SUPPLEMENTAL RESULTS
Patterns of drug intake
Of the 38 animals that were included in the present study, 9 were included in both the
Early and the Late session. The remaining animals were included in one but not both
sessions. Comparisons of self-administration behavior between the Early and the Late
session were conducted in separate ANOVAs for the 9 animals that contributed to both
sessions and for the 29 animals that contributed to one but not both sessions.
For the 9 animals, a two-way repeated measures ANOVA with session and
subject as factors showed that the median maintenance inter-infusion-interval for the last
10 lever presses was significantly lower during the Late session (6.8 ± 0.2 min) than
during the Early session (9.1 ± 0.2) [F(1,8)=115.0, p<0.001]. The median cocaine level
for the same lever presses was correspondingly greater during the Late session (7.1 ±
0.4 µM/kg) than during the Early session (4.7 ± 0.3 µM/kg) [F(1,8)=80.6, p<0.001].
Of the 29 subjects that contributed recordings to one, but not both, of the two recording
sessions, 19 contributed to the Early session data and 10 contributed to the Late session
data. A one-way ANOVA with group as a factor, showed that the average median
maintenance inter-infusion-interval for the last 10 lever presses was significantly lower
during the Late session (6.7 ± 0.4 min) than during the Early session (8.8 ± 0.5 min)
[F(1,27)=6.8, p<0.02]; average median cocaine level for the last 10 presses was
correspondingly greater during the Late session (7.1 ± 1.9 µM/kg) than during the Early
session (5.03 ± 0.4 µM/kg) [F(1,27)=9.7, p<0.004].
Anatomical location of neurons
The locations of the microwire tips are shown in Figure S1.
The differential between-session change in firing was not related to a differential
between-session sampling of core versus shell neurons.
Comparisons were made of the proportions of Task-Activated and Task-Non-Activated
neurons recorded in the core versus the shell during the Early and the Late sessions.
The comparisons were made using Chi Square tests with neuron group (i.e., TaskActivated versus Task-Non-Activated) and session as factors. This analysis showed that
there was no between-session change in the relative inclusion of core and shell neurons
in the Task-Activated and Task-Non-Activated neuron groups [χ2 (6) = 1.06, p>0.05] (see
Table S1).
The differential between-session change in firing was not attributable to
differential sampling of neurons among different subjects
In each recording session, Task-Activated and Task-Non-Activated neurons were fairly
equally sampled from the various subjects. During both the Early and the Late session,
the majority of Task-Activated neurons (51-80%) and Task-Non-Activated neurons (6769%) were recorded from animals that contributed neurons to both groups.
Nevertheless, it was possible that the differential contribution of some animals to TaskActivated and Task-Non-Activated neuron groups during either the Early or the Late
session could have influenced the observed differences in firing between the two groups.
To test for an effect of subject on the between-session changes in firing, the original
ANOVA comparison was repeated with subject added as an additional factor (data not
shown). This analysis showed that there was no three-way interaction between the
source of the neurons and either 1) session and activation [F(1,140)<1.0], 2) session
and session phase (F(4,140)=1.16, p>0.05], or 3) activation and session phase
[F(4,140)<1.0]. There was also no two-way interaction between the source of the
neurons and either activation [F(1,140)<1.0], session [F(1,140)<1.0], or session phase
[F(4,140)=1.41, p>0.05]. There was no overall effect of session [F(1,140)<1.0].
However, as for the original analysis of Task-Activated and Task-Non-Activated, there
was a significant effect of activation [F(1,140)=38.82, p<0.0001], and phase
[F(4,140)=2.79, p< 0.03], as well as a significant two-way interaction between both
activation and session [F(1,140)=5.48,p=0.02] and activation and phase [F(4,140)=3.9,
p< 0.005]. These findings are consistent with the conclusion that the observed
differences between the Task-Activated and Task-Non-Activated neurons were not
attributable to sampling the two groups of neurons from different subjects.
The differential between-session change in firing was not influenced by taskrelated inhibitions or decrease + progressive reversal firing patterns
Two subtypes of neurons were not considered in the sorting of neurons into activated
and non-activated groups. These subtypes were transient, phasic, task-related
inhibitions, and the decrease + progressive reversal firing pattern. Repetition of the
original analyses with these neuron sub-types either excluded from the analysis or
treated as a factor in the ANOVA showed that neither type of firing pattern influenced the
findings of the original ANOVA (Figs S2-3).
The number of transiently-inhibited neurons in the Task-Non-Activated group was
very small (n=2 lever press inhibitions, and n=7 session-start or 1st-press inhibitions).
When the 3-factor ANOVA (activation x session x session phase) was repeated with
these neurons excluded from the analysis, there was a significant activation x session
interaction [F(2,131)= 5.09, p<0.01]. Therefore, the inclusion of these neurons in the
analysis did not affect the outcome.
