Sensory, Attentional and Perceptual Processes

The One Thing

Sensory processes receive information, attention selects which information receives further processing, and perception organises and interprets it into a meaningful understanding of the environment. Although perception is usually stable and adaptive, it is shaped by both sensory input and top-down influences such as expectations, experience, motivation, and context, and may therefore produce illusions.

Who and What

• Stimulus: Any form of energy or environmental change capable of producing a response in a sense organ.
• Sensation: The initial detection of stimulation by sensory receptors in the external or internal environment.
• Perception: The selection, organisation, and interpretation of sensory information to give it meaning.
• Sense modalities: Systems through which information is received, including vision, hearing, smell, taste, touch, balance, and body position.
• Sensory receptors: Specialised cells or nerve endings that detect light, sound, pressure, temperature, pain, or chemical stimulation.
• Absolute threshold: The minimum intensity of a stimulus that can be detected reliably, usually on about half of the trials. It is not perfectly fixed because detection is affected by attention, motivation, fatigue, alertness, and surrounding conditions.
• Difference threshold: The smallest noticeable difference between two stimuli.
• Weber’s law: The noticeable difference between stimuli depends on the proportion of the original stimulus rather than on a fixed physical amount. It is expressed as ΔI/I = k, where ΔI is the smallest noticeable change, I is the original stimulus intensity, and k is a constant for a particular sensory system.
• Sensory adaptation: A reduction in sensitivity to a constant or unchanging stimulus over time.
• Attention: The concentration of mental activity on selected information while other information is ignored or receives reduced processing.
• Selective attention: Focusing on one source of information while filtering out competing stimuli.
• Sustained attention: Maintaining focus on a task or stimulus for a prolonged period.
• Divided attention: Attempting to attend to more than one task or source of information simultaneously. Because attention is limited, performance often declines when demanding tasks are divided.
• Attentional capture: The automatic drawing of attention by a noticeable, sudden, moving, or emotionally significant stimulus.
• Cocktail-party effect: The tendency to notice personally meaningful information, such as one’s name, in a crowded setting despite competing stimulation.
• Perceptual organisation: The arrangement of separate sensory elements into meaningful patterns or objects.
• Figure-ground organisation: The distinction between a central object or figure and its surrounding background.
• Gestalt principles: Principles explaining how sensations are organised into meaningful wholes.
• Proximity: The tendency to perceive nearby elements as belonging together.
• Similarity: The tendency to group elements that resemble one another in colour, shape, size, or form.
• Closure: The tendency to mentally complete incomplete figures or patterns.
• Continuity: The tendency to perceive smooth, continuous patterns rather than disconnected parts.
• Common region: The tendency to group elements located within the same bounded area.
• Depth perception: The ability to perceive the world in three dimensions and judge distance.
• Binocular cues: Depth information obtained from both eyes, including retinal disparity and convergence.
• Monocular cues: Depth information available to one eye, including interposition, linear perspective, relative size, texture gradient, height in the visual field, and aerial perspective.
• Perceptual constancy: The tendency to perceive objects as stable in size, shape, colour, and brightness despite changes in sensory conditions.
• Size constancy: Perceiving an object as having the same size even when its retinal image changes with distance.
• Shape constancy: Perceiving an object as having the same shape when viewed from different angles.
• Colour or brightness constancy: Perceiving an object’s colour or brightness as relatively stable under different lighting conditions.
• Illusion: A mistaken or distorted interpretation of a real external stimulus.
• Hallucination: A perceptual experience occurring in the absence of an appropriate external stimulus.
• Bottom-up processing: Perception that begins with sensory information and builds towards interpretation.
• Top-down processing: Perception guided by prior knowledge, expectations, beliefs, context, and experience.

Causes and Consequences

• A stimulus activates sensory receptors, neural signals are transmitted to the brain, and the brain organises and interprets the information. This sequence distinguishes the reception of information from its interpretation.
• Vision depends mainly on light, hearing on sound waves, smell and taste largely on chemical stimulation, and touch on pressure, temperature, and pain. Different sense modalities therefore depend on different forms of sensory energy.
• Detection depends on more than stimulus strength. Expectations, decision criteria, motivation, previous experience, attention, fatigue, alertness, and surrounding conditions influence whether a stimulus is detected.
• Weber’s law, expressed as ΔI/I = k, links the smallest noticeable change to the original stimulus intensity. Consequently, the same physical change may be noticeable in one situation but not another.
• Because attention has limited capacity, selective attention allows a person to focus on relevant information while filtering out competing stimuli. The cocktail-party effect demonstrates that personally meaningful information, such as one’s name, may nevertheless capture attention.
• Sustained attention permits prolonged concentration, whereas divided attention requires simultaneous processing of more than one task. When the tasks are demanding, divided attention commonly reduces performance.
• Attentional capture occurs when a sudden, moving, noticeable, or emotionally significant stimulus automatically draws attention. This can redirect processing away from the task that was previously being performed.
• Perceptual organisation allows separate sensory elements to be understood as meaningful objects or patterns. Figure-ground organisation and Gestalt principles—including proximity, similarity, closure, continuity, and common region—explain how the mind organises sensory information into wholes.
• Depth perception depends on binocular cues, including retinal disparity and convergence, and monocular cues, including interposition, linear perspective, relative size, texture gradient, height in the visual field, and aerial perspective. These cues support judgments of distance and three-dimensional structure.
• Perceptual constancies allow objects to be recognised as stable despite changes in distance, viewpoint, illumination, or retinal image. Size constancy preserves perceived size, shape constancy preserves perceived shape, and colour or brightness constancy preserves relatively stable colour or brightness.
• Perception combines bottom-up processing with top-down processing. Sensory information provides the initial input, while expectations, beliefs, context, prior knowledge, and experience influence its interpretation.
• Gestalt organisation and perceptual constancy support adaptation and effective behaviour by making the environment appear coherent and stable. However, the same organisational processes and assumptions can produce illusions when a real stimulus is interpreted inaccurately.
• A hallucination differs from an illusion because an illusion involves a real external stimulus that is misinterpreted, whereas a hallucination occurs without an appropriate external stimulus.

What Gets Asked

• Distinguish sensation from perception and explain the sequence from stimulus energy and sensory receptors to neural transmission, organisation, and interpretation.
• Explain absolute threshold, difference threshold, signal detection, and Weber’s law, including the equation ΔI/I = k and the factors that influence detection.
• Compare selective, sustained, and divided attention, and explain attentional capture and the cocktail-party effect.
• Apply Gestalt principles—proximity, similarity, closure, continuity, and common region—and figure-ground organisation to perceptual organisation.
• Compare binocular and monocular depth cues, including retinal disparity, convergence, interposition, linear perspective, relative size, texture gradient, height in the visual field, and aerial perspective.
• Evaluate how bottom-up and top-down processing contribute to perceptual constancy, adaptation, illusions, and the distinction between illusions and hallucinations.