When monkeys observe the movements of other animals, their brain cells respond by activating neurons in different regions of the brain. These responses may contribute to perceptual performance and decision making.
To test this hypothesis, we studied the response of VLPF neurons to observation of biological movements and object motion. Videos (12deg x 12deg) showing multiple biological stimuli and object motion were displayed while the monkey maintained fixation on a fixation window centered on the video.
The premotor cortex is an area of the motor cortex that lies within the frontal lobe just anterior to the primary motor cortex. This region is involved in several different types of motor activities, including planning movement, spatial guidance, sensory guidance, understanding the actions of others, and using abstract rules to perform specific tasks.
The premotor cortex sends axons to the primary motor cortex as well as directly to the spinal cord, so it is likely that this area plays an important role in the control of motor behavior. However, it is not completely understood what its functions are.
It is a region that is primarily studied in monkeys and humans. In some studies, it is known that axons from the premotor cortex are sent to areas of the supplementary motor area (SMA), which are responsible for selecting and programming sequences of movements. These are referred to as motor programs.
Neurons in the premotor cortex are also selectively activated before a movement starts. This is consistent with their posited role in the activation sequence for voluntary movements.
Some of these neurons are mirror neurons, which discharge when the monkey grasps an object and when it observes an experimenter grasping an object in the same way. These neurons are located in ventral premotor area F5 and parietal areas PFG and AIP7,15,23.
Observation of a hand grasping a cup to drink and a hand grasping a cup to clear the table after a meal activated different sets of F5 neurons in human subjects as observed by functional MRI. This is because the movements in these conditions were based on different behavioral contexts.
These results suggest that the premotor cortex is sensitive to the behavioral context of a movement and activates more strongly when this context is present. This is because it is able to determine the purpose of the movement and to guide the appropriate motor response.
Another interesting finding is that the premotor cortex is more active when the subject is performing a task that requires a lot of planning. This is because it is a more complex and specialized area than the primary motor cortex, which is more involved in simple movements that require little planning, such as palpating an object with the hand.
Neuron activation in the postmotor cortex is a significant part of monkey motor learning and recovery following a stroke or traumatic brain injury. This area is located in the front of the brain and mainly involves the activation of pyramidal neurons which are responsible for initiating voluntary movements.
Activation of these neurons is task specific, so the engagement of these neurons is determined by the type of motor skill being learned. It is believed that in the early stages of learning a motor skill, the activation of these neurons is based on a sequence of somatomotor transformations, whereas as the skill develops it becomes more oriented towards specific motor plans to achieve desired end results.
One of the main functions of this area is to control hand movements, so it is important that it is activated during grasping tasks (Fogassi et al., 2005). Grasping-related mirror neurons were recorded from parietal areas PFG and F5 while the monkey performed a motor task and observed the same action performed by an experimenter.
The mirror neurons discharged differently in the motor and visual conditions (Figure 3) when the monkey was executing grasping-to-eat or grasping-to-place actions (action-goal-related mirror neurons). This suggests that these mirror neurons code both the action itself and the overarching goal of the action, as a kinematic representation of what is being accomplished.
This is a highly interesting finding, as it has not been known for a long time how these mirror neurons work during grasping. Several factors have been suggested to explain their behavior, including the kinematics of grasping and the grip force used during the action.
However, the most consistent finding is that these mirror neurons discharge during the execution of a motor act as a reflection of its overarching goal. Moreover, the fact that they are coded in an orderly manner suggests that they are related to an underlying motor plan (Fogassi et al., 2010).
As seen in the previous sections, the monkey’s motor system is also involved in social cognition. Grasping, for instance, is a basic action that allows the monkey to perform complex tasks in social settings. This is possible because the monkey has access to a larger neural network, encoding aspects of social life that can help it make decisions and interact with others.
