 One of our eyetrackers for human subjects Typical eyetraces for natural stimuli. |
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One of our eyetrackers for human subjects Typical eyetraces for natural stimuli. |
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In natural environments, humans select a subset of visual stimuli by directing their gaze to locations attended. In previous studies it has been found that at fixation points luminance-contrast
is higher than average. This lead to the hypothesis that luminance-contrast makes a
major contribution to a saliency map of visual overt attention, consistent with a computation
of stimulus saliency in early visual cortical areas. We re-evaluate this hypothesis by
using natural and modified natural images to uncover the causal effects of luminance-contrast to human overt visual attention:
(1) We confirm that - viewing natural images - contrasts are elevated at fixation points.
This, however, only holds for low spatial frequencies an
d in a limited temporal window after stimulus onset. (2) However, despite this correlation
between overt attention and luminance-contrast, moderate modifications of contrast in natural images do not measurably affect the selection of fixation points. Furthermore, strong local reductions of luminance-contrast do not repel but attract fixation. (3) Neither contrast nor contrast modification is correlated to fixation duration. (4) Even the moderate contrast modifications used fall into the physiologically relevant range, and subjects are well
able to detect them in a forced choice paradigm.In summary, no causal contribution of luminance-contrast
to a saliency map of human overt attention is detectable. In conjunction with recent results on the relation
of contrast sensitivity of neuronal activity to the level in the visual cortical hierarchy, the present study provides evidence that
-for natural scenes- saliency is computed not early but late during processing (Einhäuser and König, 2003). Recently, we extended this study to non-human primates (in collaboration with the lab of Prof. Klaus-Peter Hoffmann at Ruhr-University Bochum, Germany). Here we find that for intrinsic features - such as luminance-contrast, texture-contrast or model-predicted saliency - correlative effects are not different between humans and rhesus monkeys. However, monkeys are more susceptible to moderate modifications of local features. These findings suggest that - under natural conditions - low-level stimulus features differently affect attention in monkeys as compared to humans (Einhäuser et al., 2006). |