Describe the processing of sensory input and explain how it is selected, organized and interpreted.

Sensory Input: Transduction and Transmission

The process begins with sensation, the detection of stimuli by sensory receptors. These receptors are specialized neurons that respond to specific forms of energy – light, sound, heat, pressure, chemicals. This detection involves transduction, the conversion of physical energy into neural signals. For example, photoreceptors in the retina transduce light energy into electrical signals.

• Five Major Senses: Vision, Audition, Somatosensation (touch, temperature, pain), Olfaction (smell), and Gustation (taste).
• Neural Pathways: These neural signals travel along specific pathways to the brain. For instance, visual information travels from the retina via the optic nerve to the visual cortex in the occipital lobe.
• Sensory Thresholds: The minimum intensity of a stimulus that can be detected is known as the absolute threshold. The difference threshold (Weber's Law) refers to the smallest detectable difference between two stimuli.

Selection: Attention and Filtering

Not all sensory information reaches conscious awareness. Attention plays a crucial role in selecting which stimuli are prioritized for further processing. This selection can be:

• Selective Attention: Focusing on one stimulus while ignoring others (e.g., the "cocktail party effect" – being able to focus on one conversation in a noisy room). Broadbent’s Filter Model (1958) proposed a bottleneck in early processing, while Treisman’s Attenuation Model (1964) suggested that unattended information is attenuated, not completely blocked.
• Divided Attention: Attempting to attend to multiple stimuli simultaneously. This often leads to decreased performance on each task.
• Bottom-up vs. Top-down Attention: Bottom-up attention is driven by the salience of the stimulus (e.g., a loud noise), while top-down attention is driven by our goals and expectations (e.g., searching for a friend in a crowd).

Organization: Perceptual Grouping and Gestalt Principles

Once a stimulus is selected, the brain begins to organize it into meaningful patterns. Perceptual organization relies on several principles, largely described by Gestalt psychology:

• Gestalt Principles: These principles describe how we naturally group elements together:
  • Proximity: Elements close together are perceived as a group.
  • Similarity: Similar elements are perceived as a group.
  • Closure: We tend to complete incomplete figures.
  • Continuity: We perceive smooth, continuous patterns rather than discontinuous ones.
  • Common Fate: Elements moving in the same direction are perceived as a group.
• Depth Perception: The ability to perceive the world in three dimensions. This relies on both monocular cues (e.g., linear perspective, texture gradient) and binocular cues (e.g., retinal disparity, convergence).

Interpretation: Meaning and Recognition

The final stage involves interpreting the organized sensory information and assigning meaning to it. This is heavily influenced by:

• Past Experience: Our prior knowledge and experiences shape how we interpret stimuli.
• Context: The surrounding environment and situation influence our perception.
• Perceptual Constancy: The tendency to perceive objects as stable and unchanging despite variations in sensory input (e.g., size constancy, shape constancy, color constancy).
• Top-Down Processing: Using existing knowledge to interpret incoming sensory information. For example, reading a sentence with missing letters relies on top-down processing to fill in the gaps.

The process is not linear; there is constant interaction between bottom-up and top-down processing. For instance, while recognizing a face (bottom-up), our expectations about who we might see (top-down) can influence our perception.