Brightness Sensation and the Neural Coding of Light Intensity
Robert B. Barlow Jr. Institute for Sensory Research Syracuse
University We can see stars of the 8th magnitude and distinguish
objects in bright sunlight: an operating range of more than 10
log units of light intensity. We can detect with exquisite
sensitivity small changes in illumination throughout this
enormous range ( Hecht, Peskin, & Patt, 1938), but can we judge
the absolute level of illumination itself? That is, can the
visual system perform as a photometer? More than 40 years ago
Spring and Stiles ( 1948) found that our pupil reflex was graded
over at least 8 log units of light intensity. Their measurements
were later extended by Alpern and Ohba ( 1972). Using the
psychophysical technique of magnitude estimation, Stevens (
1953) found that observers could scale the brightness of test
flashes over about 4 log units of light intensity. Ten years
later, Stevens and Stevens ( 1963) showed that the range of
dark-adapted observers was at least 7 l