What condition describes a person perceiving one sensory input as another, such as 'seeing' sounds?

The condition that describes a person perceiving one sensory input as another, such as 'seeing' sounds, is known as synesthesia. This phenomenon involves the blending of sensory modalities, where stimulation of one sensory pathway leads to automatic, involuntary experiences in a second sensory pathway. For example, an individual with synesthesia might hear music and simultaneously perceive it as visual shapes or colors. This unique cross-wiring of sensory perception allows for a rich and often vivid experience of the world, where individuals might consistently associate specific colors with certain letters or sounds.

Other conditions involve sensory processing but do not fit the description provided. Hallucinations involve perceiving something that isn't present in the environment, often associated with mental health disorders or substance use, but do not imply a cross-sensory experience. Agnosia refers to the inability to recognize objects, people, or sounds despite having functional sensory abilities, indicating a disconnect between perception and recognition rather than a mixing of senses. Prosopagnosia is specific to the inability to recognize faces, which again does not encompass the cross-sensory aspect highlighted in the question. Thus, synesthesia clearly aligns with the described phenomenon of one sensory perception influencing another.