![]() ![]() Functional activity along these two types of connections is referred to as feedforward and feedback, respectively. Throughout the remainder of this paper, we refer to ascending and descending connections when discussing the anatomy of biological systems or the architecture of artificial ones ( Friston, 2005 Clark, in press). We then outline how predictive coding offers a unique perspective for re-defining what is meant by these terms. In this review, we argue that predictive coding provides a powerful conceptual framework that goes beyond the standard dichotomy of “bottom-up” and “top-down.” We first provide an overview of previous attempts at defining bottom-up and top-down processes, and we highlight their problems and limitations. In accordance with terminology commonly used in research on perception, it has thus been suggested that predictive signaling reflects top-down processes, whereas prediction-error signaling constitutes bottom-up processing ( Friston, 2009 Alink et al., 2010 Hesselmann et al., 2010). Evidence for this assumption has been collected at different levels of neural processing ( Rao and Ballard, 1999 Hosoya et al., 2005 Muckli et al., 2005 Summerfield et al., 2006 Alink et al., 2010), which suggests that predictive-coding operates across a wide range of spatial and temporal scales.Ī second important assumption in predictive coding is that predictions are transferred from hierarchically higher levels of processing to lower ones, whereas signals traveling in the opposite direction encode prediction errors ( Rao and Ballard, 1999 Serences, 2008 Friston, 2009 Grossberg, 2009). A central assumption in predictive-coding theories is that activity in the nervous system reflects a process of matching internally generated predictions to external stimulation ( Heekeren et al., 2008 Bar, 2009). ![]()
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