Saccade Gain Adaptation

(left:me right:Jordan)

I'm a graduate student. I study Neuroscience The City College of New York as a student of the CUNY Graduate Center. I work in the Biology Department of CCNY in the lab of Josh Wallman. I study a process known as Saccade Gain Adaptation.

Saccades (as I've described before in this very blog) are rapid point to point displacements of gaze. Unless you have a target which is moving that you can follow with your gaze, you make saccades. This is unique to eye movements. That is to say that there are no such constraints on arm or leg or any other kind of movements. If you want to move your arm from one location to the other, there are a tremendous number of paths to follow and speeds to employ. With eye movements, however, you have little to no control over the path taken or the speed of the action. If you've never heard of this, it's useful to try and trace a line (like a corner where walls meet) slowly with your eyes. You'll rapidly see that this is impossible, the best you can do is make small steps along the line. This is in contrast to say, holding out your hand and moving it across your field of vision while fixating on one of your finger-tips. In this case, you can make a smooth pursuit movement.

Saccade gain adaptation is a process through which the size of eye movements elicited by an abrupt change in position of a target is either increased or reduced. Below is a little flash movie that I made illustrating this procedure.

Why might the eye want to behave in this way? Let us suppose that over time the muscles in your eye become weak as a result of aging. It makes sense that the commands sent to your eye muscles must also change in order to accurately move your eyes to desired targets in the world. Josh (and I) think that this is not quite the whole story, but accept for the moment that it is possible and sensible for the brain to be able to change its saccadic gain.

Experimentally, we can induce gain adaptation in the following way: we start with "no step back" trials (runs) in which the subject fixate a small target. At some unpredictable (to the subject) time the target abruptly changes position, and the subject's instructions are simply to follow the target. After some of these "baseline" trials, we move on to the "step back" trials. In this phase, the target steps to a new position, but when the subject moves their eye to the new target position, we move the target back slightly. Interestingly, if the step is small, the subject will not even notice the second target movement, they will simply follow it with their eyes. After many hundreds of trials ("Step Back (late)"), instead of making two saccades to reach the final position of the target (after its two steps), the subject will simply make one saccade to the final target location.

(Click on the black play button to see what a single trial looks like, and the trial type buttons to change the trial type)

In the above animation, there is a graph representing the type of data that we gather. We are only interested in the horizontal (or x) position of the gaze. All of the stimuli we're using in the experiment are presented on a monitor. We use a computer to control the presentation of the stimuli, and we use a camera and a calibration procedure to record the direction of gaze from a subjects right eye. In the graph, the vertical axis represents horizontal displacement from some arbitrary zero point (I've purposefully ignored details such as this). So when the trace representing the target jumps up, that means it has moved to some new position to the right of where it formerly was. The same is true of the trace for the gaze position. When a trace steps down, it means the correspding thing (target or gze) has moved to the left. The set of gray dots merely represents the passage of time. I'm not sure if this description is complete enough, but hit the "play" button a bunch of times and puzzle over it if you're still confused and it'll make sense, or email me and I'll explain ad nauseum.

A final note that there is a rather large (~%10 of the size of the eye movement) error in most saccades, I've simply idealized the graphics to simplify the presentation.