J Comp Physiol A (1985) 156:503-509
Joumal o
Senso y,
Compa a i e
Neu al,
and
Physiology A
~h. ,o .,
Physiology
๎9 Sp inge -Ve lag 1985
Spec al sensi i i y o g ound squi el cones measu ed
wi h ERG licke pho ome y
Ge ald
H.
Jacobs, Jay Nei z, and Michael C ognale
Depa men o Psychology, Uni e si y o Cali o nia, San a Ba ba a, Cali o nia 93106, USA
Accep ed No embe 27, 1984
Summa y.
G ound squi els ha e dich oma ic
colo ision. The spec al sensi i i ies o he wo
classes o cones ound in he e inas o wo species
o g ound squi el we e measu ed using ERG
licke pho ome y. The spec al sensi i i y cu es
o hese cone classes we e closely i by cu es
om wa eleng h-dependen isual pigmen nomo-
g ams. One cone ype had an a e age peak sensi-
i i y o 518.9 nm (Cali o nia g ound squi els,
Spe nophilus beecheyi)
o 517.0 nm ( hi een-lined
g ound squi els,
Spe mophilus idecemlinea us).
The second ype o cone ound in hese g ound
squi els had an a e age peak sensi i i y o
436.7 nm. An examina ion o he a ia ion in spec-
al sensi i i y among indi idual animals sugges s
ha he sensi i i y peaks o he middle-wa e-
leng h cone co e a ange o no g ea e han 4 nm.
In oduc ion
G ound squi els a e unusual among mammals in
ha hey ha e cone-domina ed e inas. In addi ion
o a small popula ion o ods (Wes and DoMing
1975; Jacobs e al. 1976; Long and Fishe 1983),
p obably comp ising no mo e han 10% o all
pho o ecep o s, he e is bo h elec ophysiological
(Michael 1965; Gu and Pu ple 1978; Jacobs and
Too ell 1981) and beha io al (C esci elli and Pol-
lack 1965; Jacobs 1978) e idence indica ing he
p esence o wo ypes o cones in hese e inas.
One o hese has peak sensi i i y in he sho wa e-
leng hs, a pe haps abou 440 nm (Jacobs and Too-
ell 1981). Bo h elec ophysiological and beha io -
al esul s indica e ha he o he cone ype is much
mo e p e alen . The e ha e been a numbe o spec-
i ica ions o he spec al loca ion o peak sensi i i y
o his cone based on g oss-po en ial and single-
uni elec ophysiology (Tansley e al. 1961 ; C esci-
elli and Pollack 1966; Michael 1968; Jacobs and
Yol on 1972; G een and Dowling 1975; Fishe
e al. 1976; Jacobs e al. 1976; Tong 1977; Jacobs
and Too ell 1981; Raisanen and Dawis 1983).
These es ima es co e a a he subs an ial ange,
om 516 o 535 nm, and i is na u al o wonde
i ha ange e lec s species a ia ions, di e ences
in expe imen al app oaches, o some inhe en indi-
idual a ia ions in he spec al posi ioning o cone
pho opigmen s.
We ha e ecen ly de eloped an elec o e ino-
g aphic (ERG) echnique o assessing spec al
sensi i i y based on he p inciple o licke pho o-
me y ha appea s o o e a numbe o ad an-
ages o e he mo e con en ional means used o
es ima e spec al sensi i i y om elec ophysiolog-
ical measu emen s (Nei z and Jacobs 1984). The
app oach is simila in concep o ha ecen ly also
employed in a s udy o abbi cones (Nuboe e al.
1983). We ha e used his echnique on g ound
squi els, and he e epo measu emen s o he
spec al sensi i i y o he cones in he wo species
o g ound squi els ha ha e been he mos e-
quen subjec s o ision esea ch.
Me hods
Subjec s.
Adul , hi een-lined
(Spe mophilus idecemlinea us)
and Cali o nia
(Spe mophilus beeeheyi)
g ound squi els o bo h
sexes we e examined. The o me we e ob ained om a supplie
in Illinois, he la e we e apped locally. P io o es ing all
we e housed o se e al weeks unde s anda d colony condi-
ions.
Appa a us.
