The responses of neurons in the middle temporal and medial superior temporal areas of macaque cortex are suppressed during saccades compared with saccade-like stimulus movements. We utilized the short-latency ocular following paradigm to show that this saccadic suppression is followed by postsaccadic enhancement of motion responses. The level of enhancement decays with a time constant of 100 ms from saccade end. The speed of ocular following is also enhanced after saccades and decays over a similar time course, suggesting a link between the neural and behavioral effects. There is some evidence that maximum postsaccadic enhancement occurs when cells are stimulated at their optimum speeds. Latencies of motion responses are saccade dependent: 37 ms for saccade-generated motion, 45 ms for motion in the half-second after saccades, and 70 ms with no prior saccades. The finding that saccades alter response latencies may partially explain perceptual time compression during saccades and time dilation after saccades.

. Modeling of smooth pursuit-related neuronal responses in the DLPN and NRTP of the rhesus macaque. J Neurophysiol 93: 108 -116, 2005. First published August 18, 2004; doi:10.1152/ jn.00588.2004. The dorsolateral pontine nucleus (DLPN) and nucleus reticularis tegmenti pontis (NRTP) comprise obligatory links in the cortico-ponto-cerebellar system supporting smooth pursuit eye movements. We examined the response properties of DLPN and rNRTP neurons during step-ramp smooth pursuit of a small target moving across a dark background. Our neurophysiological studies were conducted in awake, behaving juvenile macaques (Macaca mulatta). We used multiple linear-regression modeling to estimate the relative sensitivities of neurons to eye parameters (position, velocity, and acceleration) and retinal-error parameters (position, velocity, and acceleration). We found that a large proportion of pursuit-related DLPN neurons primarily code eye-velocity information, whereas a large proportion of rNRTP neurons primarily code eye-acceleration information. We calculated the relative decrease in variance found when using a six-component model that included both eye-and retinal-error parameters compared with three-component models that include either eye or retinal error. These comparisons show that a majority of DLPN (14/20) and rNRTP (17/19) neurons have larger contributions from eye compared with retinal-error parameters (P Ͻ 0.001, paired t-test). Even though eye-motion parameters provide the strongest contributions in a given model, a significant contribution from retinal error was often present (i.e., Ͼ20% reduction in variance in 6-component model compared with 3-component models). Thus our results indicate that the DLPN plays a larger role in maintaining steady-state smooth pursuit eye velocity, whereas rNRTP contributes to both the initiation and maintenance of smooth pursuit.

The cortical pursuit system begins the process of transforming visual signals into commands for smooth pursuit (SP) eye movements. The frontal eye field (FEF), located in the fundus of arcuate sulcus, is known to play a role in SP and gaze pursuit movements. This role is supported, at least in part, by FEF projections to the rostral nucleus reticularis tegmenti pontis (rNRTP), which in turn projects heavily to the cerebellar vermis. However, the functional characteristics of SP-related FEF neurons that project to rNRTP have never been described. Therefore, we used microelectrical stimulation (ES) to deliver single pulses (50–200 μA, 200-μs duration) in rNRTP to antidromically activate FEF neurons. We estimated the eye or retinal error motion sensitivity (position, velocity, and acceleration) of FEF neurons during SP using multiple linear regression modeling. FEF neurons that projected to rNRTP were most sensitive to eye acceleration. In contrast, FEF neurons not activated following ES of rNRTP were often most sensitive to eye velocity. In similar modeling studies, we found that rNRTP neurons were also biased toward eye acceleration. Therefore, our results suggest that neurons in the FEF–rNRTP pathway carry signals that could play a primary role in initiation of SP.

Ono, Seiji, Vallabh E. Das, and Michael J. Mustari. Gaze-related response properties of DLPN and NRTP neurons in the rhesus macaque. J Neurophysiol 91: 2484 -2500, 2004. First published January 28, 2004 10.1152/jn.01005.2003. The dorsolateral pontine nucleus (DLPN) and nucleus reticularis tegmenti pontis (NRTP) are basilar pontine nuclei important for control of eye movements. The aim of this study was to compare the response properties of neurons in DLPN and rostral NRTP (rNRTP) during visual, oculomotor, and vestibular testing. We tested 51 DLPN neurons that were modulated during smooth pursuit (23/51) or during motion of a large-field visual stimulus (28/51). Following vestibular testing, we found that the majority of smooth pursuit-related neurons in DLPN were best classified as gaze (13/23) or eye velocity (7/23) related. Only a small percentage (3/51) of DLPN neurons responded during vestibular ocular reflex in the dark (VORd). We tested rNRTP neurons as described above and found the majority of neurons (35/43) were modulated during smooth pursuit or during motion of a large-field stimulus only (4/43). A significant proportion of our rNRTP gaze velocity neurons (10/18) were also modulated during VORd. We found that the majority of smooth pursuit related neurons in rNRTP were best classified as gaze velocity (18/35) or gaze acceleration (11/35) sensitive. The remaining neurons were classified as eye position or eye/head related. We used multiple linear-regression modeling to determine the relative contributions of eye, head and visual inputs to the responses of DLPN and rNRTP neurons. Our results support the suggestion that both DLPN and rNRTP play significant roles not only in control of smooth pursuit but also in control of gaze.

