Dynamic connectivity
TMS can also be used to explore brain function in patients. One of the recent controversies concerning visual awareness is whether the specialized secondary visual areas such as V4 and V5 are sufficient for awareness of their preferred attribute (for example, color or motion), or whether they must interact with V1 to generate awareness.
A recent study applied TMS to extrastriate visual area V5 in a patient with almost total destruction of the striate cortex in the left hemisphere. TMS over area V5 can produce the illusory perception of motion in normal individuals; the question is whether it can also elicit motion perception in the absence of V1. The patient perceived normal moving phosphenes when V5 was stimulated in the cerebral hemisphere that had an intact V1, but motion perception could not be elicited from the blind hemifield by stimulating the hemisphere without an intact V1. The importance of V1–V5 interactions was further substantiated by the production of moving phosphenes in a peripherally blind patient by stimulation over area V5. This patient had suffered traumatic destruction of the optic nerves, but V1 was intact in both hemispheres.
This combination of real and virtual lesions is still in its infancy but it is clearly a paradigm that needs further exploration.
Neurophysiological studies have recently recorded the timing of interactions between extrastriate and striate cortex by cooling V5 while recording from V1. The effect of V5 deactivation occurred remarkably early — in the first 10 ms or so of the V1 response. Evidence of similarly fast, or perhaps continuous communication may be observed if TMS can be used to study the dynamics of backprojections in humans. The usual effect of V5 stimulation is to produce a perception of moving phosphenes, but this may be weakened or even abolished if V1 is stimulated within a critical time window of the V5–V1 interaction.
This of course would not imply that movement is perceived in V1, only that it is necessary for movement perception.
