106 Foundations of Visual Perception
noise evidence
densitysignal + noise
evidence densityphenomenal evidencephenomenal evidence(B) noise evidencedensity signal + noiseevidence density(A)low energy energy energy
energynoise density signal + noisedensityCATCH TRIALSnoise density ROC CURVESSIGNAL TRIALSnoise density signal densityhigher energy energy energy
energy(C) noise density signal + noisedensity(D) d’d’phenomenal evidence phenomenal evidence phenomenal evidencephenomenal evidence phenomenal evidence phenomenal evidenceenergyenergyFigure 4.10 Revision of Figure 4.8 to show that energy thresholds are compatible with the absence of an observer threshold.reviewed by Mersch, Middendorp, Bouhuys, Beersma, &
Hoofdakker, 1999). To determine an individual’s retinal sensi-
tivity, they used a psychophysical technique called themethod
of limitsand studied the course of their dark adaptation (for a
good introduction, see Hood & Finkelstein, 1986, §4).
Terman and Terman (1999) first adapted the participants to
a large field of bright light for 5 min. Then they darkened the
room and turned on a dim red spot upon which the partici-
pants were asked to fix their gaze (Figure 4.11). Because they
wanted to test dark adaptation of the retina at a region that
contained both rods and cones, they tested the ability of the
participants to detect a dim, intermittently flashing white disk
below that fixation point. Every 30 s, the experimenter grad-
ually adjusted the target intensity upward or downward and
then asked the participant whether the target was visible.
When target intensity was below threshold (i.e., the partici-
pant responded “no”) the experimenter increased the inten-
sity until the response became “yes.” The experimenter then
reversed the progression until the subject reported “no.”
Figure 4.12 shows the data for one patient with winter
depression. The graph shows that the transition from “no”
to “yes” occurs at a higher intensity than the transition from
“yes” to “no.” This is a general feature of the method of lim-
its, and it is a manifestation of a phenomenon commonly seen
in perceptual processes called hysteresis.
167red fixation dotflashing disk
(750 ms on,
750 ms off)
Figure 4.11 Display for the seasonal affective disorder experiment
(Terman & Terman, 1999). Rules of thumb: 20° of visual angle is the width
of a hand at arm’s length; 2° is the width of your index finger at arm’s length.