- seCtIon FIVe: BoRDeRLAnDs
the two types occur at different times, for example a day apart. When they
are fused, the two senses are typically unrelated (for example, seeing the
devil while hearing the voice of a relative), suggesting independent though
overlapping mechanisms.
Other hallucinations are of bodily sensations like pain, heaviness, stretching,
palpitation, or touch, temperature, or proprioception (Kathirvel and Mor-
timer, 2013). A common tactile hallucination is formication – the feeling of
ants crawling over or under the skin. These kinds of hallucinations are much
rarer, harder to verify (especially in visceral cases where a doctor cannot
easily look inside the body to determine that nothing is there), and not well
researched. But corresponding activity in somatosensory cortices seems
present as for visual and auditory hallucinations.
Phantosmia is the hallucination of a smell, usually an unpleasant smell as of
something burning or rotten. This sometimes precedes epileptic seizures, and
sometimes occurs in only one nostril – the one with the worse olfactory ability.
It may have peripheral or neural causes, and is associated with increased activity
in contralateral frontal, insula, and temporal regions which reduces after treat-
ment of the nasal cavity.
Although there is no limit to the variety of hallucinations, there are some
remarkably common features, suggesting a consistency that reflects
underlying brain functions. Persistent visual forms include spirals, concen-
tric patterns, wavy lines, and bright colours. Meditators, who may sit for
long periods in front of a blank wall, report bright, variously coloured star-
like bursts of light, wavelike, or cobwebby patterns, as well as shimmering,
pixellation, and general brightening (Lindahl et al., 2014; Brasington, 2015).
Mandalas based on circular forms are common, especially in meditative
traditions, and Carl Jung included the mandala as one of the archetypal
forms of the collective unconscious, describing it as the symbol of a harmo-
nious self. These persistent patterns can be seen on shamans’ drums, cave
paintings, ritual designs, and clothing and artefacts from many cultures.
But why?
The reason for these similarities was first investigated in 1926 by Heinrich
Klüver at the University of Chicago while studying the effects of mescaline.
He found that the brightly coloured images the drug induced persisted with
eyes open or closed, and tended to take on four repeated forms. These ‘form
constants’ were 1) gratings and lattices, 2) tunnels, funnels, and cones, 3)
spirals, and 4) cobwebs. All are found in the hallucinations caused by drugs,
fever, migraine, epilepsy, and near-death experiences, and in hypnagogic
imagery and the imagery of synaesthetes.
The reason may lie in the way the visual system is organised, and in partic-
ular the mapping between patterns on the retina and the columnar organ-
isation of primary visual cortex (Cowan, 1982; Bressloff et al., 2002). This
mapping is well known from both monkey and human studies, and is such
that concentric circles on the retina are mapped into parallel lines in visual
cortex. Spirals, tunnels, lattices, and cobwebs map onto lines in different
directions. This means that if activity spreads in straight lines within visual
cortex, the experience is equivalent to looking at actual rings or circles.
FIGURE 14.4 • (a) Hallucinated tunnels can be simple
dark spaces leading to a bright light,
schematic tunnel patterns, or realistic
tunnels like sewers, subways, or
caverns. In his experiments with THC,
psilocybin, LSD, and mescaline, Siegel
(1977) found that after 90 to 120
minutes colours shifted to red, orange,
and yellow, pulsating movements
became explosive and rotational, and
most forms were lattice-tunnels, such
as this one which has complex memory
images at the periphery (Siegel, 1977,
p. 137). (b) An almost identical tunnel
was painted by David Howard, a man
with narcolepsy who claimed to have
been frequently abducted by aliens
(see Blackmore, 2017, p. 217).