The study of the neuron activation process in monkeys has a number of questions that need to be answered. These questions include the effect of stimulation pulses on monkey neurons and the lateralization of monkey neurons’ activation. In addition, the study of the latencies of monkey neurons in the standard detection task is also an important aspect of the investigation.
Observation of non-goal-directed actions
To date, researchers have demonstrated the use of a number of techniques to observe non-goal-directed actions in humans and monkeys. One such technique is local field potentials (LFPs). For the most part, these measures have been used to study the activity of the somatosensory cortex.
A recent study measured the scalp-EEG of adult macaques. They were presented with a set of visual stimuli including the face of a human agent. If the monkey correctly fixed the window for a defined period of time, juice was delivered. However, the experimental design did not have enough precise timing to measure the exact moment when a stimulus triggered a specific action.
The single cell recording of a neuron in the primary motor cortex of a macaque revealed that it was capable of coding a grasping gesture. Not only could the neuron code the grasping motion, it also discharged during the action. This is a very unusual event for the motor system.
Recent fMRI studies have shown that the putamen and basal ganglia are involved in the process of grasping. Additionally, recent single-cell recordings show that the F5 in the ventral premotor cortex of monkeys can be activated by somatosensory stimuli. It is unclear whether these neurons are purely motor or involve other sub-components.
In particular, the mirror neuron has been studied in a number of macaque monkeys. These neurons are thought to be responsible for desynchronizing the EEG in humans.
Evidence for a mirror matching mechanism in humans
Mirror neurons are a phenomenon spotted in some animals, including the monkey and the human. They are believed to be important in action observation and gesture understanding. These neuronal mechanisms are not yet fully understood in humans, but recent evidence points to their existence.
Mirror neurons are thought to be responsible for a lot of things, including coordinating behaviors during social interactions. These neurons are known to be able to remap other people’s actions and emotions onto the brain structure of the beholder. This may be part of a much larger neural system. However, there are no plausible models to explain this.
In recent years, studies have been conducted on mirror neurons in different animal species, from monkeys to humans. Researchers have used functional magnetic resonance imaging (fMRI) to image the activity of these neurons. The results have shown that they respond to different tasks, such as grasping and watching others do a task. Moreover, they have been found to be especially active when the monkey is performing a certain task.
Although the mirror neuron has been the subject of several studies, the actual discovery remains a mystery. However, research has found that this phenomenon can be seen in infant macaques, who have been observed to mimic the movements of their human counterparts during limited temporal windows. A few studies have suggested that this activity is induced by the sound of a task being performed.
Axon impulses propagate in both directions
During activation of a monkey neuron, axon impulses travel in both directions. Axons are the ‘wires’ of the nervous system. They form a synapse with other neurones. Neurotransmitters then diffuse across the space between the axon and the next neuron, transforming the stimulus into an impulse. The impulse is transmitted to the effector cells.
The axon is the proximal end of motor neurons. Initially, the axon has a spatially uniform resting potential of -60 mV. But once the axon has been stimulated, the resting potential drops to -50 mV. This change is called the action potential.
Axons are also leaky tubes of fluid. Unlike copper wires, axons do not conduct electricity as efficiently. Instead, the rate of conduction is affected by the axon’s size. Larger axons are more effective at transmitting nerve impulses because they are easier to stimulate. Smaller axons, on the other hand, have less leakage and are faster to conduction.
Nerve impulses have three main phases: hyperpolarisation, repolarisation and depolarisation. Each phase is approximately a millisecond long. For example, the depolarisation phase of an action potential will last for two milliseconds.
When the axon is activated, the ions and proteins that are inside the membrane become active. In the absence of protein, ions can’t flow through the membrane.
Sodium and potassium ion channels are concentrated at the Nodes of Ranvier. Depending on the axon’s size and diameter, these channels spread the wave of depolarisation along the axon.
Effects of stimulation pulses on monkey neuron activation
Microstimulation is widely used in monkeys performing behavioural tasks. These studies have contributed to our understanding of how neural activity is coded for behavior and have provided guidance for future work. In this review, we explore the relationship between microstimulation and the detection of visual stimuli.
Neurons respond selectively to complex visual stimuli. For instance, neurons in IT respond to faces. The presence of these cells in patches and their spatial arrangement suggest that they are highly sensitive to stimuli associated with a particular face. However, their precise function is unclear. This leads to the question of whether changes in the neuronal activity of these neurons alter animal perception.
To investigate the effects of stimulation pulses on monkey neuron activation, we measured the period of periodicity in evoked spike trains. We then applied this information to determine the amplitude of the stimulus. As a result, we produced neurometric and psychometric curves. They are illustrated in Figure 3E.
The neurometric curves are similar to the psychometric curves in their shape and slope. However, they differ in their amplitude modulation. Moreover, the mean psychometric curve is steeper than the neurometric curve.
Microstimulation biases perception towards weakly coherent motion stimuli. Consequently, monkeys are less likely to detect veridical visual stimuli when trained to detect microstimulation. Furthermore, microstimulation can interfere with other visual tasks.
Although it is possible to perform a standard detection task with microstimulation, this does not indicate that such a task is optimal. Instead, a more comprehensive approach is needed.
Latencies of monkey neurons during the standard detection task
When monkeys performed a standard detection task with a visual stimulus, the latencies of their neurons were similar. During the late phase of the trial, some of the neurons showed a significant increase in activity and some decreased activity. These results suggest that the primate somatosensory thalamus may play a role in perceptual performance. However, the sensitivity of the monkeys to the amplitude of the visual stimulus was similar regardless of whether they detected it or not.
In addition, a permutation test was applied to the latencies of the neurons recorded during the passive stimulation and the somatosensory detection tasks. The results showed that the amplitude modulation of the curve accounted for the variations in monkey/neuron performance in the relevant interval.
Using this approach, the latencies of monkeys’ neurons during the somatosensory detection task were compared to those during the color change task. During the latter task, a stimulus omission was required. As a result, the amplitude modulation of the curve was calculated as the difference in the detection probability between the maximum and minimum amplitudes.
Similarly, the latencies of the neurons in the dentate nucleus were compared to those in the caudate nucleus. The study showed that the dentate neuron’s firing rate was significantly lower than that in the caudate. This suggests that the caudate neuron’s latencies may be related to the omission of a stimulus.
Evidence for lateralization of monkey neuron activation
The two hemispheres of the monkey brain play a complementary role during voluntary movement. A lateralized pattern of brain activation is evident during a reach or grasping act, and a lesion of one hemisphere can produce a variety of syndromes. Some examples include aphasia, neglect, and apraxia. This review of lateralization and neuronal coding provides a summary of how this process works in nonhuman primates, as well as a discussion of its implications for motor execution.
Laterality is a controversial topic. While some evidence exists to support it, others argue that there is no such thing as lateralization. It is also not certain whether the lateralization is functional or neuroanatomical.
Several studies have examined the influence of lateralization on motor acts. These studies involved using electrodes to record activity from single neurons in the MNS during various hand-actions. Those experiments demonstrated that there were many neurons that responded to both hands equally, although some of them responded more strongly to the right hand.
Activation of a hemisphere is usually associated with a corresponding deficit in the opposite hemisphere. For example, a lesion of the ipsilateral upper limb can affect the performance of the contralateral hand. However, this is not always the case.
Another example of lateralization is the activation of the parieto-premotor circuit during a reach or grasping action. Although several studies have reported a left-lateralized pattern of activation, the details of the activation remain elusive.