Leveraging Quality Improvement Science to Reduce C. difficile Infections in a Community Hospital.

BACKGROUND: The most common infection acquired in US hospitals is Clostridium difficile, which can lead to protracted diarrhea, severe abdominal cramping, and infectious colitis and an attributable mortality of 6.5%. The mortality associated with C. difficile is of major clinical importance. The best strategy to prevent such infections is an open question. METHODS: A multiyear quality improvement initiative was performed in our community hospital to determine where hospitals should focus their resources to achieve sustainable reductions in hospital-acquired C. difficile infection (CDI). Quality improvement methodology was used to evaluate the impact of sequential interventions in environmental cleaning, infection prevention, and antibiotic stewardship over time. RESULTS: After four years, hospital-acquired CDI declined 55.5%, from 12.2 to 5.4 cases/10,000 patient-days (Poisson rate test, p = 0.002). High-risk antibiotic use declined 88.1%, from 63.7 to 7.6 days on treatment/1,000 patient-days (Student's t-test, p < 0.001). The highest-impact intervention was stewardship on diagnostics and high-risk antibiotics using home-grown decision support tools. CONCLUSION: Translating scientific evidence into clinical practice using quality improvement methods led to sustained reductions in C. difficile transmission and identified high-risk antibiotics and diagnostics as key leverage points.

Multiple time scales of the ventriloquism aftereffect.

The ventriloquism aftereffect (VAE) refers to a shift in auditory spatial perception following exposure to a spatial disparity between auditory and visual stimuli. The VAE has been previously measured on two distinct time scales. Hundreds or thousands of exposures to a an audio-visual spatial disparity produces enduring VAE that persists after exposure ceases. Exposure to a single audio-visual spatial disparity produces immediate VAE that decays over seconds. To determine if these phenomena are two extremes of a continuum or represent distinct processes, we conducted an experiment with normal hearing listeners that measured VAE in response to a repeated, constant audio-visual disparity sequence, both immediately after exposure to each audio-visual disparity and after the end of the sequence. In each experimental session, subjects were exposed to sequences of auditory and visual targets that were constantly offset by +8° or -8° in azimuth from one another, then localized auditory targets presented in isolation following each sequence. Eye position was controlled throughout the experiment, to avoid the effects of gaze on auditory localization. In contrast to other studies that did not control eye position, we found both a large shift in auditory perception that decayed rapidly after each AV disparity exposure, along with a gradual shift in auditory perception that grew over time and persisted after exposure to the AV disparity ceased. We modeled the temporal and spatial properties of the measured auditory shifts using grey box nonlinear system identification, and found that two models could explain the data equally well. In the power model, the temporal decay of the ventriloquism aftereffect was modeled with a power law relationship. This causes an initial rapid drop in auditory shift, followed by a long tail which accumulates with repeated exposure to audio-visual disparity. In the double exponential model, two separate processes were required to explain the data, one which accumulated and decayed exponentially and the other which slowly integrated over time. Both models fit the data best when the spatial spread of the ventriloquism aftereffect was limited to a window around the location of the audio-visual disparity. We directly compare the predictions made by each model, and suggest additional measurements that could help distinguish which model best describes the mechanisms underlying the VAE.

Accumulation and decay of visual capture and the ventriloquism aftereffect caused by brief audio-visual disparities.

Visual capture and the ventriloquism aftereffect resolve spatial disparities of incongruent auditory visual (AV) objects by shifting auditory spatial perception to align with vision. Here, we demonstrated the distinct temporal characteristics of visual capture and the ventriloquism aftereffect in response to brief AV disparities. In a set of experiments, subjects localized either the auditory component of AV targets (A within AV) or a second sound presented at varying delays (1-20 s) after AV exposure (A2 after AV). AV targets were trains of brief presentations (1 or 20), covering a ±30° azimuthal range, and with ±8° (R or L) disparity. We found that the magnitude of visual capture generally reached its peak within a single AV pair and did not dissipate with time, while the ventriloquism aftereffect accumulated with repetitions of AV pairs and dissipated with time. Additionally, the magnitude of the auditory shift induced by each phenomenon was uncorrelated across listeners and visual capture was unaffected by subsequent auditory targets, indicating that visual capture and the ventriloquism aftereffect are separate mechanisms with distinct effects on auditory spatial perception. Our results indicate that visual capture is a 'sample-and-hold' process that binds related objects and stores the combined percept in memory, whereas the ventriloquism aftereffect is a 'leaky integrator' process that accumulates with experience and decays with time to compensate for cross-modal disparities.

