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Two-Alternative Forced Choice Task (2AFC)
Two-alternative forced choice (2AFC) (and the variant Two-interval forced choice (2IFC)) is a psychophysical method for eliciting responses from a person about his or her experiences of a stimulus. Specifically, the 2AFC experimental design is commonly used to test speed and accuracy of choices between two alternatives given an timed interval. It has been documented in behavioral studies since 1898. The task is an established controlled measure of choice and is widely used to test a range of choice behaviors in animals and in humans. The basic components of a 2AFC task are 1) two alternative choices presented simultaneously (e.g two visual stimuli), 2) a delay interval to allow a response/choice, 3) a response indicating choice of one of the stimuli.

Behavioral Experiments with 2AFC
There are various manipulations in the design of the task, engineered to test specific behavioral dynamics of choice. In one well known experiment to test attention, 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 association 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 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 choose between two directions and the choice response is indicated by a saccade to the direction of perceived coherent motion.

Biases in decision making
[Here insert summary about cue validity, effects : Speed accuracy tradeoff and statistical patterns of experience(short term exposure vs long intervals).]

Computational models of decision making in 2AFC
The 2AFC task has yielded consistent behavioral results on decision making, which lead to the development of formal models attempting to model the dynamics of decision making . There are typically three assumptions made by computational models using the 2AFC:"i) evidence favoring each alternative is integrated over time; ii) the process is subject to random fluctuations; and iii) the decision is made when sufficient evidence has accumulated favoring one alternative over the other."

Drift-Diffusion Model (DDM)
The DDM is a model that is well defined (Smith, 2000).

Brain Areas
Parietal lobe Lateral Intraparietal (LIP) neuron firing rate in monkeys predicted the choice response of direction of motion suggesting this area is involved in decision making in the 2AFC.

Models
The evidence from neural and behavioral data supports models such as the DDM (from Bogcs page 2... e.g., Hanes & Schall, 1996; Shadlen & Newsome, 1996, 2001; Schall, 1998, 2001; Gold & Shadlen, 2002; Ratcliff et al., 2003b). The DDM implements the most optimal strategy for decision making in the 2AFC (Laming, 1968).