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.
Anomic aphasia
Although many experts have believed that damage to Broca's area or Wernicke's area are the main causes of anomia, current studies have shown that damage in the left parietal lobe is the epicenter of anomic aphasia. One study was conducted using a word repetition test as well as fMRI in order to see the highest level of activity as well as where the lesions are in the brain tissue. Fridrikkson, et al. saw that damage to neither Broca's area nor Wernicke's area were the sole sources of anomia in the subjects. Therefore, the original anomia model, which theorized that damage occurred on the surface of the brain in the grey matter was debunked, and it was found that the damage was in the white matter deeper in the brain, on the left hemisphere. More specifically, the damage was in a part of the nerve tract called the arcuate fasciculus, for which the mechanism of action is unknown, though it is known to connect the posterior (back) of the brain to the anterior (front) and vice versa.