Interval Timing

Historical roots: analysis of reinforcement schedules.
Four classic instrumental schedules reviewed:
• $\text{VR}$ (Variable Ratio)
• $\text{FR}$ (Fixed Ratio) – produces run–break pattern; possibly a metronomic effect rather than literal counting.
• $\text{VI}$ (Variable Interval) – steady, low response rate, minimal post-reinforcement pause.
• $\text{FI}$ (Fixed Interval) – produces "scalloping": slow start → accelerating responses → abrupt stop at reinforcement.
Key experiment (Roberts 1981 – Phase 1)
• Light → FI20 Tone → FI40
• On 20 % of trials, food omitted ("empty" trials) to reveal internal clock independent of reinforcement.
• Response-rate curves peaked at ≈ 20 s (light) or 40 s (tone).
• Wider generalization gradient at 40 s ⇒ lower precision for longer intervals.

Both stimuli now $\text{FI}_{20\,\text{s}}$ but probability of reinforcement manipulated:
• Light: food on 80 % of trials.
• Tone: food on 20 % of trials.
Findings:
• Peak response time remained ≈ 20 s for both stimuli (unchanged clock).
• Peak response rate differed dramatically (higher under 80 % chance) ⇒ motivation affects rate, not timing perse.
Conclusion: temporal control and motivational variables can be experimentally dissociated.

Human subjects view a target duration ( $6$ , $12$ or $21$ s), then hold a button and release when they believe the same duration has elapsed.
Resulting release-probability curves mirror rat data:
• Narrow, high-precision peaks for short durations.
• Broader, noisier distributions as duration lengthens.
Demonstrates cross-species commonality in scalar property of timing.

Test paradigm (Roberts 1981)
• Training: Tone → FI20 → LightFI40
• Probe: Present tone for $5$ , $10$ or $15$ s, then switch to light.
Predictions:
• Countdown model – peak should occur at $40-\text{tone\ duration}$ (e.g., 5 s tone ⇒ 35 s peak).
• Count-up model – peak should stay at $40\,\text{s}$ irrespective of pre-light tone length.
Data: All three conditions peaked at $\approx40\,\text{s}$ ⇒ animals use stopwatch-like count-up mechanism.

Timeout experiment (Roberts 1981)
• Baseline: Tone → $\text{FI}_{40}$ (animals well-trained after ≈ 19 days).
• Test: During empty trial, lever retracted & house light off for $10\,\text{s}$ (or $5\,\text{s}$ ).
Results:
• Peak response shifted rightward by ≈ timeout length (10 s blackout → 50 s peak; 5 s → 45 s peak).
• Peak response rate unaffected ⇒ motivation stable.
Interpretation: Animals can stop accumulating time during blackout and restart at prior value.

Components (functional boxes):
1. Pacemaker – tonic pulse generator ("metronome").
2. Switch – closes when timing stimulus appears, routing pulses to…
3. Accumulator (Working Memory) – counts pulses; represents current elapsed time.
4. Reference Memory – stores mean pulse count for past reinforced intervals under that stimulus.
5. Comparator / Decision Rule – continually checks similarity between accumulator and reference; greater similarity ⇒ higher response probability.
Explains:
• Count-up direction (switch starts at 0 pulses).
• Pause-resume (opening switch freezes count).
• Scalar property (constant coefficient of variation when pulses follow Poisson process).

Pacemaker speed modulated by dopamine in cortico-striatal loops.
Methamphetamine (dopamine agonist) – accelerates pacemaker.
• Initial effect: peak times earlier (e.g., FI-40 peak shifts to ≈ 30 s; FI-20 to ≈ 10 s).
• With continued drug exposure, reference memory recalibrates ⇒ peaks drift back toward nominal interval.
• After drug removal (saline), pacemaker slows to baseline but reference memory "expects" more pulses ⇒ peaks occur later (≈ 50 s & 30 s).
Haloperidol (dopamine antagonist) – produces mirror-image pattern (slowed pacemaker, delayed peaks, then opposite rebound).
Supports link between dopamine tone and clock speed.

Real-world relevance: understanding drug-induced distortions of time (amphetamine, antipsychotics) in humans.
Motor-skill coaching: millisecond cerebellar timing insights help optimize athletic & musical training.
Animal-welfare & research ethics: recognizing precise vs flexible timing capacities informs enrichment schedules and experimental design.
Clinical translation: circadian-striatum-cerebellum distinctions guide treatment of sleep disorders, ADHD, Parkinson’s (dopaminergic timing deficits).