Sensation & Perception Prosopagnosia helps to understand the difference between sensation and perception. Frogs have neural networks with built-in 'fly detectors'. Top-down versus bottom-up. Top-down constructs perceptions from sensory input by drawing on our experience and expectations. Bottom-up processing starts at the sensory receptors and works up to higher levels of processing. Neuroscience always proceeds from bottom-up to top- down. It is always a two-way communication; editing and filtering are always occuring.

Sensation & Perception Transduction is the process of converting one form of energy into another. Ultimately, any perceptable energy-- heat, light, sound, smell, pressure--must be convertable to action potentials. Absolute thresholds are the minimum stimulation necessary to detect a particular sensation 50% of the time. This number is derived from the normal curve. Detecting a weak stimulus, or signal, depends not only on the signal's strength but also on our psychological state-- experience, expectations, motivation and alertness. However, a subliminal message--below the absolute threshold--an unnoticed image or word can prime a later response.

Sensation & Perception A masking stimulus is used that interrupts the brain's processing before conscious perception can occur. Jiang et al. (2006)(m226f17.3c220f6.3) subjects evaluated a stimulus even when not aware of it. The difference threshold (just noticeable difference) is the minimum difference a person can detect between any two stimuli half the time. (The numbers are again derived from the normal curve). Weber's law: Two stimuli must differ by a constant proportion for a person to perceive the difference. Two lights must differ by 8 percent. Two objects must differ in weight by 2 percent. Two tones must differ in frequency by 0.3 percent.

Sensation & Perception Pritchard R., 1961. (mp229f6.5 c223f17.5) A projector mounted on a contact lens makes the projected image move with the eye. Initially, the person sees the stabilized image, but soon she sees fragments fading and reappearing. These fragments, however, reappear as meaningful shapes, not random ones. This indicates that top-down processing is taking over given these random and uncertain conditions. Our sensors are alert to novelty, not to sameness. We perceive the world not exactly as it is, but as it is useful for us to perceive it.

Sensation & Perception Perceptual set are those mental tendencies and assumptions that greatly affect what we perceive, an example of top-down processing. We are hard-wired for pattern recognition, and will impose patterns on ambiguous stimuli, such as (m229f6.6 c223f17.6) We form schemas, conceptual systems that organize and interpret unfamiliar information. The interplay between context and emotional perception(m231f6.8 c225f6.8) The context makes the pursuing monster look more aggressive and bigger than the pursued. Are songs sadder now than they use to be? Many pop songs are written in minor keys or ancient modes, previously reserved for funerals.