Visual error signals from the pretectal nucleus of the optic tract guide motor learning for smooth pursuit

FEFsem neuronal response during combined volitional and reflexive pursuit

Although much is known about volitional and reflexive smooth eye movements individually, much less is known about how they are coordinated. It is hypothesized that separate cortico-ponto-cerebellar loops subserve these different types of smooth eye movements. Specifically, the MT-MST-DLPN pathway is thought to be critical for ocular following eye movements, whereas the FEF-NRTP pathway is understood to be vital for volitional smooth pursuit. However, the role that these loops play in combined volitional and reflexive behavior is unknown. We used a large, textured background moving in conjunction with a small target spot to investigate the eye movements evoked by a combined volitional and reflexive pursuit task. We also assessed the activity of neurons in the smooth eye movement subregion of the frontal eye field (FEFsem). We hypothesized that the pursuit system would show less contribution from the volitional pathway in this task, owing to the increased involvement of the reflexive pathway. In accordance with this hypothesis, a majority of FEFsem neurons (63%) were less active during pursuit maintenance in a combined volitional and reflexive pursuit task than during purely volitional pursuit. Interestingly and surprisingly, the neuronal response to the addition of the large-field motion was highly correlated with the neuronal response to a target blink. This suggests that FEFsem neuronal responses to these different perturbations—whether the addition or subtraction of retinal input—may be related. We conjecture that these findings are due to changing weights of both the volitional and reflexive pathways, as well as retinal and extraretinal signals.

Temporal dynamics of retinal and extraretinal signals in the FEFsem during smooth pursuit eye movements

Neurons in the smooth eye movement subregion of the frontal eye field (FEFsem) are known to play an important role in voluntary smooth pursuit eye movements. Underlying this function are projections to parietal and prefrontal visual association areas and subcortical structures, all known to play vital but differing roles in the execution of smooth pursuit. Additionally, the FEFsem has been shown to carry a diverse array of signals (e.g., eye velocity, acceleration, gain control). We hypothesized that distinct subpopulations of FEFsem neurons subserve these diverse functions and projections, and that the relative weights of retinal and extraretinal signals could form the basis for categorization of units. To investigate this, we used a step-ramp tracking task with a target blink to determine the relative contributions of retinal and extraretinal signals in individual FEFsem neurons throughout pursuit. We found that the contributions of retinal and extraretinal signals to neuronal activity and behavior change throughout the time course of pursuit. A clustering algorithm revealed three distinct neuronal subpopulations: cluster 1 was defined by a higher sensitivity to eye velocity, acceleration, and retinal image motion; cluster 2 had greater activity during blinks; and cluster 3 had significantly greater eye position sensitivity. We also performed a comparison with a sample of medial superior temporal neurons to assess similarities and differences between the two areas. Our results indicate the utility of simple tests such as the target blink for parsing the complex and multifaceted roles of cortical areas in behavior.NEW & NOTEWORTHY The frontal eye field (FEF) is known to play a critical role in volitional smooth pursuit, carrying a variety of signals that are distributed throughout the brain. This study used a novel application of a target blink task during step ramp tracking to determine, in combination with a clustering algorithm, the relative contributions of retinal and extraretinal signals to FEF activity and the extent to which these contributions could form the basis for a categorization of neurons.

The influence of age on adaptation of disparity vergence and phoria

A paucity of research exists to investigate whether the normal aging process influences the ability to adapt disparity vergence and phoria. Vergence eye movements and dissociated phoria were recorded from 49 healthy subjects (ages 20-70years) using an objective eye movement tracking system. Four-degree vergence responses were modified using a double-step protocol. Dynamics of vergence were quantified via peak velocity. The phoria adaptation experiment measured the magnitude (net change in phoria level) and rate (magnitude divided by the time constant) of phoria adaption during 5min of sustained fixation on a binocular target (40cm/8.44° from midline). The magnitude of phoria adaptation decreased as a function of age (r=-0.33; p=0.04). The ability to adapt vergence peak velocity and the rate of phoria adaptation showed no significant age-related influence (p>0.05). The data suggest that the ability to modify the disparity vergence system and the rate of phoria adaptation are not dependent on age; whereas, the magnitude of phoria adaptation decreases as part of the normal adult aging process. These results have clinical and basic science implications because one should consider age when assessing the changes in the magnitude of phoria adaptation which can be abnormal in those with oculomotor dysfunctions.

