Neuroimaging

We have used neuroimaging to answer specific questions that could not be satisfactorily addressed using psychophysics alone. With fMRI, we have mapped the cortical distribution of absolute and relative disparity signals across visual cortex (Neri et al 2004), and have demonstrated that they are to some extent separately encoded within dorsal and ventral streams (Neri 2005; see Fig. 1).

Figure 1 Absolute disparity (blue) reflects the distance between a given object and the point in space where the observer is looking; as such, it depends on oculomotor state. Relative disparity (red), on the other hand, reflects the distance between two objects, regardless of where the observer is looking. These two classes of disparity signals carry different significance in relation to the perception and analysis of depth; we have demonstrated that they are also represented differently across visual cortex, with an emphasis on absolute disparity in the dorsal stream and an emphasis on relative disparity in the ventral stream.

With EEG, we have estimated the timescale of natural scene segmentation (Neri 2017). A specific occipital EEG marker with coarse retinotopic properties, the N2pc wave, was found to reflect the perceptual significance of object boundaries and not their definition within the physical image (Fig. 2). The perceptual specificity of the modulation occurs very early (within 100 ms after stimulus onset), challenging compartmentalized accounts of bottom-up/top-down processing in natural scene understanding (forthcoming).

Figure 2 Probes were inserted at different locations within natural scenes (indicated by dashed circles), involving variations of physical image content (blue) or perceptual significance (green). Associated N2pc waves display perceptually-driven modulations within 100-ms after stimulus onset (right panel), and no image-driven modulation (within the range spanned by the probe insertion paradigm). See Neri (2017) for details.

Relevant publications:

• Neri P Object segmentation controls image reconstruction from natural scenes 2017 PLoS Biology 15 (8) e1002611

• Neri P A stereoscopic look at visual cortex 2005 Journal of Neurophysiology 93 1823-1826

• Neri P, Bridge H, Heeger DJ Stereoscopic processing of absolute and relative disparity in human visual cortex 2004 Journal of Neurophysiology 92 1880-1891