Our senses are our gateway to the world. From the general senses like touch and pain to special senses like vision and hearing, they help us navigate our environment. These sensory systems work through a series of stages, from detecting stimuli to processing in the brain.

Sensory processing and integration combine information from different senses to create a complete picture. The brain's ability to detect small changes in stimuli, known as just-noticeable difference, and adapt to constant stimulation helps us stay alert to important changes in our surroundings.

Sensory Systems

General vs special senses

• General senses, also known as somatosensation, include touch, pressure, temperature, pain, and proprioception (awareness of body position and movement)
  • Receptors for general senses are widely distributed throughout the body in the skin, muscles, tendons, and joints
  • Examples: feeling the warmth of a cup of coffee, sensing the texture of a fabric, experiencing muscle soreness after exercise
• Special senses include vision, hearing, taste, smell, and equilibrium (balance)
  • Receptors for special senses are localized in specific sense organs such as the eyes, ears, tongue, nose, and vestibular system (inner ear)
  • Each special sense has its own specialized receptors and neural pathways that process the specific type of sensory information
  • Examples: seeing a beautiful sunset (vision), hearing a symphony (hearing), tasting a delicious meal (taste), smelling a fragrant flower (smell), maintaining balance while walking (equilibrium)

Stages of sensory perception

1. Stimulus detection: Sensory receptors detect environmental stimuli such as light, sound, chemical molecules, or pressure

   • Receptors transduce the stimuli into electrical signals called receptor potentials
   • Example: photoreceptors in the retina detect light entering the eye

2. Sensory transduction: The process of converting the stimulus energy into electrical signals

   • Involves changes in the membrane potential of the sensory receptor cells
   • Example: in the ear, sound waves cause vibrations of the basilar membrane, which leads to the generation of receptor potentials in hair cells

3. Sensory nerve activation: If the receptor potential is strong enough, it triggers action potentials in the sensory neurons

   • The frequency of action potentials is proportional to the intensity of the stimulus
   • Example: stronger pressure on the skin leads to a higher frequency of action potentials in the sensory neurons

4. Sensory signal transmission: Action potentials travel along the sensory nerve fibers to the central nervous system (CNS)

   • The signals may be processed and modified in the spinal cord or brainstem before reaching the brain
   • Example: sensory signals from the hand travel through the spinal cord and brainstem before reaching the somatosensory cortex in the brain

5. Brain processing: Sensory signals reach the appropriate regions of the cerebral cortex

   • The brain interprets the sensory information, integrating it with memories, emotions, and other sensory inputs
   • This leads to conscious perception of the stimulus and appropriate responses
   • Example: the visual cortex processes signals from the eyes, allowing us to perceive and recognize objects, faces, and scenes

Sensory Processing and Integration

• Sensory processing involves the organization and interpretation of sensory information from various sensory modalities
  • Different sensory pathways transmit information from sensory receptors to specific areas of the brain
  • Sensory neurons play a crucial role in conveying information along these pathways
• Sensory integration is the process of combining information from multiple sensory modalities to create a coherent perception of the environment
  • This integration occurs in various brain regions, including the thalamus and cortex
  • Example: combining visual and auditory information to locate a barking dog

Just-noticeable difference in thresholds

• Just-noticeable difference (JND), also known as the difference threshold, is the smallest change in a stimulus that can be detected 50% of the time
  • Represents the minimum difference in stimulus intensity required for a person to perceive a change
  • Example: the smallest difference in weight between two objects that a person can reliably detect when holding them
• Absolute threshold is the minimum stimulus intensity required for a sensory receptor to detect a stimulus 50% of the time
  • Determines the lower limit of sensory perception
  • Example: the faintest sound that can be heard in a quiet environment
• Weber's law states that the JND is a constant fraction of the original stimulus intensity
  • The Weber fraction is calculated as: $\frac{\Delta I}{I}$, where $\Delta I$ is the JND and $I$ is the initial stimulus intensity
  • This law holds true for most sensory modalities, but the Weber fraction varies depending on the sense involved
  • Example: To detect a change in brightness, a larger difference in intensity is needed when the initial brightness is high compared to when it is low
• Sensory adaptation is the decrease in responsiveness to a constant stimulus over time
  • Allows the sensory system to remain sensitive to changes in stimuli rather than being overwhelmed by constant stimulation
  • Adaptation occurs more quickly for some senses (touch, smell) than others (pain)
  • Example: when entering a room with a strong odor, the smell may be intense initially but becomes less noticeable over time as the olfactory system adapts