Biological Sciences 330/331, Smith College | Neurophysiology
Animation of flicker stimuli
Based on Figure 2 from D. C. Kiper, K. R. Gegenfurtner and J. A. Movshon (1996) Cortical oscillatory responses do not affect visual segmentation. Vision Research 36: 539-544.
The paper describes stimuli that flicker to force synchronous firing of cortical cells. The stimulus image consists of a background field of short lines at similar angles. Within this is a rectangular area of lines at different angles. The observer's task is to identify the direction of the rectangle (vertical or horizontal) as the angles of the lines in the rectangular area are made more similar to the angles of the background lines. In the published example (Fig. 2, left), the angles of the lines in the rectangle and the background differ enough so that it is easy to perceive the vertical rectangle.
The image is presented under three flicker conditions:
- the entire image appears in one frame, preceded and followed by blank frames.
- the background is in one frame, the rectangular region is in the next frame, and a third blank frame follows.
- the three frames contain randomly chosen elements from both the rectangle and the background.
Select various stimuli to see for yourself:
(to see the sequence of frames)
1. Entire image
Significance: A controversial theory proposes that neurons in V1 that respond to individual features (the lines) of the same object (the rectangle) fire synchronously to "bind" the features of the object together. This theory predicts that stimulus three (which mixes together random elements from the rectangle and the background) should diminish perception of the rectangle, since neurons responding to the background and the rectangle will be forced to fire together by the flicker. The paper's data show that perception of the rectangle under all three flicker conditions is similar.
Details of the animation: Each frame is scheduled for 10 msec in a looping animated GIF derived from Figure 2. Computer monitors will present the frames at irregular rates, but the average flicker rate of the complete stimulus (three frames) will be under 30 Hz. This is less than the flicker rate in the actual experiment. A very slowly flickering version of each stimulus was also created to show the individual frames.
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