Bo h he appa a us and gene al p ocedu e a e de-
sc ibed in de ail elsewhe e (Nei z and Jacobs 1984). B ie ly,
s imuli we e p oduced by a h ee-beam Maxwellian- iew op ical
sys em. The es ligh o igina ed om a Bausch and Lomb
high-in ensi y g a ing monoch oma o ha ing a ungs en-halide
504 G.H. Jacobs e al. : Spec al sensi i i y o g ound squi el cones
sou ce (hal -ene gy passband = 10 nm). I s in ensi y was a ied
wi h a ci cula neu al densi y wedge (OD =0.0-3.0). The e e -
ence ligh came om a ungs en-halide lamp. A hi d beam,
used he e o accesso y adap a ion, also o igina ed om a ung-
s en-halide lamp. The cha ac e is ics o he la e wo ligh s
we e con olled wi h neu al-densi y s ep il e s and Di ic in e -
e ence il e s (hal -ene gy passband= 10 im). All lamps we e
unde un a 11 V om a egula ed DC powe supply. The ou -
pu s om hese h ee beams we e op ically supe imposed and
p esen ed o he eye in Maxwellian iew (53~ Each channel
o he op ical sys em con ained a high-speed elec omagne ic
shu e (Vincen Associa es Unibli z) d i en om a p og amm-
able digi al ime . Ligh measu emen s we e made wi h a PIN
10 DF silicon pho odiode (Uni ed De ec o Technology).
ERGs we e di e en ially eco ded wi h a Bu ian-Allen bi-
pola con ac lens elec ode. The g ound elec ode was placed
agains he inside o he cheek. In he licke -pho ome ic p oce-
du e he ERG esponses o a licke ing monoch oma ic ligh
a e compa ed o he esponses elici ed by a simila ly licke ing,
ixed e e ence ligh . The sequence o s imuli was: es ligh ,
no ligh , e e ence ligh , no ligh , e c., whe e all in e als we e
equal in du a ion. One such cycle is he e e e ed o as s imulus
equency; no e ha licke equency is wice his alue. The
ampli ied ERG elici ed by hese s imuli was passed h ough
an ac i e, na ow bandpass il e , he cen e equency o which
was se o he s imulus equency. The il e consis ed o wo
wo-pole s ages ( o each, Q = 5). The hal - ol age bandpass
o he il e was 0.2 x cen e equency. When he in ensi y
o he es and e e ence ligh we e g ea ly di e en , he ou pu
om his il e was oughly sinusoidal. A e e sal o he ela i e
in ensi ies o he wo ligh s p oduced a phase e e sal o he
sinusoidal ou pu om he il e . A in e media e in ensi ies
o he wo ligh s, he ampli ude o he esul an signal was
minimized and i s phase in e media e. To imp o e he cancella-
ion achie ed a his poin , he ou pu om he il e was passed
h ough a second na ow bandpass il e . In his case he cen e
equency o he il e was se o he licke equency. Tha
ou pu was in u n a e aged wi h an O ec Model 4623 Signal
A e age such ha he signal p oduced by he e e ence ligh
was a e aged wi h he in e ed signal om he es ligh (Nei z
and Jacobs 1984). The signal a e age ou pu was displayed
on an oscilloscope.
P ocedu e.
G ound squi els we e anes he ized wi h IP injec-
ions o sodium pen oba bi al (75 mg/kg). The pupil o he es
eye was dila ed (Phenyleph ine HC1 + 0.04% a opine sul a e).
The animal was placed in a s e eo axic ins mnen and aligned
wi h he es beam. No mal body empe a u e was main ained
h ough he use o a ci cula ing ho wa e hea e .
As no ed, spec al sensi i i y was measu ed wi h a licke
pho ome ic p ocedu e. The ollowing condi ions we e used o
measu e spec al sensi i i y o he g ound squi el middle-wa e-
leng h cone. The licke a e was 50 Hz. The e e ence ligh
was ach oma ic and had a co neal adiance o 33 gW. The
es ligh was aken om he monoch oma o as desc ibed
abo e. The measu emen s we e made in he p esence o a sho -
wa eleng h adap a ion ligh (wa eleng h = 440 nm; co neal a-
diance= 115 gW). The pu pose o his ch oma ic adap a ion
was o u he elimina e he possibili y o any con ibu ion om
he sho -wa eleng h cones o he 50 Hz licke . All he eco d-
ing was done in a ligh ed oom. As measu ed a he co nea
hese o e head ligh s yielded a luminance o 60 cd. m- 2.
