Cognitive neuroscience of visual object recognition
Agnosia is a rare occurrence and can be the result of a stroke, dementia, head injury, brain infection, or hereditary.
Apperceptive agnosia is a deficit in object perception creating an inability to understand the significance of objects.
Similarly, associative visual agnosia is the inability to understand the significance of objects; however, this time the deficit is in semantic memory. Both of these agnosias can affect the pathway to object recognition, like Marr's Theory of Vision. More specifically unlike apperceptive agnosia, associative agnosic patients are more successful at drawing, copying, and matching tasks; however, these patients demonstrate that they can perceive but not recognize.
Integrative agnosia(a subtype of associative agnosia) is the inability to integrate separate parts to form a whole image. With these types of agnosias there is damage to the ventral (what) stream of the visual processing pathway.
Object orientation agnosia is the inability to extract the orientation of an object despite adequate object recognition. With this type of agnosia there is damage to the dorsal (where) stream of the visual processing pathway.
This can affect object recognition in terms of familiarity and even more so in unfamiliar objects and viewpoints.
A difficulty in recognizing faces can be explained by prosopagnosia. Someone with prosopagnosia cannot identify the face but is still able to perceive age, gender, and emotional expression. The brain region that specifies in facial recognition is the fusiform face area. Prosopagnosia can also be divided into apperceptive and associative subtypes. Recognition of individual chairs, cars, animals can also be impaired; therefore, these object share similar perceptual features with the face that are recognized in the fusiform face area.
Asemia
Asemia is the term for the medical condition of being unable to understand or express any signs or symbols.
It is a more severe condition than aphasia, which is the inability to understand linguistic signs. Asemia is caused by damage to the areas of the brain that process communication – more specifically, when there is damage to the left side of the brain in the areas that process communication such as Broca's and Wernicke's areas. Damage can be inflicted by physical trauma to the brain, but is more commonly caused by stroke and sometimes tumors. The onset of this condition is usually quick but not permanent. Treatment of this condition is traditional speech therapy in which the individual must relearn how to read, write, and talk. Depending on the severity of the injury, recovery can take days to years. Considerable recovery is common, but often not to the full extent of baseline ability.