# Cognitive and kinematic markers of ketamine effects in behaving non-human primates

Pierre Pouget, Pierre Daye, Martin Paré

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Pierre Pouget, Pierre Daye, Martin Paré. Cognitive and kinematic markers of ketamine effects in behaving non-human primates. European Journal of Pharmacology, 2024, 987, pp.177185. 10.1016/j.ejphar.2024.177185. hal-05344214

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# Cognitive and kinematic markers of ketamine effects in behaving non-human primates

Pierre Pouget<sup>b, *</sup>, Pierre Daye<sup>b</sup>, Martin Paré<sup>a, **</sup>

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## ARTICLE INFO

Keywords:

Ketamine

Saccade

Cognition

Motor kinematic

## ABSTRACT

Ketamine is widely used to probe cognitive functions relying on the properties of methyl-D-aspartate receptor (NMDAR) synaptic transmission. Numerous works have proved that cognitive performance and adjustments in the decision or perceptual domains are affected after ketamine injection in general circulation of primates. Here, we take advantage of that in the brain stem; horizontal saccade deceleration is controlled by glycine-NMDAR-gated current, while gamma-aminobutyric acid (GABA) current controls vertical deceleration to demonstrate that despite general circulation level manipulation of NMDAR synaptic transmission, the kinematic of the saccade appeared to be in the motor brainstem generator circuit differentially maintained. The results show that the deacceleration of the saccade elicited toward a horizontal target was substantially decreased, while the deacceleration of a vertical saccade remained largely unaffected. These results provide functional distinct markers for estimating cognitive and kinematic NMDAR-gated specificity acting in the pre-frontal cortex while maintaining specificity among the GABA circuit of drugs in general circulation.

## 1. Introduction

The cognitive roles in saccade control have been extensively studied across multiple laboratories and tasks (Gill et al., 2021; Schall, 2001). Interestingly, in the pharmacological domain, it has been shown that at a subanesthetic dose (IM: 0.5–2 mg/kg), ketamine, an N-methyl-D-aspartate receptor antagonist, interferes with multiple aspects of cognition in primates, including saccade latencies (Skoblenick and Everling, 2012; Taffe et al., 2002; Weed et al., 2016). More specifically, ketamine has been shown to disturb decisions when rules have not been learned (Brunamonti et al., 2014), reduce task selectivity in macaque dorsolateral prefrontal neurons, and impair the performance of randomly interleaved prosaccades and antisaccades or task switching (Condy et al., 2005a; Skoblenick and Everling, 2012, 2014; Stoet and Snyder, 2006). At the oculomotor level, smaller doses (0.1–0.5 mg/kg) have been shown to benefit monkeys on visual search tasks and saccade-delayed tasks (Ilhame et al., 2015; Shen et al., 2010). Human subjects show a paradoxically increased performance in target tracking,

which unexpectedly changes direction (Allen et al., 2009), but the gain of smooth pursuit is strongly altered (Thaker et al., 1998). Humans under ketamine also failed to exploit environmental regularities in saccade tasks (Avila et al., 2006).

The kinematics of the saccades following ketamine have been less reported but mentioned to be not altered (the amplitude/maximum velocity relationship at the dose of 1 mg/kg) (Godaux et al., 1990; Krystal, 1994). These results were surprising because chemical lesions of the brain stem region containing glycinergic omnipause neurons (OPNs) cause saccade slowing with no change in latency. Simulation studies using a conductance-based model of premotor excitatory burst neurons (EBNs) have shown that the peak speed of a typical saccade was determined by the T- and NMDA currents in EBNs after the OPNs shut off (Miura and Optican, 2006). Furthermore, it has been demonstrated that the neurotransmitter released by OPNs and horizontal inhibitory burst neurons (IBNs) is glycine (Horn et al., 1994; Spencer et al., 1989), while vertical IBNs release GABA. Thus, EBNs in the vertical/torsional part of the saccadic system may depend upon GABA for inhibition, while EBNs

Abbreviations: BSG, BrainStem Generator; EBN, *Excitatory Burst Neuron*; FEF, Frontal Eye Field; GABA, gamma-aminobutyric acid; hIBN, horizontal *Inhibitory Burst Neuron*; IBN, Inhibitory Burst Neuron; LIP, *Lateral Intraparietal cortex*; NMDAR, *N-methyl-D-aspartate receptor*; OPN, *OmniPause Neuron*; PPRF, *Paramedian Pontine Reticular Formation*; riMLF, *rostral intertial nucelus of the Medial Longitudinal Fasciculus*; SC, *Superior Colliculus*; vIBN, vertical *Inhibitory Burst Neuron*.

