Scientific Understanding of Consciousness
Consciousness as an Emergent Property of Thalamocortical Activity

Cortical Connectivity and Sensory Coding


Nature 503, 51–58 (07 November 2013)

Cortical connectivity and sensory coding

UCL Institute of Neurology, Queen Square, London WC1N 3BG, UK

Kenneth D. Harris

UCL Department of Neuroscience, Physiology and Pharmacology, 21 University Street, London WC1E 6DE, UK

Kenneth D. Harris & Thomas D. Mrsic-Flogel

Biozentrum, University of Basel, Klingelbergstrasse 50/70, CH - 4056 Basel, Switzerland

Thomas D. Mrsic-Flogel


The sensory cortex contains a wide array of neuronal types, which are connected together into complex but partially stereotyped circuits. Sensory stimuli trigger cascades of electrical activity through these circuits, causing specific features of sensory scenes to be encoded in the firing patterns of cortical populations. Recent research is beginning to reveal how the connectivity of individual neurons relates to the sensory features they encode, how differences in the connectivity patterns of different cortical cell classes enable them to encode information using different strategies, and how feedback connections from higher-order cortex allow sensory information to be integrated with behavioural context.

Our senses are constantly bombarded with a vast amount of information. To guide behaviour, the brain must extract and amplify a relatively small number of features from this massive input; for example, features corresponding to the nature and location of objects in the world. In mammals, the cerebral cortex has a central role in this process. The cortex contains multiple hierarchically arranged areas devoted to each modality. Neuronal populations in these areas extract features of sensory scenes and integrate them with non-sensory cognitive and behavioural variables, in a progressively abstract manner. Each sensory stimulus causes a complex pattern of activity in the neuronal populations of multiple cortical areas. The relationship between sensory stimuli, and the firing patterns they evoke, defines the ‘neural code’ of the corresponding populations.

Each cortical area contains a richly interconnected array of diverse cell types, whose patterns of connectivity underlie the cortex’s ability to extract sensory features. The circuits of different cortical regions and species share striking commonalities in their constituent cell types, their intrinsic properties, and the incidence and properties of synaptic connections between them. Connections between cortex and other brain structures, and connections between cortical regions, also show a largely preserved relationship to cortical cell classes. These similarities suggest that there may be a set of general principles linking the common characteristics of cortical circuitry to the nature of cortical processing in multiple areas.


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