These systems achieved frame-by-frame temporal precision by operating within a cooperative multitasking operating system such as Mac OS (prior to Mac OS X) and running at interrupt time. ) or were limited to simple stimuli and required extensive programming and debugging in low-level C (e.g., John Maunsell’s custom LabLib). Early systems developed in the 1990s required no specialized hardware but could only draw pre-rendered stimuli and movies (e.g., early releases of the PsychToolbox: Brainard, 1997
#STIMULUS DURATION FRAMEN PSYCHOPY GENERATOR#
The main limitations of the Picasso are essential to its design as a specialized function generator – namely that it is tied to a specific (and now rare) display device, and that the range of stimuli it could produce were limited.Ĭomputers provide the ability to produce arbitrary visual stimuli, but with a new set of limitations. Furthermore, by providing BNC connections for voltage inputs, time-varying stimuli could be driven via analog outputs from the same data acquisition system being used to record responses, simplifying experimental design. Many scientists found the Picasso wonderfully easy to use, as its intuitive interface with a myriad of switches and potentiometers allowed rapid experimentation until a suitable stimulus was found. However, instead of using it as an oscilloscope display, vision scientists often controlled the 608 with an Innisfree Picasso device, a specialized function generator that creates a raster scan of X,Y positions and modulates luminance to produce a variety of simple stimuli such as sinusoidal gratings and rectangles. As a result, on most cards, as long as frames are not being ‘dropped’ (see Detecting dropped frames) you can present stimuli for a fixed, reproducible period.In the 1980s and 1990s, vision scientists frequently displayed their stimuli on a Tec-tronix 608 display, a small (∼12 cm diagonal) cathode ray tube with independent X,Y and luminance inputs originally intended for use in a high-bandwidth analog oscilloscope. Calls to Window.flip() will be synchronised to the frame refresh the script will not continue until the flip has occurred. The frame rate is extremely precise, much better than ms-precision. At 60Hz refresh you can not present your stimulus for, say, 120ms the frame period would limit you to a period of 116.7ms (7 frames) or 133.3ms (8 frames).Īs a result, the most precise way to control stimulus timing is to present them for a specified number of frames. It also might also give the false impression that a stimulus can be presented for any given period. An error of 16.7ms might be acceptable to long-duration stimuli, but not to a brief presentation. So using this method you get timing accurate to the nearest frame period but with little consistent precision. Alternatively, if the time has reached 2.001s, there will not be an extra frame drawn. If the screen is refreshing at 60Hz (16.7ms per frame) and the getTime() call reports that the time has reached 1.999s, then the stimulus will draw again for a frame, in accordance with the while loop statement and will ultimately be displayed for 2.0167s. In the above, the stimulus does not actually get drawn for exactly 0.5s (500ms). flip ()Ĭlocks are accurate to around 1ms (better on some platforms), but using them to time stimuli is not very accurate because it fails to account for the fact that one frame on your monitor has a fixed frame rate. phase += 0.1 # Increment by 10th of cycle win. getTime () < 2.0 : # Clock times are in seconds if 0.5 <= clock. autoLog = False # Or we'll get many messages about phase change # Let's draw a stimulus for 2s, drifting for middle 0.5s clock = core. autoDraw = True # Automatically draw every frame gabor. GratingStim ( win, tex = 'sin', mask = 'gauss', sf = 5, name = 'gabor' ) gabor. From psychopy import visual, core # Setup stimulus win = visual.
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