To measu e he spec al sensi i i y o he g ound squi el
sho -wa eleng h cone, he licke a e was se o 25 Hz. A
monoch oma ic sho -wa eleng h ligh (440 nm, co neal a-
diance= 71 gW) was used as he e e ence. The es ligh was
again d awn om he monoch oma o . To p o ide a long-
wa e-leng h adap a ion ligh , a high pass il e was placed in
he adap a ion beam (Kodak no. 23 A; 50% ansmi ance=
590 nm). Se e al adap a ion in ensi ies we e employed in di e -
en expe imen s (co neal adiance om 283 o 639 p.W).
To make a sensi i i y measu emen , he densi y wedge was
i s se o an a bi a y posi ion. A ain o 100 s imulus cycles
was p esen ed and he esponses o he las 60 o hese we e
a e aged. The phase and ampli ude o ha esponse was used
o indica e he change in wedge se ing equi ed o be e null
he esponse o he e e ence ligh . This p ocedu e was done
i e a i ely un il he bes null posi ion was ound. Sensi i i y
measu emen s we e usually made a successi e 10 nm s eps wi h
equen echecking h oughou he cou se o he expe imen .
To de e mine spec al sensi i i y, he inal wedge se ings
a each es wa eleng h we e co ec ed o he spec al ansmi -
ance o he wedge, o he a ia ions in he monoch oma o
ou pu a di e en wa eleng hs and o lens abso bance in hese
species (Yol on e al. 1974). These co ec ed alues we e com-
pa ed o wa eleng h-dependen isual pigmen nomog ams us-
ing he polynomial exp essions p o ided by Dawis (1981). A
compu e was used o de e mine he spec al posi ion ( o he
nea es nm) o ha nomog am which ga e he bes i o he
spec al sensi i i y alues. To do his he di e ence squa ed
be ween each sensi i i y alue and he nomog am was com-
pu ed o each o he possible nomog ams. The nomog am
p o iding he smalles a e age di e ence squa ed was selec ed
as bes i ing he da a.
In de e mining he spec al sensi i i y o he sho wa e-
leng h mechanism i p o ed impossible o comple ely isola e
he esponses mn a single cone class, and so in hose expe i-
men s wo o he nomog am i ing p ocedu es we e employed.
Bo h we e based on he assump ion ha he licke spec al
sensi i i y unc ions e lec linea summa ion o wo spec al
mechanisms, each ha ing he sensi i i y o a nomog am pho o-
pigmen . In one p ocedu e he 2m, ~ alue o one o he wo
mechanisms was speci ied by he ope a o and he compu e
de e mined he 2,~a~ alue o he o he nomog am mechanism
needed o yield he bes summed i o he wo. In he o he
a ia ion he compu e was employed o de e mine he 2max
alues o he wo spec al mechanisms equi ed (when summed)
o bes i he obse ed spec al sensi i i y unc ion. In bo h
cases, he compu e de e mined he spec al loca ion(s) o peak
sensi i i y o he nea es nm and he ela i e p opo ions o
he wo componen s, as well as he goodness o i o he
bes i ing combina ion.
Resul s
In ea lie measu emen s made on monkeys we
showed ha he licke pho ome ic p ocedu e p o-
ides a e y sensi i e and eliable ool o assessing
spec al sensi i i y (Nei z and Jacobs 1984). The
echnique p o ed o be a leas as sensi i e wi h
g ound squi els as wi h monkeys; as in ha ea lie
expe imen , changes in he wedge se ing om he
null posi ion on he o de o 0.02 o 0.04 log uni
usually p oduced clea ly disc iminable changes in
he ERG ou pu .