<footnote><sup>a</sup> Centre for Neuroscience Studies, Queen's University, Kingston, Ontario, Canada, K7L 3N6</footnote>

<footnote><sup>b</sup> Institut Du Cerveau, ICM, INSERM 1127, CNRS 7225, Sorbonne Université, Paris, France</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>** Corresponding author. Centre for Neuroscience Studies, Queen's University, Kingston, Ontario, K7L 3N6, Canada.</footnote>

<footnote>E-mail addresses: pierre.pouget@icm-institute.org (P. Pouget), martin.pare@queensu.ca (M. Paré).</footnote>
in the horizontal part depend upon glycine. Finally, in support of this
possible cognitive/kinematic distinction, chemical lesions of the brain
stem region containing glycinergic omnipause neurons (OPNs) cause
saccade slowing with no change in latency (Horn et al., 1994; Spencer
et al., 1989).

In the present study, we aimed to further evaluate the specific effect
of ketamine intake on a cognitive marker (saccade latency) and a ki-
nematic marker (saccade skewness). By estimating the potential
distinction of action of ketamine solely on the kinematic of horizontal
EBN component, we hypothesized the possibility that specificity on
NMDA peripheral intake can be monitored using saccade dynamics. Our
results confirmed a cognitive/kinematic distinction and supported pre-
vious neuro-mimetic model predictions for saccade generation in the
brainstem (Miura and Optican, 2006) and contemporary views that
channelopathies can cause specific human saccade disorders (Gjerulfsen
et al., 2024).

## 2. Materials and methods

This study compares saccade kinematics with and without intra-
muscular ketamine injection while monkeys executed a visual search
task. Shen and Paré acquired data (Shen et al., 2010). Methods from
Shen and Paré are summarized here. All surgical and experimental
protocols were approved before 2010 by the Animal Use Subcommittee
of the University of Western Ontario Council on Animal Care and were
by the Canadian Council on Animal Care guidelines.

### 2.1. Experimental setup

Stimuli were built on a personal computer running the Psychophysics Toolbox on MATLAB (The MathWorks, Natick, MA; Brainard, 1997). They were displayed on a 37-inch monitor (Mitsubishi XC-3730C Megaview Pro 37) located 57 cm from the animal. Monkeys sat in private chairs with their heads restrained. Positions of the eyes were recorded (500Hz) with an infrared camera (EyelinkII, SR Research Ltd., Mississauga, Ontario, Canada). A real-time QNX-architecture REX system displayed paradigms (Hays et al., n.d.).

### 2.2. Paradigm

Each trial began with the appearance of a central, white, 0.5°-wide
fixation spot. Monkeys were required to fixate the target for 1s. Then,
eight spots were displayed equally spaced along a 10-degree radius
circle. Seven had a low luminance of 10.6 cd/m2, while the eighth had a
higher luminance, varying randomly from trial to trial between 10.9 and
76.6 cd/m2. Within 500ms, monkeys had to look at the most luminous
dot (2 × 2 deg validation window); see Fig. 1a for a schematic display.
They had to maintain fixation for 200ms to be rewarded. A trial was
labeled as correct if the first saccade was initiated toward the target and
if the monkey stayed in the validation window for at least 200ms. If the
monkey reached the right target despite an erroneous initial saccade, the
trial was labeled incorrect (monkeys were rewarded with a smaller
amount of water). If the target was not reached within 2000ms following
its display, monkeys were not rewarded, and the next inter-trial was
lengthened. Each session consisted of two blocks of 500-800 trials,
lasting approximately 15-20 min. Injections were done between the first
and the second block of each session. Thus, block one assessed the
baseline performance of the animal, and block 2 assessed the specific
role of the injection.

