Cortical sources of vernier acuity: an EEG-source imaging study

Vernier acuity determines the relative position of visual features with a precision better than the sampling resolution of the cone mosaic. Because the vernier offset is thought to be detected by orientation-tuned mechanisms, we expect cortical areas such as V1 to respond strongly to vernier stimuli. Here we use 128-channel EEG combined with cortical source imaging to identify multiple regions of visual cortex that underlie the detection of vernier offsets. Steady-state Visual Evoked Potentials (SSVEPs) were recorded from fifteen normal vision observers under two conditions. In the test condition, vernier offsets were periodically introduced and withdrawn at 3.75 Hz (alignment/misalignment) on a 2 cpd square-wave grating. In the control condition, the offsets were displaced symmetrically with respect to the reference (misalignment/misalignment), to produce the same displacement as in the test condition. In both conditions, the size of the offset was swept from 8 to 0.5 arcmin in 10-logarithm steps within a ten-second presentation interval. The vernier-offset (test condition) elicited robust odd harmonic responses in V1 and lateral occipital cortex (LOC). Thresholds for eliciting odd-harmonic responses were lowest in V1 and LOC, followed by V3a, with higher thresholds in middle temporal (hMT+) and hV4. Importantly, the control condition elicited very weak odd harmonic activity. Both the vernier test and the lateral-motion control condition elicited similar activity at even harmonics in all cortical regions of interest (ROI), related to the lateral-motion of the offsets. Our data indicate that V1, LOC and V3a are particularly sensitive to the detection of vernier offsets, while all visual ROIs are sensitive to lateral motion. Our results support a role for V1 in vernier acuity related to its exquisite sensitivity to orientation differences. The robust activation of LOC might be related to the texture characteristics of the extended vernier, or to its sensitivity to vernier offsets. Meeting abstract presented at VSS 2015.