title: Directional bias of neurons in V1 and V2 of strabismic monkeys: temporal-to-nasal asymmetry?
abstract: OBJECTIVE
  Strabismus that develops shortly after birth is known to cause temporal-to-nasal eye movement asymmetries under monocular viewing. The neural mechanisms underlying this deficit are not well understood. In the current study, the hypothesis that this eye movement anomaly reflects a similar asymmetry in the directional response properties of neurons in the early stages of cortical processing was examined.
  
  
  METHODS
  Strabismus was simulated with optical methods in infant monkeys between 4 and 14 weeks of age. When the monkeys were mature, microelectrode recording experiments were conducted in the primary visual cortex (V1) and visual area 2 (V2). After the spatial frequency of sine wave-grating stimuli for each neuron was optimized, each neuron's responsiveness to 24 directions of stimulus movement was measured. The preferred direction and the strength of directional bias were determined by a vector summation method:
  
  
  RESULTS
  There was not an overabundance of neurons in V1 or V2 of strabismic monkeys preferring the temporal-to-nasal direction of stimulus movement. However, the average directional bias was significantly reduced in these strabismic monkeys. Interocular suppression was highly prevalent, and this suppression was stronger and more common in neurons dominated by the ipsilateral eye.
  
  
  CONCLUSIONS
  The results suggest that the eye movement asymmetries in strabismic subjects do not result from similar asymmetries in the directional properties of V1 or V2 neurons, but rather reflect impoverished cortical signals to the brain stem nuclei that control eye movements.
authors.affiliation: College of Optometry, University of Houston, TX 77204, USA.
authors.forename: Ichiro
authors.initials: I
authors.lastname: Watanabe
authors.forename: Hua
authors.initials: H
authors.lastname: Bi
authors.forename: Bin
authors.initials: B
authors.lastname: Zhang
authors.forename: Eiichi
authors.initials: E
authors.lastname: Sakai
authors.forename: Takafumi
authors.initials: T
authors.lastname: Mori
authors.forename: Ronald S
authors.initials: RS
authors.lastname: Harwerth
authors.forename: Earl L
authors.initials: EL
authors.lastname: Smith
authors.forename: Yuzo M
authors.initials: YM
authors.lastname: Chino
mesh_terms: D000818:Animals; D000831:Animals, Newborn / Q000502:physiology; D004195:Disease Models, Animal; D023882:Dominance, Ocular / Q000502:physiology; D005133:Eye Movements / Q000502:physiology; D008253:Macaca mulatta; D008839:Microelectrodes; D009039:Motion Perception / Q000502:physiology*; D009474:Neurons / Q000502:physiology*; D013285:Strabismus / Q000503:physiopathology*; D015348:Vision, Binocular / Q000502:physiology; D014793:Visual Cortex / Q000503:physiopathology*