The Precedence Effect in mice, demonstrated in the inhibitory effect of reversing the order of clicks within pairs from two speakers on the startle reflex. James Ison 1, Nathaniel Housel 1, Stephanie Yee 1, Colleen Zenczak 1, Paul Allen 1, Anita Karcz 2, Conny Kopp-Scheinpflug 2 1 Department of Brain & Cognitive Sciences, University of Rochester, Rochester, NY; 2 Institute of Biology II, University of Leipzig, Leipzig, Saxony, Germany ARO Introduction The phenomenon known as the Precedence Effect (PE: Wallach et al. JASA 1949) or “The Law of the First Wavefront” (“Das Gesetz der ersten Wellenfront” Cremer, 1948, cited in Gardner, JASA 1968) describes in humans the illusory percept in which near-coincident transients presented from two locations are heard as a single image at the leading location. The PE takes different forms with increasing lead-lag delay (LLD) in humans (Litovsky et al. JASA 1999), but we are interested in the small LLD that yield “summing-” and “dominance-localization”. This has been investigated in some animals (owls, Spitzer & Takahashi, Neurophys 2006; cats, Tollin & Yin, Neurophys 2003) by observing their orienting to the first of two transients, near coincident in time but not in location; or by studying the generalization of learned responses from single spatially distinct transients to their paired presentation with varied LLD (in rats, Kelly, JASA 1974; Hoeffding & Harrison, JEAB 1979; and in budgerigars, e.g., Dent & Dooling, JASA 2003). Here we study PE in mice using prepulse inhibition (PPI) of the startle reflex (ASR), following Allen et al. (2003). We found that reversing the order of pairs of spatially separated transients yields PPI, which depends on the number of presentations of the re-ordered click pair, and on the LLD. These data show the importance of timing cues for high-frequency hearing in mice. Results: The blue line shows PPI when the ES is coincident with the first click in Pair 1, which provides a series of control conditions. The red line shows PPI when the ES was presented on Pair 10, when the order in the click pair reversed from R leading with a certain LLD to L leading with that same LLD. PPI increased with an increase in LLD over the interval of ms to 1 ms, and was different from 0 as early as.25 ms (p < 0.01) To the experimenters the location of the “buzz” moved off the midline to the right with the longer LLD, and moved across the midline to a symmetric left location following the reversal. Discussion: While these PPI data are consistent with the idea that the perceived location of the click pair is driven by the first click in mice as it is in humans, they cannot really tell us where mice localize these sounds. However, they do show that mice are sensitive to the small temporal differences in the order of acoustic transients that are critical for the PE, which may involve interaural cues related to location and/or spectral cues. Further, these data show that the “Law of the First Wavefront” must be as important for mice as it is for the other species in which the PE is found, species with better low-frequency hearing than mice: on the basis of parsimony it seems reasonable to hypothesize that this effect is accomplished by comparable neural machinery in these several vertebrate animals. Experiment 2: “Real” location changes versus exchanging the temporal order within pairs, compared across an increasing number of pairs. Research supported by NIH (AG09524 & DC05409) and the Schmitt Foundation for Integrative Brain Research Apparatus (1)Click Stimuli (2) Experiment 1: The effect of exchanging the temporal order within the click-pair (R-L to L-R) versus no exchange, and the effect of varying the time of the ES within the inter-click interval. General procedures Eight male CBA/CaJ mice, 4 months old, served in each experiment. The startle stimulus (ES) was 110 dB SPL, 20 ms in duration. The clicks were 82 dB SPL, 0.1 ms in duration. In general clicks were presented in pairs with a LLD of 1 ms (0 to 2 ms in Exp.3), with their order/location reversed prior to the ES. The ES was presented at different times after the reversal. Trials were given on average 20 sec apart, in ten blocks of randomly ordered conditions. Experiment 3: Effect of exchanging temporal order for R-L pairs with LLD varying from 0 ms to 2 ms Results: The black dotted line shows PPI with no prior order reversal, the blue line shows PPI when the ES was coincident with or immediately followed the first pair after the reversal. The solid red line shows the PPI that accompanied the 10 th pair after the reversal. The timing of the click alters ASR expression, equally in all conditions, but the overall elevation of the red line indicates that mice also detect the reversal. The experimenters heard the “buzz” of the click train as coming first from the right, then from the left following the reversal, that is, its location was determined by the first click in the pair (which defines the PE). The behavioral data indicate that mice also are sensitive to the order of the clicks. Results: Click pairs and wide band noise (WBN) were switched from one speaker to the other from 0 to 300 ms (1 to 30 pairs) prior to the ES. The blue line shows the effect of changing both clicks presented on the same side from the right (RR) to the left (LL): PPI reached a peak at about 60 ms (6 pairs) and then slowly declined. The black dotted line shows the effect of switching WBN from the right to the left speaker: PPI reached a peak in about 10 ms and gradually declined beyond the same interval of 60 ms (data from Allen et al. 2003). The solid red line shows the effect of reversing the order within the click pair, from RL to LR: PPI increased up to ms (6-10 pairs) and then was maintained for the 300 ms duration of the trial. These data show that the reversed order of clicks was more inhibitory than the “real” change in location. PPI emerged rapidly for WBN, a sign of more rapid information transmission compared to the pulsatile stimuli. That PPI is more persistent for the reversed clicks than either “real” location change suggests the hypothesis that this stimulus condition activates some form of more complex neural processing compared to a simple change in location. 1: Showing the mouse in the test cage, an overhead startle speaker and two speakers 90° apart across the midline for delivery of click pairs. An accelerometer under the cage registers the force of the ASR. The room is lined with foam panels. 2: Showing the order of clicks from right and left, and their exchange