Oculomotor micropsia is not a new discovery,
but it is only recently utilized by McCready and others to explain the Moon illusion. I have discussed the phenomenon briefly in the previous article, and now I am going to discuss it in more details.
Briefly oculomotor micropsia is conventionally understood as this: if there is only one object,
or a dominant object, in the field of view, our eyes try to converge and
accommodate to its distance.
The muscles that control convergence and
accommodation send signals to our brain, which are important distance cues. But
when one's eyes converge and accommodate on a distance nearer than that of the
object,
that object appears to subtend a smaller angle than if one's eyes were
converged and accommodated upon it. Micropsia means "appearing small", and here
refers to the visual angle subtended by an object.
The reverse effect,
macropsia, occurs when the eyes converge or accommodate at a distance greater
than the object being judged. In summary, (1) as a rule, when the eyes focus and converge to a closer distance than a
viewed object its angular size appears to become smaller (micropsia) than it was
before the readjustment;
(2) when the eyes adjust to a farther
distance, the angular size of a viewed object appears to become larger
(macropsia) than it was before.
Why would the eyes converge or accommodate at distances different than the
object of interest in the field of view?
Several reasons have been given. A very large number of
nearby objects spread over the field of view may bias the brain towards
convergence at their distance.
For most people, the functions of convergence
(aiming the eyes to a common point) and accommodation (focus setting of the
eye's lenses) are "locked together" so if one converges to nearby objects,
the accommodation adjusts to that distance also, and
vice-versa. Proponents of this hypothesis as explanation of the moon
illusion argue that if there are a number of distant objects in the
field of view,
as there would be
when observing the rising or setting full moon, the brain adjusts accommodation
and convergence to them. But when viewing the full moon directly overhead, in a
clear sky, there are no other distance cues, and the eye adjusts to its
resting focus a distance of 1 or 2 meters.
This makes the perceived
angular size of the overhead moon seem smaller.
On the surface there are some
similarities between oculomotor micropsia and macropsia and my approach
to the Moon illusion;
but there are some fundamental differences between them. According to the proponents of oculomotor micro/macropsia,
the fact that the horizon moon is perceived as larger is
because our brain adjusts accommodation and convergence to the distant
objects in the field of view; as a result, we have oculomotor macropsia.
Put the situations where the distant objects are not visible at
all, e.g., at sea, aside, the distant objects on the horizon are
physically much closer than the horizon moon and perceived as such also.
This is against the rule of oculomotor macropsia, which is that when the eyes adjust to a farther
distance, the angular size of a viewed object appears to become larger
(macropsia) than it was before.What the proponents have observed is not the oculomotor macropsia as described by the above rule,
but is the rule of perceived size of objects. As I have stated in The Ames Room Illusion, the fundamental rule of perceived size is that when you move an observational frame of reference,
whether it is a ruler, a thumbnail or any object, away from your
eye and toward an object, the target object will appear larger; and when you
move that reference frame away from an object and toward your eye,
the
object will appear smaller. In addition, the observational frame of
reference is associated with the converging point which in turn is
associated with the visual field volume.
Hence, the larger appearance of the horizon moon is determined
by the place of the converging point and the reduced visual field
volume, not by oculomotor macropsia.
The micropsia and macropsia of
our size perception are believed to be caused by the muscles that
control convergence and accommodation sending signals to our brain,
and it is why they are called oculomotor micropsia and macropsia.
As such, the size perception induced by the oculomotor
information sent to our brain which in turn interprets as further away
and smaller, or as nearer and larger. Accordingly,
it is the subjective interpretation based on the oculomotor
information that determines our size perception, not the objective
information and rules. If accommodation and related oculomotor system
were involved in the moon illusion,
the
elderly people who have lost nearly all accommodation would not perceive the
illusion. But they do perceive the Moon illusion. And persons with eye lens implants have no accommodation, and
they do perceive the Moon illusion as well.
Covering one eye removes convergence from consideration, but
that does not eliminate the Moon illusion. Also, the convergence and
accommodation will not work for objects further than 10 meters, let
alone very far objects such as the objects on the horizon.
My proposal is that micropsia and macropsia do happen; but they
are not caused by our oculomotor system as believed by many perception
researchers,
but by the converging point which can be moved closer to the
object when we focus on it and accompanying visual field volume changes.
The understanding of oculomotor
micropsia and macropsia is quite confusing, and misleading sometimes.
For instance, according to New World Encyclopedia, oculomotor macropsia
causes objects to appear larger when they appear far away based on
distance cues such as buildings or trees,
and oculomotor micropsia makes
objects appear smaller when we perceive them to be closer. Oculomotor
micorpsia exists in order to make it easier and faster to turn our
heads the right amount to perceive a nearby object that might pose a
threat.
Oculomotor macropsia exists because it is a necessary byproduct
of using oculomotor micropsia, and there is no harm done if we
misperceive an object that is far away.
These explanations based on the evolutionary theory are
contradictory and quite confusing. On the contrary, the closer objects
that appear larger would be easier to get our attention and therefore be more
adaptive for our survival and reproduction.
And the faraway predators do not pose an immediate threat; so
their smaller appearance is not very harmful for our survival and
therefore is not unadaptive.