Modular, cement-free, customized headpost and connector-chamber implants for macaques.

BACKGROUND: Neurophysiological studies with awake macaques typically require chronic cranial implants. Headpost and connector-chamber implants are used to allow head stabilization and to house connectors of chronically implanted electrodes, respectively. NEW METHOD: We present long-lasting, modular, cement-free headpost implants made of titanium that consist of two pieces: a baseplate and a top part. The baseplate is implanted first, covered by muscle and skin and allowed to heal and osseointegrate for several weeks to months. The percutaneous part is added in a second, brief surgery. Using a punch tool, a perfectly round skin cut is achieved providing a tight fit around the implant without any sutures. We describe the design, planning and production of manually bent and CNC-milled baseplates. We also developed a remote headposting technique that increases handling safety. Finally, we present a modular, footless connector chamber that is implanted in a similar two-step approach and achieves a minimized footprint on the skull. RESULTS: Twelve adult male macaques were successfully implanted with a headpost and one with the connector chamber. To date, we report no implant failure, great headpost stability and implant condition, in four cases even more than 9 years post-implantation. COMPARISON WITH EXISTING METHODS: The methods presented here build on several related previous methods and provide additional refinements to further increase implant longevity and handling safety. CONCLUSIONS: Optimized implants can remain stable and healthy for at least 9 years and thereby exceed the typical experiment durations. This minimizes implant-related complications and corrective surgeries and thereby significantly improves animal welfare.

Spontaneous variability in gamma dynamics described by a damped harmonic oscillator driven by noise.

Circuits of excitatory and inhibitory neurons generate gamma-rhythmic activity (30-80 Hz). Gamma-cycles show spontaneous variability in amplitude and duration. To investigate the mechanisms underlying this variability, we recorded local-field-potentials (LFPs) and spikes from awake macaque V1. We developed a noise-robust method to detect gamma-cycle amplitudes and durations, which showed a weak but positive correlation. This correlation, and the joint amplitude-duration distribution, is well reproduced by a noise-driven damped harmonic oscillator. This model accurately fits LFP power-spectra, is equivalent to a linear, noise-driven E-I circuit, and recapitulates two additional features of gamma: (1) Amplitude-duration correlations decrease with oscillation strength; (2) amplitudes and durations exhibit strong and weak autocorrelations, respectively, depending on oscillation strength. Finally, longer gamma-cycles are associated with stronger spike-synchrony, but lower spike-rates in both (putative) excitatory and inhibitory neurons. In sum, V1 gamma-dynamics are well described by the simplest possible model of gamma: A damped harmonic oscillator driven by noise.

Predictive coding of natural images by V1 firing rates and rhythmic synchronization.

Predictive coding is an important candidate theory of self-supervised learning in the brain. Its central idea is that sensory responses result from comparisons between bottom-up inputs and contextual predictions, a process in which rates and synchronization may play distinct roles. We recorded from awake macaque V1 and developed a technique to quantify stimulus predictability for natural images based on self-supervised, generative neural networks. We find that neuronal firing rates were mainly modulated by the contextual predictability of higher-order image features, which correlated strongly with human perceptual similarity judgments. By contrast, V1 gamma (γ)-synchronization increased monotonically with the contextual predictability of low-level image features and emerged exclusively for larger stimuli. Consequently, γ-synchronization was induced by natural images that are highly compressible and low-dimensional. Natural stimuli with low predictability induced prominent, late-onset beta (ß)-synchronization, likely reflecting cortical feedback. Our findings reveal distinct roles of synchronization and firing rates in the predictive coding of natural images.

Surface color and predictability determine contextual modulation of V1 firing and gamma oscillations.

The integration of direct bottom-up inputs with contextual information is a core feature of neocortical circuits. In area V1, neurons may reduce their firing rates when their receptive field input can be predicted by spatial context. Gamma-synchronized (30-80 Hz) firing may provide a complementary signal to rates, reflecting stronger synchronization between neuronal populations receiving mutually predictable inputs. We show that large uniform surfaces, which have high spatial predictability, strongly suppressed firing yet induced prominent gamma synchronization in macaque V1, particularly when they were colored. Yet, chromatic mismatches between center and surround, breaking predictability, strongly reduced gamma synchronization while increasing firing rates. Differences between responses to different colors, including strong gamma-responses to red, arose from stimulus adaptation to a full-screen background, suggesting prominent differences in adaptation between M- and L-cone signaling pathways. Thus, synchrony signaled whether RF inputs were predicted from spatial context, while firing rates increased when stimuli were unpredicted from context.

Effects of acute systemic and intra-cerebral stimulation of cannabinoid receptors on sensorimotor gating, locomotion and spatial memory in rats.

RATIONALE: Cannabinoid CB(1) receptors in the brain are targets of both endocannabinoid signalling and the psychoactive compounds of the hemp plant. They mediate neuronal effects of their ligands in various corticolimbic and striatal circuits by presynaptic regulation of transmitter release. OBJECTIVES/METHODS: This study investigates acute systemic effects of the full CB(1) receptor agonist WIN 55,212-2 (WIN) on prepulse inhibition (PPI) of the acoustic startle response (ASR), locomotor activity and spatial memory retrieval in an eight-arm radial-maze task. Furthermore, we tested the effect of local intra-cerebral micro-infusions of WIN into the nucleus accumbens (NAc), ventral tegmental area (VTA), dorsal (dHIP) and ventral (vHIP) hippocampus and medial prefrontal cortex (mPFC). RESULTS: Systemic WIN (1.2 mg/kg) reduced PPI without affecting ASR, had no effect on locomotion in the open field, but impaired retrieval of spatial memory. Infusions of 5 microg/0.3 microl WIN into either NAc (core or shell), dHIP or VTA did not affect PPI and locomotion immediately afterwards. However, PPI was significantly reduced after intra-mPFC and intra-vHIP infusion of WIN. Furthermore, WIN infusion into dHIP increased the number of reference memory errors in the maze, suggesting impairment of memory retrieval. CONCLUSIONS: Our data support the notion that CB(1) receptor stimulation impairs sensorimotor gating most likely by modulation of neurotransmitter release in mPFC and vHIP. The lack of effects of local WIN infusions in NAc and VTA might be due to low receptor abundance in these regions. Additionally, CB(1) receptor activation in dHIP impairs spatial memory retrieval. Taken together, cortico-hippocampal cannabinoid receptors play an essential role in the regulation of cognitive and behavioural processes.