Two-alternative forced choice (2AFC) is a method for measuring the sensitivity of a person or animal to some particular sensory input, stimulus, through that observer's pattern of choices and response times to two versions of the sensory input. For example, to determine a person's sensitivity to dim light, the observer would be presented with a series of trials in which a dim light was randomly either in the top or bottom of the display. After each trial, the observer responds "top" or "bottom". The observer is not allowed to say "I do not know", or "I am not sure", or "I did not see anything". In that sense the observer's choice is forced between the two alternatives.
Both options can be presented concurrently (as in the above example) or sequentially in two intervals (also known as two-interval forced choice, 2IFC). For example, to determine sensitivity to a dim light in a two-interval forced choice procedure, an observer could be presented with series of trials comprising two sub-trials (intervals) in which the dim light is presented randomly in the first or the second interval. After each trial, the observer responds only "first" or "second".
The term 2AFC is often mistakenly used to describe a yes–no task, in which an observer is presented with a series of trials in which a stimulus is randomly presented in some trials and not in others. The observer responds after each trial only with "yes" or "no". The results of a yes-no task are much more likely to be affected by various response biases than 2AFC tasks. For example, with extremely dim lights, a person might respond, completely truthfully, "no" (i.e., "I did not see any light") on every trial, whereas the results of a 2AFC task will show the person can reliably determine the location (top or bottom) of the same, extremely dim light.
2AFC is a method of psychophysics developed by Gustav Theodor Fechner.
The situation is complicated by the fact that 2AFC tasks are also widely used for discrimination tasks. Instead of detecting a stimulus, the observer is instructed to discriminate between two alternative stimuli. This can be interpreted as a signal-detection problem (detecting the difference between the two stimuli). The 2AFC procedure, therefore, also allows estimating unbiased objective discrimination performance, such as the smallest difference between the stimuli that the observer can distinguish. However, such discrimination tasks are often also used to make inferences on what is known as the point of subjective equality, the stimulus level at which observers are guessing between the two options at 50%. This is taken as the stimulus difference where the two stimuli appear the same, e.g., in contextual effects like the tilt illusion. Caution is advised in this scenario because unlike measuring the objective discrimination threshold, judging the appearance of a stimulus is can be subject to response biases, just like a yes–no task, even though the procedure is designed as a 2AFC task.
Behavioural experiments
There are various manipulations in the design of the task, engineered to test specific behavioral dynamics of choice. In one well known experiment of attention that examines the attentional shift, the Posner Cueing Task uses a 2AFC design to present two stimuli representing two given locations. In this design there is an arrow that cues which stimulus (location) to attend to. The person then has to make a response between the two stimuli (locations) when prompted. In animals, the 2AFC task has been used to test reinforcement probability learning, for example such as choices in pigeons after reinforcement of trials. A 2AFC task has also been designed to test decision making and the interaction of reward and probability learning in monkeys. Monkeys were trained to look at a center stimulus and were then presented with two salient stimuli side by side. A response can then be made in the form of a saccade to the left or to the right stimulus. A juice reward is then administered after each response. The amount of juice reward is then varied to modulate choice.
In a different application, the 2AFC is designed to test discrimination of motion perception. The random dot motion coherence task, introduces a random dot kinetogram, with a percentage of net coherent motion distributed across the random dots. The percentage of dots moving together in a given direction determines the coherence of motion towards the direction. In most experiments, the participant must make a choice response between two directions of motion (e.g. up or down), usually indicated by a motor response such as a saccade or pressing a button.