Illusion
Unlike a hallucination, which is a distortion in the absence of a stimulus, an illusion describes a misinterpretation of a true sensation.
So, it should not be wrong to consider that illusions are just "misinterpretations" on how our brain perceives something that exists (unlike a hallucination where a stimulus is absent).
Each stimulus follows a dedicated neural path in the early stages of visual processing, and intense/repetitive activity or interaction with active adjoining channels (perceptual neural circuits, usually at the same level) causes a physiological imbalance that alters perception.
The Hermann grid illusion and Mach bands are two illusions that are widely considered to be caused by a biological phenomenon named lateral inhibition, where the receptor signal in the retina's receptive fields from light and dark areas compete with one another.
Thus, allowing us to recognize the complex identity of different elements, and the disparate relations between them through cognitive processes.
Selectivity refers to the identification of particular features that are relevant to recognize a specific element or object, while abstracting from other features that are not fundamental to performing the same recognition (e.g. when we see the shape of a house, certain contours that are essential for us to recognize it while other contours or image properties are not, such as color).
On the other hand, invariance refers to the ability to be indifferent to small variations of a given feature, effectively identifying all those variations as simply being different versions of the same feature (e.g. we can recognize a given handwritten letter of the alphabet, written by different people with distinct styles of calligraphy).
[4] The whole process that constructs our visual experience is extremely complex (with multiple qualities that are unmatched by any computer or digital system).
In such cases, a person may momentarily perceive time as slowing down, stopping, speeding up, or running backward.
[15] In a single-case study on a patient undergoing presurgical evaluation for epilepsy treatment, electrical stimulation at the left temporo-parietal junction evoked the percept of a nearby (illusory) person who "closely 'shadowed' changes in the patient's body position and posture".
