Agnosia and other disorders regarding perception
In some circumstances, perception may not reflect reality without this being pathological. These "failures" in perception may be illusions or hallucinations. Illusions refer to an erroneous interpretation of a real external stimulus, while hallucinations consist of an erroneous perception without the presence of a real external stimulus. These perceptual experiences can happen with any existing pathologies, they are mainly caused by physiological or cognitive characteristics of the system or altered states (substance abuse or sleep). An example of illusion would be the well-known optical illusions (perceiving two identical colours differently, perceiving movement in a static image, etc.). The most common hallucinations are hypnagogic (when you are falling asleep and perceive a figure, sound or feel like someone is touching you), hypnopompic (same sensations but when you are waking up) and the ones derived from consuming hallucinogenic drugs (such as LSD or hallucinogenic mushrooms that provoke more elaborate hallucinations). Nonetheless, illusions and hallucinations can also be pathological, related with schizophrenia, psychosis episodes, delusional ideas.
Perception can also be altered by damage to our sensory organs (for example, an eye injury), damage in the pathways that take the sensory information to the brain (for example, glaucoma) or in the brain areas in charge of perception (for example, an injury in the occipital cortex). A damage in any of these three points can alter the normal perception of stimuli.
The most common perception disorder is Agnosia. This disorder entails a difficulty in directing and controlling perception, as well as behaviour in general. There are two types: Perceptive visual agnosia (can see parts of an object but is incapable of understanding the object as a whole) and Associative visual agnosia (understands the object as a whole but can place what object is it). It's difficult to understand it through these disorders since even though they can see, for them it is a similar sensation to being blind. There are also more specific disorders, such as akinetopsia (inability to see movement), achromatopsia (inability to see colours), prosopagnosia (inability to recognize familiar faces), auditive agnosia (inability to recognize an object by sound, and, in the case of verbal information, person with agnosia wouldn't be able to recognize the language as such), amusia (inability to recognize or reproduce musical tones or rhythms). These disorders are produced by brain damages such as ictus, brain trauma or, even a neurodegenerative disease.