Maps of multiple topographic areas in rat auditory cortex. (a) Polar map (hue encodes response phase and lightness encodes response magnitude) of absolute tonotopy. The hemodynamic delay was removed by the method of the stimulus reversal. (b) Surface vessel pattern from the same animal. (c) The same map as in a plotted as a phase map with overlaid vessel pattern from b. The contours outline identified auditory fields. The dotted lines, drawn at ≈16 kHz, outline the high-frequency region. The large uniform region (appearing as green) has a fine spectral structure that is emphasized on the map of double phase (Fig. 5g). (d) Frequency representation of the rat auditory cortex constructed by using standard electrophysiological mapping technique. A, anterior; D, dorsal; P, posterior; V, ventral; VPAF, ventral posterior auditory field. (Scale bar, 500 μm.) [Image in d reproduced with permission from ref. 2 (Copyright 1988, The Japan Academy, Tokyo).]

Maps of multiple tonotopic areas in rat auditory cortex and comparison with electrophysiology in the same subject. (a) Optical phase map of tonotopy, noncyclically color coded (same subject as in Figs. 1 and 5). Symbols mark sites of microelectrode recordings: white crosses, no response; white circles, CF >40 dB SPL (intensity of stimulation for optical imaging); black crosses, CF >30 dB and <40 dB SPL; black circles, CF <30 dB SPL. Tuning curves from sites labeled as a, b, c, and d are shown in Fig. 3. (b) Map of optical response amplitude. (c) Map of CFs obtained by standard electrophysiological techniques (125 multiunit sites) in the same subject. (d) Correlation between optical imaging BFs and electrophysiological CFs. n = 120 (five sites elicited no response), slope, 1.30; intercept, -1.00 octave; R² = 0.91; RMS deviation = 0.60 octave; CF below 40 dB SPL: n = 110; slope, 1.26; intercept, -0.88 octave; R² = 0.93; RMS, 0.52 octave; CF below 30 dB SPL: n = 67; slope, 1.06; intercept, -0.31octave; R² = 0.89; RMS, 0.46 octave. Note that central regions of A1, AAF, and VAAF (as outlined in Fig. 1c) appear as islands surrounded by regions of weaker response. (e) Map of CF thresholds obtained by standard electrophysiological techniques. (f) Correlation between amplitude of optical response and electrophysiological CF thresholds. To calculate the average response, shown in red, the range of thresholds was divided by horizontal dashed lines (5-dB spacing), and both optical response and electrophysiological threshold for points inside these domains were averaged. The amplitude of optical response is a nonmonotonic function of electrophysiological CF thresholds. FC, fractional change; R, rostral; D, dorsal. (Scale bar, 500 μm.)

Comparison of optical BFs and electrophysiological measures. (a–d) (Upper) Tuning curves acquired at the sites marked as a, b, c, and d in Fig. 2a. The black cross labels CF. Horizontal line, intensity of stimulation for optical imaging; vertical solid line, optical BF; legend (shown in green), correspondence between sizes of ellipses and number of spikes. (Lower) Profiles of spike distributions at the intensity of stimulation. The cosine curve is the fundamental Fourier component fitted into the spike distribution between 2 and 32 kHz. The five short vertical lines mark the measures of RFs, same color code as in e. Long green vertical lines show lower (F_lo) and upper (F_up) frequencies used to define BF. Horizontal line, level of spontaneous activity. (e) RMS deviation of the optical BFs from the electrophysiological measures as a function of optical response threshold and optical BFs from ascending staircase map. Solid red curve, CFs with threshold below stimulation for optical imaging (40 dB SPL, n = 110); dashed red curve, all CFs (n = 120). Vertical dashed line, optical noise level. (f) Correlation between optical BFs from averaged map and electrophysiological Fourier frequencies. (g) Ascending phase map with overlaid vessel pattern used for random penetration simulations. (h) Probability density of RMS deviation of random penetrations for two values of optical response threshold: 30% (solid lines) and 60% (dashed lines). Long vertical lines, lower 95% confidence interval; short lines, RMS deviation for five electrophysiological measures.

Stability of multiple fields. Optical polar and magnitude maps, respectively, from five rats [ages 25, 50, 55 (male), 60, and 62 days] are shown. The contours were drawn on Fig. 1c, then transferred without any distortion to these figures, then translated and rotated until a match was achieved. The fields identified on Fig. 1c for P75 rat almost perfectly match the optically defined fields for all subjects. “?” labels possible additional auditory field. VPAF, ventral posterior auditory field.