Spike sorting is one of the most important data analysis problems in neurophysiology. The precision in all steps of the spike-sorting procedure critically affects the accuracy of all subsequent analyses. After data preprocessing and spike detection have been carried out properly, both feature extraction and spike clustering are the most critical subsequent steps of the spike-sorting procedure. The proposed spike sorting approach comprised a new feature extraction method based on shape, phase, and distribution features of each spike (hereinafter SS-SPDF method), which reveal significant information of the neural events under study. In addition, we applied an efficient clustering algorithm based on K -means and template optimization in phase space (hereinafter K -TOPS) that included two integrative clustering measures (validity and error indices) to verify the cohesion-dispersion among spike events during classification and the misclassification of clustering, respectively. The proposed method/algorithm was tested on both simulated data and real neural recordings. The results obtained for these datasets suggest that our spike sorting approach provides an efficient way for sorting both single-unit spikes and overlapping waveforms. By analyzing raw extracellular recordings collected from the rostral-medial prefrontal cortex (rmPFC) of behaving rabbits during classical eyeblink conditioning, we have demonstrated that the present method/algorithm performs better at classifying spikes and neurons and at assessing their modulating properties than other methods currently used in neurophysiology.

It is assumed that the claustrum (CL) is involved in sensorimotor integration and cognitive processes. We recorded the firing activity of identified CL neurons during classical eyeblink conditioning in rabbits, using a delay paradigm in which a tone was presented as conditioned stimulus (CS), followed by a corneal air puff as unconditioned stimulus (US). Neurons were identified by their activation from motor (MC), cingulate (CC), and medial prefrontal (mPFC) cortices. CL neurons were rarely activated by single stimuli of any modality. In contrast, their firing was significantly modulated during the first sessions of paired CS/US presentations, but not in well-trained animals. Neuron firing rates did not correlate with the kinematics of conditioned responses (CRs). CL local field potentials (LFPs) changed their spectral power across learning and presented well-differentiated CL–mPFC/CL–MC network dynamics, as shown by crossfrequency spectral measurements. CL electrical stimulation did not evoke eyelid responses, even in trained animals. Silencing of synaptic transmission of CL neurons by the vINSIST method delayed the acquisition of CRs but did not affect their presentation rate. The CL plays an important role in the acquisition of associative learning, mostly in relation to the novelty of CS/US association, but not in the expression of CRs.

We were interested in determining whether rostral medial prefrontal cortex (rmPFC) neurons participate in the measurement of conditioned stimulus–unconditioned stimulus (CS-US) time intervals during classical eyeblink conditioning. Rabbits were conditioned with a delay paradigm consisting of a tone as CS. The CS started 50, 250, 500, 1000, or 2000 ms before and coterminated with an air puff (100 ms) directed at the cornea as the US. Eyelid movements were recorded with the magnetic search coil technique and the EMG activity of the orbicularis oculi muscle. Firing activities of rmPFC neurons were recorded across conditioning sessions. Reflex and conditioned eyelid responses presented a dominant oscillatory frequency of ≈12 Hz. The firing rate of each recorded neuron presented a single peak of activity with a frequency dependent on the CS-US interval (i.e., ≈12 Hz for 250 ms, ≈6 Hz for 500 ms, and≈3 Hz for 1000 ms). Interestingly, rmPFC neurons presented their dominant firing peaks at three precise times evenly distributed with respect to CS start and also depending on the duration of the CS-US interval (only for intervals of 250, 500, and 1000 ms). No significant neural responses were recorded at very short (50 ms) or long (2000 ms) CS-US intervals. rmPFC neurons seem not to encode the oscillatory properties characterizing conditioned eyelid responses in rabbits, but are probably involved in the determination of CS-US intervals of an intermediate range (250–1000 ms). We propose that a variable oscillator underlies the generation of working memories in rabbits.SIGNIFICANCE STATEMENTThe way in which brains generate working memories (those used for the transient processing and storage of newly acquired information) is still an intriguing question. Here, we report that the firing activities of neurons located in the rostromedial prefrontal cortex recorded in alert behaving rabbits are controlled by a dynamic oscillator. This oscillator generated firing frequencies in a variable band of 3–12 Hz depending on the conditioned stimulus–unconditioned stimulus intervals (1 s, 500 ms, 250 ms) selected for classical eyeblink conditioning of behaving rabbits. Shorter (50 ms) and longer (2 s) intervals failed to activate the oscillator and prevented the acquisition of conditioned eyelid responses. This is an unexpected mechanism to generate sustained firing activities in neural circuits generating working memories.

