Context-dependent representations of objects and space in the primate hippocampus during virtual navigation Gulli, Roberto A., Duong, Lyndon R., Corrigan, B W, Doucet, Guillaume, Williams, Sylvain, Fusi, Stefano, and Martinez-Trujillo, Julio C. Nature Neuroscience 2020
The hippocampus is implicated in associative memory and spatial navigation. To investigate how these functions are mixed in the hippocampus, we recorded from single hippocampal neurons in macaque monkeys navigating a virtual maze during a foraging task and a context–object associative memory task. During both tasks, single neurons encoded information about spatial position; a linear classifier also decoded position. However, the population code for space did not generalize across tasks, particularly where stimuli relevant to the associative memory task appeared. Single-neuron and population-level analyses revealed that cross-task changes were due to selectivity for nonspatial features of the associative memory task when they were visually available (perceptual coding) and following their disappearance (mnemonic coding). Our results show that neurons in the primate hippocampus nonlinearly mix information about space and nonspatial elements of the environment in a task-dependent manner; this efficient code flexibly represents unique perceptual experiences and correspondent memories.
Modulation of local field potentials and neuronal activity in primate hippocampus during saccades Doucet, Guillaume, Gulli, Roberto A., Corrigan, Benjamin W., Duong, Lyndon R., and Martinez‐Trujillo, Julio C. Hippocampus 2020
Primates use saccades to gather information about objects and their relative spatial arrangement, a process essential for visual perception and memory. It has been proposed that signals linked to saccades reset the phase of local field potential (LFP) oscillations in the hippocampus, providing a temporal window for visual signals to activate neurons in this region and influence memory formation. We investigated this issue by measuring hippocampal LFPs and spikes in two macaques performing different tasks with unconstrained eye movements. We found that LFP phase clustering (PC) in the alpha/beta (8–16 Hz) frequencies followed foveation onsets, while PC in frequencies lower than 8 Hz followed spontaneous saccades, even on a homogeneous background. Saccades to a solid grey background were not followed by increases in local neuronal firing, whereas saccades toward appearing visual stimuli were. Finally, saccade parameters correlated with LFPs phase and amplitude: saccade direction correlated with delta (≤4 Hz) phase, and saccade amplitude with theta (4–8 Hz) power. Our results suggest that signals linked to saccades reach the hippocampus, producing synchronization of delta/theta LFPs without a general activation of local neurons. Moreover, some visual inputs co-occurring with saccades produce LFP synchronization in the alpha/beta bands and elevated neuronal firing. Our findings support the hypothesis that saccade-related signals enact sensory inputdependent plasticity and therefore memory formation in the primate hippocampus.
Beyond metaphors and semantics: A framework for causal inference in neuroscience Gulli, Roberto A. Behavioral and Brain Sciences 2019
The long-enduring coding metaphor is deemed problematic because it imbues correlational evidence with causal power. In neuroscience, most research is correlational or conditionally correlational; this research, in aggregate, informs causal inference. Rather than prescribing semantics used in correlational studies, it would be useful for neuroscientists to focus on a constructive syntax to guide principled causal inference.
PhD Thesis: Hippocampal function in non-human primates Gulli, Roberto A. 2019
The hippocampus is a phylogenetically ancient brain structure that has been shown to be critical for spatial navigation and memory. Decades of research have uncovered neurophysiological correlates of each function in the activity of hippocampal neurons, but debate continues over the primacy of each one. This debate exists, at least in part, because navigational and non-spatial mnemonic signals are difficult to simultaneously observe and disentangle with contemporary techniques. To address this, this thesis details the development of a novel experimental paradigm that combines virtual reality tasks with hippocampal neurophysiology in monkeys (Macaca mulatta). Single neurons in the right mid/posterior hippocampus were recorded while subjects completed a foraging task and an associative memory task in the same virtual environment. Using multiple analytical techniques, it was shown that hippocampal neurons do encode information about space in each task. However, the code for space does not generalize across tasks. This can be attributed to encoding of trial-specific features of associative memory that vary depending on position in the virtual environment. Furthermore, it is not only the objects that are relevant to associative memory that are encoded by these neurons when the monkey is looking at them. A subset of neurons is activated by each object in the environment, and these representations change as a function of behaviour. Together, the work presented in this thesis shows that some, but not all predictions of spatial and mnemonic theories of hippocampal function are corroborated in monkeys completing tasks in a virtual environment. The points of divergence from established dogmas may have important implications for neuropsychological and computational theories of hippocampal function across species.
