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Functional brain imaging and physiology studies of the visual cortex often utilize fixation on a spot as their baseline condition. It is not clear, however, whether fixation per se causes changes in activity in early retinotopic visual areas. To investigate possible changes in activity in V1 and V2 during fixation, we conducted fMRI experiments in alert monkeys. Monkeys were trained to stay still for 20 s long trials. Each trial began with 3 s in which a blank dark image was presented, followed by the presentation of a fixation spot (0.15 degrees radius) for 7 s. The animals fixated within 1 degree from the spot. A blank dark image was presented during the last 10 s of each trial. A surface coil (40 mm diameter) was positioned in proximity to the lateral part of the operculum. fMRI was conducted in a 7T vertical bore Bruker magnet, using a GE-EPI sequence.
Cross-correlation analysis revealed increased activity in response to fixation, in the central visual field representation of V1 and V2 (eccentricities 0 to 2 degrees).
To verify this response using a model-free analysis, a rotating checkers pattern bound between eccentricities of 0 and 1 degree was used as a localizer stimulus. In response to the localizer stimulus, increased activity (2.5) was observed in the foveal representation of V1 and V2. The map obtained using the localizer was used as a region of interest to sample the time-course during trials in which only the fixation spot was presented. An increased activity was detected in these trials in V2 (~1) and in V1 (~0.5). To check whether this increased activity could be caused by the small fixation spot, we varied the luminance of the fixation spot and its contrast relative to the dark background. The amplitude of the response was largely invariant to differences in the contrast of the fixation point relative to the background, whether it was 90, 50, or 10. These findings indicate the involvement of attentional mechanisms in early visual cortex during fixation.
