EFFECTS OF MORPHINE ON TEMPORAL DISCRIMINATION AND COLOR MATCHING: GENERAL DISRUPTION OF STIMULUS CONTROL OR SELECTIVE EFFECTS ON TIMING?

JOURNAL OF THE EXPERIMENTAL ANALYSIS OF BEHAVIOR
2005, 84, 401–415
NUMBER 3 (NOVEMBER)
EFFECTS OF MORPHINE ON TEMPORAL DISCRIMINATION AND COLOR MATCHING:
GENERAL DISRUPTION OF STIMULUS CONTROL OR SELECTIVE EFFECTS ON TIMING?
RYAN D. WARD AND AMY L. ODUM
UTAH STATE UNIVERSITY
Discrepant effects of drugs on behavior maintained by temporal-discrimination procedures make
conclusive statements about the neuropharmacological bases of timing difficult. The current
experiment examined the possible contribution of a general, drug-induced disruption of stimulus
control. Four pigeons responded on a three-component multiple schedule that included a fixed-interval
2-min, temporal discrimination, and color-matching component. Under control conditions, response
rates and choice responses during the first two components showed evidence of control by time, and
accuracy for color matching was high in the third component. Morphine administration flattened the
distribution of fixed-interval responding and produced a general disruption of accuracy in the
temporal-discrimination component, whereas accuracy in the color-matching component was relatively
unaffected. Analysis of the psychophysical functions from the temporal-discrimination component
indicated that morphine decreased accuracy of temporal discrimination by decreasing overall stimulus
control, rather than by selectively affecting timing. These results suggest the importance of determining
the neurophysiological bases of stimulus control as it relates to temporal discrimination.
Key words: morphine, timing, stimulus control, temporal discrimination, key peck, pigeons
_______________________________________________________________________________
The neurophysiological processes underly-
posed of pacemakers and accumulators. Based
ing temporal regulation of behavior and
on input from these systems interacting with
accurate discrimination of temporal duration
memory for recent temporal events, an organ-
have been of increasing interest in recent
ism is able to accurately discriminate and
years. Research with both humans and nonhu-
respond based on temporal stimuli.
mans has led to the formation of several
Although models of temporal processing
theoretical accounts that attempt to explain
have led to productive research aimed at
the environmental and neurophysiological
describing the neuroanatomical correlates of
underpinnings of accurate temporal discrimi-
temporal discrimination (e.g., Gibbon, Mala-
nation. For example, prominent theoretical
pani, Dale, & Gallistel, 1997; Meck, 1996),
accounts of the neurophysiological basis of
conclusive statements about the neural and
timing, such as the generalized timing model
biochemical basis of timing remain elusive
(e.g., Matell, Meck, & Nicolelis, 2003), hypoth-
(see Gibbon et al., 1997). A growing body of
esize elaborate neurologically based informa-
research has provided support for the role that
tion processing systems. According to these
dopamine and other neurotransmitters play in
models, accurate discrimination of duration is
accurate timing of relevant temporal events
governed by internal-clock mechanisms com-
(e.g., Buhusi, 2003; Hinton & Meck, 1997;
Meck, 1996). In spite of these advances,
The authors thank Jason Martin for his assistance in the
further work is needed to understand more
computer programming and conduct of this experiment,
precisely how neuropharmacological and be-
as well as Timothy Shahan and Christopher Podlesnik for
havioral processes contribute to accurate
their comments on a previous draft of this manuscript.
discrimination of temporal stimuli (see Ri-
Clive Wynne, Erin McClure, and Katie Saulsgiver provided
valuable suggestions with data analysis and helpful com-
chelle & Lejuene, 1998).
ments on a previous draft of this manuscript. Portions of
To complicate matters, a growing number of
these data were presented at the 2001 annual meeting of
discrepant findings have been reported in
the Southeastern Association for Behavior Analysis in
which the same drug, or drugs from the same
Wilmington, North Carolina and the 2002 annual meeting
of the Association for Behavior Analysis in Toronto,
pharmacological class, produced different
Canada.
