Saccade trajectories evoked by sequential and colliding stimulation of the monkey superior colliculus

Double Stimulation in a Spiking Neural Network Model of the Midbrain Superior Colliculus

The midbrain superior colliculus (SC) is a crucial sensorimotor interface in the generation of rapid saccadic gaze shifts. For every saccade it recruits a large population of cells in its vectorial motor map. Supra-threshold electrical microstimulation in the SC reveals that the stimulated site produces the saccade vector specified by the motor map. Electrically evoked saccades (E-saccades) have kinematic properties that strongly resemble natural, visual-evoked saccades (V-saccades), with little influence of the stimulation parameters. Moreover, synchronous stimulation at two sites yields eye movements that resemble a weighted vector average of the individual stimulation effects. Single-unit recordings have indicated that the SC population acts as a vectorial pulse generator by specifying the instantaneous gaze-kinematics through dynamic summation of the movement effects of all SC spike trains. But how to reconcile the a-specific stimulation pulses with these intricate saccade properties? We recently developed a spiking neural network model of the SC, in which microstimulation initially activates a relatively small set of (~50) neurons around the electrode tip, which subsequently sets up a large population response (~5,000 neurons) through lateral synaptic interactions. Single-site microstimulation in this network thus produces the saccade properties and firing rate profiles as seen in single-unit recording experiments. We here show that this mechanism also accounts for many results of simultaneous double stimulation at different SC sites. The resulting E-saccade trajectories resemble a weighted average of the single-site effects, in which stimulus current strength of the electrode pulses serve as weighting factors. We discuss under which conditions the network produces effects that deviate from experimental results.

Normal Topography and Binocularity of the Superior Colliculus in Strabismus

In subjects with alternating strabismus, either eye can be used to saccade to visual targets. The brain must calculate the correct vector for each saccade, which will depend on the eye chosen to make it. The superior colliculus, a major midbrain center for saccade generation, was examined to determine whether the maps serving each eye were shifted to compensate for strabismus. Alternating exotropia was induced in two male macaques at age 1 month by sectioning the tendons of the medial recti. Once the animals grew to maturity, they were trained to fixate targets with either eye. Receptive fields were mapped in the superior colliculus using a sparse noise stimulus while the monkeys alternated fixation. For some neurons, sparse noise was presented dichoptically to probe for anomalous retinal correspondence. After recordings, microstimulation was applied to compare sensory and motor maps. The data showed that receptive fields were offset in position by the ocular deviation, but otherwise remained aligned. In one animal, the left eye's coordinates were rotated ∼20° clockwise with respect to those of the right eye. This was explained by a corresponding cyclorotation of the ocular fundi, which produced an A-pattern deviation. Microstimulation drove the eyes accurately to the site of receptive fields, as in normal animals. Single-cell recordings uncovered no evidence for anomalous retinal correspondence. Despite strabismus, neurons remained responsive to stimulation of either eye. Misalignment of the eyes early in life does not alter the organization of topographic maps or disrupt binocular convergence in the superior colliculus.SIGNIFICANCE STATEMENT Patients with strabismus are able to make rapid eye movements, known as saccades, toward visual targets almost as gracefully as subjects with normal binocular alignment. They can even exercise the option of using the right eye or the left eye. It is unknown how the brain measures the degree of ocular misalignment and uses it to compute the appropriate saccade for either eye. The obvious place to investigate is the superior colliculus, a midbrain oculomotor center responsible for the generation of saccades. Here, we report the first experiments in the superior colliculus of awake primates with strabismus using a combination of single-cell recordings and microstimulation to explore the organization of its topographic maps.