Middle wa eleng h cones
Figu e 1 shows spec al sensi i i y unc ions ob-
ained om six 13-lined g ound squi els eco ded
G.H. Jacobs e al. : Spec al sensi i i y o g ound squi el cones 505
"i 5.0
2.5
2.0
"~ 1,5
LO
_3
I I
460 500 540 580 620 660 460 500 540 580 620 660
Wa eleng h
(nm)
Fig. I. Flicke pho ome ic spec al sensi i i y unc ions o six
hi een-lined g ound squi els ob ained unde condi ions (see
ex ) which isola e he ac i i y o he middle wa eleng h cone
in his species. Solid ci cles a e sensi i i y alues ob ained by
equa ing he e ec i eness o monoch oma ic es ligh s and an
ach oma ic e e ence ligh . These alues ha e been co ec ed
o abso bance by he lens. Solid lines a e he bes i ing isual
pigmen nomog am, he spec al peak o which is indica ed
on each unc ion. Cu es a e a bi a ily posi ioned on he sensi-
i i y axis
unde condi ions which appea o isola e he ac i i-
y o he middle wa eleng h cones. The solid ci cles
a e sensi i i y alues co ec ed as desc ibed abo e;
he solid lines a e he sensi i i y cu es o he bes
i ing isual pigmen nomog am. The
~max
alue
de e mined o each animal is indica ed on he in-
di idual cu e. All peaked a ei he 516, 517 o
518 nm. I can be seen ha wi hou excep ion hese
nomog ams p o ide e y good i s o he sensi i i-
y da a. The i ing e o s gene a ed om his p o-
cedu e we e small (mean squa ed di e ence = 2.8 x
10 .3 log uni ). Figu e 2 shows simila esul s ob-
ained om six Cali o nia g ound squi els, all o
whom we e un unde he same condi ions. In his
case he peak loca ions we e 518 o 520 nm. These
da a a e also well accoun ed o by he nomog am
cu es (mean squa ed di e ence=l.8x J0 .3 log
uni ).
Indi idual a iabili y
A no ewo hy ea u e o he esul s p esen ed in
Figs. i and 2 is he small indi idual a ia ion in
he loca ion o he spec al peak o he bes i ing
nomog am. Tha a ia ion was only 3 nm o he
13-lined g ound squi els and o he Cali o nia
g ound squi els. Those esul s a e based on ela-
i ely small samples. To gain a clea e indica ion
o he magni ude o indi idual a ia ion in his
measu e in he popula ion we an addi ional Cali-
o nia g ound squi els unde he same es condi-
ions. In each case a bes i ing nomog am was
de i ed. The esul s o his expe imen o a o al
i 3.0
0 ) 2.5
C 2.0
".~ 1.5
O
I~ LO
o,
i I I i i i i i i ) i
460 500 540 580 620 660 I I I I I I l I I I I
460 500 540 580 620 660
Wa eleng h (nm)
Fig. 2. Flicke pho ome ic spec al sensi i i y unc ions o six
Cali o nia g ound squi els. All o he de ails a e he same as
gi en in Fig. 1.
IO
,n
'~ 5
6
Z
515 520 525
Wa eleng h ( im)
Fig. 3. F equency dis ibu ion showing he spec al peaks o
he middle-wa eleng h cone o wen y Cali o nia g ound squi -
els. Each peak was de i ed om a cu e o he ype illus a ed
in Fig. 2. Fo his dis ibu ion he mean spec al peak is
518.9 nm (SD = 1.07). Bin wid h, 1 nm
o 20 g ound squi els a e summa ized in he o m
o a equency his og am in Fig. 3. These esul s
e i y ha he e a e only small indi idual a ia-
ions in he spec al posi ioning o he middle wa-
eleng h cone in he g ound squi el. Fo he 20
animals he
mean 2ma x
alue was 518.9 nm (SD =
1.07). Bo h o hese desc ip o s a e close o hose
cha ac e izing he mo e limi ed sample shown in
Fig. 2.
Figu e 3 p o ides an indica ion o he a ia ion
in he spec al sensi i i ies o he middle wa e-
leng h mechanism in he g ound squi el as mea-
su ed wi h ERG licke pho ome y. I is o in e es
o ask wha migh unde lie his a ia ion. One can
imagine a leas wo sepa able sou ces accoun ing
o his a ia ion: (I) inhe en indi idual a ia ions
in he eye ( e lec ing, o example, a ia ions in
he spec al abso bance o he pho opigmen o
indi idual a ia ion in p e e inal il e ing), o (2)
inhe en a ia ions in oduced by he measu emen
p ocedu e. To see i hese po en ial sou ces o a i-
a ion could be be e sepa a ed, a second expe i-
men was pe o med. Fo his a male Cali o nia
506 G.H. Jacobs e al. : Spec al sensi i i y o g ound squi el cones
U.
515 520 525
Wa eleng h (nm)
Fig. 4. F equency dis ibu ion showing he spec al peak o he
middle wa eleng h cone de i ed om epea ed es ing o a sin-
gle Cali o nia g ound squi el. The mean spec al peak is
520.9 nm (SD=0.93), Bin wid h 1 nm
g ound squi el was epea edly es ed o e a pe iod
o se e al mon hs un il a o al o nine comple e
spec al sensi i i y unc ions had been de e mined.