Comparison of congruence judgment and auditory localization tasks for assessing the spatial limits of visual capture.

Vision typically has better spatial accuracy and precision than audition and as a result often captures auditory spatial perception when visual and auditory cues are presented together. One determinant of visual capture is the amount of spatial disparity between auditory and visual cues: when disparity is small, visual capture is likely to occur, and when disparity is large, visual capture is unlikely. Previous experiments have used two methods to probe how visual capture varies with spatial disparity. First, congruence judgment assesses perceived unity between cues by having subjects report whether or not auditory and visual targets came from the same location. Second, auditory localization assesses the graded influence of vision on auditory spatial perception by having subjects point to the remembered location of an auditory target presented with a visual target. Previous research has shown that when both tasks are performed concurrently they produce similar measures of visual capture, but this may not hold when tasks are performed independently. Here, subjects alternated between tasks independently across three sessions. A Bayesian inference model of visual capture was used to estimate perceptual parameters for each session, which were compared across tasks. Results demonstrated that the range of audiovisual disparities over which visual capture was likely to occur was narrower in auditory localization than in congruence judgment, which the model indicates was caused by subjects adjusting their prior expectation that targets originated from the same location in a task-dependent manner.

Influence of age, spatial memory, and ocular fixation on localization of auditory, visual, and bimodal targets by human subjects.

A common complaint of the elderly is difficulty identifying and localizing auditory and visual sources, particularly in competing background noise. Spatial errors in the elderly may pose challenges and even threats to self and others during everyday activities, such as localizing sounds in a crowded room or driving in traffic. In this study, we investigated the influence of aging, spatial memory, and ocular fixation on the localization of auditory, visual, and combined auditory-visual (bimodal) targets. Head-restrained young and elderly subjects localized targets in a dark, echo-attenuated room using a manual laser pointer. Localization accuracy and precision (repeatability) were quantified for both ongoing and transient (remembered) targets at response delays up to 10 s. Because eye movements bias auditory spatial perception, localization was assessed under target fixation (eyes free, pointer guided by foveal vision) and central fixation (eyes fixed straight ahead, pointer guided by peripheral vision) conditions. Spatial localization across the frontal field in young adults demonstrated (1) horizontal overshoot and vertical undershoot for ongoing auditory targets under target fixation conditions, but near-ideal horizontal localization with central fixation; (2) accurate and precise localization of ongoing visual targets guided by foveal vision under target fixation that degraded when guided by peripheral vision during central fixation; (3) overestimation in horizontal central space (±10°) of remembered auditory, visual, and bimodal targets with increasing response delay. In comparison with young adults, elderly subjects showed (1) worse precision in most paradigms, especially when localizing with peripheral vision under central fixation; (2) greatly impaired vertical localization of auditory and bimodal targets; (3) increased horizontal overshoot in the central field for remembered visual and bimodal targets across response delays; (4) greater vulnerability to visual bias with bimodal stimuli. Results highlight age-, memory-, and modality-dependent deterioration in the processing of auditory and visual space, as well as an age-related increase in the dominance of vision when localizing bimodal sources.

Active behavioral coping alters the behavioral but not the endocrine response to stress.

Exposure to traumatic stressors typically causes lasting changes in emotionality and behavior. However, coping strategies have been shown to prevent and alleviate many stress consequences and the biological mechanisms that underlie coping are of great interest. Whereas the laboratory stressor inescapable tail-shock induces anxiety-like behaviors, here we demonstrate that permitting a rat to chew on a wooden dowel during administration of tail-shock prevented the development of anxiety like behaviors in the open field and juvenile social exploration tests. Uncontrollable stressors increase corticosterone and decrease thyroid hormone, and we hypothesized that coping would blunt these changes. While tail-shock did produce these effects, active coping did not alter hormone levels. The dissociation between behavioral resilience and circulating hormones is discussed with regard to the utility of these molecules as biomarkers for psychiatric disease.

Influence of aging on human sound localization.

