Sensory and Perception

Key Concepts in Sensation & Perception

• Sensation: The process of detecting physical stimuli (light, sound, pressure) from the environment.

• Perception: The interpretation and organization of sensory information into meaningful experiences.

• Transduction: The conversion of physical stimuli into neural signals by sensory receptors.

• Bottom-Up Processing: Perception driven by raw sensory input.

• Top-Down Processing: Perception influenced by expectations, experience, and cognition.

• Absolute Threshold: The minimum intensity of a stimulus needed for detection 50% of the time.

• Difference Threshold (Just Noticeable Difference, JND): The smallest detectable change between two stimuli.

General Pathway of Sensory Processing

1. Stimulus Detection: Sensory receptors detect a stimulus (e.g., light, sound, touch).

2. Transduction: Sensory receptors convert the stimulus into electrical signals.

3. Transmission: Signals travel via sensory neurons to the brain.

4. Processing in the Thalamus: The thalamus acts as a relay center (except for smell).

5. Cortical Processing: Signals reach the primary sensory cortex (e.g., visual cortex for vision).

6. Perception: The brain interprets the sensory input into a coherent experience.

Major Components of Vision & Auditory Pathways

Vision Pathway (Retina → Brain)

1. Light enters the eye and is focused onto the retina.

2. Photoreceptors (rods & cones) transduce light into neural signals.

3. Signals pass through bipolar & ganglion cells → form the optic nerve.

4. Optic nerve fibers cross at the optic chiasm (partial decussation).

5. Signals travel via the optic tract to the lateral geniculate nucleus (LGN) in the thalamus.

6. From LGN, signals project to the primary visual cortex (V1) in the occipital lobe.

Auditory Pathway (Ear → Brain)

1. Sound waves enter the ear and vibrate the tympanic membrane (eardrum).

2. Ossicles (middle ear bones) amplify vibrations to the cochlea.

3. Cochlear hair cells transduce mechanical movement into neural signals.

4. Signals travel via the auditory nerve to the brainstem & midbrain (inferior colliculus).

5. Signals reach the medial geniculate nucleus (MGN) in the thalamus.

6. From MGN, signals project to the primary auditory cortex (A1) in the temporal lobe.

Dorsal vs. Ventral Visual Processing Streams

Once visual information reaches the primary visual cortex (V1), it splits into two major pathways:

• Dorsal Stream ("Where" Pathway)

  • Path: V1 → Parietal Lobe

  • Function: Processes motion, depth, spatial location

  • Damage Effect: Optic ataxia (difficulty using vision for guiding actions)

• Ventral Stream ("What" Pathway)

  • Path: V1 → Temporal Lobe

  • Function: Object recognition, color, faces

  • Damage Effect: Visual agnosia (difficulty recognizing objects)

Behavioral Effects of Damage to Perceptual Areas

How Visual Information Changes as It Moves Through the System

1. Retina: Detects raw visual input (light, color, edges).

2. Optic Nerve: Sends raw signals to the brain.

3. LGN (Thalamus): Organizes and relays signals to the cortex.

4. V1 (Primary Visual Cortex): Detects basic features (edges, orientation, movement).

5. Higher Visual Areas: Process complex shapes, motion, color, and recognition (V2-V5).

6. Dorsal & Ventral Streams: Further analyze "Where" vs. "What" aspects of vision.

How External Stimuli Translate into Neural Representations

• Light Waves → Neural Signals (Vision)

  • Photoreceptors (rods/cones) in the retina transduce light into electrical signals.

  • Signals pass through the optic nerve → LGN → visual cortex, where patterns, color, and movement are processed.

• Sound Waves → Neural Signals (Hearing)

  • Vibrations in the cochlea move hair cells, converting mechanical energy into neural impulses.

  • Signals travel through the brainstem → MGN → auditory cortex for tone, pitch, and speech processing.

• Touch/Pressure → Neural Signals (Somatosensation)

  • Mechanoreceptors in the skin detect pressure, heat, and pain.

  • Signals travel through the spinal cord → thalamus → somatosensory cortex for localization and perception of touch.

• Chemicals → Neural Signals (Taste & Smell)

  • Taste buds & olfactory receptors detect chemical molecules.

  • Taste signals go to the gustatory cortex, while smell bypasses the thalamus and goes to the olfactory cortex.