Selectivity is a pervasive principle of most of the brain processes and mental processes related to these brain processes. Senses are tuned selectively to certain type of signals each (eg, visual, auditory, tactile, olfactory etc modalities). Cortical afferent areas include neural units specialized for certain types of stimuli features that can trigger responses of these neurons (slants of lines, color, movement, even more complex configurations of features, etc.). Based on input data provided by sensation systems, perception can organize this data in various ways to represent objects or events. (The most illustrious examples are ambiguous figures where an identical physical image providing invariant sense data can be perceived in alternative ways -- ie, selection of an interpretation varies with invariant sense data, such as happens with Necker cube, my-wife-and-my-mother-in-law image, duck/rabbit figure, etc.) Context also biases perception. For example, an invariant stimulus I3 can be perceived either as letter B in the context A I3 C, or as number thirteen in the context i2 I3 I4. Furthermore, attention helps to select between several perceptual objects either by way of preattentional processes working in parallel for all features (eg, leading to pop-out effects in filtering tasks) or by way of successive sampling of perceptual objects by focal attention (often in the target search by feature conjunction where some of the features of target objects are present among the background or distractor objects). Covert attention may help focus perception on certain spatial areas and overt attention (eg, by eye movements or head/body adjustments) can help maximize information intake from a designated spatial location. Bias effects make perception dependent on expectancies, habits, personal preferences and in some research even effects of values and beliefs on sensory aspects of perception have been purportedly found. (The older schools of thought where selectivity of perception has been studied: Ach, Külpe in Germany; Gestalt psychology in Germany and other European countires, New Look school in USA, theory of cognitive dissonance by Festinger, etc.) One of the most difficult methodological issues has been (and still continues to be) -- how to differentiate true selective changes of sensory sensitivity from bias effects taking place at the response selection level or stage of processing.
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