Consciousness

One possible cause of straight lines of activation in visual cortex is disinhibition.
Hallucinogenic drugs, lack of oxygen, sensory deprivation, and certain disease
states can all affect inhibitory cells more than excitatory ones, causing an excess
of activity which can spread linearly. The result is hallucinations of the four famil-
iar form constants.

There are also similarities in the movement, colour, and shapes of visual halluci-
nations. Ronald Siegel and Murray Jarvik (1975) trained volunteers to report on
their hallucinations when taking a variety of drugs, including LSD, psilocybin,
THC (from cannabis), and various control drugs and placebos. When the trained
‘psychonauts’ were given amphetamines and barbiturates, they reported only
black and white forms moving about randomly, but the hallucinogens produced
tunnels, lattices and webs, explosive and rotating patterns, and bright colours,
especially reds, oranges, and yellows.

As for more complex visual hallucinations, they vary much more widely than the
simple forms, but there are common themes too, including cartoon-like charac-
ters, scenes from childhood memory, animals and mythical creatures, fantastic
cities and buildings, and beautiful scenery. In Siegel and Jarvik’s drug studies, sim-
ple hallucinations came first, then a shift to tunnels and lattices, and finally more
complex hallucinations. During the peak hallucinatory periods, the participants
often described themselves as becoming one with the images. They stopped
using similes and described their images as real. We might say that they were no
longer having pseudo-hallucinations.

Visual hallucinations in degenerative eye disease include not only the form constants
and vivid colours, but also visions of children, animals, buildings or landscapes, dis-
torted faces with prominent eyes and teeth, and even copies of the same object
arranged in rows or columns (ffytche and Howard, 1999). In an fMRI study, several
patients with Charles Bonnet syndrome were asked to report the beginning and end

‘A hallucination is a

species of reality, as

capable of teaching you

as a videotape about

Kilimanjaro or anything

else that falls through

your life’

(McKenna, 1992, quoted in

Rowlandson, 2012, p. 53)

‘Rows of mugs fixed on a

wall (three rows of four)

for up to two minutes.

Large mugs in the top

row and cups at the

bottom.’

(ffychte and Howard, 1999,

p. 1250)

Dorsal

Left

fieldUpper

Right

field

Left

cortex

VentralRight

cortex

Lower

Y

X

FIGURE 14.5 • The mapping from eye to cortex. The visual field shown on the right is mapped onto the corresponding cortical
pattern on the left. Stripes of activity in cortex are therefore experienced as though due to concentric rings in
the visual field. Depending on the direction of the waves of cortical activity, either concentric rings or spirals are
experienced. According to Cowan, this can explain the origin of the four form constants (1982, p. 1062).