In the last post I described most of the primary features of the right brain. It may have seemed that I covered most of what it means to be human. Now I will add the rest.
The left brain is much different than the right brain. The difference is not a matter of degree but of purpose. The separation of function in the hemispheres of our brains is critical to both our survival as individuals and our success as a species.
The left brain doesn't see the world. It sees this and that and the other thing: fragments in space and time. Beyond that, it sees the world through a multi-level haze of filters. These filters come from previous experiences. We experience these filters as moods, opinions, theories, convictions, prejudices, etc. At any one point in time the left brain may be interpreting the world through any number of filters.
In the left-brain mode of perception we see what McGilchrist calls re-presentations. These are constructions of the left brain based on the filters in effect at that time. For instance, if you ask someone about her relatives, you will usually get answers based on the stories that she tells herself about them, answers that may not agree with direct perception. This is a very difficult concept that will be expanded in a later post.
The left brain doesn't perceive the flow of time. The left brain understands time intervals and events and can string those intervals and events together to create the illusion of the flow, but only as long as that string is maintained.
The left brain focuses attention. It can choose a sense to follow. With effort it can pay attention to two senses and connect their occurrences.
Specifically, the left brain can focus its attention on the central portion of the right visual field. This is a relatively small area that roughly corresponds to densest aggregation of cones in the center of the eye, and only the right side of that. The cones provide our highest resolution sight, so the brain focusing on something with this vision makes a powerful tool.
The left brains sees this area in 2D. It doesn't sense or appreciate depth. Note that this small area on the curved visual field approximates a flat surface very effectively, so there is no real loss of perception.
The primary feature of the left brain's 2D perception is the discovery and tracking of boundaries. The left brain is continually finding boundaries of what is in its visual field. In fact, it does this to the extent that the boundaries often become more noticeable and important than the entities they bound.
The primary activity of the left brain is to deconstruct and reconstruct the world. Its view of the world starts with the accumulation of sensory fragments, and it proceeds to play with these building blocks to try and make sense of the world. The result of this play is the moods, opinions, theories, convictions, prejudices, etc., mentioned above that become filters to further perception.
In effect, the primary activity of the left brain involves a feedback loop between viewing the world through filters and continually updating or reinforcing those filters. More on this later.
The secondary activity of the left brain is to be the primary agent of action. Much human action is verbal, and generally only the left brain has access to the voice. The complex manipulations of breathing and the vocal chords required for speech are so complicated (taking up brain space) that it makes sense to centralize them in one place, and that place is Broca's area in the left hemisphere. The next most common human action is touch, which involves touching something, grasp, and manipulation. These actions are also concentrated in the left hemisphere, which is why we think of most people as right handed.
Memory in the left brain is problematical. The left brain can only remember what got in past the filters, and what is remembered is a collection of fragments strung together with whatever form of organization was in effect at the time the memories were created. The recall of memories from the left brain is often found to be faulty. The filters obscure and misrepresent reality. The filters in place during recall are often different from the filters in place when the memories were created, leading to misinterpretation. And the organization of the fragments is subject to revision when the memory is recalled.
These are the primary functions of the left brain. I hope you will note that healthy human beings are not characterized by either this or the right brain description. Being human is a dance between the hemispheres that magically gives us the best of both ways of approaching the world. In the next post I will start describing the interaction.