In such saliency-based models, the selection of relevant features is partly inspired by the receptive field properties of neurons in early sensory areas and partly due to available psychophysical results. Several studies have reported a correlation between image feature values and the selection of fixated locations. For example, Reinagel and Zador (
1999) reported that luminance contrast is higher at fixated than non-fixated regions, a finding which has been replicated in many other studies (Einhäuser & König,
2003; Parkhurst, Law, & Niebur,
2002; Tatler, Baddeley, & Gilchrist,
2005). Similarly, an increase of other low-level features, like texture contrast, has been observed (Parkhurst & Niebur,
2004). These correlations explain a substantial fraction of the variance in the distribution of selected fixation points. They are, however, far from perfect (Tatler,
2007; Tatler et al.,
2005) and their relevance for the selection of fixation locations has been questioned. Three reasons against the relevance of this correlation are provided: First, some studies show that these correlations can be absent or even reversed when participants perform specific tasks such as searching for a target in an outdoor scene (Einhäuser, Rutishauser, & Koch,
2008) or walking to a target in a real-world environment (Turano, Geruschat, & Baker,
2003). If image features have a general bottom-up impact on the selection of fixation points, it is expected that the influence of the saliency on the oculomotor mechanisms must be present irrespective of the task. Second, models based on saliency miss important aspects of eye movements such as repetitive scanpaths on reviewed images (Foulsham & Underwood,
2008; Noton & Stark,
1971), saccade sequences, region-to-region saccades (Henderson, Brockmole, Castelhano, & Mack,
2007), and large variability on the fixation durations (Hollingworth & Henderson,
1998). Thus, saliency can only play a partial role in the selection of fixation locations. Third, it has not yet been decided whether these correlations represent a causal influence or arise due to correlation with salient higher order regular structures like objects. In this context, Baddeley and Tatler (
2006) reported that using a model that incorporates edge, contrast, and luminance maps on two different scales they found that not luminance contrast but high spatial frequency edge information was the most salient feature. As this feature is more directly related to higher order structures, it questions the causal influence of low-level features. Supporting the doubt about causality, Henderson et al. (
2007) reported that the higher amount of fixations on salient objects can be explained by the finding that salient regions are more informative. In summary, it is an open debate whether the correlation between image features and fixation selection is still present in real-world setups and, if a correlation exists, whether the correlation is causal or based on correlations with higher order regularities like objects.