In the

    United States Court of Appeals
                For the Seventh Circuit
                    ____________________
No. 13-3636
LATESHA MOON,
                                                 Plaintiff-Appellant,

                                v.

CAROLYN W. COLVIN, Acting
Commissioner of Social Security,
                                                Defendant-Appellee.
                    ____________________

        Appeal from the United States District Court for the
          Northern District of Illinois, Eastern Division.
        No. 12-CV-05593 — Susan E. Cox, Magistrate Judge.
                    ____________________

     ARGUED MAY 27, 2014 — DECIDED AUGUST 14, 2014
                    ____________________

   Before POSNER, EASTERBROOK, and HAMILTON, Circuit
Judges.
   HAMILTON, Circuit Judge. An administrative law judge
found in 2010 that Latesha Moon was not disabled and thus
not entitled to Social Security disability benefits. The Ap-
peals Council and the district court upheld the ALJ’s deci-
sion. Moon argues in this appeal that the ALJ improperly
discounted her evidence of chronic migraine headaches. We
2                                                 No. 13-3636

agree that the ALJ’s treatment of the evidence related to her
migraines was unreasonable and failed to build a logical
bridge between the evidence and the decision denying bene-
fits. We reverse the district court’s judgment and remand this
case to the agency for further consideration. Because Moon
has been receiving Social Security disability benefits based
on a later application, the only issue on remand will be
whether she was disabled between August 2008 and the later
date from which benefits have been paid.
    Moon is a mother who was 26 years old at the time of her
application. She has worked in the past as a cashier, bank
teller, and certified nursing assistant. For years she has suf-
fered from numerous documented health issues, including
back and joint problems, mild sleep apnea, depression, and
migraine headaches. Most of these problems are related to
the fact that Moon is exceptionally obese: at a height of 5’5”,
she weighs over 400 pounds, giving her a body mass index
(BMI) of around 67. A BMI between 18.5 and 24.9 is consid-
ered normal, and a person is considered obese when his or
her BMI reaches 30. See “About BMI for Adults,” Centers for
Disease             Control          and            Prevention,
http://www.cdc.gov/healthyweight/assessing/bmi/adult_bmi
/index.html) (visited Aug. 13, 2014).
    In support of her application for disability benefits, Moon
submitted extensive medical records. On the medical issue
that is central to this appeal—her migraine headaches—the
evidence showed that she was diagnosed as early as 2005
and that she saw doctors about her headaches many times
over the years that followed. She was prescribed Amitrypty-
line and Zomig to treat her headaches in 2005, but she did
not take these medications because she feared side effects.
No. 13-3636                                                  3

Later she was prescribed Imitrex and Motrin, both of which
she was taking at the time of her May 2010 hearing before
the ALJ.
   At the hearing, Moon testified about the seriousness of
her migraines, their effect on her activities, and their treat-
ment. She explained that the headaches had become worse
over the last two or three years than they had been in 2005.
Over that period, she had headaches on most days, and her
headaches could last up to three days. Often the pain would
be so bad that she needed to lie down in the dark until it
passed. She said she believed that the medications Motrin
and Imitrex caused side effects but that she had “no choice”
but to take them because of the severity of her pain.
    In his written decision denying Moon’s claim for benefits,
the ALJ went through the standard five-step analysis for de-
termining whether a person is disabled within the meaning
of the Social Security Act. See 20 C.F.R. §§ 404.1520(a)(4),
416.920(a)(4). He found at step one that Moon was no longer
engaged in substantial gainful activity (i.e., was not work-
ing). At step two he found that her combination of impair-
ments—specifically back problems, obesity, sleep apnea, and
headaches—qualified as “severe,” meaning that her ability
to perform work activities was significantly limited as a re-
sult of the impairments.
   At step three the ALJ found that Moon’s combination of
impairments did not meet the criteria for an automatic find-
ing of disability. (If durational requirements are satisfied,
conditions listed in 20 C.F.R. Part 404, Subpart P, Appendix 1,
entitle a claimant to benefits without further analysis beyond
step three.) As a result, the ALJ was required to determine
Moon’s “residual functional capacity” at step four.
4                                                 No. 13-3636