Errors in sound localization, associated with age-related changes in peripheral and central auditory function, can pose threats to self and others in a commonly encountered environment such as a busy traffic intersection. This study aimed to quantify the accuracy and precision (repeatability) of free-field human sound localization as a function of advancing age. Head-fixed young, middle-aged, and elderly listeners localized band-passed targets using visually guided manual laser pointing in a darkened room. Targets were presented in the frontal field by a robotically controlled loudspeaker assembly hidden behind a screen. Broadband targets (0.1-20 kHz) activated all auditory spatial channels, whereas low-pass and high-pass targets selectively isolated interaural time and intensity difference cues (ITDs and IIDs) for azimuth and high-frequency spectral cues for elevation. In addition, to assess the upper frequency limit of ITD utilization across age groups more thoroughly, narrowband targets were presented at 250-Hz intervals from 250 Hz up to ∼ 2 kHz. Young subjects generally showed horizontal overestimation (overshoot) and vertical underestimation (undershoot) of auditory target location, and this effect varied with frequency band. Accuracy and/or precision worsened in older individuals for broadband, high-pass, and low-pass targets, reflective of peripheral but also central auditory aging. In addition, compared with young adults, middle-aged, and elderly listeners showed pronounced horizontal localization deficiencies (imprecision) for narrowband targets within 1,250-1,575 Hz, congruent with age-related central decline in auditory temporal processing. Findings underscore the distinct neural processing of the auditory spatial cues in sound localization and their selective deterioration with advancing age.

Advancing age alters the influence of eye position on sound localization.

Vision and audition provide spatial information about the environment to guide natural behavior. Because the eyes move in the head while the ears remain head-fixed, input conveying eye position in the head is required to maintain audiovisual congruence. Human perception of auditory space was previously shown to shift with changes in eye position, regardless of the target's frequency content and spatial cues underlying horizontal and vertical localization. In this study, we examined whether this interaction is altered by advancing age. Head-restrained young (18-44 yo), middle-aged (45-64 yo), and elderly (65-81 yo) human subjects localized noise bursts under conditions of transient and sustained ocular deflection. All three age groups demonstrated a time-dependent shift of auditory space in the direction of eye position. Moreover, this adaptation showed a clear decline with advancing age, but only for peripheral auditory space (beyond ±10° from midline). Alternatively, adaptation in the periphery may occur, but is more sluggish than in the central field and therefore not fully observed in this experiment. The age-dependent effect cannot be readily explained by senescent peripheral hearing loss, suggesting a change in central processing of auditory space in relation to the control of gaze.

Patterned tone sequences reveal non-linear interactions in auditory spectrotemporal receptive fields in the inferior colliculus.

Linear measures of auditory receptive fields do not always fully account for a neuron's response to spectrotemporally-complex signals such as frequency-modulated sweeps (FM) and communication sounds. A possible source of this discrepancy is cross-frequency interactions, common response properties which may be missed by linear receptive fields but captured using two-tone masking. Using a patterned tonal sequence that included a balanced set of all possible tone-to-tone transitions, we have here combined the spectrotemporal receptive field with two-tone masking to measure spectrotemporal response maps (STRM). Recording from single units in the mustached bat inferior colliculus, we found significant non-linear interactions between sequential tones in all sampled units. In particular, tone-pair STRMs revealed three common features not visible in linear single-tone STRMs: 1) two-tone facilitative interactions, 2) frequency-specific suppression, and 3) post-stimulatory suppression in the absence of spiking. We also found a correlative relationship between these nonlinear receptive field features and sensitivity for different rates and directions of FM sweeps, dynamic features found in many vocalizations, including speech. The overwhelming prevalence of cross-frequency interactions revealed by this technique provides further evidence of the central auditory system's role as a pattern-detector, and underscores the need to include nonlinearity in measures of the receptive field.

Perception of auditory, visual, and egocentric spatial alignment adapts differently to changes in eye position.

Vision and audition represent the outside world in spatial synergy that is crucial for guiding natural activities. Input conveying eye-in-head position is needed to maintain spatial congruence because the eyes move in the head while the ears remain head-fixed. Recently, we reported that the human perception of auditory space shifts with changes in eye position. In this study, we examined whether this phenomenon is 1) dependent on a visual fixation reference, 2) selective for frequency bands (high-pass and low-pass noise) related to specific auditory spatial channels, 3) matched by a shift in the perceived straight-ahead (PSA), and 4) accompanied by a spatial shift for visual and/or bimodal (visual and auditory) targets. Subjects were tested in a dark echo-attenuated chamber with their heads fixed facing a cylindrical screen, behind which a mobile speaker/LED presented targets across the frontal field. Subjects fixated alternating reference spots (0, +/-20 degrees ) horizontally or vertically while either localizing targets or indicating PSA using a laser pointer. Results showed that the spatial shift induced by ocular eccentricity is 1) preserved for auditory targets without a visual fixation reference, 2) generalized for all frequency bands, and thus all auditory spatial channels, 3) paralleled by a shift in PSA, and 4) restricted to auditory space. Findings are consistent with a set-point control strategy by which eye position governs multimodal spatial alignment. The phenomenon is robust for auditory space and egocentric perception, and highlights the importance of controlling for eye position in the examination of spatial perception and behavior.

