![]() This type of system operates the way reflexes are assumed to operate in which neural responses are transmitted and presumably transformed but in a fixed and immutable way (outside the context of longer term reshaping of responses). This is the fundamental assumption of a number of theories of auditory processing in which a fixed cascade of neural population responses are transmitted from one part of the brain to the other (e.g., Barlow, 1961). Figure 1A illustrates a passive process in which a pattern of inputs (e.g., basilar membrane responses) is transmitted directly over the eighth nerve to the next population of neurons (e.g., in the auditory brainstem) and upward to cortex. A passive process is an open loop sequence of transformations that are fixed, such that there is an invariant mapping from input to output ( MacKay, 1951, 1956). The distinction between active and passive processes comes from control theory and reflects the degree to which a sequence of operations, in this case neural population responses, is contingent on processing outcomes (see Nusbaum and Schwab, 1986). In principle, active systems can generate hypotheses to be tested as new information arrives or is derived ( Nusbaum and Schwab, 1986) and thus provide substantial cognitive flexibility to respond to novel situations and demands. By comparison, active cognitive systems however have a control structure that permits “information contingent processing” or the ability to change the sequence or nature of processing in the context of new information or uncertainty. However it is important to note that cognitive automatization does not have strong implications for the nature of the mediating control system such that various different mechanisms have been proposed to account for automatic processing (e.g., Logan, 1988). Automatized cognitive systems ( Shiffrin and Schneider, 1977) behave as though passive, in that stimuli are mandatorily mapped onto responses without demand on cognitive resources. A passive process is one in which inputs map directly to outputs with no hypothesis testing or information-contingent operations. Active cognitive processing is contrasted with passive processing in terms of the control processes that organize the nature and sequence of cognitive operations ( Nusbaum and Schwab, 1986). In order to achieve flexibility and generativity, spoken language understanding depends on active cognitive processing ( Nusbaum and Schwab, 1986 Nusbaum and Magnuson, 1997). ![]() ![]() In doing so, this may provide new insights into ways in which hearing disorders and loss may be treated either through augementation or therapy. Theories of speech perception need to go beyond the current corticocentric approach in order to account for the intrinsic dynamics of the auditory encoding of speech. It is important to understand how the ambiguity of the speech signal and constraints of context dynamically determine cognitive resources recruited during perception including focused attention, learning, and working memory. Recent research has suggested that speech perception is the product of both feedforward and feedback interactions between a number of brain regions that include descending projections perhaps as far downstream as the cochlea. Although theories of speech perception have begun to incorporate some active processing, they seldom treat early speech encoding as plastic and attentionally guided. Active processing assumes that attention, plasticity, and listening goals are important in considering how listeners cope with adverse circumstances that impair hearing by masking noise in the environment or hearing loss. Note that this does not mean consciously guided but that information-contingent changes in early auditory encoding can occur as a function of context and experience. An alternative view is that speech recognition, even in early stages, is an active process in which speech analysis is attentionally guided. This kind of pattern matching can be termed a passive process which implies rigidity of processing with few demands on cognitive processing. This process can be taken as a statistical pattern-matching problem, assuming realtively stable linguistic categories are characterized by neural representations related to auditory properties of speech that can be compared to speech input. One view of speech perception is that acoustic signals are transformed into representations for pattern matching to determine linguistic structure. Department of Psychology, The University of Chicago, Chicago, IL, USA.
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