effects on behavior maintained by a variety of
Address correspondence to Ryan Ward or Amy Odum
temporal-discrimination
procedures
(e.g.,
at the Department of Psychology, 2810 Old Main Hill,
Chiang et al., 2000; Frederick & Allen, 1996;
Utah State University, Logan, Utah 84322-2810 (e-mail:
Knealing & Schaal, 2002; Odum, Lieving, &
RyanWard@cc.usu.edu or Amy.Odum@usu.edu).
doi: 10.1901/jeab.2005.94-04
Schaal, 2002; Santi, Coppa, & Ross, 2001). For
401

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402
RYAN D. WARD and AMY L. ODUM
example, some researchers have reported that
distinguish selective effects of drugs on timing
amphetamine results in overestimation of time
from effects that occur as a result of a more
(e.g., Chiang et al., 2000; Meck, 1996), whereas
general disruption of stimulus control.
some have found generalized disruption of
Few studies have attempted to assess simul-
timing or even underestimation of time (e.g.,
taneously the effects of drugs on accuracy of
Chiang et al., 2000). The reasons for these
temporal and other types of discriminations.
discrepant results are unclear. To date, anal-
Santi, Weise, and Kuiper (1995) assessed the
yses have suggested that variables such as
effects of amphetamine on performance on
species, sex, route of drug administration,
a temporal discrimination and a visual sym-
and procedural variations cannot fully account
bolic matching-to-sample procedure in pi-
for the discrepant outcomes (C
¸ evik, 2003;
geons. Amphetamine disrupted accuracy for
Odum, 2002; Odum et al., 2002; Odum &
temporal discrimination more than accuracy
Ward, 2004).
for symbolic matching-to-sample. In addition,
One possible explanation for these discrep-
contrary to prominent theoretical predictions
ant results is that drug administration results
(e.g., Meck, 1996), amphetamine did not
in a general decrease in discrimination of all
produce overestimation of time. Santi et al.
types of stimuli, rather than in selective
suggested that their results were due to
changes in the neuropharmacological me-
disruption of attention to the temporal sample
chanisms responsible for timing. If this were
stimuli rather than selective changes in timing.
the case, then, following drug administration,
The present experiment further examined
choice responses in temporal-discrimination
how drugs simultaneously affect the stimulus
procedures may no longer be under the
control engendered by temporal and color
functional control of the presented sample
samples. We used a multiple schedule in which
durations. In fact, drugs have been reported to
we assessed the effects of morphine on
affect performance on a variety of discrimina-
performance during fixed-interval, temporal
tion procedures. For example, in one early study
discrimination, and color-matching compo-
Berryman, Cumming, Nevin, and Jarvik (1964)
nents. Morphine was used because it has been
reported that sodium pentobarbital dose-de-
shown to disrupt performance in a temporal-
pendently decreased accuracy in a color match-
discrimination procedure (e.g., Odum &
ing-to-sample task in pigeons. In a more recent
Ward, 2004) and also has been shown to
study, Andrews and Holtzman (1988) assessed
disrupt performance in a visual discrimination
the effects of morphine and amphetamine on
procedure by decreasing the discriminability
performance in a visual discrimination pro-
of the sample stimuli (Koek & Slangen, 1984).
cedure in rats. In their procedure, responses to
We reasoned that if the neuropharmacological
one of two levers were reinforced if a stimulus
effects of morphine are specific to timing,
light had been briefly flashed above the lever at
then we should see clear disruption of
the beginning of the trial. In this procedure,
temporal discrimination, with little or no
although amphetamine had relatively little
disruption of color matching. If, however,
effect, morphine produced a dose-dependent
morphine produces a general disruption in
decrease in accuracy.
stimulus control, then we should see changes
In addition to these studies, others have
in accuracy for color matching as well as
reported that drugs from a variety of pharma-
temporal discrimination. In addition, perfor-
cological classes have disrupted performance
mance during the temporal-discrimination
on discrimination procedures in rats (e.g.,
component was analyzed using a method
Grilly, Genovese, & Nowak, 1980; Koek &
suggested by Blough (1996), which can distin-
Slangen, 1983, 1984), pigeons (e.g., Berryman,
guish drug effects due to changes in timing
Jarvik, & Nevin, 1962; Eckerman, Lanson, &
from those effects due to disruption of
Berryman, 1978; Picker, Massie, & Dykstra,
stimulus control.
