Occipital lobe
Anatomy & Function: In the occipital lobe, which is located in the posterior fossa above the cerebellum, lies the visual center, i.e. part of the visual system. The information comes from the retina via the optic nerve (2nd cranial nerve) into the optic chiasm (optic nerve crossing), where the information of the outer (lateral) visual field crosses to the opposite side. The fibers continue to run in the optic tract, whereby the right optic tract contains information about the inner (medial) visual field of the right eye and the lateral visual field of the left eye due to the crossing in the chiasma.
The fibers extend through the Corpus geniculatum mediale in the thalamus and as visual radiation to the primary visual cortex in the occipital lobe. It is located in the area of the sulcus calcarinus. When the information from the eye reaches the primary visual cortex, humans become aware that they are seeing something, but the visual impression is not yet interpreted.
This only happens in the secondary visual cortex, which is adjacent to the primary one. Clinical background: Lesions in the area of the visual system can be caused by a variety of factors, such as inflammation, trauma or tumors. The symptoms of such a lesion can also vary greatly depending on its location.Thus, a lesion of the primary visual cortex on only one side leads to a loss of vision on the medial side of one eye and the lateral side of the other eye (homonymous hemianopsia).
Lesions in the peripheral area of the visual cortex also lead to visual field loss, which is usually relatively characteristic. However, if the secondary visual cortex is affected, this does not lead to visual field loss or blindness. Patients can still see, but are no longer able to interpret what they see (visual agnosia). One example is the failure to recognize faces when vision is intact (prosopagnosia).
Temporal lobe
Anatomy & Function: The temporal lobe is the central part of the auditory system, i.e. the hearing. The information is transmitted via the auditory nerve cells in the inner ear to nerve cell nuclei in the medulla oblongata (nucleus cochleares). Here there is a tonotopic classification, i.e. a classification of the information according to height and frequency.
This is also found in the cerebral cortex. After passing the medulla oblongata, the majority of nerve fibers cross over to the opposite side on their way into the cerebral cortex, while the smaller portion continues on the same side. The path continues to the upper olive stones and then, as the lemniscus lateralis, to the inferior colliculi of the four mound plate of the midbrain.
From here the nerve fibers continue to the Corpus geniculatum mediale of the thalamus and from there as auditory radiation to the primary auditory cortex in the area of the transverse windings of the temporal lobe. In this way, smaller fiber bundles cross from one side to the other, so that the primary auditory cortex on one side receives information about what is heard from the cochlea on both sides, which is essential for directional hearing. In the primary auditory cortex, the hearing person becomes aware of what he or she is hearing, but without interpreting it.
This happens only in the secondary auditory cortex. Once the information has arrived here and been processed, the sounds heard are recognized as words, melodies or sounds, for example. It is interesting to note that the secondary auditory cortex of the dominant hemisphere, in which Broca’s speech center is located, is primarily responsible for processing and recognizing and understanding speech.
It is therefore also called the sensory speech center or Wernicke area. In contrast, the secondary auditory cortex of the non-dominant hemisphere is more likely to process things like melodies. It is therefore crucial for the understanding and recognition of music.
Clinical reason: A lesion of the primary auditory cortex of one side does not lead to deafness but to a reduction of hearing in both ears. This is due to the fact that the nerve fibers on their way from the inner ear to the cerebral cortex cross over several times to the opposite side and one half of the brain thus receives information about what is heard from both ears. This also means, however, that if the primary auditory cortex on one side is disturbed, directional hearing is significantly more difficult.
In the case of a lesion of the secondary auditory cortex, the symptoms depend very much on whether the dominant or non-dominant hemisphere of the brain is affected. If the Wernicke’s area, i.e. the secondary auditory cortex of the dominant hemisphere, is damaged, speech comprehension is severely impaired. They often talk a lot (logorrhea) but without a sense for the outsider.
They are not aware that what they say makes no sense. A lesion of the secondary auditory cortex in the non-dominant hemisphere, on the other hand, results in a loss of musical understanding, but does not impair speech.