Basic stimuli presented in the "Visual Stimuli" section can be repeated and pasted across space and time to produce multi-element stimuli.

Psykinematix offers the possibility of creating multi-element stimuli that consist of either dynamic (RDK) or static (MEF) element fields:

Random-Dot Kinematogram (RDK)
Multi-Element Field (MEF)

To create a Multi-Element Stimulus (as illustrated in the example below), create a new group event in the "Experiment Designer" window, set its category to "Multi-Elements", and display its properties by clicking on the "Info" icon (or press Apple-I). Once a RDK or MEF stimulus has been selected, changing it by clicking on other tabs is disabled unless the Control key is pressed simultaneously. This is to prevent accidental changes because stimulus settings are then reset to default.

For each type of multi-element stimuli listed above, it is possible to create a list of different visual elements, hence producing heterogeneous multi-element stimuli commonly used in visual search, spatial integration, etc. The spatial spread of the elements is indicated by the preview in the bottom right-hand corner of the Area or Grid Properties.

Example:

Random-Dot Kinematogram

Random-dot kinematogram (RDK) is a special class of visual stimuli for investigating how local motion is integrated into global motion. Such a stimulus consists of a population of similar elements that move either coherently or randomly. These elements were originally only small dots, but Psykinematix is flexible enough to allow any type of visual stimulus to be used so that several types of elements can be combined to produce heterogeneous RDKs as illustrated below.


Limited lifetime RDK

Rotating and expanding RDKs

As illustrated in the panel above, the RDK properties are of two types, those describing the motion properties (on the left) and those describing the spatial properties of the motion area (on the right):

  • The motion properties consist of:

    • the motion type (uniform, radial, or angular),
    • the number of dots,
    • the coherence level in percent (i.e. proportion of dots that move coherently),
    • the speed of each individual dot in deg/s,
    • the motion direction of coherent dots,
    • the lifetime of each element expressed either in seconds or number of frames (use 'inf', 'infinity' or '∞' for infinite lifetime). Note that the temporal phase for limited lifetime elements is chosen randomly at onset time,

    Note that the motion direction is described differently from the motion type:
    • For uniform motion, the direction is an angle expressed in degrees (0 deg for left to right motion, 90 deg for bottom to top motion);
    • For radial motion, the direction is selected through a pop-up menu as either inward (periphery to center) or outward (center to periphery),
    • For angular motion, the selectable directions are clockwise or anticlockwise.

    Note that when using a mathematical expression or a variable to specify the direction for radial or angular motion, a negative direction indicates an inward or anticlockwise motion, and a positive direction indicates an outward or clockwise motion. Both speed and direction can also be specified for individual dots by using an expression between { } symbols: for example, the expression '{0:180}' for the direction indicates that the direction for individual dots is randomly and uniformly chosen between 0 and 180 degrees. Without the { } symbols, the expression indicates that the same random direction applies to all dots.

  • The area properties specify the spatial field on which the moving dots are drawn, and consist of:

    • the surface shape (square or disk),
    • its size (side for square, diameter for disk),
    • its position in the visual field (expressed either in Cartesian or polar coordinates).

    The drawing area at the bottom depicts the shape and location of the area used by each type of dot (filled area for currently selected types, outline for unselected types). Control-clicking inside this area displays a contextual pop-up menu that provides an OpenGL preview that can be optionally exported to a Quicktime movie.

  • The duration of the motion presentation is specified above the area properties either in seconds or by the number of frames.

Note that the element types cannot be added, removed, or reordered directly from this panel. These actions should be performed through the "Experiment Designer" window. However, the motion or area properties can be assigned to a group of entries by selecting them from the list and changing their properties.

RDK stimuli can appear as a node inside these categories:

Multi-Element Field

Multi-element fields (MEFs) are static or dynamic versions of RDKs where the position of each element is constrained to a polar or Cartesian grid. This type of stimulus category is useful for investigating visual searches, pop-out effects, etc. The grid can be populated with several types of non-overlapping elements as illustrated by the following examples:


Oblique target embedded
in a field of vertical distracters

High contrast target embedded
in a field of low contrast distracters

As illustrated in the pane above, the MEF properties are of two types, those describing the spatial properties of the element sets (on the left) and those describing the spatial properties of the grid (on the right):

  • The spatial properties of each element set consist of:

    • the type of spatial distribution (random by default),
    • the contribution of the specified set expressed either as the percentage or number of elements (note that the total number of elements can be fewer than the grid permits thus leaving some "holes" in the grid),
    • the spatial and orientation jitters (in degree) to be applied to each element of the set,
    • the lifetime of each element expressed either in seconds or number of frames (use 'inf', 'infinity' or '∞' for infinite lifetime). Note that the temporal phase for limited lifetime elements is chosen randomly at onset time and that the presentation of dynamic elements is also always asynchronous (ie the starting point of the dynamic sequence is randomly chosen),
    • the lifetime mode ('On-Off' or 'Field Update') that specified what happens once the life-time of each element has expired: in the 'On-Off' mode, the elements disappear for the same life-time duration before reappearing; in the 'Field Update', the elements have their position and orientation reset based on the specified position and orientation jitters (but at the same grid position!),
    • the rotation (in deg) to be applied onto each element of the set. When put between { }, the rotation expression is evaluated on an element basis: for example, the expression '{90*round(rnd)}' would specify a rotation of 0 or 90 deg on a random basis. This rotation can even be specified as function of the element position relative to the grid center: in such expression, variables x, y, r, and theta between { } indicate the Cartesian and polar coordinates of each element before the position and orientation jitters are applied. For example, to create concentrically-aligned Gabor elements, set the element rotation to '{theta}' if the element is defined as a vertical Gabor.

  • The grid properties specify:

    • the geometry (Cartesian or polar),
    • the spatial field in degree,
    • the number of samples (i.e. maximum number of elements) on the grid,
    • its position in the visual field (expressed either in Cartesian or polar coordinates). Mathematical expressions can be used to specify the Cartesian or polar coordinates.

    The drawing area at the bottom depicts the shape and location of the grid. Control-clicking inside this area displays a contextual pop-up menu that provides an OpenGL preview that can be optionally exported to a Quicktime movie.

  • The duration of the field presentation is specified above the grid properties either in seconds or by the number of frames.

Note that the element types cannot be added, removed, or reordered directly from this panel. These actions should be performed through the "Experiment Designer" window. However, the distribution properties inside the grid can be assigned to a group of entries by selecting them from the list, and changing their properties.

Finally, MEF stimuli may embed dynamic elements as illustrated below:


Field of drifting Gabors

Oscillating Elements

MEF stimuli can appear as a node inside these categories: