Reaction Time Experiments

Timing Mental Processes

• Helmholtz (1850) accidentally invented the reaction time (RT) experiment while studying nerve conduction speed.

• He stimulated a frog's nerve-muscle preparation and measured the time between nerve stimulation and muscle twitch.

• He then calculated conduction velocity by dividing the distance by the extra time.

• In human experiments, subjects pressed a key upon feeling an electric shock; Helmholtz aimed to calculate sensory nerve conduction velocity.

• Simple reaction times are about 200 milliseconds, with nerve conduction accounting for less than 10% of this time.

Decomposing Decision Times

• Donders (1868) created three reaction time experiments: 'a' (simple), 'b' (choice), and 'c' (go-no go).

• 'A' reaction: determination of signal occurrence for pre-prepared response.

• 'B' reaction: discrimination and choice among five sounds.

• 'C' reaction: stimulus discrimination with a pre-prepared response to one of five sounds.

• Subtractive logic (Discrimination time = 'c'-'a'; Choice time = 'b'-'c') calculates the time for discrimination and choice.

• Wundt established the first psychology lab in 1879, using the reaction time method.

• Wundt designed the 'd' reaction to address criticisms of the 'c' reaction, but issues arose concerning discrimination time and preparation.

Recent Developments in Stages of Information Processing

• Sternberg (1969, 1975) supported the assumption of non-overlapping serial stages in information processing.

• Subjects respond 'yes' or 'no' to whether a test stimulus was in a target set (memory scanning).

• Latency of decision response is the main recorded variable.

• Each additional item in the target set prolongs search by a constant amount (35-40 milliseconds).

• Search among memory set items proceeds serially.

• Scanning rate depends on material type (digits are fastest, nonsense syllables slowest).

• Slopes of latency functions for positive and negative responses are identical, indicating an exhaustive search.

• Additive effects on response latencies suggest different processing stages, while interacting effects suggest the same stage.

• Sternberg established non-overlapping information processing stages, elaborating on Donders' earlier postulates.

Reaction Times and Extent of Choice

• Reaction time increases with the number of stimuli and responses (Merkel, 1885).

• Hick's Law (1952) relates reaction time to degree of choice.

• Information theory quantifies communications systems.

• Psychologists applied Shannon's (1949) information theory to behavior leading to the information-processing approach.

Information Theory

• Information theory measures 'how much' information, not 'what' it means; it focuses on uncertainty reduction.

• Information is directly related to the unlikelihood of an event.

• The average amount of information is conveyed by all possible messages, weighted by relative frequency:
  H = \Sigma pi \log2{\frac{1}{p_i}}

• Average information is highest when possible messages are equally likely.

• For equiprobable alternatives, H = \log_2{n}, where n is the number of alternatives.

• The unit of information is the BIT (Binary Digit).

Reaction Times and Amount of Information Processed

• Merkel (1885) found response latency increased with choice extent but the function's slope decreased.

• Hick (1952) plotted latency against the logarithm of choices, log n.

• Hick's Law: Linear relation between choice RT and log_2(n+1), where n is the number of alternatives.

• The +1 in Hick's equation explicitly allows for that temporal uncertainty.

• Hyman (1953) varied event frequency and sequential dependency, establishing information theory as a predictor of average reaction time (Hick-Hyman Law).

Transmitted Information

• Transmitted information adjusts for inconsistencies in subject behavior, accommodating the speed-accuracy trade-off.

• It remains a sound reaction time predictor even with error rates up to 10%.

Stimulus Discrimination Versus Response Selection

• Reaction time is a linear function of transmitted information; the slope represents working speed.

• Morin and Forrin (1963) manipulated the mapping ratio to resolve contributions of stimulus discrimination and response selection.

• Response uncertainty is about twice as influential as stimulus uncertainty in determining reaction time.

• Sternberg's paradigm shows a linear relation between reaction time and memory set size versus Hick's logarithmic relation.

• Increasing stimulus alternatives linearly affects reaction time; response alternatives logarithmically affect it.

• Separate stages determine incoming stimuli and decide upon response: stimulus determination is a linear function, response selection is a logarithmic function.