TMS disruption of V5/MT+ indicates a role for the dorsal stream in word recognition

Although word recognition is a skill commonly expected to rely more on ventral rather than dorsal stream processing, there is some evidence for a magnocellular/dorsal impairment in dyslexia. The early rapid feedforward/feedback loop through the dorsal stream seen in primate has been suggested to allow an initial global analysis, and in human early activation of parietal attention mechanisms for detecting salient stimuli, facilitating more local level detailed ventral stream processing. To test this model in humans, transcranial magnetic stimulation (TMS) was used to probe the role of early visual cortex (V1/V2) and V5/MT+ in single word identification. TMS over V1/V2 between word onset and 36 ms post word onset disrupted accurate word discrimination, with disruption also evident at approximately 99 ms. TMS over V5/MT+ also disrupted accuracy following stimulation at approximately the same time as word onset and again at 130 ms post word onset. Thus, a role for V5/MT+ in accurate single word identification is apparent suggesting rapid triggering of attention to salient exogenous stimuli may be required prior to processing in primary and temporal cortical regions.