The p ocedu e used o each such de e mina ion
was iden ical o ha used in he expe imen s jus
desc ibed. The animal selec ed as a subjec o his
expe imen was one who on ini ial examina ion
had a spec al sensi i i y unc ion ep esen ing a
clea de ia ion om he sample mean o Fig. 3
-
he bes i ing nomog am o his animal on
his ini ial es had a peak a 521 nm. The esul s
o his expe imen a e summa ized in Fig. 4.
This dis ibu ion had a mean alue o 520.9 nm
wi h an SD=0.93. Al hough he absolu e di e -
ence in he es ima e o he peak o he spec al
sensi i i y unc ion o his animal ob ained om
epea ed es s om ha o he la ge sample is
small, i is s a is ically e y eliably di e en (one
sample - es , =6.79, d =8, P<0.001). Thus i
may be concluded ha eliable indi idual a ia-
ions in spec al sensi i i y can be ound in he
g ound squi el.
Gi en ha he e a e eal di e ences in spec al
sensi i i y among g ound squi els, is i possible
o u he pa i ion ha a iabili y? I he indi id-
ual a ia ions we e due o some ixed ea u e o
he eye, i would be expec ed ha he a iabili y
in he peak measu ed o he indi idual animal
should be smalle han he a iabili y in he same
measu e ac oss animals. To examine his we com-
pa ed he a iance measu e o he sample o Fig. 3
and he a iance de e mined o he epea ed mea-
su es on he indi idual g ound squi el. The a io
o hese wo a iances is 1.33. An F es was used
o ob ain he con idence in e al o he a io o
hese wo measu es in he popula ion. Those limi s
we e 0.32 and 2.58 (d =19,8; P<0.05), and hus
he hypo hesis ha hese wo a iances a e he
same canno be ejec ed. To he ex en ha he
a iance om he measu emen s on he indi idual
animal ep esen e o in he measu emen p oce-
du e, and he a iance om he measu emen s on
he wen y animals e lec measu emen e o plus
:~
2.8
2.0
1,6
$
1.2
~
0.8
0.4
520,'27~
i i , i i i i ~ i i I i i
450
490 530
570 6 0 650
Wo eleng h (nm)
Fig. 5. Flicke pho ome ic spec al sensi i i y unc ions ob-
ained om a Cali o nia g ound squi el. The unc ion a he
op was ob ained unde condi ions which isola e he ac i i y
o he middle wa eleng h cone (see ex ). The con inuous line
is he cu e o he bes i ing isual pigmen nomog am, he
spec al peak o which is indica ed. The bo om unc ion shows
a ede e mina ion o spec al sensi i i y eco ded unde condi-
ions which a o a con ibu ion om he sho -wa eleng h
cone (see ex ). The con inuous line used o i he bo om
unc ion ep esen s he summa ion o he wo nomog am pig-
men cu es which ga e he bes i o he expe imen al da a.
The peaks o hese cu es and hei ela i e p opo ions a e
indica ed. The e ical posi ioning o he wo unc ions accu a-
ely e lec s he h eshold ele a ion p oduced by he adap a ion
condi ion used o ob ain he da a shown a he bo om
inhe en indi idual a iabili y, i may be concluded
ha a leas 0.32 o he a iabili y in he spec al
peak loca ion among he la e animals can be a -
ibu ed o measu emen e o . Since he o al
ange o spec al peak loca ions o he middle
wa eleng h cone is no ap o exceed 6 nm (ca.
6 SDs), his analysis implies ha he ac ual a ia-
ion in spec al peak loca ion is no g ea e han
4 nm.
Sho wa eleng h cones
In ea lie expe imen s we ound ha a signal om
he g ound squi el sho -wa eleng h cones could
be eadily de ec ed in ERGs eco ded in he p es-
ence o in ense long-wa eleng h adap a ion (Ja-
cobs and Nei z 1984). In his in es iga ion we used
ERG licke pho ome y o y o de e mine mo e
p ecisely he spec al sensi i i y o his mechanism.
The subjec s we e Cali o nia g ound squi els.
A a ie y o ch oma ic adap a ion and licke
a e combina ions we e i s explo ed o ind hose
condi ions which maximized he con ibu ion o
he ERG om he sho -wa eleng h mechanism.