Customizable cap implants for neurophysiological experimentation Blonde, Jackson D., Roussy, Megan, Luna, Rogelio, Mahmoudian, Borna, Gulli, Roberto A., Barker, Kevin C., Lau, Jonathan C., and Martinez-Trujillo, Julio C. Journal of Neuroscience Methods 2018
Several primate neurophysiology laboratories have adopted acrylic-free, custom-fit cranial implants. These implants are often comprised of titanium or plastic polymers, such as polyether ether ketone (PEEK). Titanium is favored for its mechanical strength and osseointegrative properties whereas PEEK is notable for its lightweight, machinability, and MRI compatibility. Recent titanium/PEEK implants have proven to be effective in minimizing infection and implant failure, thereby prolonging experiments and optimizing the scientific contribution of a single primate.
We created novel, customizable PEEK ‘cap’ implants that contour to the primate’s skull. The implants were created using MRI and/or CT data, SolidWorks software and CNC-machining.
Three rhesus macaques were implanted with a PEEK cap implant. Head fixation and chronic recordings were successfully performed. Improvements in design and surgical technique solved issues of granulation tissue formation and headpost screw breakage.
Comparison with existing methods
Primate cranial implants have traditionally been fastened to the skull using acrylic and anchor screws. This technique is prone to skin recession, infection, and implant failure. More recent methods have used imaging data to create custom-fit titanium/PEEK implants with radially extending feet or vertical columns. Compared to our design, these implants are more surgically invasive over time, have less force distribution, and/or do not optimize the utilizable surface area of the skull.
Our PEEK cap implants served as an effective and affordable means to perform electrophysiological experimentation while reducing surgical invasiveness, providing increased strength, and optimizing useful surface area.
Flexible coding of memory and space in the primate hippocampus during virtual navigation Gulli, Roberto A., Duong, Lyndon R., Corrigan, Benjamin W., Doucet, Guillaume, Williams, Sylvain, Fusi, Stefano, and Martinez-Trujillo, Julio C. bioRxiv 2018
Hippocampal maps of space change across tasks. The mechanisms of this effect remain unclear. To examine this, we recorded activity of hippocampal neurons in monkeys navigating the same virtual maze during two different tasks: a foraging task requiring only cue guided navigation, and a memory task also requiring context-object association. Within each task, individual neurons had spatially-selective response fields, enabling a linear classifier to decode position in the virtual environment in each task. However, the population code did not generalize across tasks. This was due to sensory and mnemonic coding of non-spatial features and their associations by single neurons during each period of the associative memory task. Thus, sensory and mnemonic representations of non-spatial features shape maps of space in the primate hippocampus during virtual navigation. This may reflect a fundamental role of the hippocampus in compressing information from a variety of sources for efficient memory storage.
Characterizing eye movement behaviors and kinematics of non-human primates during virtual navigation tasks Corrigan, Benjamin W., Gulli, Roberto A., Doucet, Guillaume, and Martinez-Trujillo, Julio C. Journal of Vision 2017
Virtual environments (VE) allow testing complex behaviors in naturalistic settings by combining highly controlled visual stimuli with spatial navigation and other cognitive tasks. They also allow for the recording of eye movements using high-precision eye tracking techniques, which is important in electrophysiological studies examining the response properties of neurons in visual areas of nonhuman primates. However, during virtual navigation, the pattern of retinal stimulation can be highly dynamic which may influence eye movements. Here we examine whether and how eye movement patterns change as a function of dynamic visual stimulation during virtual navigation tasks, relative to standard oculomotor tasks. We trained two rhesus macaques to use a joystick to navigate in a VE to complete two tasks. To contrast VE behavior with classic measurements, the monkeys also performed a simple Cued Saccade task. We used a robust algorithm for rapid classification of saccades, fixations, and smooth pursuits. We then analyzed the kinematics of saccades during all tasks, and specifically during different phases of the VE tasks. We found that fixation to smooth pursuit ratios were smaller in VE tasks (4:5) compared to the Cued Saccade task (7:1), reflecting a more intensive use of smooth pursuit to foveate targets in VE than in a standard visually guided saccade task or during spontaneous fixations. Saccades made to rewarded targets (exploitation) tended to have increased peak velocities compared to saccades made to unrewarded objects (exploration). VE exploitation saccades were 6% slower than saccades to discrete targets in the Cued Saccade task. Virtual environments represent a technological advance in experimental design for nonhuman primates. Here we provide a framework to study the ways that eye movements change between and within static and dynamic displays.