1987), and monkeys (e.g., Dykstra, 1979;
Ridley, Baker, & Weight, 1980). These results,
METHOD
when considered along with the discrepant
results in the timing literature mentioned
Subjects
above, suggest the importance of experimental
Four experimentally naive White Carneau
preparations and methods that clearly can
pigeons served as subjects. Pigeons were

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TEMPORAL DISCRIMINATION AND COLOR MATCHING
403
maintained at 80% 6 15 g of their free-
effect for six sessions prior to multiple-sched-
feeding weight by postsession feeding as
ule training.
needed.
Between
sessions,
pigeons
were
Multiple schedule training.
The procedure
individually
housed
in
a
temperature-
was a three-component multiple schedule that
controlled colony under a 12:12 hr light/
included FI 2-min, temporal discrimination,
dark cycle and had free access to water and
and color-matching components. During ini-
digestive grit.
tial training, these components were pre-
sented in random order with the requirement
Apparatus
that each be presented 14 times during the
Four BRS/LVE sound-attenuating chambers
session and no component occur more than
were used. Chambers were constructed of
two times in a row. Components were separat-
painted metal with aluminum front panels.
ed by a 30-s intercomponent interval (ICI)
The
chambers
measured
35 cm
across,
during which all keylights and the houselight
30.7 cm deep, and 35.8 cm high. Each front
were extinguished. Pecks to the keys during
panel had three translucent plastic keys that
the ICI had no programmed consequences. To
could be lit from behind with red, green,
allow time for drug absorption prior to
yellow, and blue light. Keys also could be lit
selected sessions, all sessions began with a 10-
with a variety of horizontal and vertical line
min chamber blackout. Following the black-
stimuli, and required a force of at least 0.10 N
out, the houselight and center key were lit to
to record a response. Keys were 2.6 cm in
begin the session. Temporal discrimination
diameter and 24.6 cm from the floor. A lamp
and color-matching components were preced-
ed by the lighting of the center key with three
(28 V, 1.1 W) mounted 4.4 cm above the
black horizontal lines on a white background.
center key served as a houselight. A rectangu-
This key served as a trial-ready stimulus to
lar opening 9 cm below the center key pro-
ensure that the pigeon was attending to the
vided access to a solenoid-operated hopper
sample. A peck to the center key randomly
filled with pelleted pigeon chow. During
produced either a temporal discrimination or
hopper presentations, the opening was lit with
color-matching trial.
white light and the houselight and keylights
Temporal-discrimination component.
A peck
were extinguished. White noise and chamber
to the center key darkened the keylight and
ventilation fans masked extraneous noise.
turned off the houselight for a period of 2 or
Contingencies were programmed and data
8 s. This blackout duration constituted the
collected by a microcomputer located in an
temporal sample for each trial. Sample dura-
adjacent room using Med AssociatesH interfac-
tions were randomly selected each trial with
ing and software.
the constraint that each sample duration be
presented an equal number of times during
Procedure
the session. Following the sample presenta-
Pretraining.
Experimental
sessions
oc-
tion, the left and right keys were lit different
curred 7 days a week at approximately the
colors, each color corresponding to either
same time. Following magazine training, the
a short or a long sample duration. The
pigeons were exposed to an autoshaping
location of each color (left or right key) was
procedure (Brown & Jenkins, 1968). During
randomly determined from trial to trial (e.g.,
these sessions, all key colors and stimuli were
Stubbs, 1968). A peck to the key that was the
presented in the key locations in which they
color that corresponded to the duration of the
would appear during the experiment. Follow-
temporal sample (short or long) resulted in 3-s
ing three sessions of autoshaping, the pigeons
access to food. A peck to the key that was the
reliably pecked all key colors and stimuli to be
other color produced a 3-s blackout. Key colors
used in the experiment. Key pecking was then
were counterbalanced across pigeons in case
maintained on a fixed-interval (FI) schedule of
of a systematic drug-induced color bias (see,
food delivery of progressively increasing dura-
e.g., Wenger, McMillan, Moore, & Williamson,
tion until an FI 2-min schedule was reached.
