Occipital lobe

Named for the bones of the cranium under which they lie, the lobes are conspicuously defined by prominent sulci of the cortex, which have a relatively constant position in human brains. Each lobe is specialized for different activities (see Figure 6.3). Located in the anterior portions of the hemispheres, the frontal lobes are responsible for voluntary motor activity, speaking ability, and higher intellectual activities. The parietal lobes, which are posterior to the frontal lobes, process and integrate sensory information. The occipital lobes, located in the posterior-most aspects of the cerebrum, process visual information, and the temporal lobes, located laterally, process auditory information. 

The posterior multimodal association area is located at the junction of the parietal, temporal, and occipital lobes. It pools and integrates somatic, auditory, and visual stimuli for complex perceptual processing. As such, this area is involved primarily with visuospatial localization, language, and attention. Lesions here interfere with awareness of one s body position and of the space in which it moves as well as the ability to integrate and make sense of elements of a visual scene. In other words, these patients have normal visual acuity but cannot focus on an object of interest. 

Occipital lobe One of four, together with the frontal, parietal and temporal, lobes of the cerebral hemispheres. 

Simister, R. J., McLean, M. A., Barker, G. J., Duncan, J. S. A proton magnetic resonance spectroscopy study of metabolites in the occipital lobes in epilepsy. Epilepsia 44 550-558, 2003. 

As we move forward, it will prove helpful to get some basic aspects of the human nervous system in place. An enormous amount of work has gone into making associations between brain anatomy and function. Starting with the three main parts of the brain, we know that the cerebrum is the seat of consciousness. It is divided into two hemispheres, which are linked by the corpus callosum. In a very general sense, the left hemisphere is associated with intellectual and the right hemisphere with emotional responses. Within the cerebrum, one can associate a number of brain areas (the prefrontal, frontal, temporal, parietal, and occipital lobes, for example) with functions including vision and hearing. One can make crude maps in which function is mapped onto brain structure. 

The cerebral cortex is conventionally subdivided into four main regions that may be delineated by the sulci, or large clefts, termed the frontal, temporal, parietal and occipital lobes. These names are derived from the bones of the skull which overlay them. Each lobe may be further subdivided according to its cellular structure and composition. Thus Brodmarm has divided the cortex into approximately 50 discrete areas according to the specific cellular structure and function. For example, electrical stimulation of the strip of cerebral cortex in front of the central sulcus (see Figure 1.3) is responsible for motor commands to the muscles. This is termed the primary motor cortex and can be further subdivided according to which muscles are controlled in different parts of the body. 

In the case of visual hallucinations, similar studies have shown that these are correlated with activity in the occipital lobes (Ffytche et al., 1998). The localisation of activity was associated with the specific phenomenological characteristics of the hallucination for example hallucinations of unfamiliar faces were found by Kanwisher et al. (1997) to be accompanied by increased activity in the left middle fusiform gyrus, an area related to unfamiliar face stimuli. As with auditory hallucinations, increased activity in the affected area preceded the onset of the hallucination. 

Figure 1 In vivo spectra acquired from the human occipital lobe at 4 T (top)and 7 T (bottom) using the same experimental setup and NMR acquisition parameters (TR=3 s and 128 signal averages). PE, phosphoethanolamine PC, phosphocholine Pi, inorganic phosphate GPE, glycerophosphoethanolamine GPC, glycerophosphocholine ...

The vision center is located in the occipital lobes. It is there that visual images are coalesced into meaningful wholes. 

Alexia without agraphia in left occipital lobe infarction and the splenium of the corpus callosum. Transfer of read words from the functional right visual cortex to the left sided language center is impossible due to interruption of the splenium. Transfer of primary language information for writing or speech is not impaired. 

Jongen JCF, Franke CL, Ramos LMP et al (2004) Direction of flow in posterior communicating artery on magnetic resonance angiography in patients with occipital lobe infarcts. Stroke 35 104-108... 

Global l NAA levels in patients prior to beginning antipsychotic medication no changes in NAA in the FL, occipital lobe, caudate nucleus or cerebellum after beginning treatment... 

NAA, ml, f Glu,Gln in FL, TL and occipital lobe of elderly patients with schizophrenia 4 NAA and Cr in anterior cerebellar vermis of schizophrenic patients no relation between NAA levels and Dl or medication dose in patients 4 NAA in left and right thalami of patients no differences in Cho and Cr concentrations 4 NAA in Hippo of patients no correlation between Hippo volume and NAA levels in patients or controls 4 NAA in left FL of patients no association between NAA and medication dose or Dl no differences in Cho and Cr levels... 

Fig. 7. Anatomical organization of dopamine Di mRNA expression in the adult human brain (whole hemisphere horizontal images) at a dorsal (A) and ventral (B) level. Notice strong cortical expression of this dopamine receptor subtype in addition to the intense expression levels in the striatum (CN, Pu and NAc). Adapted from Hurd et al. (2001). aCg, anterior cingulate Amy, amygdala Cb, cerebellum cc, corpus callosum CN, caudate nucleus Cun, cuneus F, frontal lobe Hip, hippocampus hyp, hypothalamus I, insular cortex mPFC, medial prefrontal cortex mm, medial mammillary nucleus NAc, nucleus accumbens O, occipital lobe Phg, parahippocampal gyrus Pu, putamen SN, substantia nigra T, temporal lobe U, uncal gyrus.

The posterior cerebral artery encircles the midbrain close to the oculomotor nerve at the level of the tentorium and supplies the inferior part of the temporal lobe, and the occipital lobe (Marinkovic et al. 1987). Many small perforating arteries arise from the proximal portion of the posterior cerebral artery to supply the midbrain, thalamus, hypothalamus and geniculate bodies. Sometimes a single perforating artery supplies the medial part of each thalamus, or both sides of the midbrain. In approximately 15% of individuals, the posterior cerebral artery is a direct continuation of the posterior commrmicating artery, its main blood supply then coming from the internal carotid artery rather than the basilar artery. 

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