05 mechanisms of perception

What is the difference between sensation and perception?

• Sensation: The process of detecting the presence of a stimulus (e.g., "there is a light/sound").

• Perception: The higher-order process of integrating, recognizing, and interpreting complete patterns of sensation (e.g., "colour/music").

• Key point: Sensation is the raw input; perception is making sense out of what you see.

• Example: You detect a light (sensation) → you recognize it as a red traffic light (perception).

• Experience matters: Perception is shaped by experience – e.g., in the "convex/concave circles" example, your brain interprets depth based on lighting cues learned from experience

What are the three types of sensory cortex? What are the three principles of sensory system organization?

• Three types of sensory cortex:

  1. Primary sensory cortex: Receives most input directly from the thalamus.

  2. Secondary sensory cortex: Receives most input from the primary sensory cortex of that system.

  3. Association cortex: Receives input from more than one sensory system → interactions.

• Three principles:

  1. Hierarchical organization: Sensory structures are organized in a hierarchy based on specificity and complexity. Each level receives most input from lower levels and adds another layer of analysis.

  2. Functional segregation: Different functions are handled by different areas (e.g., color vs. motion in vision).

  3. Parallel processing: Multiple pathways process information simultaneously (e.g., "what" and "where" pathways in the auditory system).

• Key point: The brain processes sensory information in a hierarchy, in specialized modules, and in parallel – not all in one place.

What happens when different levels of a sensory hierarchy are damaged?

• Destruction of a sensory system's receptors:

  • Complete loss of ability to perceive in that sensory modality (e.g., total blindness, total deafness).

• Destruction of an area of association or secondary sensory cortex:

  • Complex and specific sensory deficits, while leaving fundamental sensory abilities intact.

• Example – Oliver Sacks' Dr. P. ("The man who mistook his wife for a hat"):

  • He mistook inanimate objects for people.

  • He was unable to read written words.

  • He had a problem recognizing people by face.

  • However, he had intact vision and was fluent in conversation.

  • This is a rather high-level visual disorder than a simple visual deficit.

• Key point: Damage at lower levels = complete loss; damage at higher levels = specific, complex deficits.

What is visual agnosia? What are its subtypes?

• Visual agnosia: Difficulty recognizing an object because the person is unable to "see" the object properly, regardless of the fact that vision in the eye is intact.

  • Inability to recognize shapes or written words and inability to copy drawings or letters.

  • Often due to damage to the posterior occipital cortex and/or temporal lobe(s).

• Subtypes:

  1. Prosopagnosia: Inability to recognize human faces; "face blindness." Patients rely on other features (e.g., voice, hair).

  2. Simultanagnosia: Inability to recognize multiple objects in a scene, despite being able to identify individual details. Inability to grasp content of a scene and relations among objects. Patients are better at identifying the global letter when using a finger to passively trace the global shape.

  3. Alexia: Inability to recognize written words; to read; also difficulties in copying numbers/words.

• Key point: Visual agnosia is a high-level perceptual disorder, not an eye problem.

What are the physical and perceptual dimensions of sound? What is the anatomy of the ear? How does sound localization work?

• Physical vs perceptual dimensions:

  • Amplitude (physical) → Loudness (perceptual)

  • Frequency (physical) → Pitch (perceptual)

  • Complexity (physical) → Timbre (perceptual)

• Anatomy of the ear (5 steps):

  1. Sound waves travel down the auditory canal.

  2. Sound waves strike the tympanic membrane (eardrum).

  3. Sound waves cause the tympanic membrane to vibrate the three tiny bones: malleus (hammer), incus (anvil), and stapes (stirrup).

  4. The three tiny bones convert the sound waves into stronger vibrations in the cochlea.

  5. These vibrations displace the hair cells along the basilar membrane in the cochlea.

• Sound localization:

  • You want to know what produced it (friend or stranger) and where it came from (approach vs. escape).

  • Based on: time differences between ears, intensity differences between ears, and spectral cues.

• Major pathways in the auditory cortex:

  • "What" pathway: Identifying the sound.

  • "Where" pathway: Locating the sound.

• Key point: The ear converts sound vibrations into neural signals; the brain then determines what and where the sound is.

What are the types of deafness? What is tinnitus?

• Deafness: Most prevalent human disability. Complete deafness is rare – about 99% of hearing-impaired people have at least some response to loud noises.

• Types:

  1. Conductive deafness (middle-ear deafness):

     • Associated with damage to the ossicles (hammer, anvil, stirrup).

     • Caused by diseases, infections, or tumors near the middle ear.

     • Sometimes temporary.

     • Can be corrected by surgery or hearing aids that amplify the stimulus.

  2. Nerve deafness (inner-ear deafness):

     • Associated with damage to the cochlea, the hair cells, or the auditory nerve.

     • Can occur in any degree and may be confined to one part of the cochlea.

     • Hearing aids cannot compensate for extensive nerve damage, but they help people who have lost receptors in part of the cochlea.

     • Can be inherited or can develop from prenatal problems.

• Tinnitus: Frequent or constant ringing in the ears.

  • Causes: damage to hair cells, impaired blood circulation, earwax blockage, age-related hearing loss, exposure to loud noises/music.

  • Treatment depends on the condition associated with symptoms.

• Key point: Conductive deafness is often fixable; nerve deafness is more serious.

What is olfaction? How does the olfactory system work? What are the two major olfactory pathways?

• Olfaction: The sense of smell; the detection and recognition of chemicals that contact the membranes inside the nose.

• Function:

  • We rely on olfaction for deciding whether food is good or bad (adaptive role!).

  • Many smells evoke highly emotional memories.

