Interaural Time Difference Discrimination Thresholds for Single Neurons in the Inferior Colliculus of Guinea Pigs

Abstract

Sensitivity to changes in the interaural time difference (ITD) of 50 msec tones was measured in single units in the inferior colliculus of urethane-anesthetized guinea pigs. ITD functions were measured with 100 repeats and fine spacing (100 points per cycle). The just noticeable difference (jnd) for ITD was determined using receiver operating characteristic (ROC) analysis of the spike-count distribution at each ITD. The jnd became progressively smaller as the signal frequency increased from 50 to 800 Hz but became unmeasurable above 1 kHz. The lowest jnds (30 microsec) were comparable with human jnds, indicating that there is sufficient information in the firings of individual neurons to permit discrimination without obligatory pooling. ROC analysis requires the choice of a reference ITD from which the jnd may be found by stepping the target ITD through the ITD function. For each neuron the reference was chosen to minimize the jnd. The lowest jnd was usually for ipsilateral leading references, near the minimum of the ITD function where the variance was also low, but where the slope was nearing its steepest. This was despite the peak of the ITD function occurring for contralateral leading stimuli. When the reference ITD was on midline, a jnd could be obtained by looking for firing rates either greater or smaller than the firing rate at midline. The lower jnd was usually obtained by looking for a decrease in firing rate. As duration increased, jnds either decreased or increased, depending on unit type, whereas when level increased, jnds generally increased.

Contributions of Intrinsic Neural and Stimulus Variance to Binaural Sensitivity

“…We found that the best single neurons in the guinea pig inferior colliculus (IC) are capable of ITD discrimination sensitivity equivalent to humans (Skottun et al 2001;Shackleton et al 2003). However, in measurements of IAC sensitivity (Shackleton et al 2005), we found that the neural thresholds did not match the behavioral thresholds, being at least three times worse.…”

“…When a unit was isolated the characteristic frequency (CF: frequency at which lowest discharge rate threshold was obtained) and threshold were obtained audiovisually. Frequency response areas, discharge rate versus level functions (RLFs) and peristimulus response histograms (PSTHs) were obtained by using diotic pure tones to enable units to be characterized and their location in central IC verified physiologically (see Shackleton et al 2003 for details). RLFs were obtained by using binaurally uncorrelated, narrowband noise to determine the noise threshold.…”

“…Receiver operating characteristic (ROC) analysis (Cohn et al 1975;Bradley et al 1987;Green and Swets 1988;Shackleton et al 2003) was used to determine the smallest change in IAC and ITD, which the neuron could correctly indicate by a change in its firing rate. The details of, and justification for, this procedure are discussed at length by Shackleton et al (2003); however, we will give a short summary here based on Figures 2 and 3.…”

“…The details of, and justification for, this procedure are discussed at length by Shackleton et al (2003); however, we will give a short summary here based on Figures 2 and 3.…”

“…This means that if it is desired to discriminate whether a sound has moved from, say, straight ahead (zero ITD) to one side, then firing rate will only give the correct answer on a proportion of trials, and this proportion will increase as the separation increases. This qualitative argument is quantified in the neurometric function (Shackleton et al 2003), the proportion of trials in which a correct discrimination from a reference can be made as a function of ITD. In Figures 2K and 3K are shown the pooled neurometric functions for noise and tone ITDs with a reference of zero.…”