The superior colliculus and the steering of saccades toward a moving visual target

Following the suggestion that a command encoding current target location feeds the oculomotor system during interceptive saccades, we tested the involvement of the deep superior colliculus (dSC). Extracellular activity of 52 saccade-related neurons was recorded in three monkeys while they generated saccades to targets that were static or moving along the preferred axis, away from (outward) or toward (inward) a fixated target with a constant speed (20°/s). Vertical and horizontal motions were tested when possible. Movement field (MF) parameters (boundaries, preferred vector, and firing rate) were estimated after spline fitting of the relation between the average firing rate during the motor burst and saccade amplitude. During radial target motions, the inner MF boundary shifted in the motion direction for some, but not all, neurons. Likewise, for some neurons, the lower boundaries were shifted upward during upward motions and the upper boundaries downward during downward motions. No consistent change was observed during horizontal motions. For some neurons, the preferred vectors were also shifted in the motion direction for outward, upward, and "toward the midline" target motions. The shifts of boundary and preferred vector were not correlated. The burst firing rate was consistently reduced during interceptive saccades. Our study demonstrates an involvement of dSC neurons in steering the interceptive saccade. When observed, the shifts of boundary in the direction of target motion correspond to commands related to past target locations. The absence of shift in the opposite direction implies that dSC activity does not issue predictive commands related to future target location.NEW & NOTEWORTHY The deep superior colliculus is involved in steering the saccade toward the current location of a moving target. During interceptive saccades, the active population consists of a continuum of cells ranging from neurons issuing commands related to past locations of the target to neurons issuing commands related to its current location. The motor burst of collicular neurons does not contain commands related to the future location of a moving target.

Normal correspondence of tectal maps for saccadic eye movements in strabismus

The superior colliculus is a major brain stem structure for the production of saccadic eye movements. Electrical stimulation at any given point in the motor map generates saccades of defined amplitude and direction. It is unknown how this saccade map is affected by strabismus. Three macaques were raised with exotropia, an outwards ocular deviation, by detaching the medial rectus tendon in each eye at age 1 mo. The animals were able to make saccades to targets with either eye and appeared to alternate fixation freely. To probe the organization of the superior colliculus, microstimulation was applied at multiple sites, with the animals either free-viewing or fixating a target. On average, microstimulation drove nearly conjugate saccades, similar in both amplitude and direction but separated by the ocular deviation. Two monkeys showed a pattern deviation, characterized by a systematic change in the relative position of the two eyes with certain changes in gaze angle. These animals' saccades were slightly different for the right eye and left eye in their amplitude or direction. The differences were consistent with the animals' underlying pattern deviation, measured during static fixation and smooth pursuit. The tectal map for saccade generation appears to be normal in strabismus, but saccades may be affected by changes in the strabismic deviation that occur with different gaze angles.

Response normalization in the superficial layers of the superior colliculus as a possible mechanism for saccadic averaging

How does the brain decide where to look? Neuronal networks within the superior colliculus (SC) encode locations of intended eye movements. When faced with multiple targets, the relative activities of neuronal populations compete for the selection of a saccade. However, the computational principles underlying saccadic choices remain poorly understood. We used voltage imaging of slices of rat SC to record circuit dynamics of population responses to single- and dual-site electrical stimulation to begin to reveal some of the principles of how populations of neurons interact. Stimulation of two distant sites simultaneously within the SC produced two distinct peaks of activity, whereas stimulation of two nearby sites simultaneously exhibited a single, merged peak centered between the two sites. The distances required to produce merged peaks of activity corresponded to target separations that evoked averaging saccades in humans performing a corresponding dual target task. The merged activity was well accounted for by a linear weighed summation and a divisive normalization of the responses evoked by the single-site stimulations. Interestingly, the merging of activity occurred within the superficial SC, suggesting a novel pathway for saccadic eye movement choice.

A flexible user-interface for audiovisual presentation and interactive control in neurobehavioral experiments

A major problem facing behavioral neuroscientists is a lack of unified, vendor-distributed data acquisition systems that allow stimulus presentation and behavioral monitoring while recording neural activity. Numerous systems perform one of these tasks well independently, but to our knowledge, a useful package with a straightforward user interface does not exist. Here we describe the development of a flexible, script-based user interface that enables customization for real-time stimulus presentation, behavioral monitoring and data acquisition. The experimental design can also incorporate neural microstimulation paradigms. We used this interface to deliver multimodal, auditory and visual (images or video) stimuli to a nonhuman primate and acquire single-unit data. Our design is cost-effective and works well with commercially available hardware and software. Our design incorporates a script, providing high-level control of data acquisition via a sequencer running on a digital signal processor to enable behaviorally triggered control of the presentation of visual and auditory stimuli. Our experiments were conducted in combination with eye-tracking hardware. The script, however, is designed to be broadly useful to neuroscientists who may want to deliver stimuli of different modalities using any animal model.

