The spike discharge regularity of 254 tonically firing units in the in
ferior colliculus (IC) of the anesthetized guinea pig was studied in r
esponse to tones presented at best frequency (BF) to the ear contralat
eral to the recorded IC. Regularity of firing was measured by calculat
ing the coefficient of variation (CV) as a function of time over the c
ourse of a unit's response. Two hundred and fifteen units (56 under ur
ethan and 159 under chloralose anesthesia) in the central nucleus of t
he IC (CNIC) were studied in detail. In response to tones at 15-25 dB
above threshold, 80% of units in the urethan sample fired regularly (C
V < 0.5) during their sustained response, and 46% were highly regular
(CV less than or equal to 0.35). For chloralose the values were 68% an
d 23%, respectively. Units recorded under urethan were significantly m
ore regular than those recorded under chloralose. For units in the sam
ple with a measurable onset CV, 63% were regular and 44% highly regula
r under urethan, and 73% were regular and 54% highly regular under chl
oralose. The units' peristimulus time histogram (PSTH) patterns were c
lassified into subdivisions of four categories: choppers [9%: chop-sus
tained (C-s), chop-onset (C-o)]; pausers [42%: pauser-chop-sustained (
P/C-s), pauser-chop-onset( P/C-o), pauser-no-chop]; ON-sustained (43%:
primary-type, L-type, h-type); and sustained (6%). The presence of ch
opping was a reliable predictor of regularity: C-s and P/C-s units wer
e highly regular throughout their response, whereas C-o and P/C-o unit
s were highly regular at onset and became less regular. Some units in
the other PSTH categories were highly regular despite the absence of c
hopping, and units with virtually identical PSTHs showed very differen
t sustained CVs. Regularity was measured as a function of firing rate
in 71 units. Tn 23%, regularity remained constant when firing rate cha
nged with stimulus level. Forty-six percent fired more regularly as fi
ring rate increased, 8% fired less regularly, and 23% of units showed
no consistent relationship between CV and firing rate. Regularity did
not correlate with the neurons' frequency response areas or BFs. Regul
ar firing was also found in a smaller sample of units recorded in cort
ices surrounding the CNIC. We conclude that regular firing is a charac
teristic feature of most neurons in the IC. Regularity is a specific f
eature correlated with four PSTH types (C-s, C-o, P/C-s, and P/C-o). O
ther PSTH types may or may not exhibit regularity.