This is a course in sensation and perception. Sensation is stimulation of our sense organs by external stimuli, while perception is the organization of sensory information. Sensation is bottom-up information conducted from the periphery to the brain, and perception is top-down information that imposes an interpretation on sensation.

In the two videos in this week’s Learning Resources (in Content >> Course Content >> Week 1), Professor Kihlstrom makes two major points: Sensation may change while perception remains the same; and sensation may remain the same while perception changes. That is, bottom-up and top-down understanding are not just the beginning and end of a single process. It’s a dual process.

There’s a duality in perception called “bottom-up” and “top-down” processing. What we see depends on what the outside world tells us, bottom-up, and also what we expect to see, top-down.

Everything we see is ambiguous at first.  Shapes, luminances, distances and colors always can be duplicated by alternative stimuli.  Consequently, every percept is an interpretation, a choice among alternatives.

Usually we “disambiguate” what we see too fast to be aware of our analysis.  Sometimes a pattern may be so blatantly ambiguous that we alternate between interpretations, as with the Necker cube and the Young-girl/Old-woman illusion*.

A raw sensation supplies “bottom-up” information.  Our mental interpretation is called “top-down”.   In the Ponzo illusion below, the equal length of the yellow bars is bottom-up information.  The unequal appearance of the bars that results from our analysis of linear perspective is a top-down perception**.

It is hard to turn off either bottom-up or top-down processes when both are active.  We find it hard to enjoy the bottom-up delights of a dessert if they remind us of an illness that once accompanied them; and despite knowing that only animals have faces, we sometimes see top-down faces in inanimate objects.

Questions (please answer one)

· Can you think of a pure bottom-up or pure top-down perception?

· How would you classify the perception of the difference between living and nonliving? Is it bottom-up or top-down? Explain your answer.

· Is it trivial to distinguish sensation and perception? Most sensory input is ambiguous. Sometimes we confront sensory stimuli that can’t be interpreted in one best way, but usually the brain sets to work and we quickly dispel the fog. If we can’t do that, we suffer and persist.

*These figures generate bistable perceptions; that is, we see one or the other.  Tristable alternations are also possible.

**Not convincing?  Try this other illusion of perspective (explanation here).  Can you separate the bottom-up from the top-down influences?

Note: Links in the discussion topics are optional; they will not appear in tests, although the discussions are intended to clarify the readings and videos in the syllabus and weekly Course Content.

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