Fluctuations in expectation may be described by a model that actually calculates expectation using a weighted combination of new and old information. According to this model, when the probability of a target's appearance changes abruptly, a smooth change occurs that encodes prior probability. This model even predicts small changes in expectation even when there is a constant probability of appearance of the target (Anderson & Carpenter, 2006). The experience-based techniques employed that are used to predict probability of the appearance of a stimulus requires that certain pieces of information are held in storage over several trials, which requires an additional number of neurons for the process. Anderson & Carpenter (2006) explain how "the main virtue of (their) model is its simplicity and ease with which its exponential decay in the effect of stimulus history can be implemented by biologically plausible means."

(2006). Changes in expectation consequent on experience, modeled by a simple, forgetful neural circuit. Journal of Vision, 6(8), 822-35.
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Macknik, S. (2006). Chapter 11 - Visual masking approaches to visual awareness. Progress in Brain Research, 155, 177-215.

Min-Shik, K., Cave, K. (1999). Grouping effects on spatial attention in visual search. Journal of General Psychology, 126, 326-52.

Muller, M., Anderson, N., Trujillo, N., Valdes-Sosa, P., Malinowski, P., Hillyard, S. (2006). Feature-selective attention enhances color signals in early visual areas of the human brain. Proceedings of the National…