Fig. 2.

Two- and three-layer dynamic neural field models of visual working memory (VWM). The thin, solid horizontal line in each field marks the activation threshold (conventionally set to be 0), the point at which interactions among neurons within and between layers become engaged. The two-layer model (a) consists of a single population of feature-selective excitatory neurons coupled to a similarly tuned population of inhibitory neurons. This simulation depicts the formation of a peak of activation following localized input to the excitatory layer. Input takes the form of a Gaussian distribution that is centered at a particular field location and has a specified strength and width. Once activation goes above threshold (i.e., 0) in the excitatory layer, activation is passed to the inhibitory layer, which, in turn, passes broad inhibition back to the excitatory layer. Locally excitatory interactions among neurons in the excitatory layer (solid, curved arrow) keep neurons in a highly active state, whereas inhibitory feedback from the inhibitory layer keeps excitation localized by preventing the diffusion of activation throughout the field. The three-layer model (b) contains two populations of excitatory neurons (perceptual and VWM fields) reciprocally coupled to a single population of inhibitory neurons (inhibitory field). Input is applied to both excitatory fields, but input to the perceptual field is much stronger than input to the VWM field. Once activation in the perceptual field goes above 0, strong activation is propagated to both the inhibitory and the VWM fields. The VWM field also projects excitatory activation to the inhibitory field, which projects inhibition to both the perceptual and the VWM fields. The model also contains a response layer consisting of two nodes: one that receives summed excitatory input from the perceptual field and is responsible for generating “different” (“Diff”) responses, and a second that receives summed excitatory input from VWM and is responsible for generating “same” responses. The nodes in the response layer have self-excitatory connections and are mutually inhibitory. Note that only above-threshold activation (i.e., activation > 0) in the perceptual field or VWM is propagated to the response nodes at test.