Forebrain, Modularity, and Lashley Experiments

Forebrain, Midbrain, Hindbrain: Shared Architecture

• All vertebrates (fish, amphibians, reptiles, birds, mammals, humans) possess the same three major brain divisions:
  • Forebrain
  • Midbrain
  • Hindbrain
• Differences across species are largely morphological (shape, size, neuron count), not categorical.
• Human distinctiveness stems from the extraordinary development of the forebrain:
  • Highest neuron count among animals
  • Seat of higher‐order cognition (language, explicit & working memory, complex planning, abstract thought)

What Makes Us “Uniquely Human”

• The forebrain’s advanced structure and neuron density enable:
  • Language processing
  • Conscious problem solving and reasoning
  • Episodic (explicit) memory formation and retrieval
  • Sustaining and manipulating information in working memory
• Midbrain & hindbrain remain crucial (sensory routing, arousal, autonomic functions) but do not confer our unique cognitive abilities.

Distributed Processing vs. Modularity

• Foundational question: “Are cognitive functions localized in specific anatomical modules or distributed across the entire brain?”
• Two theoretical poles:
  1. Distributed Model – Functions arise from activity spread widely throughout the cortex; damage anywhere degrades performance in proportion to lesion size.
  2. Localization (Modularity) Model – Distinct functions are housed in identifiable, relatively circumscribed neural circuits (e.g., a face‐recognition module).

Historical Evidence: Karl Lashley’s Search for the Engram

• Goal: Pinpoint the physical location (engram) of memory in the brain.
• Method:
  • Trained rats to run a specific maze until performance was flawless (implicit procedural memory for the route).
  • Surgically created brain lesions of varying size & location using cauterization.
  • Allowed recovery, then retested maze performance.
• Findings:
  • Location‐independent: Rats could still solve the maze regardless of which specific cortical region was lesioned.
  • Size‐dependent: Larger lesions produced greater memory impairment; small lesions produced minimal or no deficit.
• Conclusion:
  • Memory is widely distributed; no single cortical spot houses the engram.
  • Inspired the adage, “Mass Action”: total amount of tissue destroyed predicts behavioral deficit, not the precise site.
• Caveat/Modern View:
  • Lashley’s mazes tapped mainly procedural/spatial habits that rely heavily on subcortical circuits (e.g., striatum, hippocampus) he often spared.
  • Some cognitive domains (e.g., language, face perception) do show focal localization; Lashley’s paradigm could not reveal these.

Modern Evidence for Localization: Dr. Kanwisher (“Mind’s Machinery”)

• Presents a counter‐argument emphasizing localized functional architecture ("localization of function").
• Example previewed in class discussion:
  • Fusiform Face Area (FFA): A patch of cortex in ventral temporal lobe that responds selectively to human faces.
  • Discovered via fMRI by Nancy Kanwisher (the “Kenwisher/Kanmacher” in transcript; professor at MIT, formerly UCLA grad mentor).
• Key message: Certain high‐level perceptual & cognitive processes recruit specialized, consistent neural territories.

Example of Modularity (Face Recognition)

• Recognition of familiar faces (e.g., “That’s Stacy”) does not derive from an undifferentiated cortical swarm.
• Instead, relies on:
  • FFA, Occipital Face Area (OFA), Superior Temporal Sulcus (STS) regions.
  • Damage to these sites can produce prosopagnosia (face blindness) despite intact intelligence and vision—powerful evidence for localization.

Reconciling the Two Views

• Brain exhibits both properties:
  • Localized hubs/modules: Specialized processors (language, faces, place navigation, motion detection, etc.).
  • Distributed networks: Integration across modules; robustness to small lesions; co‐operation among regions during complex tasks.
• Rule of thumb: Higher‐order, evolutionarily recent skills (language, social cognition) often show sharper localization.

Engram Terminology

• “Engram” = physical substrate of a specific memory trace.
• Lashley used maze learning as proxy but searched cortex; modern neuroscience shows:
  • Hippocampus critical for forming episodic engrams.
  • Neocortex gradually incorporates distributed memory representations via consolidation.

Pedagogical & Institutional Context Mentioned

• Large Research Universities (e.g., UCLA, Berkeley, Princeton):
  • Classes often 300–800 students; lecture captured on giant video screens.
  • Faculty incentivized to secure grants & produce research; teaching load can be “bought out.”
  • Grading & many discussion sections handled by Graduate Teaching Assistants (TAs).
• Teaching‐Focused Universities (e.g., Fresno State):
  • Smaller classes, direct professor–student interaction.
  • Research still required but secondary to instruction.
• Dr. Kanwisher (professor featured in TED Talk):
  • Valued teaching despite research expectations.
  • Ultimately moved to Princeton (later MIT) to balance world‐class research with pedagogical outreach.
  • Renowned for clear communication and public science advocacy.

Take-Home Messages & Exam Cues

• The forebrain’s expansion underlies human uniqueness; remember its association with higher cognition.
• Know definitions:
  • Engram
  • Mass Action (Lashley)
  • Localization of Function vs. Distributed Processing
• Lashley’s rat lesion data: lesion size ≫ lesion site for maze memory.
• Modern counter‐evidence: FFA and other modules show precise localization.
• Recognize the dual truth: specialized modules embedded within distributed, resilient networks.
• Institutional anecdote underscores how research priorities can shape teaching environments—relevant if essay touches on scientific culture.