Donald Broadbent's filter theory (1958), presented in his book Perception and Communication, was the first comprehensive information-processing model of attention and one of the founding documents of cognitive psychology. The theory proposed that the brain processes information through a series of stages, with a selective filter early in the sequence that passes only attended information for full analysis while blocking unattended input.
The Model
Broadbent's model specified several processing stages: a sensory buffer that briefly holds all incoming information, a selective filter that passes only one channel based on physical characteristics (location, pitch, loudness), a limited-capacity processing channel that performs semantic analysis, and a response system. The filter was necessary because the central processing channel could only handle one stream of information at a time.
Filter selects based on physical characteristics (ear, pitch, location)
Unselected information is blocked before semantic analysis
Supporting Evidence
Broadbent's theory was motivated by Cherry's dichotic listening findings showing that participants could report little about the content of the unattended ear, and by his own split-span experiments. In the split-span task, three digits were presented simultaneously to each ear (e.g., "4, 9, 3" to the left ear and "6, 2, 7" to the right ear), and participants preferred to report by ear (4, 9, 3 then 6, 2, 7) rather than by temporal pair (4, 6 then 9, 2 then 3, 7). This suggested that the filter grouped information by physical channel (ear) rather than by temporal organization.
Challenges and Modifications
The filter theory was challenged by findings showing that unattended information sometimes does get processed semantically. Moray's (1959) finding that one's name on the unattended channel is sometimes detected, and Treisman's (1960) finding that participants sometimes followed meaningful content as it switched ears, both suggested that the filter was not completely impervious. These findings led to Treisman's attenuation model and later to late-selection models.
Regardless of whether its specific mechanism is correct, Broadbent's filter theory was enormously influential. It established the information-processing framework that dominated cognitive psychology for decades, introduced the idea of studying attention as a selection mechanism operating within a multi-stage processing system, and demonstrated the value of building explicit, testable models of cognitive processes. It transformed the study of attention from philosophical speculation into experimental science.
Modern Perspective
The early vs. late selection debate that Broadbent's theory initiated has largely been resolved by recognizing that the locus of selection is flexible. Lavie's perceptual load theory proposes that early selection occurs when perceptual demands are high (consuming all available capacity) and late selection occurs when perceptual demands are low (leaving spare capacity that processes distractors). This framework preserves the core insight of Broadbent's theory — that capacity is limited and selection is necessary — while accommodating the evidence for flexible selection.