89-Perception & imagery #5
Early V5 processing decoupled from awareness with fast but not slow moving stimuli: TMS evidence for dynamic deployment of parallel pathways depending on motion velocity Paolo A. Grasso1,2,3, Elisabetta Làdavas1,2, Caterina Bertini1,2, Serena Caltabiano2, Gregor Thut3,4, Stephanie Morand3,4 1
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CsrNC, Centre for Studies and Research in Cognitive Neuroscience, University of Bologna, Viale Europa 980, Cesena, 47521, Italy 2 Department of Psychology, University of Bologna, Viale Berti Pichat 5, Bologna, 40127, Italy 3 CCNi, Centre for Cognitive Neuroimaging, University of Glasgow, 58 Hillhead Street, G12 8QB, Glasgow, UK 4 School of Psychology, University of Glasgow, Glasgow, UK
INTRODUCTION Studies on animals and V1 lesioned patients revealed that motion information can reach V5 through two parallel pathways (Gross, 1991; Rodman et al., 1990; Ffytche et al., 1996). One pathway conveys information to V5 via projections through V1 (retino-geniculo-striate pathway), while another pathway reach V5 through a subcortical route (retino-colliculo-extrastriate pathway) possibly mediating implicit motion processing in the absence of V1 (blindsight). Here we tested the hypothesis that the deployment of the two pathways in the normal brain strictly depends on motion velocity, with fast motion stimuli being mainly processed by the subcortical pathway, while slow moving stimuli requiring the activation of the geniculo-striate pathway (dynamic parallelism hypothesis; Ffytche et al., 1996). MATERIALS AND METHODS
RESULTS
Participants Twelve participants took part in Experiment 1 (two males; mean age: 25.1, SD: 3.9), and twelve participants in Experiment 2 (three males; mean age 25.1, SD: 3.8).
Experiment 1 (V5 stimulation) The ANOVA conducted on fast motion and slow motion trials revealed a significant Stimulation (TMS vs SHAM) x Velocity (Fast vs Slow) x SOA interaction. Two separate ANOVAs where then conducted both revealing a significant Stimulation x SOA interaction (fast motion p = 0.024; slow motion p = 0.028). V5TMS impaired fast motion processing on 2nd (53.3ms; p = 0.035) and 6th SOA (160ms; p = 0.019) and slow motion processing on 3rd SOA (80ms; p = 0.011). Impaired performance on 3rd SOA of slow motion trials correlated significantly with a subjective awareness of impairment (r = -0,62; p = 0.031) as revealed by rating scale scores but this was not true for impairments on fast motion trials. The processing of static trials was not impaired by TMS.
Functional Localization V5 and V1/V2 areas were individually localized based on perception of moving (V5) or static (V1/V2) phosphenes. During the experimental session, the stimulator output was set 10% below the individual phosphene threshold.
EXPERIMENT 2
EXPERIMENT 1
V1/V2 STIMULATION
V5 STIMULATION SLOW MOTION
Inverse Efficiency Score
(RT/Acc)
Rating Scale Score
(sum of rating score/n° of trials)
FAST MOTION
Correlations
Experimental Paradigm Participants were presented with a patch of dots that could move at fast (23°/s) or slow (4.4°/s) velocity. As a control condition, static stimuli were also included. After stimulus appearance, real or SHAM double pulse TMS over V5 (Experiment 1) or V1/V2 (Experiment 2) was applied at one of six possible stimulus onset asynchronies (SOAs) or, alternatively, no TMS was delivered. Participants were asked to report the perceived direction of motion (rightward v.s. leftward) and to rate their awareness of motion on a four point scale (1 - “I did not perceive any motion at all”, 2 - “I might have perceived motion but I did not have any idea of its direction”, 3 - “I did not actually see the direction of the motion, but I may have been able to sense or guess its direction”, 4 - “I saw the direction of the motion”).
* *
FAST MOTION
SLOW MOTION
SOA (ms)
SOA (ms)
*
SOA (ms)
SOA (ms)
2° SOA
3° SOA p = 0.03
n.s.
n.s.
n.s.
Experiment 2 (V1/V2 stimulation) The ANOVA conducted on fast motion and slow motion trials revealed a significant main effect of Stimulation (p = 0.028) and no Stimulation x Velocity x SOA interaction (p = 0.20). No significant correlations with subjective awareness of impairment were found. Also the ANOVA conducted on static trials revealed a significant main effect of Stimulation (p = 0.045), suggesting that V1-TMS did not selectively interfere with motion processing.
CONCLUSIONS Our results support the idea that parallel processing of motion signals through segregated routes is dynamically deployed depending on motion velocity (dynamic parallelism). One possibility is that fast motion information is mainly conveyed by a direct route connecting the superior colliculus (SC) to V5 bypassing V1 (i.e. colliculo-extrastriate pathway) while slow motion may reach V5 only after processing in V1 (i.e. geniculo-striate pathway). Interestingly, V5TMS induced decreases in motion processing correlated with subjective awareness of impairment for slow but not fast motion stimuli suggesting that the colliculo-extrastriate pathway could be sufficient to implement implicit sensorimotor transformations necessary to quickly respond to the presence of a moving target, but not to provide a complete conscious visual analysis. TMS applied over V1/V2 showed a timely unspecific decrease of performance irrespective of stimulus velocity. TMS applied over this area could have interfered with general visual processing as also suggested by a significant decrease in the performance on control static trials. Indeed, even if V1 exhibit responses to patterns of motion, the role of this area is not strictly confined to the analysis of a motion signal. It is thus reasonable to think that TMS applied over V1 could cause a broader interference with visual processing. REFERENCES Ffytche, D. H., Guy, C. N., and Zeki, S. (1996). Motion specific responses from a blind hemifield. Brain 119, 1971–1982 Gross, C. G. (1991). Contribution of striate cortex and the superior colliculus to visual function in area MT, the superior temporal polysensory area and inferior temporal cortex. Neuropsychologia 29, 497–515. Rodman, H. R., Gross, C. G., and Albright, T. D. (1990). Afferent basis of visual response properties in area MT of the macaque. II. Effects of superior colliculus removal. J. Neurosci. 10, 1154–1164. e-mail:
[email protected] 
