Rethinking the episodic-semantic distinction from a gradient perspective

In almost 50 years since Tulving's seminal episodic/semantic distinction, advances in cognitive neuroscience have fostered a dramatic change in our understanding of declarative memory. Functional neuroimaging evidence reveals considerable overlap in the neural substrates of episodic and semantic memory systems, while studies in clinical populations underscore the blurred boundaries between them. Leveraging new insights on 'gradients' of macro-scale and region-specific cortical organisation, we propose that instead of localisations based on categorical cognitive constructs, gradual transitions in neural engagement offer a more parsimonious account of declarative memory. Positioned at the apex of this macro-scale organisation, the default mode network may provide a common neural substrate upon which both semantic and episodic memory representations are anchored.