A striking feature of visual processing in the primate brain is the existence of neurons that are reactive to pictures of highly specific stimuli, such as faces or complex objects with which an animal or human is familiar. While the existence of these neurons is clearly related to learning and memory, as a large body of lesion experiments have shown, they also sit at the end of a complex general processing stream whose physiological principles have yet to be completely elaborated. Perhaps for this reason, their precise role of such feature- or object-responsive neurons in visual perception, picture memory, and more general object recognition has been difficult to ascertain. ? ? Conceptual frameworks for how the brain represents a large number of similar complex objects, such as faces, suggest that the visual system is able to tailor itself to the specific statistics of a stimulus category. One framework that has its roots deep in the human psychophysics literature, but which has received relatively little attention, is sometimes called norm-based coding. Considering again the example of faces, much evidence suggests that the brain processes faces by comparing them to an internally stored average prototype. This average face would serve as a norm for interpreting the structure of other faces. A face with larger than average eyebrows and a thinner than average nose would be encoded in those relative terms, rather than according to its absolute metrics. Our previous neurophysiological and psychophysical work supports this view, though research in this area is only beginning to take hold. ? ? In the laboratory, we are presently starting projects that attempt to elucidate coding principles in the inferotemporal cortex. We are particularly interested in how selective responses come about with experience, and whether changing the nature of the faces to which one is exposed (e.g. race, gender) can systematically shape the way in which neurons in this part of the brain participate in face analysis. We are using computer programs to generate morphed stimulus sets consisting of both face and non-face patterns. The goal is to observe whether changes in neural tuning occur over short and/or long time scales that reflect the pattern of learning that characterizes primate face perception. Recent technological advances have made it possible to monitor a single neuron, of which there are over 100 billion in the brain, for several weeks at a time. During this period, we hope to establish whether a single neuron has an unwavering role in the analysis of complex images, or whether the specific patterns lead to continual adjustments in its selectivity.? ? In a related subproject, we are investigating whether the adaptation to one face for several seconds affects the way that a subsequent face is processed. The motivation for this study is the observation that face misperceptions can be the psychological result of this stimulus sequence. These perceptions, commonly known as adaptational aftereffects, can make one face appear as another, with the changes being systematically related way each of the faces deviates from the average prototype. The form of these aftereffects has generated hypotheses about how face-encoding neurons might respond under similar adaptation conditions. Our preliminary evidence shows that neurons in the inferior temporal cortex of the monkey brain are likely to be directly involved in this process. We are trying to understand whether such adaptation paradigms can provide insight into the nature of the neural representations in the temporal lobe of the primate brain, and specifically whether the role of such neurons in various aspects of object perception and recognition can be identified.
| Rhodes, Gillian; Jeffery, Linda; Clifford, Colin W G et al. (2007) The timecourse of higher-level face aftereffects. Vision Res 47:2291-6 |
| Leopold, David A; Bondar, Igor V; Giese, Martin A (2006) Norm-based face encoding by single neurons in the monkey inferotemporal cortex. Nature 442:572-5 |
