Are normal external stimuli still processed by the brain during a hallucination? I was wondering if hallucinations could radically override real external stimuli. After all every sensory organ responds to external stimuli, but when a hallucination occurs, I suppose, the external stimulus and the false stimulus (hallucination) combine. Or do they?

For example: Let's say I'm looking at an apple and in the meantime I hallucinate an orange, theoretically the informations about the apple should still reach my brain together with the hallucination of the orange, therefore I should be able to see the apple and the orange at the same time.

Something analogous happens in the phenomenon of afterimage where the photochemical activity in the retina continues with less intensity even without the original stimulus. The new and the old stimuli combine.

Is that what happens during hallucinations too?
Q: Are normal external stimuli still processed by the brain during a hallucination?

A: Yes, normal stimuli are still processed during a hallucination, but some of them may be distorted or obliterated. This is evident clinically. For example,


When a schizophrenia patient has an auditory hallucination (such as voices talking to him/her) [Ref 1] while he/she is doing something in the public, the patient still hears sounds in the surroundings and what other people are talking normally.
When a migraine or epileptic patient has a visual aura (a kind of visual hallucination, which usually occurs as flashing lights [scintillation], zigzag patterns [fortification spectra], or blind spots [scotomas]) [Ref 2] or a drug addict has a visual hallucination (such as insects crawling on the floor, a pink elephant coming into the room, or a stranger coming to attack him/her) from psychedelic drugs, he/she still can see the visual image of the surrounding place.[Ref 3] But, the part of visual field that the visual aura/hallucination is occurring may be normal, distorted, or obliterated, as in the figures below.


Figure 1.Visual aura of migraine. [Ref 4]



Figure 2. This is what an aura is like to one migraine sufferer. [Ref 5]




Some patients with a tumor, abscess, cyst, or other lesions at the parietal somatosensory area have sensory seizure (a kind of somatic sensory or tactile hallucinations) and some schizophrenics have tactile hallucinations, which can occur as insects crawling on the skin, the skin over the head being stretched, the patient being cuddled or kissed, etc. [Ref 6]. These patients still have intact sensory perception while they have these tactile hallucinations because they can feel touch, pain, hot, cold, proprioception, etc. as usual while they're having the hallucinations. But, although these sensations are still normal in the unaffected areas, they may be altered, diminished, or absent in the affected areas.


So, normal stimuli are still processed by the brain during a hallucination, but they may not be processed normally, especially in the affected area, and the resulting perceptual experience is the experience of a mixture of normal perception and hallucination.

References:


Larøi F, Sommer IE, Blom JD, Fernyhough C, H. ffytche D, Hugdahl K, et al. The Characteristic Features of Auditory Verbal Hallucinations in Clinical and Nonclinical Groups: State-of-the-Art Overview and Future Directions Schizophrenia Bulletin. 2012 Jun; 38 (4): 724–733. https://doi.org/10.1093/schbul/sbs061
Schott GD. Exploring the visual hallucinations of migraine aura: the tacit contribution of illustration. Brain. 2007 Jun; 130(6):1690–1703.
Teeple RC, Caplan JP, Stern TA.Visual Hallucinations: Differential Diagnosis and Treatment Care Companion J Clin Psychiatry.2009; 1(1): 26–32.
Falardeau J. Visual aura of migraine. American Academy of Ophthalmology.
Bekkie’s Wonderland. Migraine and migraine art drawn by patients.
Wikipedia. Tactile hallucination