30th Annual meeting. Society for Nueroscience Abstracts (SfN2000), pp. 670

Authors:

  • Masashi Tanigawa
  • Kenji Yoshikawa
  • Keisuke Toyama
  • Yoshio Ohtani
  • Yoshimichi Ejima
  • Shiro Ikeda
  • Shigeki Kajihara

Abstract:

Signals of contextual modulation (CM) underlying perception of motion from contour (CFM) have been studied as difference between cortical responses to segmented motion (SM) where random dots (RDs) in the neighboring segments moved in the opposite direction and homogenous motion (HM) where all RDs moved in the same direction (Lamme et al., 1994). Using a magnetometer-spatial filter system (Shimadzu SBI-100) capable of resolving individual visual cortical activities, we found similar CM signals localized in human V2/3. The CM signals occurred behind V5 responses, suggesting that CFM is conducted in V2/3, instructed by global motion signals detected in V5 and back-conveyed to V2/3 (Toyama et al., 1999). The present study reports two findings supporting this view. First, a fine parallelism was found between CM signals in V2/3 and CFM determined by the forced choice estimate for mixed SM (MSM) where SM was modified to contain variable fractions (7-50%) of oppositely moving RDs. Visual stimuli were presented to the right visual field of 7 male subjects, and MEGs were recorded from the left occipital cortex. In all subjects, CFM started to decrease with 10% MSM and reduced to 35% with 40% MSM. There was a corresponding reduction (35%) in CM signals in V2/3. This parallelism was even preserved after adaptation to HM (for 3min.). CFM decreased dramatically after adaptation (65% reduction for 40% MSM), paralleled by a similar robust reduction (35%) of the CM signals. Second, there was also a dramatic reduction of motion perception (80% for 70% MHM) for mixed HM (MHM) where HM was modified to contain various fractions (70 – 90%) of incoherent RD motion, and a reduction of V5 response (40%), but practically no reduction in V1 and V2/3 responses ($<$10%). These findings support the above view and further indicate that V5 is the primary site for motion perception and adaptation.