Patient MW: transient visual hemi-agnosia

The concept of functional modularity in human visual processing was proposed 25 years ago with the distinction between a ventral pathway for object recognition and a dorsal pathway for action processing. Lesions along these pathways yield selective deficits. A 15-year-old patient (MW) presented with a seizure due to a lesion in the left occipitotemporal cortex. Surgical resection of the lesion was performed with sparing of the classic language areas and visual fields. Postoperatively MW had great difficulty reading and had a specific agnosia for more complex visual stimuli in the right hemifield. No deficit was seen for lower level visual discrimination tasks. Gradual improvement of hemi-agnosia was paralleled by slower reaction times reflecting a speed-accuracy trade-off. Absolute reading speed improved markedly over time, doubling at 6 weeks. MW fully recovered after 18 months. Postoperative functional Magnetic Resonance Imaging (fMRI) illustrated an overlap of the lesion with object and word processing areas. Diffusion Tensor Imaging showed damage to the white matter tracts [inferior fronto-occipital fasciculus and inferior longitudinal fasciculus (ILF)] interconnecting ventral temporal areas. A transient higher order deficit can result from a disruption of the neural network supporting visual word and object processing. Most visual system research has focused on cortical areas, while the underlying subcortical network received much less attention. We believe that white matter tracts, in particular the ILF, play a critical role in object perception by connecting visual areas along the ventral visual stream. Lesions of the ILF should be taken into consideration in agnosia.