Cross-species 3D virtual reality toolbox for visual and cognitive experiments Doucet, Guillaume, Gulli, Roberto A., and Martinez-Trujillo, Julio C. Journal of Neuroscience Methods 2016
Background Although simplified visual stimuli, such as dots or gratings presented on homogeneous backgrounds, provide strict control over the stimulus parameters during visual experiments, they fail to approximate visual stimulation in natural conditions. Adoption of virtual reality (VR) in neuroscience research has been proposed to circumvent this problem, by combining strict control of experimental variables and behavioral monitoring within complex and realistic environments. New method We have created a VR toolbox that maximizes experimental flexibility while minimizing implementation costs. A free VR engine (Unreal 3) has been customized to interface with any control software via text commands, allowing seamless introduction into pre-existing laboratory data acquisition frameworks. Furthermore, control functions are provided for the two most common programming languages used in visual neuroscience: Matlab and Python. Results The toolbox offers milliseconds time resolution necessary for electrophysiological recordings and is flexible enough to support cross-species usage across a wide range of paradigms. Comparison with existing methods Unlike previously proposed VR solutions whose implementation is complex and time-consuming, our toolbox requires minimal customization or technical expertise to interface with pre-existing data acquisition frameworks as it relies on already familiar programming environments. Moreover, as it is compatible with a variety of display and input devices, identical VR testing paradigms can be used across species, from rodents to humans. Conclusions This toolbox facilitates the addition of VR capabilities to any laboratory without perturbing pre-existing data acquisition frameworks, or requiring any major hardware changes.
Exercise restores insulin, but not adiponectin, response in skeletal muscle of high-fat fed rodents. Gulli, Roberto A., Tishinsky, Justine M., MacDonald, Tara, Robinson, Lindsay E., Wright, David C., and Dyck, David J. American journal of physiology. Regulatory, integrative and comparative physiology 2012
High-fat (HF) diets impair skeletal muscle response to the insulin-sensitizing adipokine adiponectin (Ad) in rodents, preceding the development of insulin resistance. Skeletal muscle insulin response in HF-fed rats can be restored with chronic exercise; whether recovery of skeletal muscle Ad response is necessary for the exercise-induced recovery of insulin-stimulated glucose transport is not known. In the current study, insulin and Ad resistance were induced in rodents with 4 wk of HF feeding (HF(4); low-fat fed animals used as control). Rats were then treadmill-exercised (HF(5)EX(1), HF(6)EX(2)) or supplemented orally with the pharmacological agent beta-guadinoproprionic acid (GPA; HF(5)GPA(1), HF(6)GPA(2)) for 1 or 2 wk with continued HF feeding. Insulin and Ad responses (glucose transport and palmitate oxidation, respectively) were assessed 48 h after the last exercise bout ex vivo in isolated solei. Insulin response was impaired following 4 wk of HF feeding and improved with 1 and 2 wk of exercise and beta-GPA supplementation (HF(5)EX(1), HF(6)EX(2), HF(5)GPA(1), and HF(6)GPA(2)). The recovery of insulin response generally coincided with improved Akt Thr(308) phosphorylation in HF(5)GPA(1), HF(6)EX(2), and HF(6)GPA(2), although not in HF(5)EX(1). Ad-stimulated palmitate oxidation was not restored with either treatment. Total protein contents of AdipoR1, AdipoR2, APPL1, and APPL2, as well as total and phosphorylated AMPK and ACC were unaltered by diet, exercise, and beta-GPA at the assessed time points. We conclude that the exercise and pharmacologically (beta-GPA)-induced recovery of skeletal muscle insulin response after HF feeding is not dependent on the restoration of Ad response, as assessed ex vivo.
Fish oil prevents high-saturated fat diet-induced impairments in adiponectin and insulin response in rodent soleus muscle. Tishinsky, Justine M., Gulli, Roberto A., Mullen, Kerry L., Dyck, David J., and Robinson, Lindsay E. American journal of physiology. Regulatory, integrative and comparative physiology 2012
High saturated fatty acid (SFA) diets contribute to the development of insulin resistance, whereas fish oil-derived n-3 polyunsaturated fatty acids (PUFA) increase the secretion of adiponectin (Ad), an adipocyte-derived protein that stimulates fatty acid oxidation (FAO) and improves skeletal muscle insulin response. We sought to determine whether fish oil could prevent and/or restore high SFA diet-induced impairments in Ad and insulin response in soleus muscle. Sprague-Dawley rats were fed 1) a low-fat control diet (CON group), 2) high-SFA diet (SFA group), or 3) high SFA with n-3 PUFA diet (SFA/n-3 PUFA group). At 4 wk, CON and SFA/n-3 PUFA animals were terminated, and SFA animals were either terminated or fed SFA or SFA/n-3 PUFA for an additional 2 or 4 wk. The effect of diet on Ad-stimulated FAO, insulin-stimulated glucose transport, and expression of Ad, insulin and inflammatory signaling proteins was determined in the soleus muscle. Ad stimulated FAO in CON and 4 wk SFA/n-3 PUFA (+36%, +39%, respectively P \textless/= 0.05) only. Insulin increased glucose transport in CON, 4 wk SFA/n-3 PUFA, and 4 wk SFA + 4 wk SFA/n-3 PUFA (+82%, +33%, +25%, respectively P \textless/= 0.05); this effect was lost in all other groups. TLR4 expression was increased with 4 wk of SFA feeding (+24%, P \textless/= 0.05), and this was prevented in 4 wk SFA/n-3 PUFA. Suppressor of cytokine signaling-3 expression was increased in SFA and SFA/n-3 PUFA (+33 and +18%, respectively, P \textless/= 0.05). Our results demonstrate that fish oil can prevent high SFA diet-induced impairments in both Ad and insulin response in soleus muscle.