1995). For Pigeons P211 and P212, the colors
During these sessions, the center key was lit
during the temporal-discrimination compo-
with three black vertical lines on a white
nent were green and red. For P211, green
background. The FI 2-min schedule was in
corresponded to a short sample duration and

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404
RYAN D. WARD and AMY L. ODUM
red to a long duration. This color assignment
incorrect side key was followed by a 3-s
was reversed for P212. For Pigeons P213 and
blackout. The entire trial was then repeated,
P214, the colors during the temporal-discrim-
with the same sample duration or color, and
ination component were blue and yellow. For
the side keys lit with the same colors in the
P213, yellow corresponded to a short sample
same positions. This process continued until
duration and blue to a long duration. This
a correct choice ended the trial in food. All
color assignment was reversed for P214.
pigeons experienced the correction procedure
Color-matching component.
A peck to the
at some point during training. The correction
center key extinguished the trial-ready stimu-
procedure was not in effect during the drug-
lus and lit the key with a color sample for 2 s.
testing phase.
The center key then was extinguished and the
side keys were lit different colors. The location
Morphine Tests
of each color (left or right key) varied
Drug testing began for individual pigeons
randomly from trial to trial. A peck to the
when accuracy in the temporal discrimination
key that matched the sample color led to 3-s
and color-matching components was high and
access to food, and a peck to the key that did
stable and rates of responding and the index
not match the sample color led to a 3-s
of curvature (a measure of temporal pattern-
blackout. Key colors during this component
ing; Fry, Kelleher, & Cook, 1960) during the FI
were counterbalanced. For Pigeons P211 and
were stable (without any evident trend or
P212, the colors during the color-matching
unusual variability) as judged by visual in-
component were blue and yellow. For P213
spection over the last 10 sessions. Responding
and P214, the colors were green and red.
met these criteria within 112 to 164 sessions,
FI component.
The center key was lit with
across pigeons.
three black vertical lines on a white back-
Morphine sulfate (Sigma) was dissolved in
ground. The first peck after 2 min resulted in
0.9% saline and administered in a volume of
3-s access to food.
1.0 ml/kg of the 80% free-feeding body
After color-matching and temporal discrim-
weight. Morphine and vehicle were adminis-
ination accuracies were at least 80% over the
tered via intramuscular injections into the
last 10 sessions, intermediate sample durations
breast immediately before the pigeon was
of 3, 4, 4.5, 5.5, 6, and 7 s were inserted into
placed in the experimental chamber. To
the temporal-discrimination component. Sam-
accustom the pigeons to the injection pro-
ple durations of less than 5 s were considered
cedure, they were given one to three pre-
short and sample durations of more than 5 s
liminary injections of saline. Results of these
were considered long. Correct categorization
injections were excluded from the analyses.
of the intermediate sample durations was
Following the preliminary injections, mor-
reinforced. A 3-min limited hold was instituted
phine and vehicle were given in the following
during each component. If a response did not
order: 1.0 mg/kg, 3.0 mg/kg, 0.56 mg/kg,
occur within 3 min, all keylights and the
5.6 mg/kg, and saline. These doses were
houselight were extinguished and a 30-s ICI
chosen because they produce a wide range of
occurred, after which a new component was
effects of morphine (e.g., Odum & Ward,
randomly selected. The number of component
2004). Tests were separated by at least three
presentations of each type was changed to
consecutive baseline sessions not preceded by
eight FI, 40 temporal discrimination, and eight
an injection. The session immediately pre-
color-matching components. Each temporal
ceding a morphine or vehicle session was
sample duration was thus presented five times
designated as a control session. Dose-effect
during each session. Sessions ended after 56
curves were determined with all doses before
trials or 90 min, whichever occurred first.
any dose was repeated. The effects of saline
Sessions usually ended after 56 trials in
and each drug dose were determined four
approximately 70 min.
times for each pigeon. Data from the color
Correction procedure.