### 2.3. Drug

Three doses of ketamine (0.25, 0.5, 0.75 mg/kg) and a saline solution (0.9% NaCl) as a control were used in this study. All were injected intramuscularly. Although ketamine is widely known and used as a dissociative anesthetic, numerous studies reported that doses <1.0 mg/

<figure><img src="image_2.png" /><figcaption>**Fig. 1.** Experimental task and extracted saccade parameters. a) and b) are extracted inset from Shen et al., (2010). c) position (top) and velocity (bottom) traces of a 10° typical saccade. The horizontal red bar on the bottom plot shows the velocity threshold defining both saccades' beginning and endpoints. The vertical red bar indexes the point of peak-velocity. The ratio of the number of data points in segment A over the length of D thereby defines the skewness index. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)</figcaption></figure>

kg do not cause debilitating effects on behavior (Condy et al., 2005b; Shiigi and Casey, 1999). Sub-anesthetic doses of ketamine (0.8 mg/kg - 1.5 mg/kg) could induce a tolerance if injections are frequently repeated (Pouget et al., 2010), while very low doses of ketamine were not reported as having similar tolerance effects (Ilhame et al., 2015; Shen et al., 2010) without a fully comprehensive inter-individual, species, sex and dose of tolerance mechanism being reported (Pouget et al., 2010).

Thus, monkeys were injected once a week, except for the lowest
dose, which was sometimes tested twice weekly. Three injections per
dose were performed in a randomized order (12 injections per monkey).

### 2.4. Behavior analysis

Eye movements from three female rhesus monkeys (Macaca mulatta,
G, H, and F) were recorded (Shen et al., 2010). H was excluded from the
present study because she was tested with one medium dose of
ketamine. Queen's University Animal Care Committee approved animal care and experimental protocols by the Canadian Council of Animal Care guidelines. Data were analyzed offline using a custom-made Matlab script. Saccades were detected using a velocity threshold criterion (30°/s). Eye velocity was computed by differentiation and low-pass filtering of digitized eye position (Usui and Amidror, 1982). The time interval of the ketamine effect was chosen from the second minute to the twentieth. Indeed, latency was significantly affected approximately 2 min after the injection (see (Shen et al., 2010), Fig. 2A–B). The effect lasted approximately 20 min. Our post-doc analysis included the first saccade of correct trials occurring within this time window (18 min wide) for the treatment block of each session. Eight thousand two hundred sixty-four saccades were included in this study, which corresponds to 363 ± 115 trials per orientation, per monkey, and dose.

As shown in Fig. 1, we computed each saccade's latency, peak and average velocity, amplitude, duration, and skewness. To evaluate the skewness, we computed the ratio between the duration of acceleration (between saccade onset and time of peak velocity) and deceleration (between the time of peak velocity and saccade offset). Thereby, a skewness of 1 indicates a symmetrical velocity profile. A skewness

smaller than 1 corresponds to a more extended deceleration phase and asymmetrical velocity profiles. Finally, skewness is more significant than one and corresponds to a more extended acceleration phase.

## 2.5. Model and simulations

Simulations were performed with SIMULINK on a portable computer. We used a fixed-step Runge-Kutta-4 simulation solver with a step size of 1µs. We made a simple adjustment of NMDA currents to model the effects of a decreasing horizontal skewness with an increasing dose of ketamine. Their outputs ($I_{NMDA}$) were multiplied by a fixed gain, hypothesized to be the degree of blockade of the NMDA channels. Four linearly decreasing gains were applied to the output of NMDA channels: 1.0, 0.66, 0.33, and 0.0. We simulated three sizes of saccades: 10°, 15°, and 20°.

