The Visual angle
The visual angle is the angle subtended by an object at the eye of an observer. Visual angles are generally defined in degrees, minutes and seconds. (A minute is 1/60 degree and a second is 1/60 minute). As a general rule, a thumbnail held at arm's length subtends about 1 degree of visual angle. Another useful fact is that a 1-cm object viewed at 57 cm has a visual angle of approximately 1 degree.

Two-stage model of human visual information processing

Stage 1 Processing includes:
- Rapid parallel processing
- Extraction of features, orientation, color, texture and movement patterns
- Transitory nature of information, which is briefly held in an iconic store
- Bottom-up, data driven model of processing

Stage 2 Processing includes:
- Slow serial processing
- Involvement of both working memory and long-term memory
- More emphasis on arbitrary aspects of symbols
- Top-down processing
- Different pathways for object recognition and visually guided motion

Psychophysics and Cognitive Psychology

Psychophysics is the set of techniques that are based on applying the methods of physics to measurements of human sensation.

In cognitive psychology, the brain is treated as a set of interlinking processing modules.

Sensory and Arbitrary Representations
- Sensory is one for which the meaning is perceived without additional training
- Arbitrary is used to define aspects of representation that must be learned, having no perceptual basis.