During training, when
matching and temporal-discrimination com-
temporal discrimination or color-matching
ponents during sessions preceded by drug
accuracy was low because of a pronounced
administration were included in analyses only
color or side bias, a correction procedure was
if at least half of the presented components of
instated. In this procedure, a peck to the
each type were completed. For Pigeon P211,

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TEMPORAL DISCRIMINATION AND COLOR MATCHING
405
data were excluded on these grounds for one
session following administration of 3.0 and
5.6 mg/kg. For Pigeon P212, data were ex-
cluded for one session following administra-
tion of 3.0 mg/kg and for two sessions
following administration of 5.6 mg/kg. For
Pigeon P213, data were excluded for 5.6 mg/
kg, as this pigeon responded during only one
session following administration of this dose.
Data from all sessions were included for
Pigeon P214.
RESULTS
Figures 1 and 2 show the effects of mor-
phine on FI performance. Figure 1 shows
overall response rates during the FI during
control sessions and as a function of mor-
phine. Saline had no systematic effect on
overall rates of key pecking. Morphine de-
creased rates of pecking somewhat, although
in some cases the decreases were relatively
small. Figure 2 shows the index of curvature
(Fry et al., 1960) for all pigeons during control
sessions and as a function of increasing
morphine dose. The index of curvature is
a measure of the proportional distribution of
responses across fixed intervals. Fixed intervals
were divided into ten 12-s bins. The number of
responses that occurred in each bin was
summed across the session for the FI compo-
nent. The index of curvature then was
calculated for each session using the following
formula from Fry et al. (1960):
I ~ 9R10 { 2 (R1 z R2 z R3
z . . . R9)=10R10,
where R1 is the total number of responses
occurring in the first bin, R2 is the total
number of responses occurring in the first and
Fig. 1.
Mean pecks per minute during the FI 2-min
second bin, R3 is the total number of
component as a function of morphine for each pigeon.
responses occurring in the first, second, and
Unconnected points show means for all control (C) and
third bins, and so on until R10, which is the
saline (S) sessions. Lines connect points showing the mean
total number of responses occurring in all
across doses of morphine. Vertical bars represent one
bins. Calculated in this way, the possible range
standard deviation above and below the mean.
of the index is 20.90 (if all responses occurred
in the first bin) through 0 (if an equal number
Morphine dose-dependently decreased the
of responses occurred in each bin) to +0.90 (if
index of curvature for all pigeons, with the
all responses occurred in the last bin).
largest decreases occurring for Pigeons P212
During control sessions, the index of curva-
and P213. The decrease in the index indicates
ture was positive, indicating that relatively
that, under morphine, relatively more re-
more responses occurred later in the interval.
sponses occurred earlier in the intervals
Saline had no systematic effect on the index.
compared to control performance.

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406
RYAN D. WARD and AMY L. ODUM
Fig. 2.
Mean index of curvature (degree of temporal
Fig. 3.
Proportion correct during the color matching
patterning) during the FI 2-min component as a function
(unfilled circles) and portions of the temporal-discrimina-
of morphine for each pigeon. See text for calculation.
tion (filled circles) components as a function of morphine
Other details as in Figure 1.
for each pigeon. Points showing accuracy for temporal
discrimination reflect only accuracy for trials with 2- and
8-s sample durations. Points for temporal discrimination
Although control accuracy in both the color
and color-matching accuracy are offset slightly on the x
matching and temporal-discrimination com-
axis for clarity. Other details as in Figure 1. Data are not
ponents was above 0.8, accuracy for temporal-
shown for Pigeon P213 following administration of
5.6 mg/kg morphine because this pigeon responded
discrimination was slightly lower than for color
following only one administration of this dose.
