Sensation Thresholds and the Eye

The Five Senses??

Basic Principles Sensation – process by which our sensory receptors and nervous system receive and represent stimulus energies from our environment Perception – process of organizing and interpreting sensory information, enabling us to recognize meaningful objects and events

A Demonstration of Perception

Types of Processing Bottom-up Processing – analysis that begins with the sensory receptors and works up to the brain’s integration of sensory information Top-down Processing – information processing guided by higher-level mental processes, as when we construct perceptions drawing on our experience and expectations.

Top-Down vs. Bottom-Up

Thresholds Psychophysics – the study of relationships between the physical characteristics of stimuli and our psychological experience of them. Absolute Threshold – the minimum stimulation needed to detect a particular stimulus 50 percent of the time (school test for hearing)

Thresholds (cont’d) Signal Detection Theory – A theory predicting how and when we detect the presence of a faint stimulus (the signal) amid background stimulation (noise). Assumes there is no single absolute threshold and that detection depends partly on a person’s experience, expectations, motivation, and level of fatigue.

Examples of Signal-Detection Theory New mothers and newborns Wartime vs. peacetime Life or death consequences (radar detection, ICU nurses, air traffic controllers, airport )

Signal Detection Theory Correctly identifies stimulus as absent Response Yes Response No Signal Present HitMiss Signal Absent False AlarmCorrect Negative

Thresholds (cont’d) Subliminal – any signal below absolute threshold for conscious awareness (by definition you will detect it some percent of the time) Priming – the activation, often unconsciously, of certain associations, thus predisposing one’s perception, memory, or response.

Thresholds (cont’d) Difference Threshold – the minimum difference a person can detect between two stimuli (50% of the time). This is experienced as a “just noticeable difference” or JND.

Thresholds (cont’d) Let’s do a demonstration. I need a volunteer! Weber’s Law – to perceive a difference, two stimuli must differ by a constant minimum percentage (rather than a constant amount).

Sensory Adaptation Sensory Adaptation – diminished sensitivity as a consequence of constant stimulation. (cold swimming pools and hot showers….) Our perceptions are organized by the meanings our minds impose. Sensory adaptation reduces sensitivity, but it enables us to focus on informative changes in our environment without being distracted by constant stimulation.

The Eye

Eye Movements