Abstract
A new study reveals the importance of feedback connections for human visual awareness [Pascual-Leone, A. and Walsh, V. (2001) Science 292, 510–512]. The researchers used transcranial magnetic stimulation (TMS) to excite primary visual cortex, V1, and the visual motion area, V5. This allowed them to work out the relative timing of events in the two areas. Stimulation of visual areas evokes ‘phosphenes’, or unstructured visual sensations. V5 was stimulated above the threshold for phosphenes, and V1 below threshold. V5 stimulation thereby evoked moving phosphenes, whereas V1 stimulation acted as a temporary lesion by introducing local electrical noise. Disrupting V1 activity before V5 stimulation had no effect on the perception of visual motion. However, disrupting V1 activity 5–45 ms after V5 stimulation abolished or reduced the sensation of visual motion. Walsh commented, ‘the work follows earlier TMS studies of blind and blindsighted patients by myself and Cowey, which showed that V5 phosphenes could not be elicited in the absence of V1.’ The study also highlights the strength of TMS as a technique for determining the timing of neural events, as it provides an estimate of the time for signals to travel from one cortical region to another. HJB
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