Vision Psychophysics

Getting Started

1. Install the BonVision package from the Bonsai Community feed in the package manager.
2. Go through the basic stimuli tutorial at the BonVision website.

Make sure the latest version of the BonVision package is installed for this worksheet

Orientation Discrimination

In this worksheet you will build the skeleton of an orientation discrimination vision psychophysics task. In this variant of the task we will present two test gratings in quick succession at different random orientations, and ask the participant to report which of the gratings had the more clockwise orientation. Orientations for each grating will be drawn from a random uniform distribution, and feedback of whether the response was correct or incorrect will be provided visually.

The following set of exercises are to be developed in a single workflow, so do not remove the elements from the previous exercise from subsequent exercises, unless it is specifically mentioned.

Exercise 1: Random Orientation Grating

To allow sharing screen calibration for all displayed task elements, we start by defining a common BonVision render pipeline.

• Insert a CreateWindow source and set the ClearColor property to Gray.
• Insert the BonVisionResources and LoadResources operators to preload all built-in BonVision shaders.
• Insert the WorkflowOutput operator after LoadResources to ensure the workflow terminates when the shader window is closed.
• Insert a RenderFrame source. This source will emit a notification when it is time for a new frame to be drawn on the screen.
• Insert a NormalizedView operator. This will specify that our stimulus dimensions are resolution independent, aspect ratio corrected, and normalized to the range [-1,1].
• Insert a PublishSubject operator and set its Name property to Draw. We will use these events whenever we need to draw any element on the screen.

The first step in developing our task will be to display a grating in the center of the screen at a random orientation for a specified period of time, and store the value of the orientation, so we can use it later to test the participant.

• Insert a CreateRandom source.
• Insert a CreateContinuousUniform and set its Lower and Upper properties to -1 and 1, respectively.
• Insert an AsyncSubject and set its name property to AngleDistribution.

For now, we start by displaying a repeating sequence of random orientation gratings.

• Insert a Timer (Shaders) source and set its DueTime property to 2 seconds.
• Insert a SelectMany operator and set its name to ReferenceGrating.
• Insert a Repeat operator.

Note: The Timer (Shaders) source works exactly like the default Timer (Reactive) source, but it counts the time by using the screen refresh time, rather than the operating system time. This can be important for precise timing of screen stimuli, as it avoid clock drift and jitter when synchronizing multiple visual elements, and should be in general preferred when specifying the various intervals used to control elements in the BonVision or Shaders packages.

To implement the ReferenceGrating state, we will need to sample a random angle from the angle distribution, use it to initialize the angle property of the gratings, and present the gratings for a specified period of time. At the end, we need to send out as a result the value of the random orientation which was generated.

ReferenceGrating:

• Use the Sample (Numerics) operator to sample a random orientation value from the AngleDistribution subject and store it in a new AsyncSubject named Angle. This will allow us to reuse the sampled value when drawing the gratings later.
• Subscribe to the Draw subject we defined previously and insert a DrawGratings operator.
• Externalize the Angle property from the DrawGratings node and connect the Angle subject we created to it.
• Insert a Timer operator and set its DueTime property to 1 second.
• Insert a TakeUntil operator and connect the DrawGratings node as the source, and the Timer as the trigger.
• Insert a Last operator. This will ensure we will get a notification whenever the Timer stops the presentation of the stimulus.
• Insert a Sample operator following the Angle declaration, and connect the Last operator as a trigger. This will store the sampled angle value until it is time to return.
• Insert a WorkflowOutput operator to specify the final output of the state.

Run the workflow and verify whether the behaviour of the system is correct. Are different orientation values being used for each subsequent presentation of the gratings?

Exercise 2: Reusing stimulus definitions

The second step in defining the contrast discrimination task is to display a second randomly oriented grating in each trial, with a small blank (or masking) period in between. To do this, we want to avoid repeating the entire workflow we designed for our reference grating, so we will make use of the IncludeWorkflow operator to reuse our stimulus presentation logic.

ReferenceGrating:

• Inside the ReferenceGrating state, select all nodes before WorkflowOutput, right-click the selection, and choose the Save as Workflow option. Choose RandomOrientationGrating as the name for the extension.

After we have our new reusable operator, we can extend the workflow to include the blank period and the second grating stimulus.

• Insert a SelectMany operator after the ReferenceGrating state and set its Name property to Blank.
• Insert another SelectMany operator after Blank with the name TestGrating.
• Insert a Repeat operator.

