📖 In-Depth Theory
What is Reaction Time?
REACTION TIME is the time between a stimulus being detected and a response being made.
It involves the complete nervous system pathway:
Stimulus detected by receptor → electrical impulse along sensory neurone → relay neurone in CNS → motor neurone → effector responds.
Typical human reaction time: 0.2–0.3 seconds for a simple stimulus.
Reaction time varies between individuals and is affected by several factors:
AGE — reaction time generally increases (worsens) with age.
GENDER — some research suggests small differences between males and females.
STIMULUS TYPE — auditory (sound) reactions are slightly faster than visual (light) reactions.
PRACTICE — repeated performance improves reaction time.
FATIGUE — tiredness slows reaction time significantly.
DRUGS AND ALCOHOL — depressants (alcohol, sedatives) slow reaction time; stimulants (caffeine) may slightly improve it.
DISTRACTION — dividing attention (e.g. using a phone while driving) significantly increases reaction time.
Measuring Reaction Time — The Ruler Drop Test
The RULER DROP TEST is a simple method to measure reaction time:
METHOD:
1. Person A holds a ruler vertically with the 0 cm mark at the bottom.
2. Person B places their thumb and finger at the 0 cm mark, not touching the ruler.
3. Without warning, Person A drops the ruler.
4. Person B catches it as quickly as possible.
5. The distance the ruler falls before being caught is measured.
6. Use the distance to calculate reaction time using the formula: d = ½ × g × t²
Rearranging: t = √(2d/g) where g = 10 m/s².
7. Repeat multiple times and calculate the MEAN to reduce the effect of random variation.
IMPROVING RELIABILITY:
Conduct multiple repeats and calculate a mean.
Ensure no anticipation of the drop (no pattern or warning).
Keep conditions consistent (same ruler, same position, same person).
COMPUTER-BASED TESTS:
More accurate than the ruler test — use a computer screen stimulus and measure time to press a key.
Remove human error in reading the ruler distance.
Factors Affecting Reaction Time — Investigations
Common investigations into reaction time:
EFFECT OF CAFFEINE:
Caffeine is a stimulant — it increases the release of neurotransmitters at synapses.
Hypothesis: caffeine will decrease reaction time (improve performance).
Method: measure reaction time before and after consuming caffeine. Compare to a placebo group.
Controls: same person, same test, same time of day, account for practice effect.
EFFECT OF DISTRACTION:
Using a phone or listening to music divides attention.
Hypothesis: distraction will increase reaction time.
Method: test reaction time with and without a distractor (e.g. mental arithmetic task simultaneously).
IMPORTANT: the ruler drop test has limitations:
Random variation — each trial can differ.
Subject may anticipate the drop.
Measuring exactly where the ruler is caught introduces error.
This is why multiple repeats and means are essential.
⚠️ Common Mistake
When calculating reaction time from the ruler drop test — distance must be in METRES (not cm). g = 10 m/s² (or 9.8 m/s² for more precision). Rearrange d = ½gt² to get t = √(2d/g). Always take multiple repeats and use the MEAN — single measurements are unreliable.