Sensory, Attentional and Perceptual Processes
This domain tests how physical energy becomes experience: sensory thresholds, attention, perceptual organisation and the sources of error called illusions.
Sensation and thresholds
Sensation is the detection of stimulus energy; perception is its interpretation. The absolute threshold is the weakest detectable stimulus; the difference threshold (just noticeable difference) is the smallest change detected. Weber's law states that the JND is a constant fraction of the original stimulus. Signal detection theory separates sensory sensitivity from response bias.
Sensory systems
Vision uses rods (dim light, peripheral, no colour) and cones (colour, detail, fovea). Audition encodes pitch mainly by place on the basilar membrane and loudness by amplitude. Touch, taste, smell and kinesthesis complete the set.
Attention and perception
Attention can be selective, divided or sustained. Perception organises input by Gestalt laws: proximity, similarity, continuity, closure and figure–ground. Depth uses monocular cues (linear perspective, relative size, interposition, texture gradient) and binocular cues (retinal disparity, convergence). Perceptual constancy maintains size, shape and brightness across changes; perceptual set is expectation-driven.
Worked example
Railway tracks appear to meet in the distance. That is linear perspective, a monocular pictorial cue, not a failure of the eye. The brain reads convergence as depth.
Traps
Rods are not for colour. The absolute threshold is not the same as the difference threshold. Illusions are systematic, not random errors, and often reveal normal perceptual rules.