Abstract
Purpose: :
Cortical area MT and MSTd mediate smooth pursuit (SP) in primates. Inter-hemispheric connections between these areas pass through splenium of the corpus callosum (SCC). The directional organization of MSTd is ipsiversive. Thus, area MT must send feed forward projections representing rightward vs. leftward motion from each visual hemi field to the appropriate MSTd, presumably through SCC. To test this hypothesis we studied SP a) before callosotomy, b) partial callosotomy through SCC c) complete callosotomy: disrupts callosal connections between all visual areas including frontal eye fields and d) complete callostomy with hemi-retinal ablation: disrupts visual input to one hemisphere in non-human primates. Callosotomy would be expected to disrupt the foveofugal (FF) and spare foveopetal (FP) SP. Also, complete callosotomy with hemi-retinal ablation will preclude simultaneous bilateral activation of cerebral hemispheres thus disrupting vertical pursuit.
Methods: :
Horizontal SP was recorded in a 1 year old monkey for conditions a, b, c as outlined above for FP and FF tasks. Target steps were 5 or 10 deg and target velocity were either 13 or 27 degs/sec. Vertical SP was recorded similarly after c and d. Step ramp trials were analyzed for open loop (OL) gain (first 150 msec of SP), closed loop (CL) gain (>150 msec of SP), saccade error prior to pursuit initiation.
Results: :
Horizontal SP for a, b and c showed decrease OL gain for FF (mean:1.2,0.95,1.0; stdev:0.27,0.34,0.18 p<0.05) but not for FP (mean:0.98,1.0,1.0; stdev:0.26,0.34,0.21 p>0.05) tasks. No change in CL gain for FF (mean:0.9,0.92,0.89; stdev:0.14,0.3,0.12 p>0.05) or FP (mean:1.02,1.03,1.0; stdev:0.16,0.23,0.11 p>0.05) tasks was seen. SE for a, b and c showed hypermetric saccades for FF (mean:-1.42,1.11,1.9; stdev:2.0,2.6,1.6 p<0.05) but not for FP (mean:-4.2,-4.1,-4.0; stdev:2.1,2.1,1.8 p>0.05) tasks. Vertical SP for c showed decrease upward gain compared to downward gain (mean: 0.7, 1.0; stdev: 0.9, 0.5 p <0.05). Vertical SP for d showed marked decline in upward and downward gain (mean: 0.17, 0.42; stdev: 0.14, 0.15 p <0.05).
Conclusions: :
Partial and complete callosotomy have underwhelming effects on horizontal pursuit thereby suggesting a role of other inter-cortical MT/MST connections like tapetum/major forceps in providing information to MST from ipsilateral visual field. Complete callosotomy and hemi-retinal ablation precludes bilateral simultaneous activation of cerebral hemispheres thus decreasing vertical pursuit.
Keywords: eye movements: saccades and pursuits • visual cortex • ocular motor control