Recovered insulin response by 2 weeks of leptin administration in high-fat fed rats is associated with restored AS160 activation and decreased reactive lipid accumulation. Stefanyk, Leslie E., Gulli, Roberto A., Ritchie, Ian R., Chabowski, Adrian, Snook, Laelie A., Bonen, Arend, and Dyck, David J. American journal of physiology. Regulatory, integrative and comparative physiology 2011
Leptin is an adipokine that increases fatty acid (FA) oxidation, decreases intramuscular lipid stores, and improves insulin response in skeletal muscle. In an attempt to elucidate the underlying mechanisms by which these metabolic changes occur, we administered leptin (Lep) or saline (Sal) by miniosmotic pumps to rats during the final 2 wk of a 6-wk low-fat (LF) or high-fat (HF) diet. Insulin-stimulated glucose transport was impaired by the HF diet (HF-Sal) but was restored with leptin administration (HF-Lep). This improvement was associated with restored phosphorylation of Akt and AS160 and decreased in reactive lipid species (ceramide, diacylglycerol), known inhibitors of the insulin-signaling cascade. Total muscle citrate synthase (CS) activity was increased by both leptin and HF diet, but was not additive. Leptin increased subsarcolemmal (SS) and intramyofibrillar (IMF) mitochondria CS activity. Total muscle, sarcolemmal, and mitochondrial (SS and IMF) FA transporter (FAT/CD36) protein content was significantly increased with the HF diet, but not altered by leptin. Therefore, the decrease in reactive lipid stores and subsequent improvement in insulin response, secondary to leptin administration in rats fed a HF diet was not due to a decrease in FA transport protein content or altered cellular distribution.
A single prior bout of exercise protects against palmitate-induced insulin resistance despite an increase in total ceramide content. Thrush, A. Brianne, Harasim, Ewa, Chabowski, Adrian, Gulli, Roberto, Stefanyk, Leslie, and Dyck, David J. American journal of physiology. Regulatory, integrative and comparative physiology 2011
Ceramide accumulation has been implicated in the impairment of insulin-stimulated glucose transport in skeletal muscle following saturated fatty acid (FA) exposure. Importantly, a single bout of exercise can protect against acute lipid-induced insulin resistance. The mechanism by which exercise protects against lipid-induced insulin resistance is not completely known but may occur through a redirection of FA toward triacylglycerol (TAG) and away from ceramide and diacylglycerol (DAG). Therefore, in the current study, an in vitro preparation was used to examine whether a prior bout of exercise could confer protection against palmitate-induced insulin resistance and whether the pharmacological [50 muM fumonisin B(1) (FB1)] inhibition of ceramide synthesis in the presence of palmitate could mimic the protective effect of exercise. Soleus muscle of sedentary (SED), exercised (EX), and SED in the presence of FB1 (SED+FB1) were incubated with or without 2 mM palmitate for 4 h. This 2-mM palmitate exposure impaired insulin-stimulated glucose transport (-28%, P \textless 0.01) and significantly increased ceramide, DAG, and TAG accumulation in the SED group (P \textless 0.05). A single prior bout of exercise prevented the detrimental effects of palmitate on insulin signaling and caused a partial redistribution of FA toward TAG (P \textless 0.05). However, the net increase in ceramide content in response to palmitate exposure in the EX group was not different compared with SED, despite the maintenance of insulin sensitivity. The incubation of soleus from SED rats with FB1 (SED+FB1) prevented the detrimental effects of palmitate and caused a redirection of FA toward TAG accumulation (P \textless 0.05). Therefore, this research suggests that although inhibiting ceramide accumulation can prevent the detrimental effects of palmitate, a single prior bout of exercise appears to protect against palmitate-induced insulin resistance, which may be independent of changes in ceramide content.
