Representation of Tone in Fluctuating Maskers in the Ascending Auditory System

Abstract

Humans and animals detect low-level tones masked by slowly fluctuating noise very efficiently. A possible neuronal correlate of this phenomenon is the ability of low-level tones to suppress neuronal locking to the envelope of the fluctuating noise ("locking suppression"). Using in vivo intracellular and extracellular recordings in cats, we studied neuronal responses to combinations of fluctuating noise and tones in three successive auditory stations: inferior colliculus (IC), medial geniculate body (MGB), and primary auditory cortex (A1). We found that although the most sensitive responses in the IC were approximately isomorphic to the physical structure of the sounds, with only a small perturbation in the responses to the fluctuating noise after the addition of low-level tones, some neurons in the MGBandallA1neuronsdisplayedstrikingsuppressiveeffects.Theseneuronswerehypersensitive,showingsuppressionalreadywithtonelevels lower than the threshold of the neurons in silence. The hypersensitive locking suppression in A1 and MGB had a special timing structure, starting Ͼ75 ms after tone onset. Our findings show a qualitative change in the representation of tone in fluctuating noise along the IC-MGB-A1 axis, suggesting the gradual segregation of signal from noise and the representation of the signal as a separate perceptual object in A1.

Non-Gaussian Membrane Potential Dynamics Imply Sparse, Synchronous Activity in Auditory Cortex

“…Membrane potential dynamics in the auditory cortex of intact animals have been examined previously by several groups (De Ribaupierre et al, 1972;Volkov and Galazyuk 1992;Ojima and Murakami, 2002;Wehr and Zador, 2003;Zhang et al, 2003;DeWeese and Zador, 2004;Tan et al, 2004;Las et al, 2005), and the present data appear consistent with that body of work. In particular, examples of subthreshold activity presented in those reports typically appeared to consist of brief excursions from rest, similar to the bumps that dominate the dynamics we observed.…”

“…As expected from previous results Zhang et al, 2003;DeWeese and Zador, 2004;Tan et al, 2004;Las et al, 2005), tone-evoked responses were brief and often occurred with a short latency immediately after the tone (Figs. 2a1, 3a).…”

Representation of Sounds in Auditory Cortex of Awake Rats (Dissertation)

“…Studies conducted in auditory cortex of anesthetized animals (DeWeese and Zador, 2006;Las et al, 2005;Ojima and Murakami, 2002;Tan et al, 2004;Zador, 2003, 2005;Zhang et al, 2003) all support the general impression that subthreshold activity in auditory cortex consists mainly of brief and infrequent voltage excursions, as either stated explicitly in the text, or inferred from the data presented.…”

“…Experimental preparations for studying the auditory system at the level of single neurons include non-human primates (Beaton and Miller, 1975;deCharms et al, 1998;Durif et al, 2003;Hocherman et al, 1976;Miller et al, 1972;Populin, 2006;Ryan and Miller, 1978;Ryan et al, 1984), ferrets , guinea pigs , gerbils (Ohl and Scheich, 1997), cats (Las et al, 2005;Oatman, 1971Oatman, , 1976Tollin et al, 2005;Ulanovsky et al, 2003), bats (Suga and Ma, 2003), songbirds (Hahnloser et al, 2002;Sen et al, 2001;Theunissen and Doupe, 1998), chinchillas (Langner et al, 2002), mice (Linden et al, 2003), and others.…”

Sound frequency representation in primary auditory cortex is level tolerant for moderately loud, complex sounds

“…6, B -E.) stimulus levels for spectrotemporally dense sounds may be important for their representation, as well as for discriminating between them. Las et al (2005) showed that the neural representation of a low-level tone in a high-level fluctuating noise masker is enhanced along the afferent auditory pathway from the inferior colliculus (IC) to MGB to AI. Whereas the tonotopy of the maps does not improve from IC to AI, the increased lateral inhibition from IC to AI allows more efficient suppressing of the masker noise by the tones.…”