Fig. 5 shows esul s om an expe imen which
achie es his goal.
A he op o Fig. 5 is a spec al sensi i i y
unc ion eco ded unde condi ions hough o
G.H. Jacobs e al. : Spec al sensi i i y o g ound squi el cones 507
2.5
8
1.5 2**
1~1 ' %
;,69~
'&o
~' ' '5;' ''
;o'; ;' a .......
4 0 460 0 540 5 6 0 4 0 4 0 500
540 580 620
Wa eleng h
(nm)
Fig.
6. Flicke pho ome ic spec al sensi i i y unc ions ob-
ained om Cali o nia g ound squi els unde condi ions a o -
ing con ibu ion om he sho -wa eleng h mechanism. In each
o he six cases he con inuous cu e is ha o he bes i ing
linea summa ion o wo nomog am pigmen cu es. In each,
one o he componen s had a peak se a 519 nm. The wa e-
leng h and ela i e p opo ion o he sho -wa eleng h compo-
nen equi ed o comple e he bes i a e indica ed on each
unc ion. No e ha al hough he deg ee o isola ion o he
sho -wa eleng h componen a ied subs an ially in hese ex-
pe imen s, he de i ed peak o he sho -wa eleng h mechanism
a ies nonsys ema ically o e a small spec al ange. The cu es
a e a bi a ily posi ioned on he sensi i i y axis
isola e he middle wa eleng h cone. As be o e, he
da a a e well i ed by a nomog am ha ing a peak
a 520 nm. The lowe cu e was ob ained om
he same animal using 25 Hz licke , a 440 nm e -
e ence ligh , and concu en long-wa eleng h ad-
ap a ion. This adap a ion p oduced a la ge de-
c ease in sensi i i y (ca. 1.3 log uni s a 520 nm)
and yielded a sensi i i y unc ion in which he con-
ibu ion om he sho -wa eleng h mechanism
p edomina es. The cu e d awn h ough he da a
poin s is he bes i ing linea summa ion o wo
nomog am cu es. To de i e his cu e, all pai -
wise combina ions o nomog ams ha ing
/~max
d awn om he anges o 435 o 445 nm and 514
o 521 nm espec i ely we e examined. The bes
i ing unc ion so de e mined had he ollowing
componen s: 440 nm (73%)+ 520 nm (27%). The
i o his combina ion o he da a is excellen
(mean squa ed di e ence = 2.30 x 10 .4 log uni ).
Addi ional animals we e es ed unde condi-
ions simila o hose jus desc ibed. In each o
se en such expe imen s he de i ed peak o he
middle wa eleng h mechanism was close o ha
ound in he ea lie expe imen s (mean = 519.8 nm,
SD = 0.7). Consequen ly, in la e expe imen s he
cu e i ing was simpli ied by speci ying he peak
o one componen a 519 nm and hen sea ching
o he second nomog am which p o ided, in com-
bina ion, he bes i o he da a poin s. The esul s
a e shown o six g ound squi els in Fig. 6. No e
ha al hough he e ec i eness o he isola ion o
he sho wa eleng h cone a ied om expe imen
o expe imen , he peak o ha mechanism was
always loca ed o e a sho spec al ange, om
435 nm o 440 nm. Fo a o al o wen y-one such
de e mina ions, he p opo ion o he sho wa e-
leng h componen equi ed in he bes i ing unc-
ion was no co ela ed wi h he posi ion o i s
spec al peak ( =0.10, d = 19, n.s.). Mos g ound
squi els we e es ed unde se e al condi ions. Fo
en animals he bes es ima e o he spec al peak
o he sho wa eleng h componen a ied om
433 o 440 nm, wi h a mean alue o 436.7 nm
(SD=2.6).
Discussion
The licke pho ome ic p ocedu e yields a clea
and consis en pic u e o he spec al p ope ies
o he middle wa eleng h cones in he g ound
squi el. The a e age peak loca ion o he Cali o -
nia g ound squi el is 518.9 nm (N=20); o he
hi een-lined g ound squi el i is 517.0 nm (N=
7). In bo h cases he shapes o he spec al sensi i i-
y unc ions a e well accoun ed o by cu es de-
i ed om wa eleng h-dependen isual pigmen
nomog ams.
