Modulations of Depth Responses in the Human Brain by Object Context: Does Biological Relevance Matter?

Depth sensitivity has been shown to be modulated by object context (plausibility). It is possible that it is behavioral relevance rather than object plausibility per se which drives this effect. Here, we manipulated the biological relevance of objects (face or a non-face) and tested whether object relevance affects behavioral sensitivity and neural responses to depth-position. In a first experiment, we presented human observers with disparity-defined faces and non-faces, and observers were asked to judge the depth position of the target under signal-noise and clear (fine) task conditions. In the second experiment, we concurrently measured behavioral and fMRI responses to depth. We found that behavioral performance varied across stimulus conditions such that they were significantly worse for the upright face than the inverted face and the random shape in the signal-to-noise (SNR) task, but worse for the random shape than the upright face in the feature task. Pattern analysis of fMRI responses revealed that activity of fusiform face area (FFA) was distinctly different during depth judgments of the upright face versus the other two stimuli, with its responses (and to a stronger extent, those of V3) appearing functionally-relevant to behavioral performance. We speculate that FFA is not only involved in object analysis, but exerts considerable influence on stereoscopic mechanisms as early as in V3 based on a broader appreciation of the stimulus' behavioral relevance.