Alexander Kraskov

Object selectivity of local field potentials and spikes in the macaque inferior temporal cortex.

Local field potentials (LFPs) arise largely from dendritic activity over large brain regions and thus provide a measure of the input to and local processing within an area. We characterized LFPs and their relationship to spikes (multi and single unit) in monkey inferior temporal cortex (IT). LFP responses in IT to complex objects showed strong selectivity at 44% of the sites and tolerance to retinal position and size. The LFP preferences were poorly predicted by the spike preferences at the same site but were better explained by averaging spikes within approximately 3 mm. A comparison of separate sites suggests that selectivity is similar on a scale of approximately 800 microm for spikes and approximately 5 mm for LFPs. These observations imply that inputs to IT neurons convey selectivity for complex shapes and that such input may have an underlying organization spanning several millimeters.

Corticospinal Neurons in Macaque Ventral Premotor Cortex with Mirror Properties: A Potential Mechanism for Action Suppression?

Summary The discovery of “mirror neurons” in area F5 of the ventral premotor cortex has prompted many theories as to their possible function. However, the identity of mirror neurons remains unknown. Here, we investigated whether identified pyramidal tract neurons (PTNs) in area F5 of two adult macaques exhibited “mirror-like” activity. About half of the 64 PTNs tested showed significant modulation of their activity while monkeys observed precision grip of an object carried out by an experimenter, with somewhat fewer showing modulation during precision grip without an object or grasping concealed from the monkey. Therefore, mirror-like activity can be transmitted directly to the spinal cord via PTNs. A novel finding is that many PTNs (17/64) showed complete suppression of discharge during action observation, while firing actively when the monkey grasped food rewards. We speculate that this suppression of PTN discharge might be involved in the inhibition of self-movement during action observation.

Latency and selectivity of single neurons indicate hierarchical processing in the human medial temporal lobe

Neurons in the temporal lobe of both monkeys and humans show selective responses to classes of visual stimuli and even to specific individuals. In this study, we investigate the latency and selectivity of visually responsive neurons recorded from microelectrodes in the parahippocampal cortex, entorhinal cortex, hippocampus, and amygdala of human subjects during a visual object presentation task. During 96 experimental sessions in 35 subjects, we recorded from a total of 3278 neurons. Of these units, 398 responded selectively to one or more of the presented stimuli. Mean response latencies were substantially larger than those reported in monkeys. We observed a highly significant correlation between the latency and the selectivity of these neurons: the longer the latency the greater the selectivity. Particularly, parahippocampal neurons were found to respond significantly earlier and less selectively than those in the other three regions. Regional analysis showed significant correlations between latency and selectivity within the parahippocampal cortex, entorhinal cortex, and hippocampus, but not within the amygdala. The later and more selective responses tended to be generated by cells with sparse baseline firing rates and vice versa. Our results provide direct evidence for hierarchical processing of sensory information at the interface between the visual pathway and the limbic system, by which increasingly refined and specific representations of stimulus identity are generated over time along the anatomic pathways of the medial temporal lobe.

Sparse Representation in the Human Medial Temporal Lobe

Recent experiments characterized individual neurons in the human medial temporal lobe with remarkably selective, invariant, and explicit responses to images of famous individuals or landmark buildings. Here, we used a probabilistic analysis to show that these data are consistent with a sparse code in which neurons respond in a selective manner to a small fraction of stimuli.

M1 Corticospinal Mirror Neurons and Their Role in Movement Suppression during Action Observation

Summary Evidence is accumulating that neurons in primary motor cortex (M1) respond during action observation [1, 2], a property first shown for mirror neurons in monkey premotor cortex [3]. We now show for the first time that the discharge of a major class of M1 output neuron, the pyramidal tract neuron (PTN), is modulated during observation of precision grip by a human experimenter. We recorded 132 PTNs in the hand area of two adult macaques, of which 65 (49%) showed mirror-like activity. Many (38 of 65) increased their discharge during observation (facilitation-type mirror neuron), but a substantial number (27 of 65) exhibited reduced discharge or stopped firing (suppression-type). Simultaneous recordings from arm, hand, and digit muscles confirmed the complete absence of detectable muscle activity during observation. We compared the discharge of the same population of neurons during active grasp by the monkeys. We found that facilitation neurons were only half as active for action observation as for action execution, and that suppression neurons reversed their activity pattern and were actually facilitated during execution. Thus, although many M1 output neurons are active during action observation, M1 direct input to spinal circuitry is either reduced or abolished and may not be sufficient to produce overt muscle activity.

Interactions between areas of the cortical grasping network.

Highlights ► The anterior intraparietal area (AIP) is crucial for the processing of grasp-related object properties. ► AIP receives visual information about graspable objects from both the dorsal and ventral stream. ► Reciprocal interactions between the ventral premotor (PMv) and primary motor cortex (M1) allow the motor command to be grasp-specific. ► AIP plays a causal role in influencing interactions between PMv and M1.

