Two major classes:

1. Bottom-up: perception builds up hierarchically from a set of primitive "features" to our internal representations.
2. Top-down: perception starts with a set of primitives, but our perceptual experience is influenced by higher-level processes, such as knowledge and context.

All bottom-up theories rely on the notion that perception builds upwards from a foundation of primitives to a representation our cognitive system can use.

This takes place without any influence from higher cognitive processes.

Five main theories:

Direct perception

Precursor to behaviorism — Perception is a direct result of stimulus energy affecting receptor cells. No higher cognitive processes or internal representations are necessary

Template/Exemplar theory

We store examples of all the objects we have seen as exemplars or templates.

We compare a perceived object to this set of exemplars until we find a match.
 
 

Prototype theory

Instead of storing many exemplars or rigid templates, we store a prototype, which is kind of like the average of an object.

We compare a perceived object to these prototypes until we find the closest match.
 
 

Feature theory

Perception starts with the identification of basic features that are put together into more complex objects, which are put together into more complex objects, etc. until we identify an object.

Example: Pandemonium
 
 

Using single-cell recording (remember that?), Hubel & Wiesel found neurons in the primary visual cortex (occipital lobe) that respond to visual features such as lines and corners.

These feature detectors are a result of how the early visual system is wired.

Structural description theory

Kind of like a three-dimensional version of feature theory, where rather than having lines and corners as the basic features, simple geometric shapes, called geons, are the basic features.

Top-down theories posit varying degrees of influence of higher cognitive processes on what we actually perceive.
 
 

The primary example of this is the effect of context on perception, such as in the Word Superiority Effect.