As presented in Fig. 2, we replicated Miura's model (Miura and Optican, 2006), which seemed to have a reasonable level of detail (Gerstner et al., 2012). This is a hybrid control model with feedback on the current eye position. It incorporates two one-compartment conductance-based neurons integrated into a simple and standard lumped

<figure><img src="image_3.png" /><figcaption>**Fig. 2.** Block diagram of the original model. s denotes the Laplace variable. K is a constant set to 4.5. Tin denotes the time constant for the glutamate input to EBN unit, the average time order of synaptic transmission. T1 is set equal to Tin. Red lines with flat endings projecting to EBNs are inhibitory inputs sent to their glycine channels. The “choke” input to the ipsilateral EBN is equal to the drive of the contralateral EBtN, and vice versa. The input $e_d$ is the desired eye displacement. The output is the Eye Position. The eye plant is not modeled. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)</figcaption></figure>
model of saccade generation. Therefore, the focus falls on the contri-
bution of the membrane potential of this neuron to the eye movement,
which is the output of the whole system. It is worth noting that this
model generates only horizontal eye movements.

Apart from the comparator (on the left of the previous diagram) and
the latch circuit (lying inside the OPN unit), which have yet to find an
anatomical substrate, every block of this diagram has a functional basis.
Although, a significant part of the BSG needs to be more concise or
present in the model. For example, the neural integrator for eye position
is perfect, contradicting experimental data (Becker and Klein, 1973).
The integrated eye position is the same signal as the muscle command
and the feedback signal. Finally, the viscoelastic properties of the
muscles are not incorporated.

**Input and feedback to the EBN:** The model input is the desired eye displacement (ed). That command is fed to a comparator, which subtracts the efferent copy eye position to this input, sending to the downstream structures a dynamic estimate of instantaneous motor error (ê_m). The model is symmetric downstream to the comparator. The two branches contain the same elements, differing only by the input signal they receive: one receives a positive drive if the instantaneous motor error is positive, i.e., the eye has not reached the target yet; the second receives a positive drive if the instantaneous motor error is negative, which appeared to be the case when the eye overshoots the target. Negative drives are rectified to zero, and positive drives are fed to the EBN unit through a low-pass filter (5ms time constant), reflecting the "sluggish" development of the activity of LLBN (providing the actual drive to the EBN)(Scudder et al., 1988).

The EBN unit is simulated as a conductance-based membrane whose output is a transfer function generating spike trains. The following classical equation describes the membrane voltage: Apart from the notable exception of the leak current, whose conductance stays constant with voltage variations, dynamical systems define other currents. Dynamical equations of the currents can be found in the original paper (Miura and Optican, 2006). The EBN output transforms the membrane potential in a signal reflecting its state. Thus, whenever the EBN potential is over -10mV, the EBN output is set to 1.

The OPN unit: This system includes one OPN unit and projects to both EBNs. The state of the OPN unit is defined by the sum of a constant bias (=1), a brief inhibitory trigger (20 ms inhibitory pulse = -2) starting when the drive input reaches the EBNs, and an inhibitory latch signal. The magnitude of the latch signal is defined as the sum of outputs of the two EBNs multiplied by 100 and fed through a low-pass filter (time constant 50 ms). The state was rectified as either positive or null, and then this signal was fed to the glycine channels of both EBNs. This output of the two EBNs is summed and then multiplied by a factor K, reflecting the exploration of the eye caused by one EBN spike. It has been set to 4.5. Eventually, this signal is fed to a mathematical integrator, issuing an eye displacement signal. This last signal is used as feedback and input to the extraocular muscles.

All data were subsequently sub-sampled to 1000Hz. Eye position
data issued by the model were treated with the same differentiation
methods and filter algorithms as those used on our second data set (see
Methods). EBN currents and potentials were filtered after clipping off
the variations related to the action potentials, as described in the orig-
inal paper, with a digital three-pole Butterworth low-pass filter (cut-off
frequency of 30Hz).

All statistical tests were two-tailed t-tests. The difference in skewness
is significant (p < 0.001, Bonferroni-corrected).

3. Results

This section briefly describes the results obtained in Shen's covert visual search task (Shen et al., 2010). We did not reanalyze the performances. As stated in Shen and Paré and presented in Fig. 3, ketamine at subanesthetic dose (0.25, 0.5, 0.75 mg/kg) seemingly has a similar effect on reaction times, regardless of the target's position on the screen