Large Identified Pyramidal Cells in Macaque Motor and Premotor Cortex Exhibit “Thin Spikes”: Implications for Cell Type Classification

Recent studies have suggested that extracellular recordings of putative cortical interneurons have briefer spikes than those of pyramidal neurons, providing a means of identifying cortical cell types in recordings from awake monkeys. To test this, we investigated the spike duration of antidromically identified pyramidal tract neurons (PTNs) recorded from primary motor (M1) or ventral premotor cortex (area F5) in 4 awake macaque monkeys. M1 antidromic latencies (ADLs) were skewed toward short ADLs (151 PTNs; 0.5–5.5 ms, median 1.1 ms) and significantly different from that of F5 ADLs (54 PTNs; 1.0–6.9 ms, median 2.6 ms). The duration of PTN spikes, recorded with a high-pass filter of 300 Hz and measured from the negative trough to the positive peak of the spike waveform, ranged from 0.15 to 0.71 ms. Importantly, we found a positive linear correlation between ADL and spike duration in both M1 (R2 = 0.40, p < 0.001) and F5 (R2 = 0.57, p < 0.001). Thus PTNs with the shortest ADL (fastest axons) had the briefest spikes, and since PTN soma size is correlated with axon size and conduction velocity, it is likely that the largest pyramidal neurons (Betz cells in M1) have spikes with short durations (0.15–0.45 ms), which overlap heavily with those reported for putative interneurons in previous studies in non-primates. In summary, one class of physiologically identified cortical pyramidal neuron exhibits a wide variety of spike durations and the results suggest that spike duration alone may not be a reliable indicator of cell type.

A category-specific response to animals in the right human amygdala

The amygdala is important in emotion, but it remains unknown whether it is specialized for certain stimulus categories. We analyzed responses recorded from 489 single neurons in the amygdalae of 41 neurosurgical patients and found a categorical selectivity for pictures of animals in the right amygdala. This selectivity appeared to be independent of emotional valence or arousal and may reflect the importance that animals held throughout our evolutionary past.

Selectivity for grasp in local field potential and single neuron activity recorded simultaneously from M1 and F5 in the awake macaque monkey

The selectivity for object-specific grasp in local field potentials (LFPs) was investigated in two awake macaque monkeys trained to observe, reach out, grasp and hold one of six objects presented in a pseudorandom order. Simultaneous, multiple electrode recordings were made from the hand representations of primary motor cortex (M1) and ventral premotor cortex (area F5). LFP activity was well developed during the observation and hold periods of the task, especially in the beta-frequency range (15–30 Hz). Selectivity of LFP activity for upcoming grasp was rare in the observation period, but common during stable grasp. The majority of M1 (90 of 92) and F5 (81of 97) sites showed selectivity for at least one frequency, which was maximal in the beta range but also present at higher frequencies (30–50 Hz). When the LFP power associated with grasp of a specific object was large in the beta-frequency range, it was usually of low power in the higher 30–50 Hz range, and vice-versa. Simple hook grips involving flexion of one or more fingers were associated with large beta power, whereas more complex grips involving the thumb (e.g., precision grip) were associated with small beta power. At many M1 sites, there was a highly significant inverse relationship between the tuning of spikes (including those of identified pyramidal tract neurons) and beta-range LFP for different grasps, whereas a positive correlation was found at higher frequencies (30–50 Hz). High levels of beta LFP and low pyramidal cell spike rate may reflect a common mechanism used to control motor set during different types of grasp.

Local Field Potentials and Spikes in the Human Medial Temporal Lobe are Selective to Image Category

Local field potentials (LFPs) reflect the averaged dendrosomatic activity of synaptic signals of large neuronal populations. In this study, we investigate the selectivity of LFPs and single neuron activity to semantic categories of visual stimuli in the medial temporal lobe of nine neurosurgical patients implanted with intracranial depth electrodes for clinical reasons. Strong selectivity to the category of presented images was found for the amplitude of LFPs in 8% of implanted microelectrodes and for the firing rates of single and multiunits in 14% of microelectrodes. There was little overlap between the LFP- and spike-selective microelectrodes. Separate analysis of the power and phase of LFPs revealed that the mean phase was category-selective around the frequency range and that the power of the LFPs was category-selective for high frequencies around the rhythm. Of the 36 microelectrodes with amplitude-selective LFPs, 30 were found in the hippocampus. Finally, it was possible to readout information about the category of stimuli presented to the patients with both spikes and LFPs. Combining spiking and LFP activity enhanced the decoding accuracy in comparison with the accuracy obtained with each signal alone, especially for short time intervals.