V1 lesion projection zone signals in a subject with tunnel vision

Abstract

Introduction. Visual cortex that is deprived of retinal input by macular lesions can respond to visual stimulation (Baker et al, 2005). These lesion projection zone (LPZ) signals depend on the task, not the stimulus (Masuda et al., 2008). It is unknown whether these LPZ signals arise only when the macula (or fovea) is lesioned. To address this question, we examined the V1 signals in a subject with tunnel vision, i.e. a peripheral visual field loss with a spared central region including the fovea. Methods. We measured responses in visual cortex (fMRI) that were elicited by drifting contrast patterns. The subjects either viewed the stimuli passively or performed a stimulus judgment (one-back) task. One subject had retinitis pigmentosa (RP, approximate age of onset: 6); control subjects were presented with stimuli to simulate the tunnel vision of the RP subject. Results. In all subjects we observed stimulus-synchronized fMRI signals in the posterior calcarine sulcus. This is the cortical region that normally represents the foveal projection zone. In the normal control subjects we did not observe any stimulus-synchronized signals in the anterior calcarine sulcus in either passive viewing or during the stimulus judgment task. When the RP subject performed the stimulus judgment, the stimulus-synchronized responses spread into the LPZ located in anterior calcarine sulcus. Discussion. Task dependent signals can be elicited in the LPZ of subjects with an intact fovea. These results are consistent with the hypothesis that these task dependent signals originate from an imbalance of the feed-forward and the feed-back signals in the LPZ.