    Residual functional capacity is the extent to which a per-
son can still work despite having health problems. See 20
C.F.R. §§ 404.1545(a)(1), 416.945(a)(1). The ALJ found that
Moon, though suffering from a severe combination of im-
pairments, was still capable of doing sedentary work if she
would be permitted to sit or stand at will. He found she
could lift 20 pounds occasionally and 10 pounds frequently
and could also stoop occasionally, but should not climb
ropes, ladders, or scaffolding. The ALJ based his assessment
largely on the opinions of two doctors who had reviewed
Moon’s medical records but had not examined her. Dr.
Young-Ja Kim, a state agency medical reviewer, provided a
report. Dr. Anthony F. Francis, a medical expert, provided
interrogatory answers that the ALJ described as being “in
full agreement” with Dr. Kim’s report.
   On the specific issue of Moon’s headaches, the ALJ did
not explain in his decision how they influenced his determi-
nation of her residual functional capacity, but he implied
that their role was minimal. He noted that the “alleged
headaches” had begun in 2005, three years before Moon
stopped working. He also pointed out that she had not taken
the medications she was prescribed in 2005 and that a 2008
MRI of her head was “unremarkable,” showing “no cause
for headaches.” Moreover, the ALJ explained, Moon had
“denied backaches, headaches, and depression” at a visit to
her doctor in March 2009. Regarding Moon’s testimony
about her headaches, the ALJ highlighted a perceived ten-
sion between her testimony that her current medications
caused side effects and her medical records, which did not
show that she had reported these side effects to her doctors.
No. 13-3636                                                   5

   Based on Moon’s residual functional capacity, the ALJ
found at step four that she was not capable of doing her past
work. At step five, however, he found that Moon was not
disabled because sedentary jobs with a sit/stand option exist
in significant numbers. After the Appeals Council agreed
with the ALJ’s assessment, Moon sought judicial review in
the district court, which also upheld the ALJ’s decision.
    Moon argues on appeal that the ALJ, in determining her
residual functional capacity, did not sufficiently account for
her migraines. Like the district court, we review the ALJ’s
determination to see if it is supported by “substantial evi-
dence,” meaning “‘such relevant evidence as a reasonable
mind might accept as adequate to support a conclusion.’”
Moore v. Colvin, 

, 1120–21 (7th Cir. 2014), quot-
ing Richardson v. Perales, 

, 401 (1971). This defer-
ential standard of review is weighted in favor of upholding
the ALJ’s decision, but it does not mean that we scour the
record for supportive evidence or rack our brains for reasons
to uphold the ALJ’s decision. Rather, the ALJ must identify
the relevant evidence and build a “logical bridge” between
that evidence and the ultimate determination. 

.
   In this case, the ALJ’s analysis of the evidence related to
Moon’s migraines is not logically connected to his determi-
nation of her residual functional capacity. He found at step
two that her headaches were part of a severe combination of
impairments. At step four, though, he seemed to imply that
she did not actually suffer migraines when he addressed the
specifics of her “alleged headaches,” despite the undisputed
record of years of treatment for migraines. Even if the ALJ
did not mean to imply that Moon had no migraines, though,
6                                                  No. 13-3636

his analysis of the migraine evidence is problematic in sev-
eral respects.
    First, the ALJ’s description of Moon’s March 2009 visit to
the doctor was mistaken. He wrote that she “denied head-
aches” as if she denied ever having them. The record of that
visit shows that Moon, who was there to receive test results,
denied currently having a headache. The fact Moon did not
have a headache at the time of that visit is no reason to con-
clude anything about the frequency or severity of her mi-
graines.
    Second, the ALJ reliance on Moon’s “unremarkable” 2008
MRI as evidence that her migraines were not a significant
problem is not supportable. No doctor ever suggested that
the MRI evidence meant anything about Moon’s migraines,
and for good reason. Doctors use MRIs to rule out other possi-
ble causes of headache—such as a tumor—meaning that an un-
remarkable MRI is completely consistent with a migraine
diagnosis. See “Migraines: Tests and Diagnosis,” Mayo Clin-
ic, http://www.mayoclinic.org/diseases-conditions/migraine-
headache/basics/tests-diagnosis/con-20026358 (visited Aug.
13, 2014). This mistaken reading of the evidence illustrates
why ALJs are required to rely on expert opinions instead of
determining the significance of particular medical findings
themselves. See Blakes ex rel. Wolfe v. Barnhart, 

