Local-global processing theory
Explanation
Stanislas Dehaene and collaborators have experimentally characterised the distinction between local and global processing as key to understanding what makes a representation conscious. Local processing occurs in specific brain regions and is typically rapid, automatic and unconscious. Global processing involves the ignition of broad and far-reaching networks, associated with consciousness.
The typical methodology uses stimuli at the threshold of consciousness. A stimulus presented briefly and masked may or may not become consciously perceived. By comparing brain activity between seen (conscious) and unseen (non-conscious) trials with identical physical stimulation, the specific correlates of consciousness are identified.
The results are consistent. Non-conscious stimuli activate relevant sensory areas, but processing remains local and early. Conscious stimuli additionally activate a broad network (prefrontal, parietal, temporal) with late propagation (from ~250-300 ms post-stimulus). This difference is robust across sensory modalities and experimental paradigms.
The attentional blink paradigm illustrates this distinction. When two target stimuli are presented in rapid succession, the second is often not consciously perceived (it blinks out). Brain studies show that during the blink, the second stimulus activates local sensory areas but fails to achieve global ignition. When global processing is restored, the conscious percept reappears.
This local/global distinction underlies the global neuronal workspace (GNW) theory of Baars-Dehaene-Changeux. Consciousness arises when information is broadcast across the global network, becoming available to multiple systems (memory, language, motor control, evaluation). Without global propagation, it remains unconscious even though it can influence behaviour indirectly (subliminal priming).
Clinical implications are important. In vegetative and minimally conscious states, local processing may be preserved but global processing is broken. Techniques such as transcranial magnetic stimulation with EEG can measure the brain's capacity for global propagation and estimate levels of consciousness, even in patients unable to communicate. This is becoming an important clinical tool.
Strengths
- Empirically distinguishes access and phenomenality.
- Specific neural mechanism (local recurrence).
- Coherent with IIT and with evidence of the posterior hot zone.
- Productively criticises frontal centrality.
Main critiques
- Difficult to study experience without some form of report.
- The P-A distinction is contested.
- Mixed evidence.
- Specific mechanism not entirely clear.