Agnosia is a inability to recognize or identify an object despite he have the knowledge about the object. It may due to defect in sensory stimulus whether visual, auditory or tactile.

Let see this video first;

On this video we can see the doctor holding a glasses and asking the patient to identify what it is. The patient fail to identify it, but only describing characteristic of the object. But when the doctor ask him to hold and touch it, he really know that it is his own glasses and he looks so frustrated. This frustrated reaction show that he exactly know the object but he just can't identify it when he looking at it. And the same things happen when the doctor show a bunch of key in the second part of video.

Agnosia patient will failed to identify somethings if the stimuli comes from the defect sensory stimulus. If the patient have problems in visual agnosia, just asking him to touch or hear, or smell it and for sure he will answer correctly.

p/s: Correct me if I wrong