The term agnosia defines an impairment of stimulus recognition, limited to one modality and not explainable in terms of sensory deficits or general mental deterioration. Visual object agnosia refers to the inability to recognize objects and prosopagnosia to the failure to recognize faces that are well familiar to the patient, when stimuli are visually perceived. Both deficits may appear in an apperceptive form, where it is the internal and external structure of the stimulus to be unrecognized and an associative form where the patient achieves a good percept, but cannot assign it a meaning. Apperceptive forms are preferentially associated with bilateral occipital damage, object associative agnosia with left occipital damage and associative prosopagnosia with right occipital damage.
失认症这一术语指的是刺激识别受损，仅限于一种感觉模式，且无法用感觉缺陷或一般精神衰退来解释。视觉物体失认症是指无法识别物体，而面孔失认症是指当视觉感知到刺激时，患者无法识别自己非常熟悉的面孔。这两种缺陷都可能以感知形式出现，即无法识别刺激的内部和外部结构，也可能以联想形式出现，即患者能够形成良好的感知，但无法赋予其意义。感知形式优先与双侧枕叶损伤相关，物体联想性失认症与左侧枕叶损伤相关，联想性面孔失认症与右侧枕叶损伤相关。
