title: Evidence for fast signals and later processing in human V1/V2 and V5/MT+: A TMS study of motion perception.
abstract: Evidence from human and primate studies suggests that fast visual processing may utilize signals projecting from primary visual cortex (V1) through the dorsal stream, to area V5/MT+ or beyond and subsequently back into V1. This coincides with the arrival of parvocellular signals en route to the ventral pathway and infero-temporal cortex. Such evidence suggests that the dorsal stream region V5/MT+ is activated rapidly through the traditional hierarchical pathway and also via a less-well-established direct signal to V5/MT+ bypassing V1. To test this, 16 healthy humans underwent transcranial magnetic stimulation (TMS) of V1/V2 and V5/MT+ while performing a motion-direction detection task. A three-alternate forced-choice design (left/right motion, stationary) allowed analysis of the quality of errors made, in addition to the more usual performance measures. Transient disruption of V1/V2 and V5/MT+ significantly reduced accuracy when TMS was applied at or near motion onset. Most participants also showed disrupted performance with TMS application over V1/V2 approximately 125 ms post motion onset, and significantly reduced accuracy at 158 ms with V5/MT+ stimulation. The two periods of disruption with V1/V2 TMS are suggestive of feedforward/feedback models, although the earlier period of disruption has not been reported in previous TMS studies. Very early activation of V5/MT+, evidenced by diminished accuracy and reduced perception of motion after TMS may be indicative of a thalamic-extrastriate pathway in addition to the traditionally expected later period of processing. A profound disruption of performance prestimulus onset is more likely to reflect disruption of top-down expectancy than disruption of visual processing.
authors.affiliation: School of Psychological Science, La Trobe University, Bundoora, Victoria 3086, Australia. r.laycock@latrobe.edu.au
authors.forename: Robin
authors.initials: R
authors.lastname: Laycock
authors.forename: David P
authors.initials: DP
authors.lastname: Crewther
authors.forename: Paul B
authors.initials: PB
authors.lastname: Fitzgerald
authors.forename: Sheila G
authors.initials: SG
authors.lastname: Crewther
mesh_terms: D000328:Adult; D000818:Animals; D005260:Female; D006801:Humans; D007700:Kinetics; D008297:Male; D009039:Motion Perception / Q000502:physiology*; D010775:Photic Stimulation; D011323:Primates; D011930:Reaction Time; D015398:Signal Transduction / Q000502:physiology; D013997:Time Factors; D014793:Visual Cortex / Q000502:physiology; D014796:Visual Perception / Q000502:physiology*