Bill Goss asks:
>In the intro text I am using (Weiten) there is a statement that the absolute
>threshold for human vision is " a candle flame seen at 30 miles on a dark,
>clear night." Can someone tell me how this was determined? Did someone
>really do a field study on this?
>Thanks
>
The minimal quantity of light required to produce a detection response
under optimal (fully dark-adapted) conditions was established by Hecht,
Shlaer, & Pirenne (1948) in a classic and elegant piece of research. They
determined that the minimal stimulus that could be detected was one in
which the light flash emitted 54-148 quanta of light, most of which get
lost through either reflection or absorbtion by the cornea, the lens, and
the fluids in the eye & others just flat-out pass right by the rods (these
stimuli must be aimed at rods-only regions of the eye to get this level of
sensitivity) without being absorbed. Through some elegant use of
probability, Hecht et al. conclude that approximately 10 quanta must be
absorbed to produce a visual sensation and, because the rods converge &
summate to produce a single neural output from the retina in this region,
they conclude that a single quantum is enough to trigger an impulse from a
single rod (but we need 5-14 stimulated rods to summate enough to trip the
next neuron in the chain). (I can't find my copy of Hecht, Shlaer, &
Pirenne. But Woodworth & Schlosberg, 1965, ps.377 or Kling & Riggs, 1971,
p. 284-285 have good summaries of this work.)
I have heard the candle example (possibly from W. A. H. Rushton, who
offered a seminar on vision when I was in grad school in which we examined
the Hecht et al. work in detail) but I can't identify the original source.
It might be in the Hecht et al. article (which I read a _long_ time ago and
can't put my hands on now). Light is measured in foot-candles, so you
could figure out how much diffussion etc. you would have over that distance
to end up with a stimulus that puts only 54-148 quanta at the eye. They
like to make these familiar object/distance examples of how little energy
is needed to produce a threshold experience (the touch of bee's wing for
tactile thresholds, Bownian movement of air molecules for auditory
thresholds, etc.) Woodward & Schlossberg cite Pirenne claiming that the
energy in a threshold-level light stimulus is equivalent to the mechanical
energy of a pea falling one inch. (I think the candle on a mountain example
is more vivid.)
Hecht, S., Shlaer, S., & Pirenne, M. H. (1942). Energy, quanta, and
vision. Journal of General Physiology, 25, 819-840.
Kling, J. W., & Riggs, L. A. (1971). Woodworth & Schlosberg's
Experimental Psychology (3rd ed). New York: Holt, Rinehart and Winston.
Woodworth, R. S., & Schlosberg, H. (1965). Experimental Psychology (rev.
ed). New York: Holt, Rinehart and Winston.
________________________________________________________
Claudia J. Stanny, Ph.D. e-mail: [EMAIL PROTECTED]
Department of Psychology Phone: (850) 474 - 3163
University of West Florida FAX: (850) 857 - 6060
Pensacola, FL 32514 - 5751
Web: http://www.uwf.edu/~psych/stanny.html
