What's Happening in the Brain
An impaired sense of smell might reflect involvement of a structure called the enterorhinal cortex that is related both to sense of smell and sense of memory . Apart from this, with the other senses, the problem is not so much in the sensory organ, (e.g. eyes and ears) but in the brain's interpretation of the information that it gets from the sensory organs.
'Apraxia' is the term used to describe the problem in which a person cannot complete a motor task (e.g. buttoning a shirt) even though all the component parts of the brain required to carry out that task are working (e.g. the person can move their hands and arms, is not clumsy in the movement). Apraxia therefore is a problem of how the higher ("thinking") parts of the brain integrate the information needed to carry out the motor activity. The prefrontal motor cortex and the parietal lobes of the brain are particularly associated with apraxia.
As a person ages, it is natural for them to have difficulty with their vision due to disorders of the eye, such as cataracts or glaucoma . Cataracts which cause a person's vision to become blurry occur because of a change in the lens of the eye. Glaucoma causes vision loss and possible blindness due to increasing pressure in the eye. Poor vision is not always caused by problems with the eye, but can also result from problems in the brain.
Alzheimer's disease does not cause these disorders of the eye. Instead, Alzheimer's disease affects the ability of our brain to understand and recognize what we see. This problem is known as 'agnosia'. The disease damages areas in the brain that organize visual information, especially what is known as the occipital and temporal lobes. These areas of the brain send and receive messages to and from the eyes. This miscommunication can cause a person to be able to see and describe an object, but they cannot tell you what the object is. The link between a past experience with an object and the sight of an object is lost. For example, the person can see a face, but cannot recognize that it belongs to their spouse.