Psykinematix

I would like to have a left/right motion discrimination task with contrast varied according to a staircase method. This is just like the contrast sensitivity lesson 1 but with with motion discrimination rather than H/V discrimination. I am having trouble with the decision variables in the columns "correlated with expression" and "value". I have been trying to use a phase expression such as 0:360(360) but get an error that this is not a supported decision statement.
Any suggestions would be greatly appreciated.

The expression '0:360(360)' does not specify a value for a variable (it should be only used to specify the time-varying properties of a stimulus parameter). You should instead define an independent variable (named for example 'dir') with 2 random values (using the format '-1,1' where -1 would specify a left motion and 1 a right motion), then use 'dir' as the decision variable in the procedure panel ("correlated with expression" column) and -1 and 1 as the decision values ("Value" column) for the left and right choice inputs. If your stimulus is a vertical Gabor, to specify its motion simply set its phase to '0:360(360)' and its orientation to '90*[dir:1]' : it will drift to the right when the 'dir' takes the value '1' and to the left when it takes the value '-1'.

Have a look also at the following demo in the 'Storage' area:

Demos, Examples & Tutorials 1.2 / Experiments / RDK / Contrast Threshold / Staircase

It is using RDK instead of a Gabor but the experiment design is very similar to yours.

Hope this helps!

-- The Psykinematix Support Team

In addition to this effect due to the vertical part of the rotational effect, there is a horizontal effect, which is usually explained by describing the motion of a pendulum.
If you set a pendulum to swinging, the only forces normally acting on it are the force of gravity, which is downwards, and the force of the string supporting the pendulum bob, which is up and to the side. These two forces define a vertical plane, and if no other forces act on the pendulum bob the pendulum should swing back and forth in this plane without veering to either side of the plane.