There are also scientific studies showing physiological
differences between alters:

1: J Am Optom Assoc. 1996 Jun;67(6):327-34. Visual function
in multiple personality disorder. Birnbaum MH, Thomann K.
State College of Optometry, State University of New York, NY
10010, USA. "Physiologic differences across alter
personality states in MPD include differences in dominant
handedness, response to the same medication, allergic
sensitivities, autonomic and endocrine function, EEG, VEP,
and regional cerebral blood flow. Differences in visual
function include variability in visual acuity, refraction,
oculomotor status, visual field, color vision, corneal
curvature, pupil size, and intraocular pressure in the
various personality states of MPD subjects as compared to
single personality controls."

Clin Electroencephalogr. 1990 Oct;21(4):200-9. Brain mapping
in a case of multiple personality. Hughes JR, Kuhlman DT,
Fichtner CG, Gruenfeld MJ. Department of Neurology,
University of Illinois, Chicago 60612. "Brain maps were
recorded on a patient with a multiple personality disorder
(10 alternate personalities)....Findings that were
replicated in the second session showed differences from 4
personalities, especially in theta and beta 2 frequencies on
the left temporal and right posterior regions.... Maps from
S acting like some of her personalities or from a
professional actress portraying the different personalities
did not reveal significant differences."

Arch Gen Psychiatry. 1982 Jul;39(7):823-5. EEG studies of
two multiple personalities and a control. Coons PM, Milstein
V, Marley C. "These data suggest that EEG differences among
personalities in a person with multiple personalities
involve intensity of concentration, mood changes, degree of
muscle tension, and duration of recording, rather than some
inherent difference between the brains of persons with
multiple personalities and those of normal persons."