Bloch’s law and a temporal integration model for simple reaction time to light

Hildreth, James D. · 1973 · OpenAlex-citations

DOI: 10.3758/bf03211177

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Summary

This 1973 study by James D. Hildreth investigates whether Bloch’s law (luminance-duration reciprocity) applies to simple reaction time (RT) to low-energy visual stimuli. Previous research yielded conflicting results, often attributed to asymmetrical experimental designs where luminance was fixed within trial blocks while duration varied. Hildreth aimed to resolve this by employing a symmetric design where both luminance and duration varied randomly, and by testing a "temporal integration model" as an alternative explanation for RT data. The study comprised three experiments involving two subjects, including the author. Stimuli were white light flashes presented via a tachistoscope with durations ranging from 1 to 128 msec. Three distinct luminance ranges were tested, spanning factors of 4, 10, and 64 in intensity. To ensure symmetry, stimulus parameters were randomized across trials, and 25% of trials were catch trials with no stimulus. Subjects were dark-adapted for 12 hours and instructed to respond as quickly as possible while maintaining a false alarm rate below 4%. Each experiment consisted of six sessions, yielding approximately 2,160 observed RTs per subject per experiment. The results demonstrated that Bloch’s law holds only approximately and under specific conditions. When the range of luminances spanned a factor of four, mean RTs for stimuli with equal luminance-duration products were similar, supporting reciprocity. However, when the luminance range expanded to a factor of ten or more, Bloch’s law failed; RTs for equal energy stimuli diverged significantly. The analysis of variance confirmed significant main effects for luminance and duration, as well as a strong interaction between them. Additionally, the context of the experiment influenced RTs; identical luminance levels produced different latencies depending on whether they were the highest or lowest intensities within a given experimental set, suggesting effects of dark adaptation or response set. To account for these findings, Hildreth proposed a temporal integration model. This model posits that a light stimulus generates a "visual response function" (VRF) that rises quickly to a maximum and then decays exponentially after the flash offset. Detection occurs when the time integral of the VRF reaches a fixed criterion. Because the subject integrates the VRF throughout its decay phase (visual persistence), detection can occur after the stimulus has ended. This model successfully accounts for RT data across wide luminance ranges where Bloch’s law fails, implying that temporal integration of a decaying visual response, rather than simple energy reciprocity, is the more general mechanism governing simple reaction time to light.

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• time perception