Working Memory

The rostral brainstem receives both “bottom-up” input from the ascending auditory system and “top-down” descending corticofugal connections. Speech information passing through the inferior colliculus of elderly listeners reflects the periodicity envelope of a speech syllable. This information arguably also reflects a composite of temporal-fine-structure (TFS) information from the higher frequency vowel harmonics of that repeated syllable. The amplitude of those higher frequency harmonics, bearing even higher frequency TFS information, correlates positively with the word recognition ability of elderly listeners under reverberatory conditions. Also relevant is that working memory capacity (WMC), which is subject to age-related decline, constrains the processing of sounds at the level of the brainstem. Turning to the effects of a visually presented sensory or memory load on auditory processes, there is a load-dependent reduction of that processing, as manifest in the auditory brainstem responses (ABR) evoked by to-be-ignored clicks. Wave V decreases in amplitude with increases in the visually presented memory load. A visually presented sensory load also produces a load-dependent reduction of a slightly different sort: The sensory load of visually presented information limits the disruptive effects of background sound upon working memory performance. A new early filter model is thus advanced whereby systems within the frontal lobe (affected by sensory or memory load) cholinergically influence top-down corticofugal connections. Those corticofugal connections constrain the processing of complex sounds such as speech at the level of the brainstem. Selective attention thereby limits the distracting effects of background sound entering the higher auditory system via the inferior colliculus. Processing TFS in the brainstem relates to perception of speech under adverse conditions. Attentional selectivity is crucial when the signal heard is degraded or masked: e.g., speech in noise, speech in reverberatory environments. The assumptions of a new early filter model are consistent with these findings: A subcortical early filter, with a predictive selectivity based on acoustical (linguistic) context and foreknowledge, is under cholinergic top-down control. A prefrontal capacity limitation constrains this top-down control as is guided by the cholinergic processing of contextual information in working memory.

"This Monograph aims to contribute to the integration of the information processing, the differential, and the developmental modelling of the mind into an integrated theory. In the sake of this aim, the Monograph presents a longitudinal study which investigated the relationships between processing efficiency, working memory and problem solving from the age of 8 to 16 years. This study involved 113 participants, about equally drawn among 8-, 10-, 12-, and 14-year olds at the first testing: these participants were tested for two more times spaced one year apart. These participants were tested individually with a large array of tasks addressed to processing efficiency (that is, speed of processing and inhibition), working memory (that is, in terms of Baddeley´s model, phonological storage, visual storage, and the central executive of working memory), and problem solving (that is, quantitative, spatial, and verbal reasoning).
Confirmatory factor analysis validated the presence of all of the above dimensions and indicated that they are organized in a three-stratum hierarchy. The first stratum included all of the individual dimensions mentioned above. These dimensions were organized, at the second stratum, in three constructs, namely processing efficiency, working memory, and problem solving. Finally, all second-order constructs were strongly related to a third-order general factor. This structure was stable in time.
Structural equation modelling indicated that the various dimensions were interrelated in a cascade fashion so that more fundamental dimensions proved to be part of more complex dimensions. That is, speed of processing was the most important aspect of processing efficiency and it fully determined the condition of inhibition, indicating that the more efficient one was in stimulus encoding and identification, the more efficient one was in inhibition. In turn, processing efficiency extensively influenced the condition of executive processes in working memory, which, in their turn influenced the condition of the two modality-specific stores (phonological and visual). Finally, problem solving was influenced by both processing efficiency and working memory, the central executive in particular.
All dimensions appeared to change systematically with time. Growth modelling suggested that there were significant individual differences in attainment in each of the three aspects of the mind investigated here. Moreover, development affected differently each of the three aspects of the mind as well as their interrelationships. Mixture growth modelling suggested that there were four types of developing persons, each being defined by a different combination of performance in the aspects of the mind. Some types were more efficient and stable developers than others. These analyses indicated that processing efficiency is a factor that explains developmental differences in problem solving whereas working memory explains individual differences. Modeling by logistic equations uncovered the rates and form of change in the various dimensions and their reciprocal interactions during development. These findings are discussed from the point of view of information processing, differential, and developmental models of thinking and an integrative model is proposed. 
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