Look Ma! No Network!: PCA of Gabor Filters Models the Development of Face Discrimination

Look Ma! No Network! : PCA of Gabor Filters Models the Development of Face Discrimination Lingyun Zhang and Garrison W. Cottrell lingyun,gary@cs.ucsd. edu UCSD Computer Science and Engineering 9500 Gilman Dr., La Jolla, CA 92093-0114 USA Abstract In previous work, we modeled Mondloch’s behav- ioral data [Mondloch et al., 2002] on adult face dis- crimination [Zhang and Cottrell, 2004]. We found that our standard model [Dailey and Cottrell, 1999, Dailey et al., 2002] was overly holistic and that by adding local feature processing we could qualitatively match the human data. However, further investigation has lead us to reconsider our conclusions. In particu- lar, we have found that the form of the Gabor filters we used were not biologically realistic, in that the spatial frequency range was overly limited. In this paper, we show that biologically realistic Gabor filters allow us to match the adult data using only the first two processing steps of our model. No neural network is necessary. In addition, we find that we can also model the develop- mental data qualitatively using a very simple manipu- lation of this preprocessing, i.e., how many people the model knows, and how many principal components it uses to represent the data. These two variables are suf- ficient to account for the slower development of configu- ral processing compared to featural processing, and how the child’s performance improves with age. Introduction We have developed a model of visual categorization that accounts for a number of important phenomena in face and object processing and visual expertise [Dailey and Cottrell, 1999, Cottrell et al., 2002, Dailey et al., 2002, Joyce and Cottrell, 2004, Tran et al., 2004, Tong et al., 2005]. Here, we investigate the model’s ability to ac- count for human sensitivity to variations in faces that are considered theoretically important for face identifi- cation. Face processing is typically described as holis- tic or configural. Holistic processing is typically taken to mean that the context of the whole face has an im- portant contribution to processing the parts: subjects have difficulty recognizing parts of the face in isolation, and subjects have difficulty ignoring parts of the face when making judgments about another part. Configural processing means that subjects are sensitive to the re- lationships between the parts, e.g., the spacing between the eyes. Holistic processing can easily be captured by a model that uses whole-face template-like representations as ours does: interference from incongruent halves of a face occurs when making judgments about a different part (e.g, expression on top when a different expression is on bottom [Cottrell et al., 2002]). However, configu- ral effects related to spacing information are attenuated by the alignment procedure that we typically use, which warps the image so the eyes and mouth are always in the same three positions. Diamond and Carey [Diamond and Carey, 1986] were among the first to discriminate between the types of processing involved in face/object perception and recog- nition. Based on studies looking at the inversion ef- fect to faces, landscapes and dogs in both dog novices and dog experts, they proposed that first-order rela- tional information, which consists of the coarse spa- tial relationships between the parts of an object (i.e. eyes are above the nose), is sufficient to recognize most objects. By contrast, second-order relational informa- tion, which is needed for face recognition and recogni- tion of individuals within categories of expertise, is re- served for visually homogeneous categories where slight differences in configuration must be used to distinguish between individuals (e.g. a slight change in the dis- tance between the eyes and the nose). Diamond and Carey [Diamond and Carey, 1986] suggest that experi- ence allows people to develop a fine-tuned prototype and to become sensitive to second-order differences between that prototype and new members of that category (e.g. new faces). One implication of the Diamond and Carey study is that the inversion effect (a large reduction in same/different performance on inverted faces, com- pared to inverted objects) is based on a relative re- liance on second-order relational information, and that perhaps this characteristic distinguishes face/expert- level processing from regular object recognition. Farah et al. [Farah et al., 1995] found that encouraging part-based processing eliminated the inversion effect, whereas allowing/encouraging non-part-based process- ing resulted in a robust inversion effect. Thus Farah et al. conclude that the inversion effect, in faces and other types of stimuli, is associated with holistic pattern perception. However, this emphasis on holistic and configural pro- cessing has led to less consideration of the obvious fact that subjects are also quite sensitive to changes in the features themselves – substitutions of different eyes or mouths can make the face look quite different. The Thatcher illusion [Thompson, 1980] suggests that parts are processed somewhat independently, and only loosely connected to the representation of the whole face. A study by Mondloch et al. (2002) that varied these dif- ferent aspects of a face (configuration, feature changes,