matching. Because the level of stimulus con-
trol during control conditions has been shown
to moderate the disruptive effects of drugs
accuracy. Figure 3 shows the proportion
(e.g., Ksir, 1975), we compared the effects of
correct for the temporal discrimination and
morphine on temporal discrimination and
color-matching components as a function of
color matching at similar levels of control
morphine dose for each pigeon. Data shown

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TEMPORAL DISCRIMINATION AND COLOR MATCHING
407
from the temporal-discrimination component
functions are different, averaging functions
reflect only trials with 2 and 8 s sample
can lead to a reduction of the slope of the
durations. Control proportion correct in both
mean function. To avoid this artifact, we
components was between 0.8 and 1.0 for all
averaged the proportion long response data
pigeons. There was no systematic difference in
from each determination of each dose of
accuracy between the color matching and
morphine and fit one Gaussian function to
temporal-discrimination components. Saline
the average data from each dose for each
had no systematic effect on accuracy in either
pigeon.
component. Overall, morphine decreased ac-
In an analysis of these functions first in-
curacy during temporal-discrimination trials,
troduced by Heinemann, Avin, Sullivan, and
whereas accuracy for color matching was
Chase (1969) and described in detail by
relatively unaffected. This effect was less
Blough (1996), the parameters of the model
apparent for Pigeon P211. Examination of
reflect three sources of error in discrimination
the accuracy data with all temporal-discrimi-
procedures: overall stimulus control, sensitivi-
nation trials included showed that the overall
ty, and bias. The degree of overall stimulus
effects of morphine were the same, although
control is reflected in the range of the
the overall level of accuracy for temporal
function (upper asymptote – lower asymp-
discrimination remained below that for color
tote). In the current temporal-discrimination
matching at all doses of morphine.
procedure, if stimulus control was perfect, the
To assess whether the effects of morphine
values of the upper and lower asymptotes
on accuracy during the temporal discrimina-
would be 1 and 0, respectively. The resulting
tion and color-matching components were
range of the function would be 1.0, indicating
statistically significantly different, a line was
perfect discrimination of the endpoints (2 and
fit using linear regression to the data relating
8 s) of the temporal sample continuum. The
the proportion correct in the temporal dis-
SD is a measure of the slope of the function
crimination and color-matching components
and reflects the degree of sensitivity to the
to the dose of morphine. Control and saline
differences between the sample stimuli in the
data were excluded from this analysis. Data
short and long categories, with greater SDs
were pooled across pigeons. The slope of the
indicating decreased sensitivity. The mean of
function relating proportion correct in the
the function is the duration at which the
temporal-discrimination component to the
proportion of responses to the long key color
dose of morphine was 20.055, whereas the
is .5 (the point of subjective equality; PSE).
slope of the function for the proportion
This parameter is a measure of the degree of
correct in the color-matching component was
bias, and is affected by shifts in the psycho-
20.011. These slopes were significantly differ-
physical curve. Leftward and rightward shifts
ent, F(1,112) 5 12.60, p 5 0.00057, when
in the curve change the mean of the function
compared using analysis of covariance as
and indicate bias for the key color associated
described by Zar (1999). These results show
with short and long sample durations, re-
that morphine decreased accuracy of discrim-
spectively.
ination of the temporal sample endpoints,
Figure 4 shows that the control data (top
whereas accuracy of color matching was
row) were well described by the cumulative
relatively unaffected.
Gaussian functions, which accounted for an
Figures 4 and 5 present a detailed analysis of
average of 99.3% of the variance across
the effects of morphine on performance in the
pigeons. The functions had an average mean
temporal-discrimination component. The data
of 4.7 s, indicating that pigeons accurately
in Figure 4 are expressed as the mean pro-
discriminated the passage of time. Morphine
portion of responses to the long key color as
(lower rows) tended to dose-dependently de-
a function of sample duration. The data were
crease the proportion of long responses
fit using a cumulative Gaussian function with
following long sample durations, with an
four parameters: upper and lower asymptotes,
increase in variability in the functions and
standard deviation (SD), and mean. The fits
across determinations at higher doses. Partic-
were obtained with the SOLVER tool of the
ularly at higher doses, morphine also in-
EXCEL 5.0 spreadsheet program. Blough
creased the proportion of long responses
(1996) noted that if the means of the fitted
following short sample durations. In most

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408
RYAN D. WARD and AMY L. ODUM
Fig. 4.
