A typical scene contains many different objects that compete for neural representation owing to the limited processing capacity of the visual system. The natural visual scenes are cluttered, and they contain many different objects that cannot all be processed at the same time. At the neural level, the competition among multiple stimuli is evidenced by the mutual suppression of their visually evoked responses and it occurs most strongly at the level of the receptive field. The competition among multiple objects can be biased by both bottom-up sensory driven mechanisms and top-down influences, such as selective attention. Functional brain imaging studies reveal that biasing signals because of selective attention can modulate neural activity in visual cortex not only in the presence but also in the absence of visual stimulation.
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