Other questions we can ask about binocular rivalry are more theoretical. As long
ago as 1901 the theory was proposed that neurons encoding the two versions act
on each other with reciprocal inhibition. The currently dominant version inhibits
the suppressed version, but the cells involved begin to habituate, weakening the
inhibition and allowing the suppressed version to win out again until the same
process happens in reverse. This would predict that sensitivity in the dominant eye
should gradually decrease while that in the suppressed eye increases. Alais, Cass,
and O’Shea (2010) investigated this by presenting a brief probe stimulus (a change
in contrast at either the bottom or the top of the image) at random times, on aver-
age every three seconds, and timed how quickly participants responded to it. The
results clearly showed that at the start of a dominance period, probe sensitivity is
higher for the dominant image, and lower for the suppressed image, but that this
difference reduces towards equal sensitivity by the end of the epoch. This supports
the ‘adapting reciprocal inhibition model’. It is interesting to note that there are no
corresponding changes in the experience of rivalry: although some people seem
to briefly see composite images at moments of transition, the dominant stimulus
doesn’t seem to fade before being replaced by the suppressed one.
Looking at what the suppressed eye does, these experiments also showed that
observers are apparently able to respond to a stimulus they are not consciously
aware of. One explanation is that the probe presentation itself causes a reversal
of dominance that makes the suppressed image detectable, an idea compatible
with Dennett’s multiple drafts theory of consciousness (see Chapter 5), in which
an answer to the question ‘what was I conscious of?’ is created only by how that
experience is probed (by an explicit question, or a particular task, for example).
More generally, this finding might indicate that visual awareness manifests
either after or at the same time as the planning of motor responses to the probe
(Baker, 2010), meaning that it cannot be playing a role in that planning.
Work on binocular rivalry has also been incorporated into a hierarchical model
of levels of processing, based on whether the suppressive effects of one tempo-
rary blinding method (e.g. binocular rivalry, backward masking, attentional blink)
functionally precede or follow those of another method. The neuroscientist Bruno
Breitmeyer (2015) locates binocular rivalry at the very lowest, i.e. earliest, level of
this hierarchy. He notes that we must bear in mind that this functional hierarchy
does not necessarily map readily onto cortical anatomical levels of processing –
as a vastly complex network rather than a neatly serial processor, the brain is far
too complicated for this to be possible.
Breitmeyer’s mapping of the ‘functional hierarchies of unconscious processing’
can be applied to contexts like the reading process. When reading, we are able to
process information not just about the word our eyes are currently fixated on, but
also about words we have not yet read. Though we may not be able to identify
subsequent words in a sentence before reading them, there may be ‘unconscious
previewing’ of those words with measurable priming effects (Prioli and Kahan,
2015). Breitmeyer (2015) remarks that ‘a word stimulus can reveal a type and level
of unconscious processing that is higher than that of its basic visual features such
as the orientations or curvatures embedded in its graphemic structure’ (p. 240).
He claims that all visual processing in the subcortical retinal and LGN processing
levels is unconscious, but that at the cortical level where processing gets more
widely distributed in complex interactions among ‘bottom-up’, ‘same-level’, and
