Spontaneous and stimulus-evoked intrinsic optical signals in primary auditory cortex of the cat

Spontaneous and tone-evoked changes in light reflectance were recorded from primary auditory cortex (A1) of anesthetized cats (barbiturate induction, ketamine maintenance). Spontaneous 0.1-Hz oscillations of reflectance of 54 0- and 690-nm light were recorded in quiet. Stimulation with tone pips evok ed localized reflectance decreases at 540 nm in 3/10 cats. The distribution of patches "activated" by tones of different frequencies reflected the kno wn tonotopic organization of auditory cortex. Stimulus-evoked reflectance c hanges at 690 nm were observed in 9/10 cats but lacked stimulus-dependent t opography. In two experiments, stimulus-evoked optical signals at 540 nm we re compared with multiunit responses to the same stimuli recorded at multip le sites. A significant correlation (P < 0.05) between magnitude of reflect ance decrease and multiunit response strength was evident in only one of fi ve stimulus conditions in each experiment. There was no significant correla tion when data were pooled across all stimulus conditions in either experim ent. In one experiment, the spatial distribution of activated patches, evid ent in records of spontaneous activity at 540 nm, was similar to that of pa tches activated by tonal stimuli. These results suggest that local cerebral blood volume changes reflect the gross tonotopic organization of A1 but ar e not restricted to the sites of spiking neurons.