REGULARITY OF FIRING OF NEURONS IN THE INFERIOR COLLICULUS

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.