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Fig. 10.8. Spurious responses to the inverted three-dot pattern. Several studies have used an inverted three-dot pattern as a non-facelike control for an upright three-dot pattern with conflicting results (e.g. Johnson and Morton 1991; Simion et al. 1998a; Valenza et al. 1996). However, the results with HLISSOM show that this pattern does not make a good control, because of the many axes of symmetry of a three-dot pattern. The first two rows in (a) and (b) are reproduced from Figure 10.7a,i, and show that HLISSOM prefers the facelike upright pattern to the control. However, the preference is sensitive to the value of the FSA threshold &theta_l and the FSA input scale &gamma_A. For instance, if &gamma_A is increased by 30%, the model FSA responds more strongly to the inverted pattern (bottom row). The inverted pattern is not as good a match for any single neuron's weights, so the FSA activity spots are always smaller for the inverted pattern. However, with a high enough &gamma_A, the FSA responds in three different places (b) compared with only one for the upright pattern (a), and together the three small responses outweigh the single larger response. Plot (c) demonstrates how such spurious FSA responses arise in the model. These responses are shown superimposed on the retinal pattern as three small dots, and the outlines indicate the three-dot patterns that they represent. Each pattern shares two dots with the inverted input, shown as three black squares; these two shared dots are enough to activate the unit. In HLISSOM, &gamma_A is set to a value low enough to prevent such spurious responses, which ensures that FSA neurons respond only to patterns that are a good match to their (upright) RFs. For humans, the &gamma_A value represents the state of contrast adaptation at a given time, which varies depending on the recent history of patterns seen (Albrecht et al. 1984; Turrigiano 1999). Thus, these results suggest that infants will have no preference (or will prefer the inverted pattern) if they are tested on the high-contrast schematic patterns while being adapted to the lower contrast levels typical of the environment. Because such adaptation is difficult to control in practice, the inverted pattern is a problematic comparison pattern -- negative results like those of Johnson and Morton (1991) may be due to temporary contrast adaptation instead of genuine, long-term pattern preferences.