Brain Anatomy Part II
Section VII. Occipital Lobe Overview
The occipital lobe is the only lobe dedicated primarily to a single functional category: vision. The functional anatomy consists of the primary visual cortex along the banks of the calcarine sulcus, surrounded by shells of increasingly higher order occipital visual association cortex. In general, visual information about object/person recognitions is relayed along the basal temporal lobe (especially fusiform gyrus) while visual information related to where in space an object is located is relayed to the parietal heteromodal/higher order association cortex.
This functional anatomic configuration helps account for certain deficits by lesion location such as difficulty recognizing faces (prosopagnosia) with bilateral lesions of the posterior fusiform gyri and hemineglect with lesions of the parietal association cortex (especially right parietal lobe). This model of ventral (what) and dorsal (where) dual stream of functional information has also been applied to other cerebral functions including language, auditory information, and attention (further detailed in functional sections).
The boundaries and sufrace anatomy of the occipital lobe is less well-defined on surface images than those of the other lobes. But, the pariet-occipital fissure is reliably identified on the surface and in all 3 standard cross-sectional planes.
Major Borders
Major borders of the occipital lobe, lateral view. The lateral parieto-temporal line extends from the pre-occipital notch to the parieto-occipital fissure. The temporal-occipital line extends from the posterior aspect of the Sylvian fissure to the parieto-occipital line. The names of these lines need not be memorized. Practically speaking, it is usually sufficient to remember there are not highly discrete borders to the lobes along the posterior lateral convexity in this region of association cortex.
Major Borders
Major borders of the occipital lobe, inferior view. Note that the lateral occipito-temporal gyrus (fusiform gyrus) (light blue) seamlessly spans the temporal and occipital lobes. Similarly, the parahippocampal gyrus of the temporal lobe (pink) is continuous with the lingula of the occipital lobe.
Major Borders
Major borders of the occipital lobe, posterior view. For practical purposes of essential neuroradiology content, the occipital lobe is arbitrarily divided by the calcarine sulcus level into a superior portion (green) and an inferior portion (light blue) on some images.
However, the occipital lobe does contain named gyri. In clinical practice it is difficult, and often not necessary, to precisely identify many of the borders of several gyri on clinical cross sectional imaging-especially the middle and inferior occipital gyri.
Key occipital lobe sulci and gyri can be identified in all three standard imaging planes. In particular, the calcarine sulcus centered in the primary visual cortex, is readily identifiable.
Axial Image, Superior Level
The parieto-occipital fissure is nestled between the two parasagittal components of the intra-parietal/intra-occipital sulci. The parieto-occipital fissure can then be traced while scrolling inferiorly according to the anterior shift sign.
Posterior to anterior, there is the occipital lobe, parietal lobe, cingulate gyrus, and corpus callosum.
Goalpost Sign
The intra-parietal sulci continue into the occipital lobes to become the intra-occipital sulci before terminating in a T-bone termination. Here, the intra-parietal/occipital sulci have a parasagittal orientation, forming a goalpost. The posterior-superior parieto-occipital fissure can be identified as a dominant fissure between the goalpost uprights.
The visualized portion of the occipital lobe medial to the goalposts corresponds to the superior occipital gyri.
Axial Image, Mid-Level, Superior to Calcarine Sulcus
The intra-occipital sulcus peters out as it comes to the termination.
Posterior to anterior, we again see the occipital lobe, parietal lobe, cingulate gyrus, and corpus callosum.
Anterior Shift Sign
Comparing the prior two images, we see that the the parieto-occipital fissure is shifts anteriorly as you scroll superior -> inferior on axial images, similar to the pars marginalis of the cingulate sulcus. This feature allows consistent identification of the parieto-occipital sulcus on axial images, starting between the goalposts and ending inferiorly at the junction with the calcarine sulcus.
Barbell Sign
Another less consistent sign that can be used to help identify the parieto-occipital fissure on axial images is the barbell sign. This configuration may be subtle or absent however.
Axial Image, Calcarine Sulcus Level
The calcarine sulcus is closest to the axial plane (nearly in-plane, but slightly oblique). Thus, it is not well seen on axial images, but the surrounding cortex has a characteristic configuration that is consistently visualized.
