconds, rewarded her right away. The next trial, I gave the two-handed, no-reward signal and quickly ended the trial with a touch on the head. We repeated this for about five minutes, and amazingly, she didn’t get bored or leave the simulator. Instead, her posture and attentiveness improved. Her head positioning became more consistent, and her eyes were fixed in attention on my hands. Now when I showed the reward signal, I could see her pupils dilate, indicating a high level of positive arousal. And she remained motionless.

VR2 was a success! If Callie could catch on so quickly, I was sure McKenzie would too. And with her pupils dilating, it was clear that Callie now cared about the hand signals.

If this didn’t work, nothing would. We were ready.

18

Through a Dog’s Eyes

WE DIDN’T HAVE MUCH TIME to get Callie and McKenzie trained on the new version of the task. We could have taken longer with the training, but the logistics of finding a day when Mark, Melissa, Rebeccah, and the scanner were all available dictated the schedule, and the next available time that everyone could meet again was only two weeks away. If we missed the window in two weeks, we would have to wait another month to book a large chunk of time at the scanner. The pressure was on.

At least we knew the dogs could do this. Each time we had gone to the scanner, we had accomplished more than I had expected, and I was counting on this next time to be no different. The dogs knew what they had to do. The real uncertainty was how much data we would be able to collect and whether it would be enough to demonstrate caudate activity.

The fMRI signal is very weak. We measure activity as the relative change in signal intensity from some baseline level. But even in the best of circumstances the signal intensity rises by less than 1 percent. To make matters worse, the fMRI signal is noisy. The noise, which comes from heart rate variability, breathing, and even the electronics of the scanner, causes fluctuations in the signal that are ten times as much as the thing we are looking for. The signal-to-noise ratio (SNR) of fMRI is therefore quite low. Fortunately, the noise is random. If we collected enough repetitions during the experiment, we could average the fMRI signals from each, and the effects of noise would be diminished.

Often, when doing an experiment for the first time, you don’t know how big the signal is, so you have to make an educated guess at how many repetitions will be required to detect it. The Dog Project was on the verge of moving beyond a cute dog trick and into the realm of legitimate science. But to make this jump, we would first need to figure out how many repetitions would be required.

Andrew and I took a close look at what we had collected on the first scan day. Even though we had failed to find any differences between peas and hot dogs, there was still useful information in the data. We could estimate the SNR of the dog brain and, from that, determine how many repetitions Callie and McKenzie would have to do at the next scan.

Andrew zoomed in on the caudate of McKenzie’s brain. He pulled up a graph of the level of activity in the caudate for each scan that we had acquired. The first few scans had no signal because McKenzie hadn’t placed her head in the head coil until about the twentieth scan. But then it looked like noise. It was hard to tell how much of the noise was because of the usual sources or her moving slightly during the scan. The size of the fluctuations measured about 15 percent of the overall signal. This was much higher than in human studies.

I let out a sigh and said, “We would need a thousand repetitions to get the SNR up to a reasonable level.” Neither dog nor human would sit still for that long. “That has to be from movement.”

“It is,” Andrew said. “Check this out.” Andrew scrolled through the sequence of McKenzie’s images. This had the effect of creating a movie of her brain. It compressed the five-minute scan session into thirty seconds. Even though we had captured only half of her brain, the movie made clear that although McKenzie had been in the head coil for the whole session, she was still moving. Not much. But just enough to cause artifacts. Callie’s brain movie looked similar.

In fairness to the dogs, they had done what we asked of them. The degree of movement we were talking about was a matter of millimeters. During the stress of the scan session, neither Melissa nor I noticed it. Not that we could have done much about it on the fly.

“Well,” I said, “we have two weeks to train them not to move.”

Andrew looked skeptical.

The dogs would have to move less than two millimeters, but they would have to hold still only during the period from putting their head on the chin rest through the duration of the hand signal. After they got their hot dog, they could take their time swallowing and getting resettled in the chin rest. We needed enough time only for the fMRI signal to reach a peak and begin to decay—roughly ten to fifteen seconds. If the dogs remained motionless for that length of time, we calculated that twenty repetitions might be enough to get the SNR up to a usable level. That still left the structural scan, which we hadn’t been able to obtain on either Callie or McKenzie. That scan would require the dogs to stay motionless for thirty seconds.

Thirty became the magic number. During training, we would have to gradually lengthen the time between the hand signal and the reward until the dogs could hold absolutely still for half a minute. If they could do that, we would be able to get the structural scan and plenty of functional repetitions to boot.

Callie didn’t seem to mind the change in training procedures. At first, I felt a twinge of guilt every time I put up the signal for “no hot dog.” Callie would stare at me impassively from her position in the mock head coil. I’m in the head coil, why no hot dog? Sometimes I would touch her lightly on the top of her head, indicating that I wanted her to try again. But this soon became superfluous.

It seemed cruel to withhold rewards, but I trusted Mark’s advice and stuck to the VR2 training schedule.

Mark was right. After switching to variable reinforcement, Callie really started paying attention. She had no choice. With peas and hot dogs, she got rewarded every repetition, so there was no need to pay attention to what I was doing. Now she noticed every little movement. If my shoulder twitched, Callie’s eyes darted to the side. It was so fast that had we not been staring directly at each other I would never have noticed.