• Anatomy:

  • Olfactory mucosa: Contains the olfactory cells (neurons responsible for smell). Dendrites are located in the nasal passage.

  • Olfactory bulbs: Olfactory receptor cells synapse on neurons that project via the olfactory tracts to the brain (temporal lobes, including amygdala and piriform cortex).

• Two major pathways leave the amygdala-piriform area:

  1. Limbic pathway (diffusely): Emotional response to smell.

  2. Orbitofrontal-thalamic pathway: Conscious perception of smell.

• Key point: Smell is unique – it goes directly to the limbic system, which is why smells trigger strong emotions and memories.

What is the gustatory system? How does it work? What are the primary tastes?

• Gustation: The sense of taste.

• Anatomy:

  • Taste receptor cells: Responsible for taste.

  • Taste buds: Clusters of taste receptor cells on the tongue.

  • Papillae: Structures on the surface of the tongue.

• Primary tastes:

  • Sweet, sour, salty, bitter, umami (savory).

• Gustatory pathways (4 steps):

  1. Gustatory neurons leave the mouth and carry information from the tongue and oral cavity.

  2. Fibers terminate in the medulla (solitary nucleus).

  3. Fibers synapse on neurons that project to the thalamus.

  4. Gustatory axons of the thalamus project to the primary gustatory cortex.

• Key point: Taste information travels from the tongue → medulla → thalamus → primary gustatory cortex.

What are anosmia, specific anosmia, and ageusia?

• Anosmia: General lack of olfaction (inability to smell).

  • Caused by damage to the olfactory system.

• Specific anosmia: Inability to smell a specific chemical.

  • Caused by damage to the olfactory system.

• Ageusia: The inability to taste.

  • Caused by damage to the gustatory system.

• Key point: Damage to the olfactory system → smell loss; damage to the gustatory system → taste loss.

What are the three parts of the somatosensory system? What cutaneous receptors exist? What are the two major somatosensory pathways?

• Three parts of the somatosensory system:

  1. Exteroceptive system: Senses external stimuli applied to the skin.

  2. Proprioceptive system: Monitors information about the position of the body from receptors in muscles, joints, and organs of balance.

  3. Introceptive system: Provides general information about conditions within the body (e.g., temperature and blood pressure).

• Cutaneous receptors:

  • Free nerve endings: temperature change and pain.

  • Pacinian corpuscles: vibration (large receptive fields).

  • Merkel's disks & Ruffini endings: skin stretch, skin indentation, pressure.

  • Meissner's corpuscles: vibration (small receptive fields).

• Two major somatosensory pathways:

  1. Dorsal-column Medial Lemniscus System: Mainly touch and proprioception.

  2. Anterolateral System: Pain and temperature.

• Key point: Different pathways carry different types of somatosensory information.

What is the somatosensory homunculus? What are the effects of damage to the primary somatosensory cortex? What are the types of somatosensory agnosia?

• Somatosensory homunculus: A map of the body in the primary somatosensory cortex. The greatest part is dedicated to receiving input from the parts of the body we use to make tactile discriminations (e.g., hands, lips).

• Effects of damage to primary somatosensory cortex:

  • Often remarkably mild – because both the auditory system and the somatosensory system feature numerous parallel pathways (alternative pathways).

  • Can lead to somatosensory agnosia.

• Two types of somatosensory agnosia:

  1. Astereognosia: Inability to recognize objects by touch.

  2. Asomatognosia: Failure to recognize parts of one's body.

     • Usually unilateral (affecting only the left side of the body).

     • Usually associated with damage to the right temporal and posterior parietal lobe.

     • Common in contralateral neglect.

• Case – Aunt Betty: Damage to right temporal and posterior lobe → anosognosia (failure to recognize one's own symptoms).

  • "Lack of insight" → may lead to refusing medications or treatment.

• Rubber-hand illusion: The feeling that an extraneous object (e.g., rubber hand) is actually a part of one's own body – can be experimentally induced.

• Key point: The brain's body map can be damaged or even tricked.

What are the key facts about pain? What are the two characteristics of selective attention? What is the difference between endogenous and exogenous attention?

• Facts about pain:

  • Pain is adaptive: Extremely important for survival – warns us to stop engaging in harmful activities or to seek treatment.

  • No clear cortical representation: Painful stimuli activate many areas: thalamus, somatosensory cortex, insula, anterior cingulate cortex (and more).

  • Endorphins: Natural pain and stress fighters; interact with opiate receptors to reduce pain perception.

  • Pain can be suppressed by cognitive and emotional factors!

• Neuropathic (chronic) pain: Severe chronic pain in the absence of a recognizable pain stimulus.

  • Typical case develops after an injury → injury heals but the patient continues to experience chronic pain.

  • Can be triggered by gentle touch.

  • Problem: pain is caused by abnormal activity in the CNS. Cutting nerves often brings little relief; medication is usually ineffective.

• Selective attention – two characteristics:

  1. Selective attention improves the perception of the stimuli that are its focus.

  2. Selective attention interferes with the perception of the stimuli that are not its focus.

• Endogenous attention: Focused by internal cognitive processes (e.g., searching for one's key). Mediated by top-down (higher to lower) neural mechanisms.

• Exogenous attention: Focus by external events (e.g., cat jumped on desk). Mediated by bottom-up (lower to higher) neural mechanisms.

• Cocktail-party phenomenon: You focus on one conversation but your name in another conversation immediately gains access to consciousness – you still unconsciously monitor blocked-out stimuli.

• Change blindness: Failure to notice changes in a visual scene (e.g., the "invisible gorilla" experiment).

• Key point: Attention is selective – it enhances what we focus on and filters out what we don't.