For the Blank state we will use a simple gap interval where nothing is drawn on the screen. We can do this easily by delaying the transmission of the result of the previous state, before we move on to the next state.

Blank:

• Insert a Delay (Shaders) operator between the input and the output of the state workflow.

Note: Similar to Timer (Shaders), the Delay (Shaders) operator works exactly like the Delay (Reactive) operator, but using the screen refresh clock instead of the operating system clock. This also ensures that any delayed notifications are resynchronized with the render loop, in case they were emitted from other external devices.

To implement the TestGrating state, we want to reuse our previous RandomOrientationGrating extension workflow and simply combine the random generated angle with the angle from the reference grating.

TestGrating:

• Insert a new RandomOrientationGrating operator from the toolbox and combine it with the input by using the Zip combinator. This will generate a pair where the first value is the random angle from the first reference grating, and the second value is the random angle for this test grating.

Run the workflow and validate the random angle pairs are distinct and valid from trial to trial.

Exercise 3: Collect test response and compute trial outcome

Now that we have our two randomly generated gratings, we need to gather the response from the participant and compare it with the actual situation to determine whether the trial was successful.

• Insert a new Response state after the TestGrating state using the SelectMany operator.

To implement the response gathering state we will use key presses from the participant. We will use the left and right arrow keys to indicate which stimulus had the most clockwise orientation and compare the response with whether or not the first stimulus was more clockwise than the second stimulus.

Response:

• Connect the Draw subject from the toolbox to a new DrawText operator and set its Text property to a suggestive question (e.g. A or B?). Also edit the Font property and make sure the size is at least 72pt for readability.
• Insert a DelaySubscription (Shaders) operator and set its DueTime property to 1 second.

Note: As before, the difference with DelaySubscription (Reactive) is that DelaySubscription (Shaders) will use the screen refresh time and make sure that all effects of subscription are synchronized with the render loop.

• Insert a LessThan operator after the input source node. This will compare the value of the randomly sampled angles for the first and second gratings, respectively, and will return true if the first grating is more clockwise than the second grating (i.e. its angle in radians is smaller than the second grating).
• Insert a KeyDown (Shaders) source and set its Key property to Left.
• Insert a KeyDown (Shaders) source and set its Key property to Right.
• Insert a Boolean operator after each of the key press sources and set the Value property to True for the operator following the left key press.
• Combine the results of both key presses with a Merge operator.
• Insert a First operator since we are only interested in the first response from the participant.
• Combine the comparison from LessThan with the response from the participant using the Zip combinator.
• Insert an Equal operator to check whether or not the response matches the true angle comparison. This will be the result of the Response state and after it is reported, all other effects of the state will be determined (i.e. the question display).

Exercise 4: Present trial outcome feedback to participants

The only step left for finishing our experimental prototype is to report the feedback of each trial back to the participants. We will do this by drawing a colored square, indicating green for a correct response, and red for an incorrect response.

• Insert a new Feedback state after the Response state using the SelectMany operator.

This final state will simply display a quad for a certain period of time, where the color will be modulated by the trial outcome value. We want to store this value until the end of the trial so we can report it for subsequent processing.

Feedback:

• Insert an AsyncSubject operator and set its Name property to Result. This will store the trial outcome result so it can be used to compute the color value of the quad.
• Subscribe to the Draw subject and insert a DrawQuad operator.
• Externalize the ColorR, ColorG, and ColorB properties from the DrawQuad node.
• Subscribe to the Result subject and create a new ExpressionTransform operator.

In the Expression property of the ExpressionTransform operator, create a structure holding the RGB color value using the following script:

it ? new(0 as R, 1 as G, 0 as B) : new(1 as R, 0 as G, 0 as B)

• Connect the ExpressionTransform to the externalized properties.
• Insert a Timer operator and set its DueTime property to 1 second.
• Insert a TakeUntil operator and connect the DrawQuad node as the source, and the Timer as the trigger.
• Insert a Last operator. This will ensure we will get a notification whenever the Timer stops the feedback presentation.
• Insert a Sample operator following the Result declaration, and connect the Last operator as a trigger. This will store the trial outcome value until it is time to return.
• Connect it to the WorkflowOutput operator to specify the final output of the state and trial.

Run the workflow and verify the visual feedback indeed matches the perceived results from each trial.

Exercise 5 (Optional): Measure psychometric data

What is the minimal discrimination threshold for humans in this task? How would you extend the previous workflow in order to assess this?