I was brainstorming about how can I conceptualize my project so that it makes close sense to the idea of synaesthesia and the word “brain” strikes me. I was thinking of mapping out the different conditions of synaesthesia into parts of the brain where synaesthesia occurs.
One of them holds that synaesthetes have unusual connections between different sensory regions of the cerebral cortex.Scans have shown that different areas of the brain are active for synaesthetes experiencing a cross-modal association than for non-synesthetes engaged in the same task
What are the causes of Synaesthesia?
In the 19th century scientists thought that Synesthesia was caused by a defect or immature nervous system and the circuitry of the brain didn’t function properly. Such kinds of theories were formularized from Bleuler & Lehmann (“Degeneracy Theory”, 1881) and Downey (“Compensation Theory”, 1912). The activity of the brain was suppressed with relative activation of the emotions and that could be a
11“linkage” to the synesthetical perception. These theories also suggested that synesthesia occurred in the central nervous system (CNS).
Before we start to explain what might be the causes of synesthesia, we will make a reference to how human brain works and which parts are responsible for specific processes and functions.
The brain is organized in three sections:
1. the cerebral cortex
2. the central core
3. the limbic system
The cerebral cortex directs the brain’s higher cognitive and emotional functions. It is divided in two halves, the cerebral hemispheres. Every hemisphere has four subsections, the lobe sections and these are:
The frontal lobe is positioned, as the name suggests, in the front part of the hemisphere. It assists the cognitive activities (decision making) and relates present to the future through decisive behavior (setting goals, planning).
The occipital lobe(in the back part of the brain) processes the visual information and passes the processed information to the parietal and temporal lobe.
Parietal lobe is responsible for the sensory processes, the attention, as well as the language comprehension. The auditory perception, language comprehension and visual recognition are located in the temporal lobe.
The limbic system is concerned with emotional experience and forming memories. Here, conscious experiences are associated with emotions. The limbic system has three regions:
• The hippocampus controls emotions, the process of learning and memory. It influences other cortical regions of the brain.
• The amygdala controls aggression, eating, drinking and sexual behavior. • The hypothalamus is connected to the autonomous nervous and endocrine systems. It observes and controls the glucose and salt levels in the blood, the pressure and, of course, the hormones. Limbic stimulations are here integrated.
There are many theories supporting that the limbic system and its regions, especially hippocampus, have significant influence on synesthetic processes.
In his book “Union of the Senses”, Cytowic states that there are two reasons why hippocampus is the most likely part of the brain involved in the production of synesthesia:
1. there are persons who have synesthetic experiences during epileptic seizures, without being synesthetes and
2. hypothalamus is able to receive signals from different areas of the brain. During sensory neural processing it is possible that information from different stimuli (color, shape, texture) might be recombined in limbic area. In Cytowic’s researches show that conscious perception of synesthesia causes fluctuations of blood flow in the hippocampus.
But what really happens in the brain during synesthesia? Let’s say that we have the form of colored letters or numbers. How we percept an image? The lens of our eye focuses an image on the retina, a membrane in the back of the eye. The retina transforms the light into neuronal signals which are transmitted through the optic nerve to our brain in different areas. The information about color are transmitted to an areal and from there to other color processing areas like TPO (temporo – parieto – occipital) or posterior cerebral area, another name for the occipital lobe. It is known that the signal processing for letters and numbers are located in the areal V4. Intersection, cross activity or feedback between these sections could generate this type of synesthesia (colored letters/numbers). This process could explain also the color – hearing synesthesia, since the center of hearing is located near the TPO region.
There is also the possibility that neural cortex briefly stops functioning and in the same time fusion of senses are permitted, but somehow this process never arises consciousness. The forms that the synesthetes describe are very similar to those of middle sleep state before or after sleep (Eichmeier / Höfer, 1974).
Another theory states that synesthesia might happen, when some areas of the brain stop develop properly since infanthood. The fact that most synesthetes have more than one type of synesthesia and that there are different combinations of senses involved, support this theory.
In 1988 Charles and Daphne Maurer proposed that every newborn is synesthetic. In later publications Daphne Maurer and Catherine J. Mondloch (Maurer, 1993; Maurer & Mondloch, 1996) suggested the neonatal synesthesia hypothesis. It is stated that all babies up to 4 months of age are synesthetic because of immature cortex. Most of their cortical modules do not function sufficiently and the baby can not differentiate stimuli from modalities. The baby recognizes patterns but it is unaware which modality produced the pattern. Sensory input, such as sound, triggers both auditory and visual experience. Why synesthesia disappears in most of the newborns after the period of approximately 4 months it is not clear. A
15possible explanation is that the cortex starts developing properly and the baby can differentiate stimuli. Most recent theories come to the conclusion that synesthesia occurs in the left hemisphere, it is not cortical and involves temporal lobe and limbic structures.