Sequences and loops
The loop and sequence items are two special items that add structure to your experiment. Understanding how loops and sequences work is one of the trickier aspects of working with OpenSesame.
Sequence items
A sequence is a list of items that is executed sequentially. For every item in a sequence there is also a ‘Run if’ condition, which specifies the conditions under which an item should be executed (by default this is ‘always’). A sequence does not repeat automatically: For this, you will need to combine it with a loop.
A typical situation where a sequence is used is as a trial: A single trial will often correspond to a single sequence item. This illustrated in the screenshot below.
Figure 1. A single trial often corresponds to a single sequence item.
In this example trial, there is a trial_sequence, which consists of a fixation dot (a sketchpad item), a target display (another sketchpad), a response collection item (a keyboard_response), a green fixation dot (another sketchpad), a red fixation dot (another sketchpad), and a data logging item (a logger).
Most items are called ‘always’. However, the green and red fixation dots have specific ‘Run if’ conditions. The green fixation dot is only called when the variable correct has the value 1. The red fixation dot is only called when the variable correct has the value 0. Effectively, this means that the color of the fixation dot provides the participant with immediate feedback on every trial: Green means correct, red means incorrect.
The variable “correct” is set automatically by the keyboard_response item. For more information about variables and conditional statements, see:
loop items
Loop items repeatedly call a single other item, the ‘item to run’. Loops are also used to control independent variables, so that every time that the item-to-run is called, the independent variables have different values. A loop only calls a single other item. To call multiple items, will need to combine a loop with a sequence.
A typical situation where a loop is used is to form a block of trials. In that case, the item-to-run is a trial sequence, which is called multiple times. This is illustrated in the screenshot below.
Figure 2. A loop item provides a table in which you can define your independent variables.
In this example loop, three independent variables have been defined: object, orientation, and correct_response. There are eight different cycles, or combinations (‘knife, left, z’, ‘knife, right, z’, etc.). Because ‘repeat’ is set to 3, every combination is called three times. Therefore, the item trial_sequence is called 8 x 3 = 24 times. The ‘order’ is set to ‘random’, which means that a random cycle is selected (without replacement) for every call of the item trial_sequence.
Combining loops and sequences
Loops and sequences are often combined to create a structure in which multiple items are repeated. As we’ve seen, a typical example of a loop-sequence structure is a single block of trials. Here a single trial is a sequence, which is called repeatedly by a loop to form a block of trials, as shown in the screenshots below.
Figure 3. A block of trials often corresponds to a loop item, which in turn calls a sequence item that corresponds to a single trial.
One level up in the hierarchy of the experiment, there is another loop-sequence structure, which corresponds to multiple blocks of trials. Here, a sequence (the block_sequence in the figure) calls a single block of trials (the block_loop), followed by a feedback item. This sequence is repeatedly called by a loop (the experimental_loop), so that there are multiple blocks of trials, each followed by feedback.
Figure 4. You can use nested loop-sequence structures to implement trials, blocks of trials, blocks of blocks, etc.
The structure displayed in the screenshot above might look a bit confusing at first sight, but it becomes clearer when you think about it as a two nested loop-sequence structures. The first one (block_loop - trial_sequence) corresponds to a single block of trials. The second one (experimental_loop - block_sequence) corresponds to multiple blocks of trials, each followed by feedback to the participant. Many experiments will contain a structure of this kind.