<figure><img src="image_4.png" /></figure>

<figure><img src="image_5.png" /><figcaption>**Fig. 3.** Effect of ketamine on saccade latency. a) saccade latencies are the same for horizontal, oblique, and vertical targets: the higher the injected dose, the higher the reaction time. The distributions come from directionally pooled data of the two monkeys. b) Effects of ketamine on saccade skewness. Skewness means (± SEM) against the four doses of ketamine. Dashed lines are the results from monkey G; dotted lines are the results from monkey F; solid lines are the monkey average. Skewness has been defined as the ratio of the duration of the acceleration phase over the duration.</figcaption></figure>

(Fig. 3a). However, as shown in Fig. 3b for horizontal saccades, the ketamine decreases the saccade skewness in both tested monkeys in a dose-dependent fashion. All statistical tests were two-tailed t-tests. The difference in skewness is significant (p < 0.001, Bonferroni-corrected) for each pair of ketamine doses. In oblique saccades, the ketamine also decreases significantly the saccade skewness in both tested monkeys in a dose dependent fashion, but with a lower influence (p < 0.001 Bonferroni-corrected, for each pair of ketamine doses, except between the lower doses significant at p < 0.01; statistics tables not shown). However, the ketamine strikingly appeared not to influence vertical saccades regarding skewness (one significant difference between the
two middle doses, $p < 0.01$) in both monkeys. These results contrast sharply with the cognitive effect of ketamine, as shown by latency plots: we are facing a cognitive-motor distinction of ketamine effects. Eventually, we report a well-known effect of ketamine on the amplitude of the saccades, which is accompanied by a post-saccadic drift. Both results are not further studied.

Finally, a very common way to compare abnormal saccades metrics is to plot the main sequence of these saccades (Bahill et al., 1975; Zee et al., 1976). Because a parametric relationship holds between saccade amplitude and peak velocity and saccade duration, any difference on a main sequence plot reveals an underlying anomaly in the BSG. Fig. 4a-c shows an actual plot of the main sequence of these data. For each amplitude bin (0.5°-wide), peak-velocity and mean velocity of saccades are significantly decreased in the high-dose condition, compared to the saline control test. Saccade duration is significantly increased after ketamine injection. The previous results of the modulation of saccade skewness by ketamine led us to conclude a direct modulation of pre-motor activity. Blocking the current flowing through the NMDAR has consequences for smoothing and curving the trajectory of the saccade, regardless of its programmed amplitude. The more the NMDAR is blocked, the curvier the saccade is. As illustrated in Fig. 5, the outcomes of reducing the same current on the overall form of the EBN spikes train show how this train is related to the eye velocity. In 10° saccades, the adjunction of an NMDA channel blocker, like ketamine, has relatively little impact on both the saccade velocity and the EBN spikes train. In this model, for 10° saccades, the contribution of NMDA-gated currents to the total EBN current inflow is probably too small for an alteration of its function to seriously affect the overall dynamics. For 15°, though, blocking the NMDAR exhibits some noticeable outcomes on EBN firing and, thus, saccade velocity. The increase of NMDA block causes the spikes-train to somewhat "stretch": the number of spikes emitted by the EBN unit remains steady, but their temporal display expands. The firing rate decreases, defined as the inverse of the Inter-Spike Interval (ISI). The original paper showed that the EBN firing rate was related to the velocity. Here, we can note a decrease in peak-velocity of the simulated saccade due to ISI's lengthening. The last observation is the increase in duration caused by an overall longer activity of the EBN. The same results are observed for larger amplitudes when blocking the NMDAR: ISI lengthening with constant spikes, decreasing peak-velocity, and

<figure><img src="image_6.png" /><figcaption>**Fig. 4.** Effect of ketamine on saccade kinematic parameters. Consequences of the injection of the two extreme doses of ketamine (no ketamine, bright lines, vs. 0.75 mg/kg of ketamine, dim lines). The horizontal axes of all panels are the saccadic amplitude. a) Effect of ketamine on the saccade peak-velocity. b) Effect of ketamine on the mean velocity. c) Ketamine effect on the amplitudes (binned by 0.5°). Only bins containing more than 25 points were plotted. a), b), and c). Solid lines represent the mean effect, and dotted lines are the mean ± SEM.</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>**Fig. 5.** Effects of blocking NMDA channels. Colors: dimmer curves for higher block of NMDA channels (from 0% block to 100%). a) Position traces for the four NMDA degrees of the block. b) Eye velocity computed from the position traces shown in the previous figure. c) "Rotor Plot" of the EBNs for the three simulated sizes and four NMDAR degrees of block. d) NMDA currents were observed for the four NMDAR degrees of the block. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)</figcaption></figure>

increasing duration.

