Two-alternative forced choice

Two-alternative forced choice (2AFC) is a method for measuring the subjective experience of a person or animal through their pattern of choices and response times. The subject is presented with two alternative options, only one of which contains the target stimulus, and is forced to choose which one was the correct option. Both options can be presented concurrently or sequentially in two intervals (also known as two-interval forced choice, 2IFC). The term 2AFC is often mistakenly used for describing the more common yes-no task, where a subject is presented with one option only and is forced to choose whether it belongs to one or another category. 2AFC is a method of psychophysics developed by Gustav Theodor Fechner.

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.

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