Contributions of Intrinsic Neural and Stimulus Variance to Binaural Sensitivity

Abstract

The discrimination of a change in a stimulus is determined both by the magnitude of that change and by the variability in the neural response to the stimulus. When the stimulus is itself noisy, then the relative contributions of the neural (intrinsic) and stimulus induced variability becomes a critical question. We measured the contribution of intrinsic neural noise and interstimulus variability to the discrimination of interaural time differences (ITDs) and interaural correlation (IAC). We measured discharge rate versus characteristic frequency (CF) tone ITD functions, and CF-centered narrowband noise ITD and IAC functions in interleaved blocks in the same units in the inferior colliculus of urethaneanesthetized guinea pigs. Ten Bfrozen^tokens of noise were synthesized and the responses to each token were separately analyzed to allow the relative contributions of intrinsic and stimulus variability to be assessed. ITD and IAC discrimination thresholds were determined for a simulated two-interval forcedchoice experiment, based on the firing rate distributions, using receiver operating characteristic analysis. On average, between stimulus variability contributed 19% (range, 1.5-30%) of the variance in noise ITD discrimination and 27% (range, 3-50%) in IAC discrimination. Noise ITD thresholds were slightly higher than tone ITD thresholds. Taking the mean of the thresholds for individual noise tokens gave a similar result to pooling across all noise tokens. This implies that although the stimulus induced variability is measurable, it is insignificant in relation to the intrinsic noise in ITD and IAC discrimination.

Responses of Auditory Nerve and Anteroventral Cochlear Nucleus Fibers to Broadband and Narrowband Noise: Implications for the Sensitivity to Interaural Delays

“…Interestingly, the best ITD jnds of single IC neurons are comparable to the jnds found here (Shackleton et al 2003 , 2006 ), even though an IC neuron presumably represents only one sampling point, i.e., one internal delay value of τ . The limited analysis shown here (Fig.…”

Interaural fluctuations and the detection of interaural incoherence. IV. The effect of compression on stimulus statistics

“…For the internal variability, Shackleton and Palmer (2006) showed that although there was a measurable proportion of stimulus variability between different reproducible stimuli, it was quite small compared to the intrinsic neural variability when measuring discharge rates in inferior colliculus neurons in the guinea pig. However, as admitted by Shackleton and Palmer (2006), the population response to reproducible incoherent stimuli may be different than the single-unit responses that they measured. If internal neural variability truly dominates detection of incoherence, the result from this study that P c scores varied from guessing to perfect performance (see Figs.…”

Responses of neurons in the inferior colliculus to binaural disparities: Insights from the use of Fisher information and mutual information

“…On the other hand, a stimulus with BC = −1 is similar to the stimulus used to evoke the response at worst ITD (e.g. Shackleton and Palmer, 2006, the difference between the stimuli evoking minimal responses for the ITD and BC experiments stems from the use of narrow noise bands rather than pure tones, so that inverting the stimulus to achieve a correlation of −1 is not equivalent to any shift in time). As a result, the responses of a given neuron to ITD and to BC are expected to span approximately the same range of spike counts (as illustrated in Fig.…”

“…The fact that ITD is computed by a mechanism approximating cross-correlation in MSO causes neurons in MSO (and their targets in IC) to be sensitive to changes in BC as well (Albeck and Konishi, 1995;Coffey et al, 2006;Joris et al, 2006;Keller and Takahashi, 1996;Palmer et al, 1999;Shackleton and Palmer, 2006;Yin et al, 1987). Fig.…”