The visual cortex is a large region of the brain that receives information from the retina. It contains a complex map of the visual world. In mammals, the visual cortex is composed of multiple areas. Areas V1, V2, and V4 are known to be responsible for form recognition and object representation. In addition, the visual cortex is responsible for processing the speed of movement in motor tasks.
A key to understanding the function of the visual cortex is its cytological organization. Neuroanatomist Korbinian Brodmann first identified the neocortical surface as a mosaic of functionally distinct modules (Brodmann, 1909). Each module is separated by areal borders. These are defined by Nissl substance (cresyl violet) staining. The neocortical surface is also subdivided into individual regions, each with its own unique cytoarchitecture and functions.
These areas are arranged in an anatomically retinotopic manner, meaning that axons from specific parts of the visual field terminate at locations in the primary visual cortex. This enables us to compare the visual responses of different areas.
We used a series of spatially selective tests to determine how neurons are activated in the visual cortex when an animal is performing a task. We recorded the impulse activity of single cells in the visual cortex while subjects performed a fixational saccade to move an illusory stimulus over and around a stationary target.
Our results suggest that the neural activity of the visual cortex is enhanced when a receptive-field activating region (AR) is moved onto or off a stationary stimulus during a fixational saccade. In contrast, position/drift cells are not activated when an AR is swept across the stimulus during a fixational saccade.
To better understand the function of the visual cortex, we compared the activation patterns of V1 and V2 when a monkey performs a line detection task and a circle and wave detection task. We found that the population response shifted significantly between the two tasks. Under the line detection task, a sharp peak of collinear facilitation dominated the population response.
However, under the circle and wave detection tasks, a more diffused peak of geometric facilitation remained in the population response. This shift in the averaged population response correlated with the shift in the three-bar tuning surfaces of the individual neurons in the network (S7; Figs. S3, S4; Figure 5).
The averaged population response is therefore not the optimum stimulus shape preferred by individual neurons in the visual cortex. Rather, the optimum stimulus shapes are determined by the animal’s cognitive state. For example, in a line detection task, the animal prefers near-collinear contours, while in a circle or wave detection task, the animal prefers curved contours.
The sensory cortex is important for interpreting salient environmental events and prompting appropriate behavioral responses. This is a complex, dynamic task that requires the ability to quickly and accurately process transient and intense sensory inputs. This task is crucial for the efficient and coordinated control of a monkey’s behavior in an unpredictable environment. The sensory cortex has been linked to several neuroanatomical mechanisms that support this function.
One of these is a “saliency network” (SN) of cortical areas that detects and prioritizes sensory inputs that are relevant to the current behavior. The SN includes the primary sensory cortices, and the somatosensory cortex. Activation in the SN occurs as a result of a direct input from the primary sensory cortex. The SN also receives indirect input from other cortical regions, such as the prefrontal cortex.
Activation in the SN can be modulated by specific visual stimuli. These are known as modality-specific activations. These activate the SN as much as the primary sensory cortices do (see Methods;26).
We compared neuronal activity in the SN to that in the primary sensory cortex when monkeys were actively engaged in motor tasks. During these tasks, monkeys moved their arms to different target locations that varied in direction and distance from the center.
Neuronal activity in the SN was strongly correlated with the direction and distance of movement, suggesting that the SN is involved in both the physical movement of the arm and the mental rehearsal of this movement. This correlation was similar to the relationship between somatosensory and primary motor cortex, indicating that the SN is the key brain region for encoding movement direction and distance.
To test this idea, we presented video clips depicting a variety of biological stimuli and object movements. The videos were centered on a 6 deg x 6 deg fixation window and the monkey was required to fixate them for a randomized time interval.
For each video stimulus, we calculated two indexes that indicate the degree of selectivity by the neurons. The first one is the depth of tuning index (di), which measures the difference between a neuron’s maximal and minimal response; the second is the selectivity index (si), which measures the extent to which a neuron’s activity in non-preferred stimuli deviates from its maximal activity.