Consciousness - An Introduction (ch12)

What is the influence of brain damage on consciousness? - Chapter 12

How is it to note that you see only half of the world? What is it like to be blind, but to believe that you can see? or to be paralyzed, but to be convinced that you can move? These and many other questions are motivated by neuropsychological changes caused by brain damage. Although almost all types of brain damage affect consciousness, only three specific examples that are relevant to the subject of consciousness will be given in this chapter.

Amnesia

Korkasoff's syndrome is the most common form of memory loss (also called amnestic syndrome) and is caused by the toxic effects of alcohol and by a deficiency of thiamine caused by malnutrition.

There are two forms of memory loss:

1. anterograde amnesia
2. retrograde amnesia.

In the case of anterograde amnesia, patients can no longer save new, long-term memories. The short-term memory functionality remains intact. Retrograde amnesia occurs when someone has lost all his memories about the past. This form of memory loss often occurs after accidents that cause brain injuries.

With Karsakoff's syndrome, episodic memory, which is in charge of the memory of the events of someone’s life,  remains intact for  events that occurred in the far past. However, new episodic memories are no longer stored.  New information can still be stored through classical conditioning and procedural learning. However, they themselves do not realize that they are learning.

People with memory loss  are conscious. They are awake, respond to Stimuli, talk, laugh and have emotions. But who exactly has a consciousness? Since the capacity to store new episodic memories is lost, amnesic patients’ consciousness is trapped in the past. They can relate to the events that happened before but fail to make sense of their present experiences. Due to these problems, it is very common to hear amnestic patients claiming that they have just waken up or regained consciousness. Furthermore, people with memory loss also lack of memories of a continuous self.

Neglect

We speak of neglect when people is unable to be aware of items that are in half of space. This often occurs after a brain injury that affected brain’s right hemisphere. An example of this is paralysis and how some patients are not aware of their situation. This is called anosagnosis.

Anosagnosis only occurs when certain parts of the right parietal lobe are damaged. Damasio says that the core of consciousness remains intact, but that the extended consciousness, which goes beyond the here and now, is damaged. Another example is people with Anton's syndrome are blind but are convinced that they can see. When they bump into something, they invent excuses for it.

People with 'hemifield neglect' (also called 'unilateral neglect') are unaware of the existence of everything that happens at their left side. This disorder, like anosagnosis, occurs when the right hemisphere is damaged. For example, people with this disorder only apply make-up on the right side of their face. When people with this disorder are asked to  draw a clock, they leave out the left side of the clock (the numbers six to twelve) or they put all the numbers in the right half of the clock. If they are asked to make mental representations of places, they also leave out the left side of the places. This disorder can be seen as a lack of attention. Patients simply do not pay attention to the left side of everything they see. As a treatment, they can be trained to pay more attention to the left side of objects. However, the left side is  completely ignored.

Blindsight

Blindsight is common in people who have damage to parts of the visual cortex on one side. This causes degeneration of cells, while other visual pathways remain intact in the cortex. blindsight Patients that suffer from blindsight seem to not be aware of what is in their visual field. It is possible that someone with blindsight does not 'see' one half of the field of vision, but they can often report what is presented in this field of vision. A patient with blindsight therefore has vision without consciousness. Blindsight  can be tested in many different ways. For example, people with this disorder deny that they are conscious of what they see, but still make eye movements when they see stimuli, point at the location of objects and mimic the movement of objects presented in the blind field.

People with blindsight can also often 'guess' the color of stimuli that is shown in the blind area. So they can notice stimuli, but they find it difficult to determine shapes or designate similar objects. We know for sure that blindsight exists, but what this disorder tells us about consciousness is still unclear. People with blindsight are aware of fast moving stimuli with a lot of contrast (which fall into their blind area). This is also called the Riddoch phenomenon.

It is thus possible to experience movement of a stimulus, while you are not aware that you perceive the stimulus. Milner and Goodale share the opinion that blindsight represents a number of visual capacities that are mediated by the dorsal route and are linked to subcortical structures. This view fits in with the fact that the accuracy of reactions of people with blindsight differ. The proper functioning part of the eyes seems to be occupied with object identification, while the blind area deals with the detection of a stimulus.

Blindsight and consciousness

Does the existence of blindsight really prove that there is such a thing as qualia? Some scientists think so. Holt believes, for example, that blindsight and superblindsight are evidence of the existence of qualia. Block, however, has a different opinion. He states that there are two concepts of consciousness:

1. 'phenomenal consciousness', and
2. 'acces consciousness'.

The first is about the experience of consciousness, so the 'what is it like to be ...?' question. Access consciousness stands for the availability of information to be able to reason, talk and act. Block believes that stimuli in people with blindsight do not go hand in hand with both forms of consciousness. There is no access consciousness, because the patient cannot use the information and there is phenomenal awareness, because the patient is not aware of what he or she perceives.

Block states that people often confuse both forms of consciousness and think that certain functions go together with either form of consciousness, while this is not true. Weiskrantz states that people with blindsight do not have a 'commentary stage'. This means that they do not experience a phase in which information comes in to subsequently form an opinion about it.

Resources:Blackmore; Susan. (2010). Consciousness, Second Edition An Introduction. Abingdon, Oxon: Taylor & Francis.