Heart and Caduceus Signs
The calcarine cortex is associated with a bump creating an impression on the atrium of the lateral ventricle called the calcar avis. This impression is responsible for the frequently 'amputated' appearance of the occipital horns of the lateral ventricle. The cortical folds of the calcarine sulcus at this level have a curved appearance that may be likened to two opposing snakes, similar to the intertwined caduceus. The overall shape is similar to a heart.
Coronal Image, Posterior Level
Again, note the parasagittal goalpost intra-parietal/intra-occipital sulci surrounding the parieto-occipital fissure. The parieto-occipital fissure and calcarine sulcus are widely separated. As you scroll anteriorly on coronal image, these two sulci will converge.
Coronal Image, Anterior Occipital Lobe Level with Bow-Tie Sign
Coronal image, occipital lobe anterior view. At this level, the parieto-occipital fissure and the calcarine sulcus are nearly converged, together resulting in a bow-tie configuration. The precuneus is located between those two sulci. The occipital extensions of the collateral sulcus are also seen in the inferior portion of the occipital lobe, and typically have a vertical location at this level (unlike the mid-temporal lobe level). The lingual gyrus is located inferior to the calcarine sulcus and medial to the collateral sulcus. The borders of the other gyri laterally are more difficult to discern.
Midline Sagittal Image
Once again, the midline sagittal image is a key image for neuroanatomy assessment. Knowledge of the orientation and relationships of the parieto-occipital fissure and calcarine sulci is key to understanding several related signs (anterior shift, bow-tie, heart and caduceus signs).
Note that the parieto-occipital fissure is obliquely slanted relative to all 3 imaging planes while the calcarine sulcus courses close to the axial plane. These two sulci consistently converge anteriorly.
Key Signs on Midline Sagittal
Key signs of the occipital lobe on midline sagittal image. The cuneus, the medial projection of the superior occipital gyrus located between the parieto-occipital fissure and the calcarine sulcus, has a wedge shape (cuneus is latin for wedge). In distinction, the lingual gyrus (or lingula) has more of a rectangular shape. The parieto-occipital fissure can calcarine sulcus converge anteriorly. Beyond the point of convergence, the anterior segment of the calcarine sulcus continues anteriorly until it terminates at the split of the parahippocampal gyrus Y.
Parahippocampal Y Sign
The parahippocampal Y sign. The parahippocampal gyrus is contiguous with both the lingula of the occipital lobe and the isthmus en route to the cingulate gyrus. The parahippocampal gyrus, isthmus, and cingulate gyrus together form the bulk of the 'limbic lobe,' encircling the central brain region.
Acute Infarct of Cuneus
Patient with an acute infarct of the right cuneus that extended to the occipital pole. The patient had a left inferior quadrantanopsia.
Acute Lingual Gyrus Infarct
Patient with an acute infarct of the left lingual gyrus, presenting with right superior hemianopsia.
Another aspect of retinotopic organization is that central vision has relatively large cortical representation near the occipital pole and peripheral vision has relatively less cortical representation along the anterior calcarine sulcus.
In the extreme case where there is insult to the complete bilateral primary motor cortex the patient may develop Anton's Syndrome. In this condition, there is cortical blindness, but the patient may be unaware. The patient may appear to have some reflexive reactions to surroundings called blindsight due to extrageniculate visual pathways.
Lewy Body Dementia
Clinical Correlate. Surface projection images in a patient with Lewy body dementia, with FDG activity superior two rows (yellow and red indicate hypometabolism) and z-score images (comparison to normal activity) inferior two rows (green indicates hypometabolism). There is hypometabolism throughout much of the association cortex of both cerebral hemispheres with prominent involvement of the precuneus/posterior cingulate and sparing of the primary somatomotor/somatosensory areas. Note prominent involvement of the occipital lobes (remember visual symptoms of this dementia). Alzheimer disease has a similar pattern, but typically spares the occipital lobe.
Summary of Key Clinical Correlates of the Occipital Lobe
The primary visual cortex lines the calcarine sulcus and is surrounded by shells of visual association cortex.
Visual information is spatially inverted in both the RL and SI directions. For example, a lesion of the lingual gyrus can result in a contralateral superior hemianopsia.
Visual information about object/person identity is relayed to the basal temporal lobe (the 'what' pathway) and visual information about location is relayed to the parietal multimodal association cortex within the parietal lobules (the 'where' pathway).