Wi h one excep ion (Raisanen and Dawis
1983), all o he p e ious s udies o he g ound
squi el, including hose done ea lie in his labo a-
o y, yielded peak es ima es o he middle wa e-
leng h cone ha we e a somewha longe wa e-
leng hs han hose p esen ly ound. We belie e he
cu en s udy gi es a mo e accu a e indica ion o
he spec al loca ion o his cone ype han hose
done ea lie o h ee easons: (1) he numbe o
spec al poin s examined and he numbe o indi-
iduals es ed we e conside ably la ge in his
s udy, (2) none o he ea lie s udies employed au-
oma ed cu e i ing p ocedu es (in ou expe ience
i is di icul o i spec al sensi i i y da a by eye
wi h s anda d cu es o an accu acy o be e han
abou 5 nm), and, pe haps mos impo an , (3) he
licke pho ome ic p ocedu e is mo e sensi i e and
eliable han he single lash p ocedu es ypically
used in he pas . Ou esul s a e in good ag eemen
wi h hose o Raisanen and Dawis (1983) who,
al hough using di e en ERG measu es, also em-
ployed nomog am i ing echniques like hose
used in his expe imen . Fo he pho opic b-wa e
o he hi een-lined g ound squi el hei expe i-
men s yielded a spec al peak o 516 nm, while ex-
amina ion o he isola ed PIII componen o he
ERG p oduced a peak es ima e o 518 nm.
The spec al sensi i i ies o he middle wa e-
508 G.H. Jacobs e al. : Spec al sensi i i y o g ound squi el cones
leng h cones o Cali o nia and hi een-lined
g ound squi els a e e y simila . This inding is
in acco d wi h esul s om beha io al s udies in
which no signi ican di e ences we e de ec ed be-
ween he wo species in ei he colo ision o isu-
al sensi i i y (Jacobs and Yol on 1971).
Measu emen s o he spec al peak o he
g ound squi el sho -wa eleng h cone we e o ne-
cessi y made less di ec ly. Two assump ions on
which he adequacy o ha speci ica ion depends
should be no ed. The i s assump ion is ha only
wo spec al mechanism con ibu e o he licke
sensi i i y cu e. As no ed abo e, in addi ion o
he wo cone ypes, g ound squi el e inas con ain
a small popula ion o ods, and a spec al mecha-
nism wi h a 500 nm peak can be de ec ed in he
ERG eco ded unde ce ain condi ions. Al hough
we canno igo ously exclude he possibili y ha
he 500 nm mechanism is ope a i e unde he con-
di ions used o measu e he sho -wa eleng h cone,
i appea s e y unlikely o ha e had any signi ican
e ec . In addi ion o he ac ha he s imulus
condi ions would be un a o able o ob aining a
signal om ha mechanism 1, he s onge a gu-
men agains hei pa icipa ion is ha he peak
o he middle wa eleng h cone de i ed om 25 Hz
licke was he same as ha measu ed a much
highe licke a es. I he 500 nm mechanism we e
a con ibu ing ac o a he lowe licke a es one
would expec he de i ed long-wa eleng h peak o
ha e been shi ed om 519 nm owa d he sho e
wa eleng hs. As no ed, no such shi occu ed (see
Fig. 5). A second assump ion is ha he signals
om he wo cones a e linea ly summed o p oduce
he licke sensi i i y cu e. Tha same assump ion
has o en been made in o he licke measu emen s
o spec al sensi i i y (c . King-Smi h and Ca den
1976). Th ee esul s om his expe imen suppo
his assump ion: (a) he ac ha linea summa ion
o he wo nomog ams always p o ide excellen
i s o he spec al sensi i i y cu es, (b) he ac
ha he ange o de i ed peak loca ions is small,
1 Rods cons i u e a small mino i y (5-10%) o he pho o ecep-
o s in he g ound squi el e ina, and hei con ibu ions
ha e o en been di icul o see e en unde op imal condi-
ions. E en so, h ee sepa a e ea u es o he s imulus condi-
ions used in his expe imen would be expec ed o u he
mili a e agains od con ibu ions: (1) P e ious esul s om
bo h beha io al (Jacobs 1978) and ERG (Jacobs and Too ell
1977) expe imen s indica e ha he ambien ilhiminance in
he eco ding oom is well abo e ha equi ed o p oduce
a Pu kinje shi in he g ound squi el; (2) The adap ing
ligh yielded a e inal illuminance o 7.45 x 10- s sco opic lu-
men. am-Z; his alue is mo e han 0.6 log uni highe han
ha equi ed o sa u a e human ods (Aguila and S iles
1954); (3) The licke a e (25 Hz) is p obably well up owa d
he uppe limi o he od ope a ing ange
and (c) he ac ha he spec al peaks o he cone
mechanisms de i ed wi h his p ocedu e a e un e-
la ed o he ela i e p opo ions o he wo compo-
nen s equi ed o bes i he spec al sensi i i y
cu e.
