A neuronal model of a global workspace in effortful cognitive tasks

Abstract

A minimal hypothesis is proposed concerning the brain processes underlying effortful tasks. It distinguishes two main computational spaces: a unique global workspace composed of distributed and heavily interconnected neurons with long-range axons, and a set of specialized and modular perceptual, motor, memory, evaluative, and attentional processors. Workspace neurons are mobilized in effortful tasks for which the specialized processors do not suffice. They selectively mobilize or suppress, through descending connections, the contribution of specific processor neurons. In the course of task performance, workspace neurons become spontaneously coactivated, forming discrete though variable spatio-temporal patterns subject to modulation by vigilance signals and to selection by reward signals. A computer simulation of the Stroop task shows workspace activation to increase during acquisition of a novel task, effortful execution, and after errors. We outline predictions for spatio-temporal activation patterns during brain imaging, particularly about the contribution of dorsolateral prefrontal cortex and anterior cingulate to the workspace.

Figure 1

(Upper) Schematic representation of the five main types of processors connected to the global workspace (inspired from ref. 13). (Lower) Sample activation during effortful processing; a coherent link between two informationally encapsulated processors is established through the activation of distributed workspace neurons. The long-range workspace connectivity, supported by layer II/III neurons, is more prominent in Von Economo’s frontal-type cortex (left) than in sensory-type cortex (right) (14).

Figure 2

Architecture of the simulated network. (Insets) The proposed mechanisms for reward-dependent changes in workspace unit activity (Upper) and for the interaction of ascending and descending connections to a given area (Lower). Although each unit in the simulation presumably represents ≈100 neurons in an actual brain, our scheme epitomizes the basic organization of a cortical column, with intra-columnar recurrent excitation, intra-areal and mid-range excitatory connections providing excitation or inhibition (via intermediate processing inhibitory interneurons), and descending excitatory connections providing upward or downward modulation of activity (via intermediate gating inhibitory interneurons). The network is depicted in a state of activity typical of a correct trial in the effortful Stroop task. Attentional amplification reverses the relation between conflicting word and color inputs by amplifying the weaker color unit activity and suppressing the stronger word unit activity.

Figure 3

Temporal dynamics of the simulation in the course of learning the effortful Stroop task; 200 trials were simulated. The Stroop task was introduced without warning after trial 20. Note the selective activation of workspace units with a simultaneous amplification of color processors and a suppression of word processors. Workspace activity is seen in the initial phase of searching for the appropriate response rule (with considerably inter-trial variability), during the effortful execution of the task and following each erroneous response. For illustration purposes, putative brain-imaging correlates of routine and workspace activation are shown (see refs. and 25).