call ranges from 40 kHz up to 90 kHz in frequency - higher than the call ofA. p. pallidus.
P. discolor travels several miles from its sleeping quarters to its hunting grounds each night using trees as landmarks for orientation (see Chapter 2). Here, sensitivity to ITDs and IIDs is a prerequisite for evaluating the source of echoes that are used as orientation cues. The IRs of trees are complex and highly aperiodic and, depending on the foliage and shape of the tree, produce a high envelope fluctuation (see the general Introduction and Chapter 2). This envelope fluctuation gives envelope ITD cues to the listener (see Section 4.1.3.1, p. 73). Electrophysiological studies showed that neurons in the IC and AC ofP. discolor can encode echo roughness. Considering these ecological and neuro-physiological facts, we expect to find neuronal sensitivity to envelope ITDs arising from high envelope fluctuations in the IC of P. discolor. Previous studies showed a correla-tion between the temporal precision of a neuron and its roughness preference - most neurons with high temporal precision preferred stimuli with high roughness whereas most neurons with low temporal precision preferred stimuli with low roughness. In this study, we investigated neuronal sensitivity to envelope ITDs arising from high en-velope fluctuation and address the question wether temporal precision and roughness preference of neurons correlate with their sensitivity to envelope ITDs.
4.2 Material and Methods
4.2.1 Animals
2 specimens ofP. discolor, one male and one female, were used for the experiments. Their respective body weights at the beginning of the experiments were 36 and 38 gram.
4.2.2 Surgical procedure
The surgical procedure and anaesthetics were the same as described in Section 2.3.2, p. 30. All experiments were conducted under the principles of laboratory animal care and the regulations of the current version of the German Law on Animal Protection (approval 209.1/211-2531-68/03, Reg. Oberbayern).
4.2.3 Stimulus production
The experimental setup used for stimulus generation, stimulus presentation and data collection was identical to the setup described in Section 2.3.
4. Envelope ITD sensitivity for complex echoes
4.2.3.1 Envelope ITD sensitivity
The stimuli used for testing ITD sensitivity in IC units consisted of a set of 20 stimuli selected from the original 80 stimuli used in previous experiments (Chapters 2 and 3) for testing roughness sensitivity. They were divided into four groups of five stimuli each. All the stimuli from one group had the same roughness. The different rough-nesses were: 1.75, 2.25, 2.75 or 3.25 log10M4, roughness is stated as the roughness of the IR. Stimuli with different ITDs were generated by varying the stimulus onset on one ear while keeping the stimulus onset on the other ear constant. ITDs with the contralat-eral stimulus leading the ipsilatcontralat-eral stimulus were defined as negative, ITDs with the contralateral stimulus lagging behind the ipsilateral stimulus were defined as positive.
Tested ITDs varied during the experiments, but typically ranged from - 300 to + 300 µs. The range of tested ITDs always overstretched the behaviourally relevant ITD range ofP. discolor(ca. ±90 µs). The number of tested ITDs varied during experiments, but typically 11 different ITDs were tested. Stimuli were presented with an intensity of 10 dB up to 30 dB above Th and 10 repetitions each with a repetition rate of 4 Hz.
4.2.3.2 IID sensitivity
For testing IID sensitivity, pure tones at the unit’s BF with different IID combinations were used. Intensities on both ears were centered at 20 dB above the unit’s Th and ranged from -30 dB to +30 dB above or below the center intensity in 6 dB steps, result-ing in 11 tested intensities per ear. By combinresult-ing each ipsilateral intensity with every contralateral intensity, a set of 121 stimuli was generated. Stimuli were presented on both ears with a duration of 20 ms, a rise/fall time of 2 ms, five repetitions and a repe-tition rate of 6.7 Hz.
4.2.4 Recording of neural responses and reconstruction of recoding sites
Material and methods used for recording and reconstruction are identical to those used in the previous experiment and are described in Section 2.3.4, p. 32.
4.2.4.1 Unit characterization
The stimuli used for recording the FRA and the PSTH of a unit are identical to those described in Section 2.3.4.1, p. 33.
4.2 Material and Methods
4.2.5 Data analysis
4.2.5.1 Analysis of echo roughness sensitivity
The responses to the ITD test stimuli were evaluated to test the sensitivity to echo rough-ness. Spike counts were averaged over all repetitions and the median number of spikes per stimulus for each echo-roughness was calculated. Medians from the four groups of roughnesses were tested for statistically significant differences with a Kruskal-Wallis Test with a correction for multiple testing (Matlab Statistics Toolbox, Mathworks) for every single ITD (cf. Fig. 4.3, p. 80). Significance was set at p<0.05. A unit was classi-fied as rough-preferring if the response strength significantly increased with increasing roughness for at least two ITDs, classified as smooth-preferring if the response strength significantly decreased with increasing roughness for at least two ITDs and classified as band-preferring if the response strength significantly increased and decreased again with increasing roughness for at least two ITDs. If response strength did not change significantly, the unit was classified as insensitive to echo roughness. If a unit showed different response strength changes for different tested ITDs or the maximum median response strength was below 0.5 spikes per roughness group, the unit was termed as not classifiable.
4.2.5.2 Analysis of envelope ITD sensitivity
The response spike count was averaged over all repetitions. The median response spike count to one ITD was calculated for each echo roughness. Median responses from all ITDs for every single roughness were tested for significance using a Kruskal-Wallis Test with a correction for multiple testing (cf. Fig. 4.3, p. 80). Significance was set at p <
0.05. If the unit‘s response strength increased for negative ITDs, the unit was classified as preferring contra leading (PLC). If the unit‘s response strength increased for positive ITDs, the unit was classified as preferring ipsi leading (PLI). If the response strength did not change significantly, the unit was classified as insensitive to ITDs. If a unit showed an opposite ITD preference for the range of tested echo roughnesses or the unit‘s response strength decreased for a range of ITDs and increased again for others, the unit’s ITD preference was classified as special. Units with a maximum median response strength below 0.5 spikes per stimulus were excluded from the analysis.
4. Envelope ITD sensitivity for complex echoes
Figure 4.3:Analysis of ITD functions and roughness sensitivity. The upper panel shows the response of a unit to combinations of different ITDs and echo roughnesses (”ITD map”). The color bar on the right shows response strength coding. Maximum response strength isred, no response isblue.ITD sensitivity of this unit has been tested for every single roughness (y-axis) over the range of tested ITDs (x-axis). ITD sensitivity for an echo roughness of 2.75 log10M4 is marked with a white box on the ITD map, which is shown in more detail in panel A. Here, the median response strength (y-axis) for different ITDs (x-axis) has been tested for significance. Horizontal bars and asterisks mark significant differences (Kruskal-Wallis-Test, p<0.05). This test was applied for every echo roughness. See figures 4.7, p. 87, 4.8, p. 88 or 4.9, p. 89 and section 4.2.5.2, p. 79 for detailed analyses. The response strength for the unit shown in this figure increases for a leading contralateral stimulus. The unit’sroughness preferencewas classified by testing the response strength for every ITD over the range of tested echo roughnesses. The roughness preference for an ITD of -240µs is marked with a black box on the ITD map, which is shown in more detail in panel B. Here, the median response strength (y-axis) for different echo roughnesses (x-axis) was tested for significance. Horizontal bars and asterisks mark significant differences (Kruskal-Wallis-Test, p<0.05). For an ITD of -240µs, the response strength increases significantly with increasing echo roughness. A detailed analysis of roughness preference is described in Section 2.3.5.1, p. 33. Log10M4 = Base-ten logarithm of the 4th moment; BF=Best frequency; Th = Threshold