Mean proportion of responses to the long key color during control (top row) and morphine sessions (lower
rows) as a function of sample duration for each pigeon during the temporal-discrimination component. Dotted lines
indicate the bisection of .5 responses to the key color corresponding to the long sample duration, and the duration (5 s)
that was midway between the short and long sample duration categories. Vertical bars represent one standard deviation
above and below the mean. Data are not shown for Pigeon P213 following administration of 5.6 mg/kg morphine
because this pigeon responded following only one administration of this dose.
cases, the proportion of responses to the long
administration had no systematic effect on the
key color tended to increase as a function of
range. Morphine dose-dependently decreased
increasing sample duration, a result that
the range for all pigeons. This result indicates
indicates some control of choice behavior by
a dose-dependent decrease in stimulus con-
the temporal dimension of the stimuli. In
trol.
addition, in most cases the functions remained
To determine the extent to which decreases
roughly ogival, albeit somewhat flattened,
in stimulus control were responsible for the
across increasing doses of morphine.
decreased accuracy in the temporal-discrimi-
Figure 5 shows dose-effect curves of the
nation component, we examined the effects of
parameters derived from fitting the function
morphine on the measures of sensitivity and
to the mean proportion long response data
bias (i.e., SD and mean). Interpreting these
from each pigeon. The left panel shows the
measures can be problematic because, as
range of the psychophysical curves during
Blough (1996) noted, decreases in the range
control sessions and as a function of mor-
also can change the estimates of the other
phine. During control sessions, the range was
parameters. Because of this relation between
between .80 and .97, indicating a relatively
these parameters, SD changes due to de-
high level of overall stimulus control. Saline
creased stimulus control are confounded with

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TEMPORAL DISCRIMINATION AND COLOR MATCHING
409
Fig. 5.
Parameter estimates from the fit of the cumulative Gaussian function (see text for details) to mean proportion
long pecks from the temporal-discrimination component for each pigeon as a function of morphine. The left column
shows the range (upper asymptote – lower asymptote) of the function. The center column shows corrected estimates of
the standard deviation (SD) of the function. The right column shows corrected estimates of the mean of the function, or
the time at which .5 of the pecks were to the key color corresponding to the long category (point of subjective equality;
PSE). Unconnected points show parameter estimates for control (C) and saline (S) data. Lines connect data points
showing parameter estimates across doses of morphine. Data are not shown for Pigeon P213 following administration of
5.6 mg/kg morphine because this pigeon responded following only one administration of this dose.
those due to changes in sensitivity (see e.g.,
failure of the asymptotes to fall at these
Blough, 1996). To determine the actual effects
expected values indicates that choice behavior
of morphine on sensitivity of temporal dis-
on some trials is not governed by the
crimination in this procedure, it is necessary to
presented stimuli but is instead governed by
obtain an unconfounded estimate of the slope
some other, unspecified stimuli. The asymp-
of the function (i.e., SD). To obtain this
tote correction gives the probability of a re-
estimate, it is necessary to remove the in-
sponse with the assumption that choice behav-
fluence of changes in the range from the
ior on the current trial is governed by the
estimate of the SD. Accordingly, we employed
sample stimuli. In other words, the correction
an asymptote correction first introduced by
gives an estimate of the parameters of the
Heinemann et al. (1969).
model with the assumption of perfect stimulus
In calculating this correction, Heinemann
control (i.e., range value of 1.0). The resulting
et al. (1969) assumed that, given perfect
estimate of the SD is considered to represent
stimulus control, the asymptotes of the psy-
a more accurate measure of the effects of
chophysical functions should fall at zero
morphine on sensitivity of temporal discrimi-
and
unity.
They
further
suggested
that
nation.

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410
RYAN D. WARD and AMY L. ODUM
Table 1
Mean latency (in seconds) for responses to trial-ready stimuli and temporal choice comparisons
during all control and saline sessions and as a function of morphine dose for all pigeons.
Numbers in parentheses are standard deviations of the mean. Data are not shown for Pigeon
P213 following administration of 5.6 mg/kg morphine because this pigeon responded following
only one administration of this dose.