The skewness of these saccades is modified dose-related, as expected from the behavioral experiments. However, the model shed new light on the influence of NMDAR for large amplitude saccades: its participation in the total current increases non-linearly with the saccade amplitude (Miura and Optican, 2006). The decrease in skewness is also amplitude-dependent. An additional plot of skewness modulation seemed unnecessary since it can directly be read on the velocity plots.

## 4. Discussion

At the oculomotor level, small doses of ketamine (0.1-0.5 mg/kg) have been shown to improve performances on visual search tasks and saccade-delayed tasks independently of movement execution (Ilhame et al., 2015; Shen et al., 2010). Here, we have confirmed that if cognitive markers vary independently of the direction in which the decision and responses are made, saccade kinematics are affected differently regarding their directions. Indeed, the skewness, an index characterizing
the shape of the velocity profile of saccades, is mainly perturbed in horizontal saccades and unchanged for vertical saccades.

These results are essential because the cognitive effect of ketamine has been linked to the modulation of single-neuron properties of perceptual accumulators. The activity of these neuro-accumulators has been recorded in FEF and parietal areas of macaques (Pouget et al., 2011; Schall, 1991; So and Shadlen, 2022). Xiao-Jing Wang and colleagues were among the first to propose the idea that the accumulation properties of a network could be seen as an emergent property of the particularity of NMDAR synaptic transmission (Wang, 1999, 2002). Indeed, former neural decision-making models were frequently constructed as bistable state networks. The bistability of these networks could accurately capture the temporal transition between two cognitive states but fail to capture the transition between the previous state and the subsequent one (Goldman, 2003). This transition is the essence of accumulator models, which display a *gradually increasing* activity. Deciding on a noisy, spiking, conductance-based network relies on the dynamics of NMDAR (Wang et al., 2013): they open and close with a slow dynamic, enabling a gradual gating of neuronal excitability. Increasing a neuron's excitability makes it prone to respond with a higher discharge rate, as it permits maintaining activity by making excitatory feedback connections inside the network. As with other works, the findings that ketamine acts on omnidirectional saccade do not demonstrate that ketamine directly modifies the gain of NMDAR current and decreases the firing rate of simulated neurons in a decision-making network but provide substantial clues.

Besides this modulation of the NMDAR gain in a realistic network, previous works have shown similar results in a sensory discrimination task. Moreover, the distinction observed between the *unidirectionality* of cognitive ketamine effects and the direction-dependent effect on saccade kinematics led us to the hypothesis that this difference should emerge in brain areas where saccades are not encoded as a unified desired displacement vector (like FEF, LIP or SC) but as separate horizontal and vertical components (Schall, 2001). We provided a hypothesis to explain this distinction. In the brainstem, saccade components are encoded in different anatomical structures. These structures receive different inhibitory projections from other premotor brainstem areas; the horizontal burster nucleus (PPRF) receives more glycinergic inputs than the vertical one (riMLF). Yet, the glycine is a co-agonist of the NMDAR and could spill over inhibitory synapses to other ones, substantially modifying the excitability of the neurons bathed in glycine. As a result, because ketamine is a use-dependent antagonist, horizontal EBNs are more affected by NMDAR antagonization than vertical EBNs. The model was replicated to understand how the modulation of an NMDA-gated current in EBN could be sufficient to increase the asymmetry of a saccade. We could replicate most behavioral findings with a specific adaptation of a hybrid (conductance-based EBN element included in a lumped circuit) model (Miura and Optican, 2006). Indeed, blocking NMDA-current resulted in increased saccade duration, accompanied by a decrease of both peak- and mean velocity. These effects depended on the saccade size, which was absent from our analysis because only one amplitude had been tested.

