which deserve to be classed as examples of non-conscious perception. For
there are, in addition, many cases in which, while continuing to enjoy
conscious experience, I also display sensitivity to features of my environ-
ment which I donotconsciously perceive. For example, while walking
down the street, and having conscious perceptions of many aspects of my
surroundings, I may also step up and down from the kerb and make
adjustments for various irregularities and obstacles in my path of which I
have no conscious awareness. Since all the phenomena along this spectrum
involve sensitivity to changing features of the environment, and since –
most importantly – theyWt neatly into the practical reasoning model of
explanation, they deserve to be described as perceptual experiences which
are non-conscious. For it may truly be said of me that I stepped up onto
the kerb because Iwantedto avoid falling,sawthat the kerb was there, and
believedthat by stepping higher I should avoid tripping. So this is a case of
genuineseeingwhich is non-conscious.
(Some people – for example, Dennett, 1991a – have attempted to explain
such phenomena in terms of instantaneous memory loss, rather than in
terms of non-conscious experience. On this account, my percept of the
kerbwasconscious, but since no space was devoted to it in memory, I can
neither report nor remember it. Yet this explanation seems ruled out by the
existence of cases where one can respond non-consciously to changes –
such as the slowing down of a metronome – which happen too gradually to
be perceivable at an instant, and so which must presuppose an intact
memory.)
Consider, also, the striking phenomenon ofblindsight. It has been
known for some time that patients who have had certain areas of the
striate cortex damaged (area V1) will apparently become blind in a por-
tion of their visualWeld. They sincerely declare that they are aware of
seeing nothing in that region. It was then discovered that some such
patients nevertheless prove remarkably good at guessing the position of a
light source, or the orientation of a line, on their ‘blind’ side. When their
high rate of success is pointed out to them, these patients are genuinely
surprised – they really thought they were guessing randomly. But the
data show convincingly that they are capable of at least simple kinds of
non-conscious perceptual discrimination – see Weiskrantz (1986) for de-
tails. Indeed, it has been shown that some patients are capable of reach-
ing out and grasping objects on their blind sides with something like 80
or 90 per cent of normal accuracy, and of catching balls thrown towards
them from their blind sides, again without conscious awareness (Marcel,
forthcoming).
In addition to absent-minded activity and blindsight, there are also such
phenomena as sleep-walking, where subjects must plainly be perceiving, to
232 Consciousness: theWnal frontier?