Effect of stimulus contrast uncertainty on simple reaction time

Area of Science:

• Visual Neuroscience
• Human Psychophysics
• Computational Vision

Background:

• Reaction time (RT) and visually evoked potential (VEP) latency increase with spatial frequency (SF).
• A "central delay" in RT was previously observed at high SF (>5 c/deg), exceeding VEP latency increases.
• Prior studies had equipment limitations causing differing contrast constancy between RT and VEP experiments.

Purpose of the Study:

• To investigate if contrast uncertainty contributes to the "central delay" in reaction time.
• To measure RT under conditions of both constant and variable stimulus contrast.
• To determine the effect of contrast uncertainty on the relationship between RT and VEP latency across different SFs.

Main Methods:

• Measured reaction times (RT) to sinusoidal gratings (SF: 0.5–16 c/deg; contrast: 2.5–50%) under constant and variable contrast conditions.
• Recorded visually evoked potentials (VEP) to identical stimuli in fixed-contrast blocks.
• Subtracted early VEP wave latencies from RT to isolate the central processing component.

Main Results:

• Contrast uncertainty did not affect RT at low SFs (0.5, 2 c/deg).
• At high SFs (>5 c/deg), contrast uncertainty increased RT significantly.
• Constant contrast conditions reduced, but did not eliminate, the central delay component of RT increase at high SFs.

Conclusions:

• Contrast uncertainty is a contributing factor to the "central delay" observed at high spatial frequencies.
• The effect of uncertainty at high SF may stem from reduced contrast sensitivity, altered stimulus probability, or differing response strategies.
• Alternative explanations include attentional differences between low and high SF stimuli.