It was not previously envisioned that premotor regions could be functionally dissociated by injecting a pharmacological compound with such a broad activity spectrum. Thus, a challenge is to inject a drug that preferentially lowers the activity of vertical burster regions and modifies the asymmetry of vertical saccades more than horizontal ones. This drug cocktail is thought to be a GABA antagonist. Moreover, the investigation of motor consequences in a broader range of angles and eccentricities would help to characterize the specific effects of NMDAR blockade on saccade metrics. Eventually, ketamine is an excellent probe to test the reliance of decision-making on NMDAR, as supposed by modeling experiments. Sufficient behavioral and computational data arose to propose the idea of a channel-neuropathy test of this hypothesis.

This worked based on analyzing a large dataset and replicating a model for saccade generation to investigate cognitive and motor

consequences of NMDAR blocker intake. In a visual search task, general injection of the drug produced effects on latencies following the reduction in the rate of rise of a perceptual accumulator for decision-making. These cognitive effects were independent of the target's position on the display. However, the skewness of the saccades made after ketamine injection appeared to be differentially modulated: the horizontal ones were significantly decreased, while the vertical ones remained unchanged. A possible interpretation of these findings is based on local glycine levels and theories of glycine gating activity. Finally, all saccades' parameters modified by injection of ketamine found an explanation at the level of the EBN membrane. We asked about the possibility of finding the same distinction for vertical saccades with other pharmacological agents and whether build-up neurons' activity crucially relies on the NMDA receptor.

These results provided a physiological basis for dissociating ketamine effects on saccade skewness, even though it has not been injected locally. The premotor burster neuron activity correlates exceptionally well with the instantaneous saccadic velocity and the dynamic motor error (Van Gisbergen et al., 1981). The EBNs are believed to provide the saccadic pulse and velocity drive to the ipsilateral ocular motoneuron. Thus, modifying its discharge pattern could lead to modifying the saccade kinematics. Another remarkable fact about the bursters neurons lies in the difference in their neurotransmitters: EBNs use glutamate; hIBNs use glycine; vIBNs use GABA. This is a fundamental functional difference between the horizontal and the vertical/torsional saccade generator systems. Optican was among the first to propose a functional role for glycinergic transmission in the BSG (Miura and Optican, 2006; Optican, 2008). An extraordinary property of the NMDAR is to be co-activated by two different substrates: glutamate and glycine.

Furthermore, it has been proved that the glycine released in a synapse could have -enhancing- effects on the NMDAR of other synapses (Ahmadi et al., 2003; Auerbach and Zhou, 2005). Thus, glycine could modulate the excitability of a pool of neurons: the more glycinergic inputs a neuron receives, the more excitable it would be. Differences in glycine concentration "around" different neurons would mean a difference in NMDAR dynamics while receiving excitatory inputs. This is our proposal to explain the distinction of skewness modulation by ketamine. During fixation, all burster (vertical and horizontal) neurons are tonically inhibited by glycinergic projections coming from the OPN. During saccades, the EBNs (vertical and horizontal) activate the ipsilateral OMN, while their respective ipsilateral IBNs phasically inhibit the contralateral OMN. Finally, the contralateral IBNs choke off the ipsilateral EBN at the end of a saccade. Yet, this last burst of activity is conveyed by different neurotransmitters according to the direction of the saccade. Since the time constant of the glycine spillover effect is about a minute (Ahmadi et al., 2003), which is mainly superior to the time constant of EBN channel dynamics, we hypothesize that horizontal EBNs are constantly bathed in a higher concentration of glycine than vertical IBNs. Although it is a speculative hypothesis since we cannot dynamically probe GABA and glycine's extracellular concentration around EBN, it is challenging but biophysically plausible (Ahmadi et al., 2003; Burmeister et al., 2020).

## CRediT authorship contribution statement

**Pierre Pouget:** Writing – review & editing, Writing – original draft, Methodology, Formal analysis. **Pierre Daye:** Writing – review & editing, Writing – original draft, Visualization, Supervision. **Martin Paré:** Writing – review & editing, Writing – original draft, Funding acquisition, Formal analysis, Data curation, Conceptualization.

## Declaration of competing interest

P.P. and P.D. are co-inventors of several patents in eye movement tracking and co-founders of P3Lab company, which commercializes clinical eye-tracking systems. M.P. has nothing to disclose.
# Data availability

Data will be made available on request.

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