,
570 (7th Cir. 2003) (“[T]he ALJ seems to have succumbed to
the temptation to play doctor when she concluded that a
good prognosis for speech and language difficulties was in-
consistent with a diagnosis of mental retardation because no
expert offered evidence to that effect here.”); Rohan v. Chater,

, 970 (7th Cir. 1996) (“ALJs must not succumb to
No. 13-3636                                                  7

the temptation to play doctor and make their own inde-
pendent medical findings.”).
    Finally, the ALJ’s emphasis on the fact that the migraines
began long before Moon stopped working and her refusal to
take prescribed medication in 2005 failed to come to grips
with her 2010 testimony that her headaches had become
worse in recent years. Perhaps the ALJ did not credit that
portion of her testimony, but he did not actually say so or
explain why. And the reason he gave for doubting another
portion of her testimony about migraines—her statement
that she experienced side effects from her current medica-
tions—does not justify rejecting all of her testimony about
migraines without further explanation. The ALJ cited a lack
of medical documentation that she experienced side effects,
but that lack of documentation does not control the impact
migraines had on her life and ability to work.
     Perhaps the ALJ’s apparently dim view of Moon’s testi-
mony about her migraines resulted in part from his broader
skepticism of her credibility, but his more general credibility
assessment is problematic as well. For example, the ALJ
called into question Moon’s testimony that she had needed a
cane to walk since 2009 by pointing to a particular doctor’s
appointment where she did not bring a cane—an appoint-
ment that took place in 2008. He also split hairs unfairly, in
our view, when he called her testimony that she could not
lift more than 10 pounds “inconsistent” with a doctor’s find-
ing in a consultative examination that she could lift no more
than 10 to 15 pounds.
   The government attempts to support the ALJ’s analysis of
the migraine evidence by contending that the report of Dr.
Kim and the interrogatory answers of Dr. Francis (the evi-
8                                                 No. 13-3636

dence that the ALJ relied upon most heavily) took Moon’s
migraines into account sufficiently, thus relieving the ALJ of
any further obligation to account for the migraines. Since the
ALJ did not mention Moon’s migraines in his discussion of
the opinions of Drs. Kim and Francis, the government’s ar-
gument violates the principle established in SEC v. Chenery
Corp., 

, 87–88 (1943), that an agency’s lawyers
cannot defend an agency decision in court on a ground the
agency did not itself invoke. See Kastner v. Astrue, 

, 648 (7th Cir. 2012).
    Apart from Chenery concerns, the doctors’ opinions can-
not provide the needed logical bridge. Dr. Kim did not actu-
ally specify how Moon’s migraines relate to the residual
functional capacity she found. She did not address the pos-
sibility that a migraine could keep Moon at home in bed, ap-
parently understanding Moon’s “history of frequent mi-
graines” to have implications only for her ability to lift,
stoop, and stand. Dr. Francis’s report provides even less
support for the government’s position. He noted: “There is
not enough information [in the medical records] on the fre-
quency of [Moon’s] migraines to determine how much these
contribute to her overall impairment.” Unlike Dr. Kim and
Dr. Francis (who, remember, never even met Moon), the ALJ
had the opportunity to hear Moon’s own account about how
her migraines limit her activities, and as explained above, his
apparent disbelief of her testimony is not justified in his de-
cision.
   Once the ALJ’s unsupported assessment of Moon’s mi-
graines is set aside, the question becomes just how serious
her migraines were beginning in August 2008. If her testi-
mony about needing to remain in bed multiple days per
No. 13-3636                                                9

week is credited, she could not have held a full-time job. We
do not decide that her testimony must be credited. We con-
clude only that the ALJ failed to build a logical bridge be-
tween substantial evidence and his conclusion that Moon
was capable of a limited range of sedentary work after Au-
gust 2008. The judgment of the district court is REVERSED
and the case is REMANDED to the agency for further pro-
ceedings consistent with this opinion.