The present study was aimed at developing a new strategy to design and anchor custom-fitted implants, consisting of a head fixation device and a chronic recording chamber, on the skull of adult macaque monkeys. This was done without the use of dental resin or orthopedic cement, as these modes of fixation exert a detrimental effect on the bone. The implants were made of titanium or tekapeek and anchored to the skull with titanium screws. Two adult macaque monkeys were initially implanted with the head fixation device several months previous to electrophysiological investigation, to allow optimal osseous-integration, including growth of the bone above the implant's footplate. In a second step, the chronic recording chamber was implanted above the brain region of interest. The present study proposes two original approaches for both implants. First, based on a CT scan of the monkey, a plastic replicate of the skull was obtained in the form of a 3D print, used to accurately shape and position the two implants. This would ensure a perfect match with the skull surface. Second, the part of the implants in contact with the bone was coated with hydroxyapatite, presenting chemical similarity to natural bone, thus promoting excellent osseous-integration. The longevity of the implants used here was 4 years for the head fixation device and 1.5 years for the chronic chamber. There were no adverse events and daily care was easy. This is clear evidence that the present implanting strategy was successful and provokes less discomfort to the animals.

Abstract-Cortico-cortical connectivity has become a major focus of neuroscience in the last decade but most of the connectivity studies focused on intrahemispheric circuits. Little has been reported about information acquired and processed in the premotor cortex and its functional connection with its homotopic counterpart in the opposite hemisphere via the corpus callosum. In non-human primates (macaques) lateralization is not well documented and its exact role is still unknown. The present study confirms in two macaques the existence of homotopic contralateral projections and completes the picture by further exploring heterotopic (non-motor) callosal projections. This was tested by injecting retrograde tracers in the premotor cortical areas PMv and PMd (targets). Our method consisted of identifying the connections with all the homo-and heterotopic cortical areas located in the contralateral hemisphere. The results showed that PMd and PMv receive multiple low-density labeled inputs from the opposite heterotopic prefrontal, parietal, motor, insular and temporal regions. Such unexpected collection of transcallosal inputs from heterotopic areas suggests that the premotor areas communicate with other modalities through long distance low-density networks which could have important implications in the understanding of sensorimotor and multimodal integration.

Daily our central nervous system receives inputs via several sensory modalities, processes them and integrates information in order to produce a suitable behavior. The amazing part is that such a multisensory integration brings all information into a unified percept. An approach to start investigating this property is to show that perception is better and faster when multimodal stimuli are used as compared to unimodal stimuli. This forms the first part of the present study conducted in a non-human primate’s model (n = 2) engaged in a detection sensory-motor task where visual and auditory stimuli were displayed individually or simultaneously. The measured parameters were the reaction time (RT) between stimulus and onset of arm movement, successes and errors percentages, as well as the evolution as a function of time of these parameters with training. As expected, RTs were shorter when the subjects were exposed to combined stimuli. The gains for both subjects were around 20 and 40 ms, as compared with the auditory and visual stimulus alone, respectively. Moreover the number of correct responses increased in response to bimodal stimuli. We interpreted such multisensory advantage through redundant signal effect which decreases perceptual ambiguity, increases speed of stimulus detection, and improves performance accuracy. The second part of the study presents single-unit recordings derived from the premotor cortex (PM) of the same subjects during the sensory-motor task. Response patterns to sensory/multisensory stimulation are documented and specific type proportions are reported. Characterization of bimodal neurons indicates a mechanism of audio-visual integration possibly through a decrease of inhibition. Nevertheless the neural processing leading to faster motor response from PM as a polysensory association cortical area remains still unclear.

High-density scalp EEG recordings are widely used to study whole-brain neuronal networks in humans non-invasively. Here, we validate EEG mapping of somatosensory evoked potentials (SSEPs) in macaque monkeys (Macaca fascicularis) for the long-term investigation of large-scale neuronal networks and their reorganisation after lesions requiring a craniotomy. SSEPs were acquired from 33 scalp electrodes in five adult anaesthetized animals after electrical median or tibial nerve stimulation. SSEP scalp potential maps were identified by cluster analysis and identified in individual recordings. A distributed, linear inverse solution was used to estimate the intracortical sources of the scalp potentials. SSEPs were characterised by a sequence of components with unique scalp topographies. Source analysis confirmed that median nerve SSEP component maps were in accordance with the somatotopic organisation of the sensorimotor cortex. Most importantly, SSEP recordings were stable both intra- and interindividually. We aim to apply this method to the study of recovery and reorganisation of large-scale neuronal networks following a focal cortical lesion requiring a craniotomy. As a prerequisite, the present study demonstrated that a 300-mm2 unilateral craniotomy over the sensorimotor cortex necessary to induce a cortical lesion, followed by bone flap repositioning, suture and gap plugging with calcium phosphate cement, did not induce major distortions of the SSEPs. In conclusion, SSEPs can be successfully and reproducibly recorded from high-density EEG caps in macaque monkeys before and after a craniotomy, opening new possibilities for the long-term follow-up of the cortical reorganisation of large-scale networks in macaque monkeys after a cortical lesion.Electronic supplementary materialThe online version of this article (doi:10.1007/s00429-014-0776-y) contains supplementary material, which is available to authorized users.

The amplitude of the brain response to a repeated auditory stimulus is diminished as compared to the response to the first tone (T1) for interstimulus intervals (ISI) lasting up to hundreds of milliseconds. This adaptation process, called auditory sensory gating (ASG), is altered in various psychiatric diseases including schizophrenia and is classically studied by focusing on early evoked cortical responses to the second tone (T2) using 500-ms ISI. However, mechanisms underlying ASG are still not well-understood. We investigated ASG in awake mice from the brainstem to cortex at variable ISIs (125–2000 ms) using high-density EEG and intracerebral recordings. While ASG decreases at longer ISIs, it is still present at durations (500–2000 ms) far beyond the time during which brain responses to T1 could still be detected. T1 induces a sequence of specific stable scalp EEG topographies that correspond to the successive activation of distinct neural networks lasting about 350 ms. These brain states remain unaltered if T2 is presented during this period, although T2 is processed by the brain, suggesting that ongoing networks of brain activity are active for longer than early evoked-potentials and are not overwritten by an upcoming new stimulus. Intracerebral recordings demonstrate that ASG is already present at the level of ventral cochlear nucleus (vCN) and inferior colliculus and is amplified across the hierarchy in bottom-up direction. This study uncovers the extended stability of sensory-evoked brain states and long duration of ASG, and sheds light on generators of ASG and possible interactions between bottom-up and top-down mechanisms.