The cerebellum-red nucleus-facial motoneuron (Mn) pathway has been reported as being involved in the proper timing of classically conditioned eyelid responses. This special type of associative learning serves as a model of event timing for studying the role of the cerebellum in dynamic motor control. Here, we have re-analyzed the firing activities of cerebellar posterior interpositus (IP) neurons and orbicularis oculi (OO) Mns in alert behaving cats during classical eyeblink conditioning, using a delay paradigm. The aim was to revisit the hypothesis that the IP neurons can be considered a neuronal phase-modulating device supporting OO Mns firing with an emergent timing mechanism and an explicit correlation code during learned eyelid movements. Optimized experimental and computational tools allowed us to determine the different causal relationships (temporal order and correlation code) during and between trials. These intra- and inter-trial timing strategies expanding from sub-second range (millisecond timing) to longer-lasting ranges (interval timing) expanded the functional domain of cerebellar timing beyond motor control. Interestingly, the results supported the above-mentioned hypothesis. The causal inferences were influenced by the precise motor and premotor spike-timing in the cause-effect interval, and, in addition, the timing of the learned responses depended on cerebellar-Mn network causality. Furthermore, the timing of CRs depended upon the probability of simulated causal conditions in the cause-effect interval and not the mere duration of the inter-stimulus interval. In this work, the close relation between timing and causality was verified. It could thus be concluded that the firing activities of IP neurons may be related more to the proper performance of ongoing CRs (i.e., the proper timing as a consequence of the pertinent causality) than to their generation and/or initiation.

In early stages of Alzheimer's disease (AD), soluble amyloid‐β (Aβ) is a key player disrupting neuronal activity and contributing to cognitive decline in advanced stages of the disease. Although the hippocampus has been a central focus in prior research because of its susceptibility to Aβ‐induced alterations, a comprehensive understanding of the temporal progression of early AD pathology requires exploring interconnected brain regions. The posterior parietal cortex (PPC), collaborating closely with the hippocampus and involved in various memory processes, particularly spatial memory formation, holds particular significance. Investigating the function of the PPC is imperative because it may contribute to early AD characteristics and provide a more holistic perspective on disease progression. To address this gap, we examined the relationship between neural oscillations and memory processes in both the PPC and hippocampus, in a mouse model of early hippocampal amyloidosis generated by intracerebroventricular oligomeric Aβ _1‐42 (oAβ _1‐42 ) injection. By performing in vivo oscillatory activity recordings from these regions in alert animals, together with spatial and habituation memory tests (Barnes maze and open field habituation), we found oAβ _1‐42 to induce significant alterations in PPC oscillatory activity. These changes emerged several days after hippocampal disturbances showed as aberrant synaptic plasticity and network activity. Additionally, significant alterations of stereotyped behaviours were not found. Our results provide an electrophysiological substrate for persistent spatial memory deficits and the temporal progression pattern of the early deleterious effects caused by Aβ. Furthermore, investigating PPC oscillatory activity might be a valuable approach for early detection and intervention in AD. image Key points: Posterior parietal cortex (PPC), in close collaboration with the hippocampus, has been implicated in various memory processes disrupted in early Alzheimer's disease models. A mouse model of early Alzheimer's‐like hippocampal amyloidosis generated by intracerebroventricular oligomeric Aβ _1‐42 (oAβ1 42) injection was used to examine the relationship between neural oscillations and memory processes in both the PPC and hippocampus. oAβ _1‐42 induces alterations in spatial and habituation memory, associated with PPC aberrant oscillatory activity, several days after hippocampal synaptic plasticity and network activity disturbances were found. We provide an electrophysiological PPC‐mediated substrate for persistent spatial memory deficits and the temporal progression pattern of the early oscillatory deleterious effects caused by Aβ.

Cortical cytoarchitecture and lamination are believed to remain stable after completion of development. Here, we demonstrate that early postnatal and adult (P45–P60) inactivation of the Reln gene results in dispersion of layers II and III neocortical neurons which invade layer I. In addition, neuronal mispositioning was observed in the dentate gyrus, the CA1–CA3 regions and in the cerebellum. Interestingly, inactivation of the Reln gene at adult stages resulted in altered oscillatory network activity in different cortical regions and in the nucleus accumbens. Moreover, Reelin deficiency increased depression-like behavior and impaired long-term memory consolidation. Thus, besides its crucial developmental role, Reelin is important in maintaining adult cortical lamination and rhythmic neuronal synchronization. These results suggest that, in addition to physiological and synaptic plasticity, there may be a certain degree of laminar structural plasticity in adult stages, which may lead to reconsidering the origin of layering alterations observed in neuropsychiatric diseases.