Abstract
It remains controversial and hotly debated whether foveal information is double-projected to both hemispheres or split at the midline between the two hemispheres. We investigated this issue in a unique patient with lesions in the splenium of the corpus callosum and the left medial occipitotemporal region, through a series of neuropsychological tests and multimodal MRI scans. Behavioral experiments showed that (1) the patient had difficulties in reading simple and compound Chinese characters when they were presented in the foveal but left to the fixation, (2) he failed to recognize the left component of compound characters when the compound characters were presented in the central foveal field, (3) his judgments of the gender of centrally presented chimeric faces were exclusively based on the left half-face and he was unaware that the faces were chimeric. Functional MRI data showed that Chinese characters, only when presented in the right foveal field but not in the left foveal field, activated a region in the left occipitotemporal sulcus in the mid-fusiform, which is recognized as visual word form area. Together with existing evidence in the literature, results of the current study suggest that the representation of foveal stimuli is functionally split at object processing levels.
Highlights
There are two competing theories regarding the cortical representation of the foveal vision, which is crucially important for many visual tasks such as reading and face recognition
We studied a patient with a lesion in the splenium to explicitly test the BPT and split-fovea theory’’ (SFT), using various experimental manipulations and with combined use of high resolution structural and functional MRI, and diffusion tensor imaging (DTI) techniques
Neuropsychological assessments showed that the patient was unable to identify the left component of Chinese characters, a symptom similar to the cases reported by Binder et al [73] in alphabetic readers
Summary
There are two competing theories regarding the cortical representation of the foveal vision, which is crucially important for many visual tasks such as reading and face recognition. The first theory, often referred to the ‘‘bilateral projection theory’’ (BPT), proposes that the foveal information from the left and right visual fields (LVF and RVF) overlaps along the vertical meridian and two complete copies of a foveally presented visual stimulus are projected in parallel to the early visual cortex of each hemisphere. This theory is supported by a number of physiological and anatomical studies in laboratory animals [1,2,3,4,5,6,7], as well as behavioral studies in hemianopia patients showing macular or foveal sparing and in commissurotomized patients showing better performance for foveal presentation than para-foveal/peripheral presentation in tasks relying on the integration of left and right visual field information [8,9,10,11,12,13,14]. Some results of functional brain imaging studies appear to be consistent with the BPT [21,22,23]
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