Sensitivity to Interaural Correlation of Single Neurons in the Inferior Colliculusof Guinea Pigs

Abstract

Sensitivity to changes in the interaural correlation of 50-ms bursts of narrowband or broadband noise was measured in single neurons in the inferior colliculus of urethane-anaesthetized guinea pigs. Rate vs. interaural correlation functions (rICFs) were measured using two methods. These methods compensated in different ways for the inherent variance in interaural correlation between tokens with the same expected correlation. The shape of all rICFs could be best described by power functions allowing them to be summarized by two parameters. Most rICFs were best fit by a power below 2, indicating that they were only slightly nonlinear. However, there were a few fitted functions that had a power of 3Y6, indicating marked curvature. Modeling results indicate that the nonlinearity of the majority of rICFs was explicable in terms of the monaural transduction stages; however, some of the rICFs with power greater than 2 require either multiple inputs to the coincidence detector or additional nonlinearities to be included in the model. Discrimination thresholds were estimated at reference correlations of j1, 0, and +1 using receiver operating characteristic (ROC) analysis of the spikecount distribution at each correlation. Thresholds spanned the full possible range, from a minimum of 0.1 to the maximum possible of 2. Thresholds were generally highest with a reference correlation of j1, intermediate with a reference of 0, and lowest with a reference correlation of +1. Thresholds were lowest for the most steeply sloped rICFs, but thresholds were not strongly correlated to the spike rate variance. The lowest thresholds occurred using narrowband noise that was compensated for internal delays, but they were still about three times larger than human psychophysical thresholds measured using similar stimuli. The data suggest that, unlike pure tone interaural time difference, discrimination of a population measure is required to account for behavioral interaural correlation discrimination performance.

Psychophysical and Physiological Evidence for Fast Binaural Processing

“…In consequence, the degree of interaural correlation at these frequencies decreases with decreasing SMR. Because binaural neurons can be driven by the stimulus envelope (Batra et al, 1989) and are sensitive to interaural correlation per se (Shackleton et al, 2005), they could pick up these changes.…”

Binaural Unmasking with Bilateral Cochlear Implants

“…Although ITD sensitivity with low-frequency pure tones in NH improves as frequency is increased from 200 to 500 Hz (Durlach and Colburn 1978), the opposite trend is observed with transposed stimuli-a finding that has also been observed in the responses of neurons in the inferior colliculus of the guinea pig (Griffin et al 2005;Shackleton et al, 2005). In tests of ITD sensitivity to electric pulse trains presented to bilateral CIs, van Hoesel and Tyler (2003) found that performance was worse with pulse rates higher than 200 pps, compared to a rate of 50 pps.…”

“…Existing evidence for an upper limit on binaural processing comes both from research with CI listeners, and from experiments in which NH listeners are presented with transposed stimuli (Bernstein and Trahiotis 2002;van der Par and Kohlrausch 1997). Although ITD sensitivity with low-frequency pure tones in NH improves as frequency is increased from 200 to 500 Hz (Durlach and Colburn 1978), the opposite trend is observed with transposed stimuli-a finding that has also been observed in the responses of neurons in the inferior colliculus of the guinea pig (Griffin et al 2005;Shackleton et al, 2005).…”

“…First, CI users often have extensive periods without bilateral hearing, and this could lead to degeneration of the brainstem structures known to process decorrelation (Vollmer et al 2005). Second, NH studies have usually demonstrated sensitivity to binaural cues in the waveform fine structure (Smith et al 2002), whereas present CI processors only extract the envelope in each channel. Third, bilateral implants are usually fitted without care being taken to ensure that a given frequency region of the audio input is mapped on to electrodes innervating Bmatched^electrodes in the two cochleas.…”

Behavioral sensitivity to interaural time differences in the rabbit

“…In animals where sound localization is critical and available from the neural circuits, it can be used in that way. If not, the general neural architecture still supports adequate sensitivity to sound location, and in addition provides for increased separation of signals from noise and a sensitivity to auditory "spaciousness" through variations in the responses to interaural correlation (Yin et al, 1987;Shackleton et al, 2005;Coffey et al, 2006) In humans using broadband stimuli, there is a gradual increase in thresholds within the physiological range as the sound becomes increasingly lateralized towards the ear. Measurements of this increase range from about a factor of two with clicks (Hafter et al, 1975) up to a factor of about four with noise (Klumpp and Eady, 1957;Mossop and Culling, 1998).…”