Parietal Lobe

In the previous lecture we talked about the prefrontal cortex and its lesions... In this lecture we will talk about another area which is the parietal association areas. (this sheet is not included in the midterm exam) Parietal lobe The Parietal lobe extends from the central sulcus to the occipital lobe and lies superior to the temporal lobe. The anterior parietal lobe corresponds areas 3, 1, and 2and contains primary somatosensory cortices (primary and secondary sensation). Just posterior to the somatosensory areas is the posterior parietal association cortex, including areas 5 and 7. It collects multi-sensation together to give one information and it is responsible for the relations and the sequence of actions. For example: the number 3 is smaller than 5 and the difference between them is two steps… and the number 5 is smaller than 9 and the difference between them is four steps… this is what we mean in ‘’ relations and sequence ‘’ which is the main function of the posterior parietal association cortex. As the rest of the brain, the sequence and relations on the (right cerebral hemisphere) are more about music, art, spatial relationships, 3D position and location, while the more discrete type of sequence 2|Page which is mainly on the (left cerebral hemisphere) is more about analysis and math. Neglect syndrome (right parietal association cortex) Results from a unilateral parietal lobe lesion; neglect of one-half of the body or environment. Neglect is most prominent after damage to the right hemisphere of the human brain, causing a patient with neglect to behave as if the left side of sensory space is nonexistent (although they can still turn left). Patients will have no visual field deficits, so they can see. Right visual field goes to the left cortex and processed there then go to be stored in the right cortex, so it found in both brain sides. that why left side lesion will not have deficit in symptoms. The left visual field goes to the right cortex and stay at the right, so it only represented there. and hence, lesion to left side will lead to minimal neglect to the right visual field. This happens mainly when the right side is damaged since the ‘’where’’ pathway is more associated with the right hemi-sphere, which will cause severe left neglect syndrome. Neglect not only affects present sensation but memory and recall perception as well.*It is unclear, however, if this is due to a perceptive 3|Page deficit of the memory (to the patient having lost pieces of spatial information of the memory) or whether the information within the memory is whole and intact but simply being ignored, the same way portions of a physical object in the patient's presence would be ignored.* (from google) If asked to draw a clock face from memory, they will draw only the numbers 12 to 6, or all 12 numbers might be on the right side, and they might only apply make-up to the non-neglected side… so on. Patients may also deny their deficit, an anosognosia. In some cases, the lesion extends more anteriorly to damage the somatosensory cortex, so the patient doesn’t only neglect his left visual hemifield but also cannot feel his left side, so he will notice sometimes that there is a limb attached to his left side which he cannot feel (alien limb syndrome), the patient might attend to cut the alien limb since he doesn’t feel it as a part of his body. 4|Page If a lesion happened more anterior in the dorsal pathway (mainly on the right side), the movement will be normal, but the association of movement with position, navigation and 3D dimensions will be lost and patients are unable to identify specific buildings, such as their own house and famous landmarks. this is called Topographagnosia: ➢ Inability to navigate routes using familiar landmarks - deficit in familiar scene perception. ➢ Can be caused by right ventral temporo-occipital lesions like Right lingual gyrus. ➢ Also, right retrosplenial region lesions may cause Topographagnosia. ➢ Spatial relationships will be distorted. ➢ It may be part of a syndrome known as visuospatial dysgnosia. A lesion to the left parietal lobe will cause: • Acalculia: loss of the ability to perform simple calculations. • Language problems • Agraphia: a loss in the ability to communicate through writing • Apraxia: difficulty with the motor planning to perform tasks or movements when asked Bilateral Parietal Damage (Balint's Syndrome) Damage to the parietal cortex bilaterally (on the right side and small lesion on the left). 5|Page Most clinical features: ➢ Simultanagnosia: Inability to interpret the totality of a picture scene (can identify individual portions of the whole picture, like the second picture below, he will not be able to see the big H, or he will able to see only one event from the first picture). ➢ Optic ataxia: (incomplete ataxia) Defects of visually guided hand movement since the parietal eye field (responsible for the movement guided by vision) is damaged, can happen in one side depending on the lesion, the patient will have problems reaching for stuff in front of him. ➢ Oculomotor apraxia: the inability to do voluntarily guided eye movements, changing to a new location of visual fixation. The rapid eye movements are abnormal. Because of this, most patients with Oculomotor apraxia have to turn their heads in order to follow objects coming from their peripherals. 6|Page ➢ Since the right parietal is damaged the patient will have left neglect syndrome. ➢ Impaired control over the focus of visual attention due to inattentional amnesia. ➢ Complex defects in perception of visual object structure, motion and depth. NOTE: Vestibular system is the only system has not primary and secondary areas… for more understanding about this point, watch vision videos on the elearning website. Language Language is one of most complex behaviors of ours, that’s why many areas of the cortex are involved in its receiving, understanding, and production. (around 117 areas involved in language) It’s divided into two parts: 1. Receiving and understanding of language: which is the part related to sensory modalities and pathways: A. Primary sensory areas (auditory or visual) receive input and process it to letters. B. Secondary sensory areas further process this information to words and maybe interpret some meanings 7|Page C. Then association area of language understanding (Wernicke’s area) combines the processed information to understand what is said or written. 2. Production of language: this part is concerned with motor orders to tongue, lips, and vocal cords to produce spoken language. It also sends motor orders to facial muscles to produce expressions and to the hands to produce sign language and written language. A lesion in the primary motor cortex that controls tongue, lips, and vocal cords results in their paralysis and the patient becomes unable to move them, where as a lesion in the secondary motor cortex results in apraxia; the tongue, lips, and vocal cords are not paralyzed and the patient is still able to move them with full strength, yet the patient is unable to use them to do complex movements that produce some letters and without those letters the patient can’t speak properly and clearly. If the lesion is in the association areas of language, the patient is able to move their tongue, lips, and vocal cord, and do complex movements with them to produce all the letters since primary and secondary motor areas are intact (NO paralysis, NO apraxia), yet the patient is unable to produce proper and clear language and this case is called aphasia. Association area of language production is in the inferior part of the frontal lobe of the dominant hemisphere (usually the left, between prefrontal and premotor cortices), it’s includes three gyri the most important two of them are pars opercularis and pars triangularis (Brodmann’s areas 44 and 45). The first well-known case of stroke in the association area of language production was a one of Dr. Broca’s patients, this patient lost almost the whole area after a stroke and lived 15-20 years with the only sound being able to produce is “tan”, and he was unable to learn any word again, and can’t learn sign language as well. If a patient could use sign language in the past, they will lose this language or part of it if a stroke happened in Broca’s area. This happens because Broca’s area is responsible for language production by any means of communication; talking, signs, or writing, so 8|Page they’re all lost, and the loss of ability to produce written language is termed agraphia. Sarah was in an English class reading out loudly when she suddenly felt tingling in the right upper and lower limb and became unable to read. In the hospital they discovered that she had a stroke in the area of language production (Broca’s). Since the area of language understanding is intact, Sarah is able to understand what she and other people say, that’s why when she mispronounced her name as “Scott” she realized she said it wrong and then corrected it to “Sarah Scott”. When asked about her age, the only answer Sarah had is “I can’t”, she couldn’t say it so she tried to write it instead, but again she couldn’t. Even though she knows that her age in 19, she’s still unable to express it. With stroke in Broca’s area, Sarah became completely unable to express somethings (including her age), that’s why even after the doctor told her that she’s 19, Sarah only replied with “yeah”. So, a lesion in the area of language production results in Broca’s aphasia or expressive aphasia. 9|Page