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1.  Terminator Disparity Contributes to Stereo Matching for Eye Movements and Perception 
The Journal of Neuroscience  2013;33(48):18867-18879.
In the context of motion detection, the endings (or terminators) of 1-D features can be detected as 2-D features, affecting the perceived direction of motion of the 1-D features (the barber-pole illusion) and the direction of tracking eye movements. In the realm of binocular disparity processing, an equivalent role for the disparity of terminators has not been established. Here we explore the stereo analogy of the barber-pole stimulus, applying disparity to a 1-D noise stimulus seen through an elongated, zero-disparity, aperture. We found that, in human subjects, these stimuli induce robust short-latency reflexive vergence eye movements, initially in the direction orthogonal to the 1-D features, but shortly thereafter in the direction predicted by the disparity of the terminators. In addition, these same stimuli induce vivid depth percepts, which can only be attributed to the disparity of line terminators. When the 1-D noise patterns are given opposite contrast in the two eyes (anticorrelation), both components of the vergence response reverse sign. Finally, terminators drive vergence even when the aperture is defined by a texture (as opposed to a contrast) boundary. These findings prove that terminators contribute to stereo matching, and constrain the type of neuronal mechanisms that might be responsible for the detection of terminator disparity.
doi:10.1523/JNEUROSCI.3332-13.2013
PMCID: PMC3841453  PMID: 24285893
2.  Temporal evolution of pattern disparity processing in humans 
Stereo matching, i.e., the matching by the visual system of corresponding parts of the images seen by the two eyes, is inherently a 2-D problem. To gain insights into how this operation is carried out by the visual system, we measured, in human subjects, the reflexive vergence eye movements elicited by the sudden presentation of stereo plaids. We found compelling evidence that the 2-D pattern disparity is computed by combining disparities first extracted within orientation selective channels. This neural computation takes 10–15 ms, and is carried out even when subjects perceive not a single plaid but rather two gratings in different depth planes (transparency). However, we found that 1-D disparities are not always effectively combined: When spatial frequency and contrast of the gratings are sufficiently different pattern disparity is not computed, a result that cannot be simply attributed to the transparency of such stimuli. Based on our results, we propose that a narrow-band implementation of the Intersection Of Constraints (IOC) rule (Fennema and Thompson, 1979; Adelson and Movshon, 1982), preceded by cross-orientation suppression, underlies the extraction of pattern disparity.
doi:10.1523/JNEUROSCI.4318-12.2013
PMCID: PMC3711760  PMID: 23426674
3.  Eye movement sequence generation in humans: Motor or goal updating? 
Journal of vision  2010;10(14):10.1167/10.14.28 28.
Saccadic eye movements are often grouped in pre-programmed sequences. The mechanism underlying the generation of each saccade in a sequence is currently poorly understood. Broadly speaking, two alternative schemes are possible: first, after each saccade the retinotopic location of the next target could be estimated, and an appropriate saccade could be generated. We call this the goal updating hypothesis. Alternatively, multiple motor plans could be pre-computed, and they could then be updated after each movement. We call this the motor updating hypothesis. We used McLaughlin’s intra-saccadic step paradigm to artificially create a condition under which these two hypotheses make discriminable predictions. We found that in human subjects, when sequences of two saccades are planned, the motor updating hypothesis predicts the landing position of the second saccade in two-saccade sequences much better than the goal updating hypothesis. This finding suggests that the human saccadic system is capable of executing sequences of saccades to multiple targets by planning multiple motor commands, which are then updated by serial subtraction of ongoing motor output.
doi:10.1167/10.14.28
PMCID: PMC3610575  PMID: 21191134
eye movements; memory; plasticity
4.  Ocular following in humans: Spatial properties 
Journal of vision  2012;12(4):10.1167/12.4.13 13.
Ocular following responses (OFRs) are tracking eye movements elicited at ultrashort latency by the sudden movement of a textured pattern. Here we report the results of our study of their dependency on the spatial arrangement of the motion stimulus. Unlike previous studies that looked at the effect of stimulus size, we investigated the impact of stimulus location and how two distinct stimuli, presented together, collectively determine the OFR. We used as stimuli vertical gratings that moved in the horizontal direction and that were confined to either one or two 0.58° high strips, spanning the width of the screen. We found that the response to individual strips varied as a function of the location and spatial frequency (SF) of the stimulus. The response decreased as the stimulus eccentricity increased, but this relationship was more accentuated at high than at low spatial frequencies. We also found that when pairs of stimuli were presented, nearby stimuli interacted strongly, so that the response to the pair was barely larger than the response to a single strip in the pair. This suppressive effect faded away as the separation between the strips increased. The variation of the suppressive interaction with strip separation, paired with the dependency on eccentricity of the responses to single strips, caused the peak response for strip pairs to be achieved at a specific separation, which varied as a function of SF.
doi:10.1167/12.4.13
PMCID: PMC3438696  PMID: 22523400
ocular following; spatial frequency; stimulus
5.  The nonlinearity of passive extraocular muscles 
Passive extraocular muscles (EOMs), like most biological tissues, are hyper-elastic, i.e., their stiffness increases as they are stretched. It has always been assumed, and in a few occasions argued, that this is their only nonlinearity and that it can be ignored in central gaze. However, using novel measurement techniques in anesthetized paralyzed monkeys, we have recently demonstrated that EOMs are characterized by another prominent nonlinearity: the forces induced by sequences of stretches do not sum. Thus, superposition, a central tenet of linear and quasi-linear models, does not hold in passive EOMs. Here, we outline the implications of this finding, especially in light of the common assumption that it is easier for the brain to control a linear than a nonlinear plant. We argue against this common belief: the specific nonlinearity of passive EOMs may actually make it easier for the brain to control the plant than if muscles were linear.
doi:10.1111/j.1749-6632.2011.06111.x
PMCID: PMC3187914  PMID: 21950971
viscoelasticity; model; control; quasilinear; superposition
6.  The Viscoelastic Properties of Passive Eye Muscle in Primates. III: Force Elicited by Natural Elongations 
PLoS ONE  2010;5(3):e9595.
