Minimum Audible Angle Measured in Young and Old CBA Mice Using Prepulse Inhibition of Startle

Paul D. Allen, Jordan Bell, Navin Dargani, Catherine A. Moore, Carolyn M. Tyler, James R. Ison Department of Brain & Cognitive Sciences, University of Rochester, Rochester, NY

ARO 2003

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DiscussionHere we demonstrate that swapping the source of continuous noise between two speakers causes prepulse inhibition of startle in CBA mice, and since inhibition scales with angular separation, the technique can provide behavioral measures of minimum audible angle (MAA).

These data indicate that in the 3 month old CBA mouse MAA at 0 degree azimuth is between 7 and 15°, but this resolution requires 60 to 100ms of processing, with shorter ISIs yielding coarser resolution.

These results agree with previous behavioral estimates of MAA in the mouse, and the importance of sufficient ISI for inhibition to develop maximum sensitivity is analogous to the minimum integration time required for optimal MAA in human psychoacoustics.

The rapid onset of inhibition when angular separation is large, but not when it is small, indicates that monaural cues at each ear are sufficient to alert the auditory system to a change in the acoustic environment, while the longer ISI required for inhibition when angular separation and hence monaural difference cues are reduced, suggests that binaural processing of inputs to the two ears is needed to detect the change in sound source location.

The 3rd experiment using octave bands of noise shows that low frequency carriers are ineffective in producing inhibition, suggesting that ILDs provide the cues for spatial discrimination. This result is in accord with the mouse having a well developed LSO and atrophied MSO.

Alternatively, over this same frequency range the mouse pinna produces spectral notches, which might serve as a separate cue for change in spatial location, mediated by DCN processing. This could be tested using narrow band or pure tone stimuli.

There is a systematic increase with age in the ISI needed to obtain a particular level of auditory spatial resolution, and the magnitude of the inhibition produced at a given ISI and angle decline with age, particularly from 12 to 24 months of age.

This finding suggests an ageing effect which could worsen presbycusis by diminishing binaural unmasking - the aging localization system gradually ceases to provide sufficient spatial resolution in the time available for unmasking to be effective.

• Subjects: CBA/CaJ mice 3, 6, 12, 24 months old

• Mice held in acoustically transparent wire cage mounted to a platform and accelerometer

• Startle eliciting stimulus (ES) delivered overhead (120dB SPL, 15 ms broadband noise burst)

• Startle response recorded from accelerometer (RMS 100ms window, post-ES)

• Spectrally-matched high-frequency speakers (TDT ES1) located 45cm from the mouse head

• Angular separations: 180, 90, 45, 22.5, or 15° (7° in Experiments 2 and 3)

• Cage oriented with head facing mid-line of two speakers

• Continuous broadband noise (60 dB SPL, 1-50kHz) presented from one speaker (min 15s)

• Prepulse is noise-swap between speakers at set inter-stimulus interval (ISI) prior to ES

• ISI conditions: 1, 2, 5, 10, 20, 30, 40, 50, 60, 100, 150, 200, 300 ms, two no-prepulse controls, and a no-startle, no-swap activity control

• Speaker angle is fixed in each session (counterbalanced design) with 11 presentations of each condition, block randomized

• Inhibition calculated relative to no-prepulse controls

Auditory spatial acuity supports sound localization and is used to improve communication in noisy environments via unmasking of spatially separable sound sources

The CBA mouse is a successful animal model of presbycusis, displaying changes in auditory temporal processing with age analogous to those observed in humans, which have been linked to deficits in speech perception – especially in noisy backgrounds

In this study we wanted to find out if parallel changes in auditory spatial acuity occur with age in the CBA mouse, using a novel application of prepulse inhibition of acoustic startle

•Startle response amplitude is modulated by prior perturbation of the acoustic environment (inhibition)

•Does a change in spatial location of a continuous sound source cause prepulse inhibition?

•Can we estimate a minimum audible angle for the mouse in this way?

Previous behavioural measures of spatial acuity in the mouse

•7-15° in 2 month old house mouse (Ehret & Dreyer, 1984)

•19° in the 2 month old C57 mouse (Heffner, Koay, & Heffner, 2001)

How does detection of a change in spatial location depend on stimulus duration?

How do these parameters change with age?

What are the neural bases for age-related changes in spatial acuity in the mouse?

Experiment 1: Broad Band Noise Speaker Swap Experiment 2: Age Effect on Speaker Swap Experiment 3: Frequency Effect on Speaker SwapQuestion 1:

Is 180° speaker swap inhibitory?

• 180° speaker swap was compared with simple noise offset and onset (i.e. S1 or S2 alone)

• 3 month old mice

Off (N=7), On (N=6), Swap (N=13)

• Inhibition starts rapidly after noise Offset: at 1ms it is marginally significant, but highly significant from 2 to 300 ms

• Onset shows marginally significant facilitation out to 5 ms, then highly significant inhibition from 10 to 200 ms

• 180° speaker swap is significant from 5 to 300 ms, with a hint of facilitation at 1ms

• 180° speaker swap has inhibitory effect intermediate to noise offset and onset (dashed grey curve shows simple average of on- and offset data)

• The sum of monaural cues?

•Intervals with non-linear averaging may reflect binaural processing

Question 2:

Given that 180° speaker swap is inhibitory, how does inhibition depend on the angular separation of the speakers?

• Bringing speakers together reduces inhibitory effect of swap and delays ISI functions

• Smaller angles have longer ISI to peak inhibition and maximal inhibition is reduced

• Longer ISI required for significant inhibition

• Window of optimal inhibition is reduced at smaller angles: 60-100ms at 15°

• At a given ISI the effect size increases with the angular separation

• 5ms only 180° provides significant inhibition

• At 10ms, 22.5 ° is marginally significant

• At 50ms, 15° is marginally significant

• Minimum Audible Angle depends on ISI: optimum duration suggests MAA <15°

• 3 age groups:

6 month old (N=12)

12 month old (N=12)

24 month old (N=18)

• Same design as Experiment 1, except minimum angle is 7°

• None of the groups show significant inhibition for the 7° swap

• Compared with 3 month old data, ISI functions shift progressively later with age

• Size of inhibitory effect is reduced with age even when it is reliably generated by large angular separations

• Duration of optimum inhibition is reduced as longer ISI is required in old mice for inhibition, but inhibition does not persist longer

• 6 month old mice (N=12)

• 70dB octave band carriers with 2ms on/offset shaping and 50dB broadband floor to mask transients

• Swap of lower octaves (2-4 and 4-8 kHz) does not provide inhibition

• 8-16 kHz does provide inhibition, but only at large angles

• 16-32 has inhibition at this ISI that accounts for most of the response seen to swap of broadband noise, but the effect of frequency band on ISI is unclear

• Masker noise and shaping of stimuli might also reduce inhibition, and account for the difference between broadband and 16-32 kHz

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Supported by NIA Grant #AG09524 and The Schmitt Program on Integrative Brain Research