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MG

Hormones, Non-Associative Learning (Habituation & Sensitization)

Feature Detectors vs. Hormonal Cues

• Feature detectors = specialized neurons that fire only to very specific stimuli (light patterns, particular sounds, etc.)
• While feature detectors act locally within the nervous system, hormonal cues operate system-wide:
• Hormones released into the bloodstream can reach many organs simultaneously.
• Result = coordinated, species-typical (often innate) behavioral + physiological responses.

Hormone-Triggered Innate Behavior: Red Deer Case Study

• Key hormone: testosterone.
• Developmental window = the months when young males reach sexual maturity ("rutting" season).

Physiological cascade
• Rising testosterone → rapid antler growth in spring/summer.
• Antlers start covered by soft "velvet" skin.
• As rut approaches, velvet peels, revealing hard, polished bone for combat.
• Additional bodily changes:
• Musky, “rough” odor intensifies.
• Larynx enlarges → louder, deeper vocalizations.

Behavioral cascade (rutting behaviors)
• Increased mate-seeking.
• Heightened aggression & dominance displays toward rival males.
• Physical fighting with antlers for mating rights.

Significance
• Illustrates how one endocrine signal synchronizes morphology, scent, voice, and overt action—an adaptive, fully integrated package for reproductive success.


Learning (Nurture) vs. Innate Programs (Nature)

• Two broad learning categories

  1. Non-associative learning → change in response to a single, repeatedly presented stimulus. No explicit reward/punishment link.

  2. Associative learning → organism links two events (stimulus-stimulus OR behavior-consequence). Often involves reinforcement or punishment.
    • Early life experience = critical period when these learning modes strongly sculpt behavior.


Non-Associative Learning

Three principal subtypes (lecture today covers first two):

  1. Habituation

  2. Sensitization

  3. Imprinting (to be treated later)

1. Habituation

Definition
• Repeated exposure to a stimulus → progressive decrease in responsiveness.
• Organism learns “this event is harmless/background.”

Classic chick experiment
• Stimulus: predator-shaped silhouette passing over nest.
• Initial response: defensive crouch.
• After many harmless passes: crouching disappears → chicks are habituated.

Field & urban examples
• Prairie dogs:
• Initially give alarm call when hearing human footsteps.
• After repeated non-threatening encounters -> alarm call ceases for that sound only.
• National-park elk & general urban wildlife:
• Lose fear of humans; may approach for food.
• Raises risk of human–wildlife conflict.

Human parallels
• Strong odor (e.g., dirty litter box) fades from conscious notice after minutes/hours.
• City dwellers tune out continuous sirens, traffic, construction.

Ecological / ethical angle
• Habituation to humans can endanger animals (approach roads, beg for food, lose natural wariness) and humans (injury, disease transmission).

2. Sensitization

Definition
• Repeated or particularly intense stimulus → heightened (not diminished) future responsiveness.
• Nervous system becomes “primed” to over-react.

Aplysia (sea slug) model system
• Light touch on siphon → modest gill withdrawal.
• Deliver painful shock → very large gill withdrawal.
• Subsequent light touches STILL evoke large withdrawal → animal is sensitized.
• Conversely, many gentle touches without shock → gradual reduction in withdrawal (habituation). Same organism exhibits both phenomena depending on stimulus history.

Neurobiological importance
• Aplysia studies laid groundwork for molecular understanding of short-term vs. long-term synaptic plasticity (e.g., serotonin-mediated facilitation for sensitization).

Human analogues
• Being poked lightly when already stressed/in pain → exaggerated flinch or irritation.
• PTSD sufferers may show hyper-reactivity (sensitization) to loud noises reminiscent of past trauma.

Context modifiers
• Emotional state, cognition, social environment, & individual history alter whether a given person habituates or sensitizes.


Comparative Summary

• Habituation = “getting used to it” → response ↓
• Sensitization = “on edge” → response ↑
• Both forms require no explicit reward or punishment; purely stimulus-driven.


Looking Ahead

• Remaining non-associative subtype: Imprinting (time-limited "critical period" learning, e.g., ducklings following first moving object).
• Associative learning modules:
• Classical (Pavlovian) conditioning.
• Operant (instrumental) conditioning.
• Future lectures will cover mechanisms, examples, and adaptive value of each.

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