The existence of face cells, or more generally complex pattern cells, in the inferotemporal cortex is one of the most striking findings made in the primate brain. Neurons that are highly specific in their responses to a particular visual stimulus, whether that of a celebrity or a type of food, are abundant in this area. While the presence of these neurons is clearly related to recognition and stimulus memory, as evidenced by a large body of lesion experiments, 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. ? ? Our recent work has elaborated how cells may be tuned for faces in a way that facilitates the recognition of individuals. Briefly, we found that neurons in inferotemporal cortex respond not according to the absolute features that constitute a face, but rather in the extent to which a given face deviates from the average of all faces. Psychophysical experiments have shown that we process faces differentially with respect to an implicitly stored prototype. And neurons in the inferotemporal cortex appear to contribute directly to this mode of complex analysis. ? ? A fundamental question is, how does such neural selectivity come about? Are neurons innately dedicated to certain types of stimuli? Are they shaped by experience? And, if so, how permanent are their assignments? While numerous theories have been elaborated that address these and similar questions, there is relatively little empirical evidence. We have previously used a technique that permits us to isolate and monitor single neurons for days and weeks at a time. In these studies, in which monkeys were shown a set of stimuli over and over again, without any particular visual learning required, we found that neural responses in the inferotemporal cortex were remarkably stable over time, maintaining not only their general selectivity, but also their precise temporal patterning.? ? In the laboratory, we are extending this work, in the context of studies aiming to understand how neural selectivity comes about and is shaped over time. What happens in the inferotemporal cortex when one meets a group of new people, or suddenly takes up a hobby that requires visual specialization (e.g. birdwatching)? These questions are within reach, as we can train monkeys to become visual specialists in a variety of ways. One question that arises, for example, is whether neurons, once dedicated for a particular face or faces, change their selectivity when new faces are learned. A related question is, are neural responses """"""""back-compatible"""""""" -- if they start responding to a new stimulus, do they continue to respond to those which they have previously favored? ? ? Our research will be conducted in the context of a norm-based theoretical context. This context considers that visual specialization is fundamentally concerned with analyzing deviations from a """"""""prototype"""""""" stimulus. To use the example of faces, a given face is coded not by its precise metrics, but rather by its deviation from a prototype, perhaps the average of all faces. This is a subtle point, but it has important implications for how visual and other sensory processing might be done. It is also supported by recent work, which shows that neurons in this area are tuned around the average face, which is special across the population. Our present question hinges on whether we are able to shape neural tuning by changing the learning pressures in the context of a prototype-based scheme. If so, the nature of the tuning changes will be highly informative about how complex stimulus information is encoded in this region of the brain. ? ? Finally, we are investigating how the adaptation to one stimulus for several seconds (lines, curves, and faces) makes an """"""""immediate"""""""" mark on perception. Decades-old findings show that such adaptation is readily detectable by an observer attending carefully to her/his own perception (e.g. a curved line spontaneously appears to straighten ever so slightly). Such findings provide direct evidence that our perception actively """"""""tends"""""""" toward particular stimuli, perhaps to be thought of as a prototype attractor stimulus. The long term goal of this aspect of the project is to link such subjective adaptation to the well-documented adaptation in the firing rate of individual neurons at different stages of processing in the visual cortex.
| 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 |
