Investigating Factors Affecting Reaction Time

There are three types of neurones: a sensory neurone carries impulses to the Central Nervous System (CNS), a relay neurone connects neurones within the CNS, and a motor neurone carries impulses to effectors (muscles or glands). Where two neurones meet, there is a small gap called a synapse. The process of synaptic transmission occurs when an electrical impulse triggers the release of neurotransmitters, which diffuse across the gap and bind to receptors on the next neurone. This diffusion is the slowest part of the nervous pathway. Unlike automatic reflexes that bypass the conscious brain, voluntary reactions (like catching a dropped object) use the conscious brain as the coordinator.

Investigating Reaction Time (Required Practical 7)

The Ruler Drop Test is a standard method to investigate the effect of a factor (like caffeine or practice) on human reaction time. First, Person 1 sits upright and rests their forearm on a table so their hand hangs over the edge. Person 2 holds a metre ruler vertically, ensuring the 0 cm mark is exactly level with the top of Person 1's thumb. Person 2 drops the ruler at a random time without warning. Person 1 must catch the ruler as quickly as possible using only their thumb and first finger. The distance is recorded level with the top of the thumb. To ensure repeatability, the test should be repeated at least 5 to 10 times to identify any anomalies.

If you are investigating the effect of caffeine, you must first establish a baseline test before consumption. After drinking a caffeinated beverage, wait 15 to 30 minutes for absorption into the bloodstream before re-testing. A control group should drink a caffeine-free placebo version of the same drink to account for psychological effects.

Variables and Validity

To ensure your investigation is valid, you must strictly control your variables.

• Independent variable: The factor being changed (e.g., consumption of caffeine, practice, or dominant vs. non-dominant hand).
• Dependent variable: The reaction time, measured initially as the distance the ruler falls in centimetres (cm).
• Control variables: Factors kept constant to ensure a fair test. These include the starting height (0 cm at the thumb), the initial gap between the thumb and finger (just not touching), the hand used (usually the dominant hand), and environmental factors (constant light levels and background noise).

Resting the arm on the table is a crucial control variable. This prevents the participant from moving their entire arm down to "chase" the ruler, ensuring that only the reaction time is being measured.

Data Analysis and Calculations

When interpreting data from the ruler drop test, remember that a lower distance indicates a shorter (faster) reaction time. Distance is converted into time in seconds (s) using a conversion table or the free fall mathematical formula: $t = \sqrt{\frac{2d}{g}}$. Before calculating a mean from your repeats, you must identify and exclude any anomalies (results that do not fit the pattern).

Worked Example: Calculating Mean Reaction Distance

Calculate the mean reaction distance from the following results: 12 cm, 13 cm, 11 cm, 45 cm, 12 cm.

• Step 1: Identify and exclude the anomaly. The value 45 cm does not fit the pattern.

• Step 2: Add the remaining valid values together: $12 + 13 + 11 + 12 = 48 \text{ cm}$.

• Step 3: Divide by the number of valid values: $48 \div 4 = 12 \text{ cm}$.

Different factors affect reaction speeds in predictable ways. Caffeine is a stimulant that speeds up the transmission of impulses across synapses, decreasing reaction time. Repeated practice also decreases reaction time as the participant becomes more skilled. Conversely, distractions, tiredness, or alcohol (a depressant) will increase reaction time by slowing down nervous transmission.

Evaluating Methods: Ruler vs. Computer Tests

Computer-based reaction time tests (such as clicking a mouse when a screen changes colour) are often considered superior to the ruler drop test. They provide a higher resolution, measuring time precisely in milliseconds (ms). They also improve accuracy by removing human error; the participant cannot anticipate the drop based on the visual or physical cues of the person dropping the ruler.