In p e ious elec ophysiological expe imen s
on g ound squi els he e has been (appa en ly)
only one a emp o speci y he spec al p ope ies
o he sho wa eleng h cone. In ha expe imen
he spec al sensi i i ies o spec ally-opponen op-
ic ne e ibe s in he Cali o nia g ound squi el
we e measu ed (Jacobs and Too ell 1981). We ha e
now eanalyzed hose da a using he cu e i ing
p ocedu es o he p esen in es iga ion. Fo en
spec ally-opponen op ic ne e ibe s es ed in he
p esence o in ense long-wa eleng h adap a ion,
he spec al sensi i i y o sho wa eleng h mecha-
nism had an a e age peak loca ion o 441 nm. A
beha io al expe imen on golden-man led g ound
squi els yielded a peak es ima e o he sho -
wa eleng h mechanism a 440 nm (Jacobs 1978).
The conclusion om hese s udies as well as om
he cu en expe imen is ha he sho -wa eleng h
cone o he g ound squi el has a peak close o
440 nm.
We e u n, inally, o he issue o indi idual
a ia ion in cone spec al sensi i i y. In his in es i-
ga ion i p o ed possible o es ablish he deg ee
o such a ia ion in a la ge sample o subjec s,
and by making epea ed de e mina ions on an indi-
idual animal es ima e how much o ha a ia ion
migh be a ibu ed o measu emen e o , how
much migh e lec in insic di e ences be ween
animals. The conclusion was ha he ac ual ange
o a iabili y in he spec al peak loca ion o he
middle wa eleng h cone in he g ound squi el is
no ap o exceed 4 nm. This esul may seem su -
p ising i one iews i wi h he suspicion ha g oss
po en ial measu es like he ERG canno p o ide
e y p ecise indica ions o isual sys em physiolo-
gy. Howe e , his expe imen , like p e ious ones
(Nuboe e al. 1983; Nei z and Jacobs 1984), shows
his suspicion is un ounded, ha ERG licke pho-
ome y can indeed p o ide a e y eliable and sen-
si i e ool o measu ing spec al sensi i i y.
The ques ion o indi idual a ia ions in cone
spec al sensi i i y has been discussed in conjunc-
ion wi h measu emen s made on humans. Bo h
psychophysical (Alpe n and Pugh 1977; Alpe n
1981) and mic ospec opho ome ic (Da nall
e al. 1983) esul s sugges ha he e may be signi -
ican indi idual a ia ions in he spec al sensi i i-
y o wha ha e classically been conside ed single
cone ypes. Fo ins ance, measu emen s made on
human dich oma s indica e ha he spec al peak
G.H. Jacobs e al. : Spec al sensi i i y o g ound squi el cones 509
o he long wa eleng h cone may a y among indi-
iduals o e a ange o 8 o 11 nm (Alpe n and
Pugh 1977; Alpe n 1981). To he ex en ha he
p esen measu emen s and he human measu e-
men s a e compa able, i appea s ha he e is a
sou ce o indi idual a iabili y in he human no
seen in he g ound squi el. One dis inc possibili y
al eady sugges ed (Alpe n 1981; Da nall e al.
1983), and pe haps made e en mo e plausible by
he g ound squi el measu emen s, is ha wha has
been belie ed o ep esen a single spec al mecha-
nism in he human ac ually e lec s he p esence
o cones wi h mo e han one abso p ion spec um.
Whe he ha is ue emains o u u e esea ch,
bu i is a any a e clea ha he ange o spec al
a iabili y cu en ly claimed o human cones is
no a cha ac e is ic o all mammalian cones.
Acknowledgemen s.
We hank K is K ogh o expe echnical
assis ance and help ul ad ice. This esea ch was suppo ed by
a g an om he Na ional Eye Ins i u e (EY-00105).
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