Response properties of MST parafoveal neurons during smooth pursuit adaptation

Visual motion neurons in the posterior parietal cortex play a critical role in the guidance of smooth pursuit eye movements. Initial pursuit (open-loop period) is driven, in part, by visual motion signals from cortical areas, such as the medial superior temporal area (MST). The purpose of this study was to determine whether adaptation of initial pursuit gain arises because of altered visual sensitivity of neurons at the cortical level. It is well known that the visual motion response in MST is suppressed after exposure to a large-field visual motion stimulus, showing visual motion adaptation. One hypothesis is that foveal motion responses in MST are associated with smooth pursuit adaptation using a small target spot. We used a step-ramp tracking task with two steps of target velocity (double-step paradigm), which induces gain-down or gain-up adaptation. We found that after gain-down adaptation 58% of our MST visual neurons showed a significant decrease in firing rate. This was the case even though visual motion input (before the pursuit onset) from target motion was constant. Therefore, repetitive visual stimulation during the gain-down paradigm could lead to adaptive changes in the visual response. However, the time course of adaptation did not show a correlation between the visual response and pursuit behavior. These results indicate that the visual response in MST may not directly contribute to the adaptive change in pursuit initiation.

Conjugate adaptation of smooth pursuit during monocular viewing in strabismic monkeys with exotropia

PURPOSE

Humans and monkeys are able to adapt their smooth pursuit output when challenged with consistent errors in foveal/parafoveal image motion during tracking. Visual motion information from the retina is known to be necessary for guiding smooth pursuit adaptation. The purpose of this study is to determine whether retinal motion signals delivered to one eye during smooth pursuit produce adaptation in the fellow eye. We tested smooth pursuit adaptation during monocular viewing in strabismic monkeys with exotropia.

METHODS

To induce smooth pursuit adaptation experimentally, we used a step-ramp tracking with two different velocities (adaptation paradigm), where the target begins moving at one speed (25°/s) for first 100 ms and then changes to a lower speed (5°/s) for the remainder of the trial. Typically, 100 to 200 trials were used to adapt the smooth pursuit response. Control trials employing single speed step-ramp target motion (ramp speed = 25°/s) were used before and after adaptation paradigm to estimate adaptation.

RESULTS

The magnitude of adaptation as calculated by percentage change was not significantly different (P = 0.53) for the viewing (mean, 40.3% ± 5.9%) and the nonviewing (mean, 39.7% ± 6.2%) eyes during monocular viewing conditions, even in cases with large angle (18°-20°) strabismus.

CONCLUSIONS

Our results indicate that animals with strabismus retain the ability to produce conjugate adaptation of smooth pursuit. Therefore, we suggest that a single central representation of retinal motion information in the viewing eye drives adaptation for both eyes equally.

Role of MSTd extraretinal signals in smooth pursuit adaptation

The smooth pursuit (SP) system is able to adapt to challenges associated with development or system drift to maintain pursuit accuracy. Short-term adaptation of SP can be produced experimentally using a step-ramp tracking paradigm with 2 steps of velocity (double-step paradigm). Previous studies have demonstrated that the macaque cerebellum plays an essential role in SP adaptation. However, it remains uncertain whether neuronal activity in afferent structures to the cerebellum shows changes associated with SP adaptation. Therefore, we focused on the dorsal-medial part of medial superior temporal cortex (MSTd), which is part of the cortico-ponto-cerebellar pathway thought to provide extraretinal signals needed for maintaining SP. We found that 54% of the SP-related neurons showed significant changes in the first 100 ms of response correlated with adaptive changes of initial pursuit. Our results indicate that some cortical neurons in MSTd could be inside the circuit involved in SP adaptation. Furthermore, our sample of MSTd neurons started their discharge on average 103 ms after SP onset. Therefore, we suggest that extraretinal signals carried in MSTd might be due to efference copy of pursuit eye velocity signals, which reflect plastic changes in the downstream motor output pathways (e.g., the cerebellum).

Short-term adaptations of the dynamic disparity vergence and phoria systems

The ability to adapt is critical to survival and varies between individuals. Adaptation of one motor system may be related to the ability to adapt another. This study sought to determine whether phoria adaptation was correlated with the ability to modify the dynamics of disparity vergence. Eye movements from ten subjects were recorded during dynamic disparity vergence modification and phoria adaptation experiments. Two different convergent stimuli were presented during the dynamic vergence modification experiment: a test stimulus (4° step) and a conditioning stimulus (4° double step). Dynamic disparity vergence responses were quantified by measuring the peak velocity (°/s). Phoria adaptation experiments measured the changes in phoria over a 5-min period of sustained fixation. The maximum velocity of phoria adaptation was determined from an exponential fit of the phoria data points. Phoria and dynamic disparity vergence peak velocity were both significantly modified (P < 0.001). The maximum velocity of phoria adaptation was significantly correlated with the changes in convergence peak velocity (r > 0.89; P < 0.001). There was a strong correlation between the ability to adaptively adjust two different oculomotor parameters: a tonic and dynamic component. Future studies should investigate additional interactions between these parameters, and the ability to adaptively change other oculomotor systems such as the saccadic or smooth pursuit system. Understanding the ability to modify phoria, dynamic disparity vergence, and other oculomotor parameters can yield insights into the plasticity of short-term adaptation mechanisms.