Handbook of Psychology, Volume 4: Experimental Psychology

Psychophysical Methods 107

Terman and Terman (1999) overcame the problem of hys-
teresis by taking the mean of these two values to characterize
the sensitivity of the participants. The cone and rod thresh-
olds of all the participants were lower in the summer than in
the winter. However, in winter the 24 depressed participants
were more sensitive than were the 12 control participants.
Thus the supersensitivity of the patients in winter may be one
of the causes of winter depression.

Method of Constant Stimuli

Barraza and Colombo (2001) wanted to discover conditions

under which glare hindered the detection of motion. Their

stimulus is one commonly used to explore motion thresholds:

a drifting sinusoidal grating, illustrated in Figure 4.13

(Graham, 1989, §2.1.1, defines such gratings).

The lowest velocity at which such a grating appears to be

drifting consistently is called the lower threshold of motion

Figure 4.12 Visual detection threshold during dark adaptation for a patient with winter depres-

sion. The curves are exponential functions for photopic (cone) and scotopic (rod) segments of dark

adaptation. Source:From “Photopic and scotopic light detection in patients with seasonal affec-

tive disorder and control subjects,” by J. S. Terman and M. Terman, 1999, Biological Psychiatry,

46,Figure 1. Copyright 1999 by Society of Biological Psychiatry. Reprinted with permission.

period T

4

L 0

0

Position x

L 0 + mL 0

L 0 – mL 0

Luminance L

peak–trough

amplitude

(2mL 0 )

modulation

depth (mL 0 )

1

f

B. Luminance Profile of a Grating

L(x) = L 0 [1 + m cos(2πfx + θ)]

L 0 – average luminance

m – contrast

f – frequency (T = )

θ – phase

1

f

A. Luminance Grating

Figure 4.13 (A) The sinusoidal grating used by Barraza and Colombo (2001) drifted to the right or to the

left at a rate that ranged from about one cycle per minute (0.0065 cycles per second, or Hz) to about one cycle

every 3.75 s (0.0104 Hz). The grating was faded in and out, as shown in Figure 4.14. It is shown here with ap-

proximately its peak contrast. (B) The luminance profile of a sinusoidal grating, and its principal parameters.

[Image not available in this electronic edition.]