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1.  Short-latency disparity vergence eye movements: dependence on the preëxisting vergence angle 
Progress in brain research  2008;171:245-251.
We recorded the vergence eye movements that are elicited at ultra-short latencies when binocular disparities are applied to large-field patterns (Busettini et al., 1996) and determined their dependence on the preëxisting vergence angle (PVA). The search coil technique was used to record the movements of both eyes in four healthy subjects (two with presbyopia). Using dichoptic viewing, the two eyes saw identical images each consisting of a fixation cross at the center of a random dot pattern in a circular aperture. The subject fixated the crosses and then the images (crosses, random dots, windows) moved horizontally (1.5°/s) in opposite directions so as to bring the eyes to the desired horizontal vergence position without changing the accommodation demand. After a further 800-1200 ms to permit fusion at this new vergence angle (now, the PVA), a disparity step was applied and, 200ms later, the screen changed to uniform gray, marking the end of the trial. The disparity steps could have one of 6 magnitudes and four directions (crossed, uncrossed, right-hyper, left-hyper) while the PVA was varied systematically. The horizontal and vertical DVRs of one of the presbyopes consistently showed robust linear dependence on the PVA (r2>0.96). The horizontal DVRs of the other three subjects showed no sensitivity to the PVA and their vertical DVRs showed only very weak dependence. The experiment was repeated on one of the non-presbyopes after cycloplegia, but the outcome was the same, indicating that the negative findings were not due to the influence of the vergence-accommodation response. Our data indicate that the DVRs can be scaled by the PVA, but most subjects do not show this effect, perhaps because they relied on other distance cues that are uninformative in our experimental situation.
PMCID: PMC2562629  PMID: 18718308
Disparity Vergence Eye Movements
2.  Human ocular following: evidence that responses to large-field stimuli are limited by local and global inhibitory influences 
Progress in brain research  2008;171:237-243.
Large-field visual motion elicits tracking eye movements at ultra-short latency, often termed Ocular Following Responses (OFRs). We recorded the initial OFRs of 3 human subjects when vertical sine-wave gratings were subject to horizontal motion in the form of successive ¼-wavelength steps. The gratings could occupy the full screen (45° wide, 30° high) or a number of horizontal strips, each 1° high and extending the full width of the display. These strips were always equally spaced vertically. In a first experiment, the gratings always had a contrast of 32%. Increasing the number of strips could reduce the response latency by up to 20 ms, so the magnitude of the initial OFRs was estimated from the change in eye position over the initial open-loop period measured with respect to response onset. A single (centered) strip (covering 3.3% of the screen) always elicited robust OFRs, and 3 strips (10% coverage) were sufficient to elicit the maximum OFR. Increasing the number of strips to 15 (50% coverage) had little impact, i.e., responses had asymptoted, and further increasing the coverage to 100% (full screen image) actually decreased the OFR so that it was now less than that elicited with only 1 strip. In a second experiment, the contrast of the gratings could be fixed at one of four levels ranging from 8% to 64% and the OFR showed essentially the same pattern of dependence on screen coverage except that the lower the contrast, the lower the level at which the response asymptoted. This indicated that the asymptote was not due simply to some upper limit on the magnitude of the eye movement or the underlying motion signals. We postulate that this asymptote is the result of normalization due to global divisive inhibition, which has often been described in visual-motion-selective neurons in the cortex. We further suggest that the decrease in the OFR when the image filled the screen was due to the increased continuity of the gratings which we postulate would favor the local inhibitory surround mechanisms over the central excitatory ones. This study indicates that robust OFRs can be elicited by much smaller motion stimuli than is commonly supposed and that introducing spatial discontinuities can increase the efficacy of the motion stimuli even while decreasing the area stimulated.
PMCID: PMC2562005  PMID: 18718307
Ocular Following Response (OFR); Response normalization; Surround inhibition

Results 1-2 (2)