HUMAN VESTIBULOOCULAR REFLEX AND ITS INTERACTIONS WITH VISION AND FIXATION DISTANCE DURING LINEAR AND ANGULAR HEAD MOVEMENT

The vestibuloocular reflex (VOR) maintains visual image stability by g enerating eye movements that compensate for both angular (AVOR) and li near (LVOR) head movements, typically in concert with visual following mechanisms. The VORs are generally modulated by the ''context'' in wh ich head movements are made. Three contextual influences on VOR perfor mance were studied during passive head translations and rotations over a range of frequencies (0.5-4 Hz) that emphasized shifting dynamics i n the VORs and visual following, primarily smooth pursuit First, the d ynamic characteristics of head movements themselves (''stimulus contex t'') influence the VORs. Both the AVOR and LVOR operate with high-pass characteristics relative to a head velocity input, although the cutof f frequency of the AVOR (<0.1 Hz) is far below that of the LVOR (simil ar to 1 Hz), and both perform well at high frequencies that exceed, bu t complement, the capabilities of smooth. pursuit. Second, the LVOR an d AVOR are modulated by fixation distance, implemented with a signal r elated to binocular vergence angle (''fixation context''). The effect was quantified by analyzing the response during each trial as a linear relationship between LVOR sensitivity (in deg/cm), or AVOR gain, and vergence (in m(-1)) to yield a slope (vergence influence) and an inter cept (response at 0 vergence). Fixation distance (vergence) was modula ted by presenting targets at different distances. The response slope r ises with increasing frequency, but much more so for the LVOR than the AVOR, and reflects a positive relationship for all but the lowest sti mulus frequencies in the ANOR. A third influence is the context of rea l and imagined targets on the VORs (''visual context''). This was stud ied in two ways-when targets were either earth-fixed to allow visual e nhancement of the VOR or head-fixed to permit visual suppression. The VORs were assessed by extinguishing targets for brief periods while su bjects continued to ''fixate'' them in darkness. The influences of rea l and imagined targets were most robust at lower frequencies, declinin g as stimulus frequency increased. The effects were nearly gone at 4 H z. These properties were equivalent for the LVOR and AVOR and imply th at the influences of real and imagined targets on the VORs generally f ollow low-pass and pursuit-like dynamics. The influence of imagined ta rgets accounts for roughly one-third of the influence of real targets on the VORs at 0.5 Hz.