IPL Glossary

The momentary intensity perceived by an observer.

method of adaptation
[psychophysical methods] An experimental paradigm which presents comparison stimuli which change slowly until they match the standard stimulus, then continue changing until the subject detects that there is a difference, and then change in the other direction until a difference is again detected. In this way, the point of subjective equality is crossed several times in each trial.

method of adjustment
[psychophysical methods] An experimental paradigm which allows the subject to make small adjustments to a comparison stimulus until it matches a standard stimulus.

A stimulus which can be interpreted in more than one way. This example can be seen as 1. a box in the corner of a room; 2. a solid object with a cubical hole; or 3. two solid objects joined at one edge.

ascending trial
[psychophysical methods] Enlarging a small comparison stimulus until it matches or exceeds the associated standard.

ballistic (movement)
A movement which is completely determined (excepting environmental influences) before it is begun, and cannot be changed while it is occurring.

comparison stimulus
[psychophysical methods] An experimental stimulus to be compared to a standard.

control condition
The comparison condition in a within-subjects design.
-A condition designed as a standard for comparison. All variables are set at reasonable values; the test condition(s) will attempt to vary only the independent variables whose effects are to be tested.

One type of light-sensitive cell (photoreceptor) located in the human eye. (The other type is rods.) Cones allow color vision; each cone responds most strongly to either red, green, or blue wavelengths. They are concentrated in the fovea of the human eye.

method of constant stimuli
[psychophysical methods] An experimental paradigm which presents, in random order, various stimuli to be compared to previously presented standard stimulus.

context effects
A familiar object or relationship can influence the perception of a novel stimulus object.

correct rejection
[signal detection theory] The observer correctly decides that the target signal was not present.

[signal detection theory]The minimum level of activation necessary for an observer to claim detection of a signal.

d' [d-prime]
[signal detection theory] A measure of the average intensity difference perceived by an observer between samples including a signal and samples not including a signal.

-Variables whose values are generated by an experiment, which are expected to show the effect of the experimental manipulation.

dependent variable
The variable measured and recorded by the experimenter.

depth cues
Effects such as interposition, size changes, vanishing points, and height in the plane used to make 2 dimensional images seem 3 dimensional.

descending trial
[psychophysical methods] Reducing a large comparison stimulus until it matches or exceeds the associated standard.

The expected result of an action weighted by the probability of its occurence.

false alarm
[signal detection theory] The observer incorrectly claims that the target signal was presented.

Information about an action gained by an organism during or after the performance of the action.

The part of the retina which contains the highest density of cones and therefore has the best visual acuity.

framing effect
An object which seems to fill a bounding outline looks larger than the same object within a larger outline.

[signal detection theory] The observer correctly claims that the target signal was presented.

Horizontal-Vertical Illusion More description
Run experiment
A vertical line appears longer than a horizontal line of the same length.

-A random event. -Variables in an experiment whose manipulation by the experimenter is expected to explain the variance in the results of the experiment (the dependent variables). All other variables which might affect the results are either held constant or counterbalanced (randomly, or by design) to cancel out their effects.

independent variable
The variable manipulated by the experimenter.

interaction effects
Experimental results that occur when the effects of one independent variable depend on the values of other independent variables.

interval scale
A numerical scale in which a difference of one unit always reflects the same amount of physical difference. (Others are nominal, ordinal, and ratio.)

just noticeable difference [jnd]
The minimum subjective difference required for a subject to claim that two stimuli are different on half the presentations.

method of limits
[psychophysical methods] An experimental paradigm which presents comparison stimuli in either increasing (ascending) or decreasing (descending) order until the subject indicates that the standard has been matched.

main effect
The effect of one independent variable independent of the values of another independent variable.

The arithmetic average of a group of numbers (v1+v2+.../no. of values).

The center value in an ordered list of numbers.

[signal detection theory] The observer incorrectly claims that the target signal was not presented.

The most common value in a group of numbers.

A feeling or state of mind that influences one's actions.

Müller-Lyer illusion
More description
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Lines segments of equal length seem different because of inward or outward pointed arrowheads on their ends.

[signal detection theory]Any stimulus, internal or external, other than the signal to which one wishes to attend.

nominal scale
A numerical scale in which the numbers are only distinguishing labels. (Others are ordinal, interval, and ratio.)

normal distribution
The expected distribution of most variables across a population, where the values cluster near the center and are sparser at the extremes. The mean, median, and mode of a normal distribution are the same; if the distribution is skewed, the mean and median will be shifted in the direction of the skew; the mean will be shifted farther.

ordinal scale
A numerical scale in which the numbers show relative ordering. (Others are nominal, interval, and ratio.)

Poggendorff illusion
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The collinear segments of a line whose center seems to be obscured by an object appear to be misaligned.

point of subjective equality
The point at which a subject, half of the time, will call a comparison stimulus greater than the standard.

Ponzo illusion
More description
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Identical lines appear to be different sizes due to depth cues in the image.

psychophysical Methods
More description
Standard procedures used to make experimental measurements of perception.

An event whose probability of occurrence is independent of surrounding events.

A numerical, interval scale in which the zero means absence of the property of interest, and the proportion of two scale values reflects the proportional difference in the property. (Others are nominal and ordinal.)

receiver operating characteristic curves (ROC curves)
[signal detection theory] A graphical presentation of the relationship between hits and false alarms for different signal probabilities and different levels of sensitivity.

One type of light-sensitive cell (photoreceptor) located in the human eye. (The other type are cones.) Rods respond to the overall intensity of the light falling on them, producing black-and-white vision. They are located outside of the fovea.

A very fast, ballistic movement of the eye. Because the human eye can focus clearly only a very small visual angle, the eyeball must make many movements to interpret the visual world. Perception of new information is suppressed during the movement, allowing the perception of a stable world.

[signal detection theory] A measure of how strongly an observer reacts to a signal, usually called d'.

shuffled randomly
Randomly chosen from a set, without replacement.

[signal detection theory] A target stimulus, usually recurring.

Signal Detection theory
More description
Run experiment
This theory attempts to quantify the reaction of an observer to the presentation of a signal in an environment containing noise. The parameters resulting from a series of presentations are sensitivity (d') and criterion (ß) of the observer.

skew distribution
One of the tails of the distributions contains more observations than the other. The skew is toward the longer tail, and the mean and, to a lesser degree, the median shift toward that tail.

standard stimulus
[psychophysical methods] An experimental stimulus which serves as a standard for comparison for other stimuli.

A correlation exists between the given variables; they are not independent.

visual fixation
An eye movement that causes the image of an object or location to focus at the fovea of the eye.

within-subjects design
An experimental design in which each subject is tested under more than one level of the independent variable.

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