inhale, odorants excite subsets of the millions of cellular receptors in your nose, sending patterns of
neural activity to the olfactory bulb on the bottom of your brain. The olfactory bulb also receives
inputs from other brain areas, such as the hippocampus and the neocortex, so that the signals it sends
to the rest of the brain are already a combination of bottom-up sensory information and top-down
processing. Hence when you smell something, like a live duck in a barnyard or a cooked one in a
restaurant, the smell is the result of dynamic interactions of different brain areas involving both
sensory inputs and previous knowledge and expectations.
The complexity of perceptual processing in the brain shows the implausibility of the traditional
empiricist view that our sense experiences are copies of objects in the world. Without previously
acquired or inherited concepts, we would have a very difficult time dealing with the vast number of
sensory signals that our eyes, ears, and other sensors send to our brains. Perception requires brains to
be able to relate inputs from sensory organs with information they have already stored in the form of
synaptic connections between neurons. Ambiguous examples like the duck-rabbit show that
perception is not just the bottom-up processing of sensory inputs; it also involves top-down
interpretation based on what is already known. Because the brain is a parallel processor capable of
assessing many aspects simultaneously, we do not have to choose between hypotheses that perception
is primarily driven by input to sensory receptors or that it is primarily driven by top-down
interpretation. Rather, brains can perform inferences that simultaneously use both kinds of
information.
Appearance and Reality
The essential top-down contribution of previous knowledge to perception has tempted some
philosophers and psychologists to conclude that the senses do not enable us to know what objects are,
only what they appear to be. Some worry that the gap between appearance and reality cannot be
bridged, as when Kant said that we cannot know things in themselves. Some psychologists writing on
hallucinations have claimed that support for Kant's idealism comes from the brain's capability of
generating illusory perceptions that have no connection with reality. A few micrograms of a drug like
LSD can disengage your brain's perceptual apparatus from the usual sensory inputs and generate
fantastic images that have no correspondence to anything in the world. You do not have to take drugs
to hallucinate, as a similar process takes place every night when you dream. Your brain generates
complex and often compelling sensory experiences that are not directly caused by anything in the
world. Last night I dreamed that I was shopping in a supermarket and bought some delicious bread,
but morning brought the realization that the market and the bread were unreal.
Nevertheless, we should not infer from the complexity of perceptual processing and phenomena
like hallucinations and dreams that the senses fail to provide us with knowledge about how the world
actually is. Support for the reality of objects is based on inference to the best explanation, as
defended in chapter 2. There is abundant evidence that the bread I ate for lunch today exists and has
the properties I attribute to it.
First, I do not have to rely exclusively on a single sense. I see the color and shape of the loaf of
bread, but I can cross-check the shape using my sense of touch, confirming that it feels the same way
that it looks. I can also use hearing to investigate the bread by banging the loaf against a pot and
hearing the ding. Further, the bread produces pleasurable stimulation of my senses of taste and smell.
The brain has different sensory systems but can combine them to form unified perceptions. In contrast