Optimal control of saccades by spatial-temporal activity patterns in the monkey superior colliculus

A major challenge in computational neurobiology is to understand how populations of noisy, broadly-tuned neurons produce accurate goal-directed actions such as saccades. Saccades are high-velocity eye movements that have stereotyped, nonlinear kinematics; their duration increases with amplitude, while peak eye-velocity saturates for large saccades. Recent theories suggest that these characteristics reflect a deliberate strategy that optimizes a speed-accuracy tradeoff in the presence of signal-dependent noise in the neural control signals. Here we argue that the midbrain superior colliculus (SC), a key sensorimotor interface that contains a topographically-organized map of saccade vectors, is in an ideal position to implement such an optimization principle. Most models attribute the nonlinear saccade kinematics to saturation in the brainstem pulse generator downstream from the SC. However, there is little data to support this assumption. We now present new neurophysiological evidence for an alternative scheme, which proposes that these properties reside in the spatial-temporal dynamics of SC activity. As predicted by this scheme, we found a remarkably systematic organization in the burst properties of saccade-related neurons along the rostral-to-caudal (i.e., amplitude-coding) dimension of the SC motor map: peak firing-rates systematically decrease for cells encoding larger saccades, while burst durations and skewness increase, suggesting that this spatial gradient underlies the increase in duration and skewness of the eye velocity profiles with amplitude. We also show that all neurons in the recruited population synchronize their burst profiles, indicating that the burst-timing of each cell is determined by the planned saccade vector in which it participates, rather than by its anatomical location. Together with the observation that saccade-related SC cells indeed show signal-dependent noise, this precisely tuned organization of SC burst activity strongly supports the notion of an optimal motor-control principle embedded in the SC motor map as it fully accounts for the straight trajectories and kinematic nonlinearity of saccades.

A test of spatial temporal decoding mechanisms in the superior colliculus

Population coding is a ubiquitous principle in the nervous system for the proper control of motor behavior. A significant amount of research is dedicated to studying population activity in the superior colliculus (SC) to investigate the motor control of saccadic eye movements. Vector summation with saturation (VSS) has been proposed as a mechanism for how population activity in the SC can be decoded to generate saccades. Interestingly, the model produces different predictions when decoding two simultaneous populations at high vs. low levels of activity. We tested these predictions by generating two simultaneous populations in the SC with high or low levels of dual microstimulation. We also combined varying levels of stimulation with visually induced activity. We found that our results did not perfectly conform to the predictions of the VSS scheme and conclude that the simplest implementation of the model is incomplete. We propose that additional parameters to the model might account for the results of this investigation.

Order of operations for decoding superior colliculus activity for saccade generation

To help understand the order of events that occurs when generating saccades, we simulated and tested two commonly stated decoding models that are believed to occur in the oculomotor system: vector averaging (VA) and center-of-mass. To generate accurate saccades, each model incorporates two required criteria: 1) a decoding mechanism that deciphers a population response of the superior colliculus (SC) and 2) an exponential transformation that converts the saccade vector into visual coordinates. The order of these two criteria is used differently within each model, yet the significance of the sequence has not been quantified. To distinguish between each decoding sequence and hence, to determine the order of events necessary to generate accurate saccades, we simulated the two models. Distinguishable predictions were obtained when two simultaneous motor commands are processed by each model. Experimental tests of the models were performed by observing the distribution of endpoints of saccades evoked by weighted, simultaneous microstimulation of two SC sites. The data were consistent with the predictions of the VA model, in which exponential transformation precedes the decoding computation.