Morphine dose
Subject
Response
Control
Saline
0.56 mg/kg
1.0 mg/kg
3.0 mg/kg
5.6 mg/kg
P211
Trial ready
2.37 (0.30) 2.44 (0.54)
1.51 (0.18)
1.32 (0.29)
1.21 (0.31)
4.71 (3.91)
Choice
2.22 (0.21) 2.20 (0.11)
1.73 (0.06)
1.72 (0.16)
1.94 (0.27)
2.21 (0.14)
P212
Trial ready
4.28 (1.20) 4.28 (2.04)
4.32 (0.51)
4.63 (1.14)
5.63 (4.92)
6.53 (3.10)
Choice
2.59 (0.24) 2.54 (0.25)
2.47 (0.44)
2.16 (0.14)
2.10 (0.39)
1.94 (0.16)
P213
Trial ready
3.20 (0.73) 3.02 (0.38)
3.36 (0.52)
2.10 (0.20)
5.05 (5.21)
Choice
1.18 (0.13) 1.36 (0.31)
1.22 (0.11)
1.20 (0.15)
1.38 (0.21)
P214
Trial ready
1.42 (0.40) 1.30 (0.11)
1.50 (0.50)
1.10 (0.17)
1.10 (0.23)
1.22 (0.53)
Choice
1.33 (0.12) 1.33 (0.18)
1.30 (0.10)
1.31 (0.10)
1.32 (0.10)
1.40 (0.04)
The center column of Figure 5 shows the SD
To examine further the effects of morphine
of the corrected psychophysical curves during
on choice behavior in the temporal-discrimi-
control sessions and across doses of morphine.
nation component, we calculated response
The control SD was 1.26 for all 4 pigeons,
latencies. Table 1 shows the latencies to peck
indicating relatively high sensitivity to the
the trial-ready stimulus and temporal-sample
temporal distribution of the sample stimuli
comparisons during all control and saline
in the short and long categories. Saline had
sessions and as a function of morphine dose
little effect on the SD with the exception of
for all pigeons. Under control conditions,
a large increase for Pigeon P211. Morphine
pecks to the trial-ready stimulus occurred on
slightly increased the SD at some doses for
average 1.5 to 4.3 s after the stimulus was
some pigeons and decreased it for others.
presented. Saline had relatively little effect on
Overall, morphine had no systematic effect on
response latencies. Morphine increased the
the SDs. These results suggest that when the
response latency and standard deviation for 3
influence of overall stimulus control is con-
of 4 pigeons, particularly at the highest doses.
trolled for, sensitivity of temporal discrimina-
For Pigeon P214, morphine had no apprecia-
tion was not systematically affected by mor-
ble effect on the latency to respond to the trial-
phine.
ready stimulus, and for Pigeon P211 morphine
In addition to changing the SD, increases or
decreased latency at the three lowest doses but
decreases in the range also can affect the
increased latency at the highest dose. Re-
estimate of the mean. Therefore, the asymp-
sponses to the temporal sample comparisons
tote correction described above was applied
under control conditions usually occurred
to obtain unconfounded estimates of the
within 2.5 s of choice-key illumination. Saline
mean. The right column of Figure 5 shows
had relatively little effect on the latencies.
the means of the corrected psychophysical
Across doses, morphine had no systematic
curves during control sessions and across
effect on the choice-response latencies. Laten-
doses of morphine. The control means were
cies decreased slightly for 2 pigeons (P211 and
between 4.2 and 5.1 s, indicating accurate
P212), and showed little change for the other
estimation of the passage of time. Administra-
2 pigeons. Taken together, these results show
tion of saline had no systematic effect on the
that although morphine increased the latency
mean. Across pigeons, morphine had no
to peck the trial-ready stimulus at higher
systematic effect on the mean. These results
doses, the latency to peck a short or long
suggest that when the influence of stimulus
comparison key was not affected.
control was controlled for, morphine did not
systematically shift the curves either to the left
DISCUSSION
or the right, indicating no systematic bias for
the key color associated with short or long
The baseline performance in the FI and
samples.
temporal-discrimination components indicat-

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