We have recently shown that in monkey passive extraocular muscles the force induced by a stretch does not depend on the entire length history, but to a great extent is only a function of the last elongation applied. This led us to conclude that Fung's quasi-linear viscoelastic (QLV) model, and more general nonlinear models based on a single convolution integral, cannot faithfully mimic passive eye muscles. Here we present additional data about the mechanical properties of passive eye muscles in deeply anesthetized monkeys. We show that, in addition to the aforementioned failures, previous models also grossly overestimate the force exerted by passive eye muscles during smooth elongations similar to those experienced during normal eye movements. Importantly, we also show that the force exerted by a muscle following an elongation is largely independent of the elongation itself, and it is mostly determined by the final muscle length. These additional findings conclusively rule out the use of classical viscoelastic models to mimic the mechanical properties of passive eye muscles. We describe here a new model that extends previous ones using principles derived from research on thixotropic materials. This model is able to account reasonably well for our data, and could thus be incorporated into models of the eye plant.
doi:10.1371/journal.pone.0009595
PMCID: PMC2833209  PMID: 20221406
7.  The Viscoelastic Properties of Passive Eye Muscle in Primates. II: Testing the Quasi-Linear Theory 
PLoS ONE  2009;4(8):e6480.
We have extensively investigated the mechanical properties of passive eye muscles, in vivo, in anesthetized and paralyzed monkeys. The complexity inherent in rheological measurements makes it desirable to present the results in terms of a mathematical model. Because Fung's quasi-linear viscoelastic (QLV) model has been particularly successful in capturing the viscoelastic properties of passive biological tissues, here we analyze this dataset within the framework of Fung's theory.
We found that the basic properties assumed under the QLV theory (separability and superposition) are not typical of passive eye muscles. We show that some recent extensions of Fung's model can deal successfully with the lack of separability, but fail to reproduce the deviation from superposition.
While appealing for their elegance, the QLV model and its descendants are not able to capture the complex mechanical properties of passive eye muscles. In particular, our measurements suggest that in a passive extraocular muscle the force does not depend on the entire length history, but to a great extent is only a function of the last elongation to which it has been subjected. It is currently unknown whether other passive biological tissues behave similarly.
doi:10.1371/journal.pone.0006480
PMCID: PMC2715107  PMID: 19649257
8.  The Effect of Acute Superior Oblique Palsy on Torsional Optokinetic Nystagmus in Monkeys 
Purpose
To investigate the effects of acquired superior oblique palsy (SOP) and corrective strabismus surgery on torsional optokinetic nystagmus (tOKN) in monkeys.
Methods
The trochlear nerve was severed intracranially in two rhesus monkeys (M1 and M2). For each monkey, more than 4 months after the SOP, the ipsilateral inferior oblique muscle was denervated and extirpated. For M2, 4 months later, the contralateral inferior rectus muscle was recessed by 2 mm. tOKN was elicited during monocular viewing of a rotating stimulus that was rear projected onto a screen 43.5 cm in front of the animal. Angular rotation of the stimulus about the center was 40 deg/s clockwise or counterclockwise.
Results
The main findings after trochlear nerve sectioning were (1) the amplitude and peak velocity of torsional quick and slow phases of the paretic eye was less than that in the normal eye for both intorsion and extorsion, and (2) the vertical motion of the paretic eye increased during both torsional slow and quick phases. After corrective inferior oblique surgery, both of these effects were even greater.
Conclusions
Acquired SOP and corrective inferior oblique–weakening surgery create characteristic patterns of change in tOKN that reflect alterations in the dynamic properties of the extraocular muscles involved in eye torsion. tOKN also provides information complementary to that provided by the traditional Bielschowsky head-tilt test and potentially can help distinguish among different causes of vertical ocular misalignment.
doi:10.1167/iovs.07-0989
PMCID: PMC2367221  PMID: 18385059
9.  The Viscoelastic Properties of Passive Eye Muscle in Primates. I: Static Forces and Step Responses 
PLoS ONE  2009;4(4):e4850.
The viscoelastic properties of passive eye muscles are prime determinants of the deficits observed following eye muscle paralysis, the root cause of several types of strabismus. Our limited knowledge about such properties is hindering the ability of eye plant models to assist in formulating a patient's diagnosis and prognosis. To investigate these properties we conducted an extensive in vivo study of the mechanics of passive eye muscles in deeply anesthetized and paralyzed monkeys. We describe here the static length-tension relationship and the transient forces elicited by small step-like elongations. We found that the static force increases nonlinearly with length, as previously shown. As expected, an elongation step induces a fast rise in force, followed by a prolonged decay. The time course of the decay is however considerably more complex than previously thought, indicating the presence of several relaxation processes, with time constants ranging from 1 ms to at least 40 s. The mechanical properties of passive eye muscles are thus similar to those of many other biological passive tissues. Eye plant models, which for lack of data had to rely on (erroneous) assumptions, will have to be updated to incorporate these properties.
doi:10.1371/journal.pone.0004850
PMCID: PMC2660417  PMID: 19337381

Results 1-9 (9)