April 13, 1995
Binding in Episodic Memory
Professor Metcalfe's presentation was concerned with the concept of binding in memory. Binding is a term used by cognitive psychologists to describe the process of organizing lower order elements into higher order representations. Evidence from other studies has suggested that binding occurs in one type of memory but not in another. These two systems of the memory are referred to as explicit memory and implicit memory. Explicit memory is thought to involve conscious, controlled recollective processes, whereas implicit memory is thought to involve unconscious, automatic processes.
Professor Metcalfe presented research on face recognition in normal subjects. She compared her empirical results with computer simulations of human memory. Earlier research suggested that human subjects have trouble with faces which are mixtures of previously studied individual features such as eyes, nose, hair, and mouth. Subjects tend to recognize these types of faces, referred to as conjoined faces, as ones they had studied before when in fact they had not seen them at all. Apparently, changing the individual features of faces causes subjects to confuse these faces with previously studied faces which have not had their individual features changed. One reason may be that they invoke the higher order representations of faces which were studied previously.
What would happen if a face mimicked the higher order representation of a face even better than a conjoined face? This effect was produced by superimposing any two faces that subjects had studied previously to produce what Professor Metcalfe referred to as a superimposed composite face. Professor Metcalfe's research indicated that these faces, like conjoined faces, were easily mistaken for previously studies faces.
What happens when the two types of face-mixing , i.e. conjoined and composite faces, are compared? Which type of face invokes the higher order representation of a stored face in memory better? It would make the most sense for the composite faces to be most easily mistaken for unaltered faces studied before because, as discussed, they mimicked the higher order representation of a face the best. Professor Metcalfe's research indicated that this was in fact true: subjects confuse the superimposed faces more frequently with previously studied faces than they do with conjoined faces and previously studied faces.
What processes underlie this process of binding the eyes, nose, mouth, and hair into higher order representations? Computational models of memory provide very specific equations for how memory works and therefore can possibly reveal some of the processes that underlie this ability. These models have proved successful in the past in accounting for a large variety of data on human memory and learning. After inputting the faces into the four different computational models, it was found that the models that best fit the data were models that had the ability to convolve, or autoassociate, features into a larger whole. The models that did not have this additional feature were unable to provide the results that were obtained with human subjects reported earlier here.
Another feature which proved essential in producing results similar to the results obtained with the human subjects was the capacity to store individual features all together in a single storage box, which computational memory designers refer to as a memory vector . Those models that lacked this ability and designated that items be stored individualy could not provide the data Professor Metcalfe obtained. The models that fit the data obtained from research on human subjects best had the dual capacity to convolve, or autoassociate, individual features from objects together and also had the ability to store these combined features in a single storage box.
Earlier findings have suggested that binding is a process that occurs in explicit memory. This is the area from which Professor Metcalfe's research took its lead. By studying the effect of randomly mixing features of a face such as the nose, eyes, mouth and hair and comparing this with subject's recognition of faces that were superimposed on each other, it was found that binding is a process that involves the formation of higher order representations of faces such that the closer a face is to this higher order representation, the more easily it would be mistaken for this representation. Professor Metcalfe's research also revealed, through the use of computational models of memory, that binding is a process that involves a combination of these features in a concrete manner and in a single storage box or memory vector.