However, it must be noted that the CF and BF values are only within Ϯ 0.5 octave and would not necessarily be the same with a smaller- frequency step size. Note that the characteristic frequency (CF ϭ 15.6 kHz) is the same across 9 days although the threshold intensity varied between 10 and 20 dB. Figure 3 presents LFPs at threshold intensities (0–20 dB) across 9 days for another subject. Tuning can also be very similar at threshold. Note that the response range is similar (3.9–15.6 kHz) and BF10 (maximum response) is the same (7.80 kHz) across more than two weeks. Figure 2 presents examples of LFPs at 20 dB, 10 dB above threshold, from 5 of 11 recording sessions for one subject across 16 days. LFP tuning can be highly similar at suprathreshold levels across days. Significant decreasing trends were obtained for CF, BF10, and BF30 for this group and for BF30 in the half-octave group (Page test, p Ͻ 0.05) (Siegel and Castellan 1988). For example, average CF amplitudes decreased from 126 to 97 V and BF 30 decreased from 228 to 185 V over 12 days for the quarter-octave group. The absolute LFP amplitude generally decreased over days (see Figs. Note the tuning, with CF, BF10, and BF30 at 7.78 kHz, and increasing bandwidth as stimulus level increases. Figure 1B shows an example of LFPs across frequency (0.97–30.0 kHz) and stimulus level ( Ϫ 10 to 80 dB). The negative wave (“N1”) exhibits systematic frequency tuning. Figure 1A presents an example of an average LFP. The LFPs recorded from infragranular layers ( ϳ 1.1 mm depth) have a typical form consisting of a promi- nent negative component with a latency to peak of ϳ 15–20 ms, followed by a more variable positive peak at ϳ 30–40 ms. Therefore, while the recordings were obtained from tonotopically orga- nized (primary) cortical fields, we do not further consider the presumptive locations of any recording sites within this combined area. However, in the absence of detailed tuning information, we do not consider field locations to be sufficiently precise to warrant definite conclusions. The majority (16/23) of recording sites appeared to be in the anterior field, while four were indeterminate and three were in the posterior field. For both half-octave and quarter-octave groups, presumptive fields were assigned either as anterior, posterior, or high-frequency indeterminate. However, because the high- frequency regions of the two fields are adjacent to each other, one cannot draw any conclusion about the field location of electrodes exhibiting high frequency (e.g., Ͼ 20.0 kHz) tuning because there is no way to know when the border had been crossed in this experiment. For example, a subject whose most anterior electrode exhibited an average BF of 3.03 kHz and whose next electrode had a BF of 6.48 kHz was considered to have both recording sites in the anterior field. This approach was reinforced in those subjects having more than one acceptable tuned recording site. Complementary logic holds for the most posterior electrodes being tuned to low frequencies, indicative of posterior field placement. Thus, if the most anterior electrode exhibited low-frequency tuning, it may be considered to be in the anterior field because of the low-to-high-frequency, anterior–posterior organization. to be in the anterior or the posterior mirror-image field because implantation of the electrode array was guided by a prior general mapping of the two fields.
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