Sensory and motor systems are accountable for a large part of the body’s operation. The former encompasses vision, hearing, smell, taste, balance, and touch, whereas the latter comprises movements (Bowling, n.d.; Pinel & Barnes, 2022). While the sensory apparatus is afferent and goes toward the central nervous system (CNS), the motor is efferent and flows away from the CNS (Bowling, n.d.; Pinel & Barnes, 2022). Despite being quite different, the two are interconnected and together maintain various processes of receiving, analyzing, and responding to diverse signals. The sensory and motor function relies on the posterior parietal cortex (PPC) to connect the operations of distinct systems.
To understand the role of PPC, it is important to explore the body’s internal procedures. First, sensory systems (SSs) detect external stimuli and include visual, auditory, somatosensory, olfactory, and gustatory systems (Pinel & Barnes, 2022). SSs are managed within three sensory cortex areas: primary, secondary, and association, with each receiving input from distinct sources (Pinel & Barnes, 2022). Moreover, SSs have a hierarchical organization, meaning that every level gets information from lower tiers, and the association cortex is the highest one because the input is acquired from several SSs (Pinel & Barnes, 2022). A noteworthy characteristic of SSs is functional segregation, which suggests that the hierarchy’s levels each perform different types of analysis (Bowling, n.d.; Pinel & Barnes, 2022). Finally, SSs feature parallel processing, with information flowing in several pathways rather than simply one (Bowling, n.d.; Pinel & Barnes, 2022). Accordingly, the significance of the PPC can be illustrated by the fact that it can conduct diverse interactions, such as those between auditory and visual systems (Pinel & Barnes, 2022). Despite having distinguishable purposes, SSs are interconnected and can be governed within the PPC.
A part of SSs that deserves particular attention is the somatosensory system. The section is accountable for sensing touch, temperature, and pain while monitoring the body’s position alongside such internal conditions as blood pressure (Pinel & Barnes, 2022). The system can be characterized by primary somatosensory (SI), secondary somatosensory (SII), and somatosensory association cortexes (Bowling, n.d.; Pinel & Barnes, 2022). The latter receives SI and SII’s output and is affiliated with the prefrontal cortex, which oversees decision-making, attention, and affective touch, alongside the PPC, which participates in multisensory integration (Bowling, n.d.; Pinel & Barnes, 2022). In particular, the PPC has bimodal neurons that respond to the activation of different SSs (Pinel & Barnes, 2022). For instance, the concept of multisensory integration can be illustrated based on the rubber hand illusion, which refers to the feeling of an extraneous object being part of one’s own body (Pinel & Barnes, 2022). In such a situation, parietal-bimodal neurons with both somatosensory and visual fields play an influential role in the delusion’s induction (Pinel & Barnes, 2022). Consequently, SSs’ somatosensory sector governs various body perceptions and can link with other systems through the PPC.
SSs connect with the motor system (MS), forming the sensorimotor system (SMS). MS oversees the movement of the body, but the motor output is directed by sensory input (Pinel & Barnes, 2022). As a result, SMS unites the signals and processes of both SSs and MS. Notably, SMS has a hierarchical organization, with commands moving down from the sensorimotor association cortex to lower levels of secondary (MII) and primary (MI) motor cortexes (Bowling, n.d.; Pinel & Barnes, 2022). Moreover, similar to SSs, SMS is also parallel and functionally segregated, but information typically flows down in SS and goes up through the tiers in SMS (Pinel & Barnes, 2022). Therefore, MS does not necessarily operate on its own but rather depends on SSs connecting in SMS.
Furthermore, to understand the PMS’s value for MS and SSs, it is significant to explore the specifics of SMS. As noted above, the highest level of the SMS hierarchy is the sensorimotor association cortex. Accordingly, the tier encompasses the dorsolateral prefrontal and posterior parietal association cortexes, with each having distinct areas and diverse functions (Pinel & Barnes, 2022). The two sections send signals down to MII, which in turn partakes in the programming of movements’ specific patterns (Bowling, n.d.; Pinel & Barnes, 2022). MI receives input from MII, and its neurons play a substantial role in initiating body motions (Bowling, n.d.; Pinel & Barnes, 2022). As a result, since the posterior parietal association cortex is located at the top of the SMS hierarchy, the PPC is involved in managing one’s movements.
The above examination of SSs, MS, and SMS suggests that the PPC is directly connected to various processes. Consequently, the primary role of the PPC lies in uniting the body’s basic operations in SMS. The PPC is classified as an association cortex due to receiving signals from distinct SSs that are visual, auditory, and somatosensory systems since they analyze one’s placement in relation to external objects (Pinel & Barnes, 2022). The output of the PPC then goes to motor cortex regions, producing MS’s motion responses to information from SSs (Pinel & Barnes, 2022). Furthermore, to better understand the importance of the PPC, it is useful to consider findings from other sources. Li et al. (2022) propose that in addition to the PPC linking sensation and action, the cortex does not simply represent spatial target location but has a proactive role in motor planning. Chivukula et al. (2019) suggest that the PPC contributes to filtering sensory distractors and partakes in movement-related decision-making. Accordingly, the PPC interacts with diverse systems and assists in their functioning by directing signals from SSs to relevant responses in MS.
The PPC connects sensorimotor processes, but its role may be hindered if the cortex is damaged. Poor functioning of the PPC can impair the perception and memory of spatial relationships, prevent correct reaching and grasping, cause a deficit in attention, and reduce eye movement control (Pinel & Barnes, 2022). Moreover, PPC injury can lead to apraxia, which sabotages voluntary movement, and those diagnosed with it have trouble purposefully making specific motions (Pinel & Barnes, 2022). Another consequence of damaged PPC is contralateral neglect, a disturbance of one’s capability to respond to stimuli on a brain lesion on the opposite side of the body (Pinel & Barnes, 2022). Therefore, the PPC cannot accurately perform its part in uniting SSs and MS if the cortex is damaged, thus provoking substantial difficulties.
To conclude, the role of the posterior parietal cortex in sensory and motor function is to guide received information into relevant reactions within the sensory, motor, and sensorimotor systems. On the one hand, the PPC participates in multisensory integration and connects diverse signals from SSs’ visual, auditory, and somatosensory sections. On the other hand, the PPC sends its output to MS, generating movement responses. Moreover, some researchers state that the PPC plays a rather proactive role in motor planning and is involved in movement-related decision-making. However, the PPC may fail in performing its role if the cortex is damaged, as an injury can cause apraxia or contralateral neglect, hinder reaching and grasping abilities, or impede the control of eye movement. Overall, the significance of the PPC can be reflected in uniting sensation and action, yet understanding its importance requires knowing various processes that happen within the body since systems executing different functions are interconnected.
References
Bowling, N. (n.d.). Sensory and motor systems [PDF document].
Chivukula, S., Jafari, M., Aflalo, T., Yong, N. A., & Pouratian, N. (2019). Cognition in sensorimotor control: Interfacing with the posterior parietal cortex. Frontiers in Neuroscience, 13(140), 1-8. Web.
Li, Y., Wang, Y., & Cui, H. (2022). Posterior parietal cortex predicts upcoming movement in dynamic sensorimotor control. PNAS, 119(13), 1-9. Web.
Pinel, J. P. J., & Barnes, S. J. (2022). Biopsychology (11th ed.). Pearson.