A substantial portion of both Task-Activated (41%) and Task-Non-Activated
(29%) neurons showed a decrease + progressive reversal firing pattern. The prevalence
of the firing pattern did not significantly differ between the two groups [χ2=1.6, p>0.05].
The original ANOVA comparison of Task-Activated and Task-Non-Activated neurons
was repeated with the decrease + progressive-reversal firing pattern (i.e., presence
versus absence) treated as an additional factor. There was no significant three-way
interaction between either of the following: firing pattern, activation, and session phase
[F(4,138)=1.59, p>0.05], firing pattern, activation, and session [F(1,138)<1.0], or firing
pattern, session and session phase [F(4,138)=1.87, p>0.05]. There was also no
significant two-way interaction between the decrease + progressive-reversal firing
pattern and either activation [F(1,138)<1.0], session phase [F=(4,138)<1.0], or session
[F(1,138)<1.0] This analysis is consistent with the interpretation that the presence
versus absence of the decrease + progressive-reversal firing pattern did not influence
the differential between-session changes in firing exhibited by the Task-Activated and
Task-Non-Activated neurons.
The differential between-session decrease in firing of Task-Activated and TaskNon-Activated neurons was a differential change in overall firing rather than a
differential change in firing during specific task periods
The primary comparison of Task-Activated and Task-Non-Activated neurons was based
on firing rates that were sampled in short epochs that occurred just before or during the
self-administration session. The changes in firing thus potentially reflected changes in
firing during periods closely locked to task events rather than a differential change in the
overall activity of the neurons. Additional comparisons of Task-Activated and Task-NonActivated neurons were not consistent with this interpretation.
Average firing during the entire 20-min pre-session baseline period of the selfadministration session was subjected to a two-factor ANOVA with session and activation
as factors (Fig 6). This analysis showed that there was no effect of session
[F(1,141)<1.0], but there was a significant effect of activation (F(1,141)=12.2, p<0.001]
and a significant interaction between activation and session [F(1,141)=4.2, p< 0.05] on
average firing during the 20-min baseline period. An analogous test was run on a 20min post-session baseline period (the last 20 min of the recovery period in the Early
session and the last 20 min pre-CR in the Late session). This ANOVA showed that there
was no effect of session [F(1,141)<1.0], but there was a significant effect of activation
[F(1,141=27.7, p< 0.001] and a significant interaction between activation and session
[F(1,141)=4.24, p< 0.05] (Fig 6).
A test for an effect of the extinction and cue reinstatement phases of the Late
session, on average firing of Task-Activated and Task-Non-Activated neurons, was
made with an ANOVA that included activation (i.e., Task-Activated versus Task-NonActivated) and session phase as factors (Fig 9). The session phases equaled: 1) the 30
sec pre-SD, 2) the 29.5-30 min of extinction, 3) the 30 sec pre-CR, and 4) the 30 sec
post-CR (Fig. S3). The analysis showed that there was a significant effect of activation
[F(1,53)=18.85, p=0.01] but no significant activation x phase interaction [F(3,53)=1.16,
p>0.05]. In total, the various comparisons of the Task-Activated and Task-Non-Activated
neurons show that the differential between-session change in firing exhibited by the
neurons was not selective for any particular behavioral condition.
Supplemental Figure Captions
Figure S1. Histological localization of neurons recorded during the Early and the
Late sessions. The accumbens (rostral to +1.0 Bregma) is shown in each of six coronal
plates (adapted from Paxinos and Watson, 1995). The rostral-caudal position of each
coronal section is indicated by the number at the lower right-hand side of each plate.
Each point represents one to three wire tips.
Figure S2. Average firing of Task-Activated and Task-Non-Responsive neurons
during the Early and the Late session.
This analysis is analogous to that presented in Fig 6, except that the Task-Inhibited
neurons were removed from the Non-Activated group, creating a Task-Non-Responsive
group. Average firing rate of the Task-Activated and Task-Non-Responsive neurons
during various periods of the self-administration session is plotted for the Early (left plot)
and the Late (right plot) session. The periods for which firing is shown include: 1) 30
sec pre-SD, 2) 30 sec post-SD, and 3) 30 sec pre-press for each of the first 30 cocainereinforced lever presses. Horizontal lines represent average pre-SD firing.
Figure S3. Average firing of Task-Activated and Task-Non-Activated neurons
during the Early and the Late session: Comparison of decrease + progressivereversal neurons versus all other neurons.
Average firing of the Task-Activated and Task-Non-Activated neurons during various
periods of the self-administration session is plotted for the Early (left plot) and the Late
(right plot) session. Average firing rates are shown separately for decrease +
progressive-reversal neurons (top) and for all other neurons (bottom). From left to right
on the abscissa of each plot, firing is shown for the following periods: 1) 30 sec pre-SD
(horizontal lines), 2) 30 sec post-SD, and 3) 30 sec pre-press (for each of the first 30
lever presses). Horizontal lines represent average pre-SD firing.