Assignment: Creating an Adaptation Experiment
In this assignment, your task is to write an OpenSesame program to run an adaptation experiment. The purpose of the experiment is to demonstrate the existence of orientation-selective mechanisms in human vision, using a systems-analytic psychophysical approach. The experiment is also illustrative of many different types of experiments in psychology, cognitive science, and other sciences, involving the use of controls, and the presentation and interpretation of data.
Background videos on orientation-selective cells:
The experiment will demonstrate that using very simple equipment, provided some care is exercised, input/output measurements can be obtained that allow considerable insight into the functional operations of the human brain – in this instance the visual pathway.
Briefly, in the experimental condition, observers will adapt (by continuous staring) for 90 seconds to a grating (black and white bars) that is oriented at an angle of 12 degrees rotated anti-clockwise, or left of vertical (anti-clockwise is defined as negative, thus –12 deg). After adaptation observers will be required to measure the angle of grating that now looks vertical. If staring at an oriented grating affects later perception of orientation of other gratings, we are led to the conclusion that mechanisms in the visual system that are selective for orientation have been affected. The post-adaptation effect is known as the “tilt after-effect”. The psychophysical method to be used is called “method of constant stimuli”.
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(1) General instructions.
An observer will perform both the control and experimental conditions. The observer is to sit 40 cm from the display monitor and cover one eye (i.e., the baseline and experimental conditions are monocular conditions using the preferred – usually the right – eye in both adaptation and post-adaptation phases), the OpenSesame program is to present the stimuli to the observer at the centre of a computer screen, and to record the observer’s responses.
A set of 9 experimental stimuli is to be created by the program. Each stimulus is a grating; precisely, alternating black and white bars within a circular aperture or envelope against a full screen of uniform grey background. The circular aperture should measure approximately 10 cm in diameter in the full-screen mode (Note: the aperture does NOT need to be smoothed). Within the circular aperture, 4 black bars and 3 white bars should be visible (See example grating
HP4061_Assg_Grating.tiff on NTUlearn. IMPORTANT: Use OpenSesame’s own “draw gabor element” function to accurately generate the grating as described). The 9 stimuli are oriented at –4, –3, –2, –1, 0, +1, +2, +3, +4 degrees from vertical (– indicates left or anti-clockwise of vertical, + indicates right or clockwise of vertical), accordingly.
The adapting stimulus is created in the same way as an experimental stimulus, except that it is oriented at –12 degrees (12 degrees left of vertical).
(3) Baseline or control condition.
The OpenSesame program is to present the set of 9 stimuli 20 times to the observer (total: 180 trials). The observer immediately on presentation is to decide if the stimulus appears to lean left or right of vertical by pressing on the keyboard either Z or M respectively. If it looks like it is neither, a choice must be made on the basis of which way it seems to lean (forced choice). The observer should attempt to avoid bias in his/her response (i.e., always responding to the one side whenever the stimulus appears exactly vertical). Considerable thought is NOT required – the observer merely needs to look at the stimulus and make a rapid decision (within 3 seconds). The program should put the stimulus up for no more than 3 seconds, and take it down as soon as the observer responds. If the observer has not yet responded after 3 seconds, the program prompts the observer to make a guess (Z or M), with the stimulus already taken down. Only after the observer responds, the program moves on to the next trial. Between each trial, there should be a 0.5-second pause, when a uniform grey screen is presented. Each stimulus must be centred on the computer screen (No fixation cross is needed). The full set of 9 stimuli is to be presented in random order at least 20 times each. After each presentation of the set of 9 stimuli the program must re-order the stimuli for the next presentation of the 9 stimuli. No feedback should be provided to the observer on his/her performance. For each trial, the program should record the actual orientation of the stimulus (in degrees) and the observer’s response (Z or M).
For ease of plotting the psychometric function, the observer’s response is to be converted into a score of rightward response (0 for “left” or Z, 1 for “right” or M). The psychometric function is a function of the total score of rightward responses (y-axis) against orientation of experimental stimulus (x-axis). The total score of 0 means all left responses, 20 means all right responses.
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(4) Experimental condition.
The procedure above is essentially to be repeated (also 180 trials). However, prior to starting, the adapting stimulus (–12 deg) is presented at the centre of the screen. The observer is required to look at the adapting stimulus for 90 seconds and then the adapting stimulus is replaced by an experimental stimulus and a response is recorded. Between the adapting stimulus and the experimental stimulus, there should be a 0.5-second pause, when a uniform grey screen is presented. The observer should move his/her eyes around the grating while adapting. Between trials, the adapting stimulus is replaced so that the observer
can “top up” for 5 seconds. After each set of 9 stimuli have been presented, the observer should adapt for a further 15 seconds. In order to clarify the role of the stimuli, the word “Adapt” can be shown on the upper left corner of the screen when the adapting stimulus is presented. Likewise, the word “Test” is shown when the experimental stimulus is presented. Make sure these words are large enough but do not obstruct the view of the stimulus itself.
In other words, each trial starts with an adapting stimulus, followed by an experimental stimulus and response, and then a 0.5-s pause before next trial. You can consult the following presentation sequence (0.5-s pause should be inserted for each step):
A (90 s)a`S (≤ 3 s)a`A (5 s)a`S (≤ 3 s)a`…
[after 9 experimental stimuli] a`A (15 s)a`S (≤ 3 s)a`A (5 s)a`S (≤ 3 s)a`…
A: adapting stimulus, S: experimental stimulus
Find a partner now (MUST BE 2 students per group; you CANNOT have this partner again in your later projects). Both students should have equal contributions to the project. One student per group may act as the observer, who runs your OpenSesame experiment EIGHT times. On each run, you will get two psychometric functions, one for the control condition and the other for the experimental condition. Estimate the threshold for each condition (e.g., simply interpolating the data points, or fitting the data points by a curve if you prefer. After 8 runs, you will get 8 thresholds per condition, which you can use for statistical analysis using SPSS.
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