Restoration of skeletal muscle leptin response does not precede the exercise-induced recovery of insulin-stimulated glucose uptake in high-fat-fed rats. Ritchie, Ian R. W., Gulli, Roberto A., Stefanyk, Leslie E., Harasim, Ewa, Chabowski, Adrian, and Dyck, David J. American journal of physiology. Regulatory, integrative and comparative physiology 2011
Leptin administration increases fatty acid (FA) oxidation rates and decreases lipid storage in oxidative skeletal muscle, thereby improving insulin response. We have previously shown high-fat (HF) diets to rapidly induce skeletal muscle leptin resistance, prior to the disruption of normal muscle FA metabolism (increase in FA transport; accumulation of triacylglycerol, diacylglycerol, ceramide) that occurs in advance of impaired insulin signaling and glucose transport. All of this occurs within a 4-wk period. Conversely, exercise can rapidly improve insulin response, in as little as one exercise bout. Thus, if the early development of leptin resistance is a contributor to HF diet-induced insulin resistance (IR) in skeletal muscle, then it is logical to predict that the rapid restoration of insulin response by exercise training would be preceded by the recovery of leptin response. In the current study, we sought to determine 1) whether 1, 2, or 4 wk of exercise training was sufficient to restore leptin response in isolated soleus muscle of rats already consuming a HF diet (60% kcal), and 2) whether this preceded the training-induced corrections in FA metabolism and improved insulin-stimulated glucose transport. In the low-fat (LF)-fed control group, insulin increased glucose transport by 153% and leptin increased AMPK and ACC phosphorylation and the rate of palmitate oxidation (+73%). These responses to insulin and leptin were either severely blunted or absent following 4 wk of HF feeding. Exercise intervention decreased muscle ceramide content (-28%) and restored insulin-stimulated glucose transport to control levels within 1 wk; muscle leptin response (AMPK and ACC phosphorylation, FA oxidation) was also restored, but not until the 2-wk time point. In conclusion, endurance exercise training is able to restore leptin response, but this does not appear to be a necessary precursor for the restoration of insulin response.
Oral administration of a PPAR-delta agonist to rodents worsens, not improves, maximal insulin-stimulated glucose transport in skeletal muscle of different fibers. Cresser, Justin, Bonen, Arend, Chabowski, Adrian, Stefanyk, Leslie E., Gulli, Roberto, Ritchie, Ian, and Dyck, David J. American journal of physiology. Regulatory, integrative and comparative physiology 2010
Agonists targeting the nuclear receptor peroxisome proliferator-activated receptors (PPAR)-delta may be potential therapeutic agents for insulin-resistant related conditions, as they may be able to stimulate fatty acid (FA) oxidation and attenuate the accumulation of harmful lipid species in skeletal muscle. Several reports have demonstrated that PPAR-delta agonists improve whole body insulin sensitivity. However, whether these agonists exert their direct effects on glucose and FA metabolism in skeletal muscle, and specifically with different fiber types, is unknown. This study was undertaken to determine the effects of oral treatment with the PPAR-delta agonist, GW 501516, in conjunction with the administration of a high-saturated-fat diet on insulin-stimulated glucose transport in isolated oxidative (soleus) and glycolytic (epitrochlearis) rodent skeletal muscle in vitro. High-fat feeding significantly decreased maximal insulin-stimulated glucose transport in soleus, but not epitrochlearis muscle, and was associated with increased skeletal muscle diacylglycerol and ceramide content. Unexpectedly, treatment with the PPAR-delta agonist significantly reduced insulin-stimulated glucose transport in both soleus and epitrochlearis muscles, regardless of dietary fat content. The reduction in insulin-stimulated glucose transport induced by the agonist was associated with large increases in total muscle fatty acid translocase (FAT)/CD36protein content, but not diacylglycerol or ceramide contents. Agonist treatment did not alter the protein content of PPAR-delta, GLUT4, or insulin-signaling proteins (IRS-1, p85 PI3-K, Akt). Agonist treatment led to a small, but significant increase, in the oxidative capacity of glycolytic but not oxidative muscle. We propose that chronic treatment with the PPAR-delta agonist GW 501516 may induce or worsen insulin resistance in rodent skeletal muscle by increasing the capacity for FA transport across the sarcolemma without a sufficient compensatory increase in FA oxidation. However, an accumulation of diacylglycerol and ceramide, while associated with diet-induced insulin resistance, does not appear to be responsible for the agonist-induced reduction in insulin-stimulated glucose transport.