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Selective attention is defined as the ability to focus intentionally on a thought, stimulus or action while disregarding other stimuli. The state of attention influences the way the sensory information is processed, stored in memory, and acted upon. Because information processing has limitations at certain points, it can bottleneck; this happens particularly when there are competing stimuli and distractions attempting to be processed at the same time (Gazzaniga et al., 2019). As a result, the brain must be selective in which input is passed along the perceptual processing pathway.

There are two primary theories for how the brain decides what information will make it through both the primary and higher level processing of sensory input. Broadbent’s model of early selection proposed that irrelevant input would be sorted and disregarded in the primary and secondary processing stages, after the sensory input is processed (Gazzaniga et al., 2019). For example, in audition, the cochlear nerve may receive input and pass it along to the cochlear nuclei in the medulla, but only the most pertinent information. The prioritization of stimuli in this model is believed to be dictated by a top-down mechanism where the perception has already been formulated about what kind of information will be useful or pertinent for processing (Gazzaniga et al., 2019). This might occur in situations where an individual is primed to listen for a specific sound, or is expecting to hear words in a conversation. The early selection model was modified later to allow for the possibility that some of the irrelevant input may make it through these informational bottlenecks, but that the resulting data would be degraded or weaker by the time it reaches the cortex for perceptual integration (Gazzaniga et al., 2019).

From the other perspective, late selection theories propose that all sensory input is processed equally at the early stages, and prepared for integration in the thalamus and various parts of the cortex (Gazzaniga et al., 2019). The filtering then happens at this stage, where only the relevant stimulus will be sent to the higher levels of processing for the formation of memories, awareness, and a cognitive or physical response (Gazzaniga et al., 2019). This late selection model implies that all sensory information enters the mind and receives prioritization based on a bottom-up sequence; the brain forms perceptions based on the available sensory input, rather than selecting sensory input to process based on an expected perception (Gazzaniga et al., 2019). For example, a person standing in a crowded room would theoretically hear every conversation, and receive visual input from everything in their field of vision. Yet at the level of conscious perception, they may only be aware of the person standing in front of them and register the conversation they are having. Evidence of the bottom-up model may be demonstrated by the fact that in this same scenario, a person might catch snippets of another conversation or remember hearing someone say their name. In this way, all sensory input was processed, yet they only remember selective things that the brain prioritized as relevant.

Ultimately, both models demonstrate the power of attention in dictating what environmental stimuli are processed. Selective and voluntary intention enhances the possibility that stimuli will be successfully processed, while unattended stimuli may register in a primary sensory level, but may not receive conscious attention and therefore may be ignored in a sense (Gazzaniga et al., 2019).