OPTICAL IMAGING OF INTRINSIC SIGNALS IN CHINCHILLA AUDITORY-CORTEX

We have assessed sound frequency and intensity responses in primary au ditory cortex of the (ketamine) anesthetized chinchilla using optical imaging of intrinsic signals. Temporal cortex was exposed via a 10-mm craniotomy and a windowed chamber was mounted, A 4-second period of ga ted tones (10 ms rise/fall; 50 ms plateau; 10/s) was presented to the contralateral ear at levels between 0 and 80 dB SPL. The cortical surf ace was illuminated with 540 nm light and video images captured in 0.5 -second bins for 7.5 s (Imager 2001; Optical Imaging), Intrinsic signa ls were first apparent 0.5-1 s after stimulus onset, and were maximal after 3-4 s; they decayed over several seconds, The cortical area in w hich intrinsic activity was detected corresponded closely with electro physiologically defined AI cortex. Intrinsic signals can reliably be d etected to stimuli at 30-40 dB SPL? and in general, the area of intrin sic signal activity tends to expand with increasing stimulation level. Using stimulation levels of 80 dB SPL, we show that low-frequency sti muli (0.5-1 kHz) evoke intrinsic signals in anterior areas whilst post erior areas are activated by high-frequency stimuli (e.g. 16 kHz). Thu s a low-to high-frequency tonotopic organization is seen along this ax is.