Regions of human visual cortex sensitive to small vernier offsets as determined by EEG source-imaging

Abstract

Humans can determine the relative position of visual features with a precision (‘vernier acuity’) better than the sampling resolution of the cone mosaic. Here we use high-density EEG recordings combined with distributed source imaging methods to identify two regions of visual cortex that underlie the detection of near-threshold vernier offsets. Eight observers were presented with a set of 1 arc min vernier offsets that were periodically introduced and withdrawn from a 2 cpd square wave luminance grating at 3.75 Hz (alignment/misalignment). The offset regions of the grating alternated with static regions that served as a position reference. Psychophysical thresholds for this target are in the range of 15–30 arc sec (Norcia et al., 1999). This condition was contrasted with a control condition in which the 1 arc min vernier offsets displaced symmetrically with respect to the reference (misalignment/misalignment). EEG responses to each condition were recorded over 128 channels and were averaged at each electrode across observers. These “sensor-space” averages were aligned to an individual observer's head model generated from a structural MRI. Current density on the cortical surface was determined by a minimum norm inverse. Subtraction of the misalignment/misalignment condition from the alignment/misalignment condition isolates activity that is specific to the relative position of the offsets and the reference. This activity is maximal at the occipital pole (areas V1,V2,V3), but there is a secondary maximum in the middle occipital gyrus, bilaterally. This is the first evidence for a high-resolution position mechanism outside of the classical retinotopic areas.