These data indicate that the assessment of saccade disconjugacy in strabismus may yield misleading results if direction is not considered. The complex pattern of disconjugacy suggests that strabismus is associated with substantial abnormalities within the circuitry controlling saccades. Neurophysiological studies are needed to identify the specific neural substrates for these behavioral effects.

The smooth pursuit eye movement (SPEM) system is much more sensitive to target motion perturbations during pursuit than during fixation. This sensitivity is commonly attributed to a dynamic gain control mechanism. Neither the neural substrate nor the functional architecture for this gain control has been fully revealed. There are at least two cortical areas that crucially contribute to smooth pursuit and are therefore eligible sites for dynamic gain control: the medial superior temporal area (MST) and the pursuit area of the frontal eye fields (FEFs), which both project to brain stem premotor structures via parallel pathways. The aim of this study was to develop a model of smooth pursuit based on behavioral, anatomical, and neurophysiological results to account for nonlinear dynamic gain control. Using a behavioral paradigm in humans consisting of a sinusoidal oscillation (4 Hz, +/-8 degrees/s) superimposed on a constant velocity target motion (0-24 degrees/s), we were able to identify relevant gain control parameters in the model. A salient feature of our model is the emergence of two parallel pathways from higher visual cortical to lower motor areas in the brain stem that correspond to the MST and FEF pathways. Detailed analysis of the model revealed that one pathway mainly carries eye velocity related signals, whereas the other is associated mostly with eye acceleration. From comparison with known neurophysiological results we conclude that the dynamic gain control can be attributed to the FEF pathway, whereas the MST pathway serves as the basic circuit for maintaining an ongoing SPEM.

Studies of individual neurons in area MT have traditionally investigated their sensitivity to constant speeds. We investigated acceleration sensitivity in MT neurons by comparing their responses to constant steps and linear ramps in stimulus speed. Speed ramps constituted constant accelerations and decelerations between 0 and 240 degrees /s. Our results suggest that MT neurons do not have explicit acceleration sensitivity, although speed changes affected their responses in three main ways. First, accelerations typically evoked higher responses than the corresponding deceleration rate at all rates tested. We show that this can be explained by adaptation mechanisms rather than differential processing of positive and negative speed gradients. Second, we inferred a cell's preferred speed from the responses to speed ramps by finding the stimulus speed at the latency-adjusted time when response amplitude peaked. In most cells, the preferred speeds inferred from deceleration were higher than those for accelerations of the same rate or from steps in stimulus speed. Third, neuron responses to speed ramps were not well predicted by the transient or sustained responses to steps in stimulus speed. Based on these findings, we developed a model incorporating adaptation and a neuron's speed tuning that predicted the higher inferred speeds and lower spike rates for deceleration responses compared with acceleration responses. This model did not predict acceleration-specific responses, in accordance with the lack of acceleration sensitivity in the neurons. The outputs of this single-cell model were passed to a population-vector-based model used to estimate stimulus speed and acceleration. We show that such a model can accurately estimate relative speed and acceleration using information from the population of neurons in area MT.

. Smooth pursuit (SP)-related neurons in the dorsal-medial part of medial superior temporal cortex (MSTd) carry extraretinal signals that may play a role in maintenance of SP once eye velocity matches target velocity. For example, it has not been determined whether the extraretinal signals reflect volitional SP commands or proprioception. The aim of this study was to test some potential sources of extraretinal signals in MSTd pursuit neurons. We tested 40 MSTd neurons during step-ramp SP with target blink conditions to show that they carried an extraretinal signal. To examine potential contributions from eye movements that might reflect proprioceptive feedback from eye muscles, we tested MSTd neurons during rotational vestibular ocular reflex in complete darkness (VORd). Vestibular stimulation was delivered in the earth horizontal plane to elicit reflex driven smooth eye movements that matched the speed and frequency of volitional SP. We also tested VOR in the light (VORx1) and cancellation of the VOR (VORx0). Our neurons were modulated during both SP and cancellation of the VOR. In contrast, MSTd smooth pursuit neurons with extraretinal signals were not significantly modulated during VORd (sensitivity Յ 0.10 spike/s/°/s). This combination of properties is compatible with classifying these neurons as gaze-velocity related. Absence of modulation during VORd testing could be caused by cancellation of head and eye movement sensitivity or dependence of neuronal firing on volitional SP commands. Our results support the suggestion that modulation of SPrelated MSTd neurons reflects volitional SP commands rather then eye movements generated by reflex pathways.