Date of Degree
PhD (Doctor of Philosophy)
John P. Spencer
Many tasks rely on our ability to hold information about a stimulus in mind after it is no longer visible and to compare this information with incoming perceptual information. This ability relies on a short-term form of memory known as visual working memory. Research and theory at the behavioral and neural levels has begun to provide important insights into the basic properties of the neuro-cognitive systems underlying this form of memory. However, to date, no neurally-plausible theory has been proposed that addresses both the storage of information in working memory and the comparison process in a single framework. To address these limitations, I have developed a new model where working memory is realized via peaks of activation in dynamic neural fields, and comparison emerges as a result of interactions among the model's layers.
In a series of simulations, I show how the model can be used to capture each of the components underlying performance in simple visual comparison tasks--from the encoding, consolidation, and maintenance of information in working memory, to comparison and updating in response to changed inputs. Importantly, the proposed model demonstrates how these elementary perceptual and cognitive functions emerge from the coordinated activity of an integrated, dynamic neural system.
The model also makes novel predictions that were tested in a series of behavioral experiments. Specifically, when similar items are stored, shared lateral inhibition produces a sharpening of the peaks of activation associated with each item in memory. In the context of the model, this leads to the prediction that change detection will be enhanced for similar versus dissimilar features. This prediction was confirmed in a series of change detection experiments exploring memory for both color and orientation.
In addition to sharpening, shared lateral inhibition among similar items produces mutual repulsion between nearby peaks. This leads to the prediction that when similar features are held, they will be systematically biased away from each other over delays. This prediction was confirmed in a cued color recall experiment comparing memory for a "far" color with memory for two "close" colors.
Copyright 2008 Jeffrey S Johnson
Johnson, Jeffrey S.. "A dynamic neural field model of visual working memory and change detection." PhD diss., University of Iowa, 2008.