High-level visual adaptation: exploring the neural representation of faces, words and objects

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Adaptation is a technique used in psychology to study how the brain represents visual elements and objects. This involves having a person study a visual object - the adapting stimulus - for a defined period of time, following which a test stimulus is shown. The adapting stimulus alters the perception of the test stimulus, an effect that is often attributed to fatigue or habituation of the neurons that were activated by the adapting stimulus. Thus, in the waterfall effect, watching a waterfall with continuous downward movement of water will habituate neurons that code for downward motion, and when the subject is then shown a stationary image, the imbalance between fatigued downward neurons and unaffected upward neurons will cause the stationary image to appear to be drifting upwards. This technique has recently been used to examine how we code faces. Even a brief view of an adapting face for 5 seconds will alter the perception of a subsequent test face. By isolating specific properties in the face and keeping others constant, this has been used to study how we represent facial identity, expression, gender, age and so on, and to reveal specific properties that the visual system encodes about faces. Our laboratory has made significant contributions in this field over the last 7 years. In the current proposal, we aim to use the expertise we have developed with face adaptation to apply similar techniques to the study of other object representations and in particular, how our brains represent visual words. Unlike other objects, faces are held to be a particular type of visual stimulus for which nearly all humans have a special expertise. Whether this makes them different from other objects is a matter of debate. However, it is possible with interest and exposure to develop expertise for other objects, such as cars and birds. One interesting question is whether the representations that people develop for these other objects then acquire properties that make them similar to faces. Adaptation is a means of addressing this question about the nature of visual expertise, what is means to be a "visual expert". Words are of particular interest because they are also a class for which many (literate) humans have expertise, but imaging studies show that, while they do use similar parts of the brain, words involve the left hemisphere more while faces involve the right hemisphere mainly. We will use adaptation to address the question about what types of visual information in representations of words are similar to faces - indicating common properties generated by visual expertise - and what types show important differences. Differences would be important in contributing to our understanding of "hemispheric specialization", how the visual coding of the right and left hemispheres of the brain differ.