Eye position and cross-sensory learning both contribute to prism adaptation of auditory space.

Optical prisms shift visual space, and through adaptation over time, generate a compensatory realignment of sensory-motor reference frames. In humans, prism-induced lateral shifts of visual space produce a corresponding shift in sound localization. We recently reported that sound localization shifts towards eccentric eye position, approaching approximately 40% of gaze over several minutes. Given that eye position affects sound localization directly, prism adaptation may well reflect contributions of both eye position and sensory adaptation; while the visual world is shifted by the prisms, the eyes must also shift simply to gaze ahead. To test this new concept of prism adaptation, 10 young (18-27 year) adults localized sound targets before and after 4 h of adaptation to base-right or base-left prisms that induced an 11.4 degrees shift left or right, respectively. In separate sessions subjects were exposed to: (1) natural binaural hearing; (2) diotically presented inputs devoid of meaningful spatial cues; or (3) attenuated hearing to simulate hearing loss. These preliminary results suggest that the prism adaptation of auditory space is dependent on two independent influences: (1) the effect of displaced mean eye position induced by the prisms, which occurs without cross-sensory experience; and (2) true cross-sensory learning in response to an imposed offset between auditory and visual space.

Age-related hearing loss in C57BL/6J mice has both frequency-specific and non-frequency-specific components that produce a hyperacusis-like exaggeration of the acoustic startle reflex.

Auditory brainstem-evoked response (ABR) thresholds were obtained in a longitudinal study of C57BL/6J mice between 10 and 53 weeks old, with repeated testing every 2 weeks. On alternate weeks, acoustic startle reflex (ASR) amplitudes were measured, elicited by tone pips with stimulus frequencies of 3, 6, 12, and 24 kHz, and intensities from subthreshold up to 110 dB sound pressure level. The increase in ABR thresholds for 3 and 6 kHz test stimuli followed a linear time course with increasing age from 10 to 53 weeks, with a slope of about 0.7 dB/week, and for 48 kHz a second linear time course, but beginning at 10 weeks with a slope of about 2.3 dB/week. ABR thresholds for 12, 24, and 32 kHz increased after one linear segment with a 0.7 dB slope, then after a variable delay related to the test frequency, shifted to a second segment having slopes of 3-5 dB/week. Hearing loss initially reduced the ASR for all eliciting stimuli, but at about 6 months of age, the response elicited by intense 3 and 6 kHz stimuli began to increase to reach values about three times above normal, and previously subthreshold stimuli came to elicit vigorous responses seen at first only for the intense stimuli. This hyperacusis-like effect appeared in all mice but was especially pronounced in mice with more serious hearing loss. These ABR data, together with a review of histopathological data in the C57BL/6 literature, suggest that the non-frequency-specific slow time course of hearing loss results from pathology in the lateral wall of the cochlea, whereas the stimulus-specific hearing loss with a rapid time course results from hair cell loss. Delayed exaggeration of the ASR with hearing loss reveals a deficit in centrifugal inhibitory control over the afferent reflex pathways after central neural reorganization, suggesting that this mouse may provide a useful model of age-related tinnitus and associated hyperacusis.

Auditory spatial perception dynamically realigns with changing eye position.

Audition and vision both form spatial maps of the environment in the brain, and their congruency requires alignment and calibration. Because audition is referenced to the head and vision is referenced to movable eyes, the brain must accurately account for eye position to maintain alignment between the two modalities as well as perceptual space constancy. Changes in eye position are known to variably, but inconsistently, shift sound localization, suggesting subtle shortcomings in the accuracy or use of eye position signals. We systematically and directly quantified sound localization across a broad spatial range and over time after changes in eye position. A sustained fixation task addressed the spatial (steady-state) attributes of eye position-dependent effects on sound localization. Subjects continuously fixated visual reference spots straight ahead (center), to the left (20 degrees), or to the right (20 degrees) of the midline in separate sessions while localizing auditory targets using a laser pointer guided by peripheral vision. An alternating fixation task focused on the temporal (dynamic) aspects of auditory spatial shifts after changes in eye position. Localization proceeded as in sustained fixation, except that eye position alternated between the three fixation references over multiple epochs, each lasting minutes. Auditory space shifted by approximately 40% toward the new eye position and dynamically over several minutes. We propose that this spatial shift reflects an adaptation mechanism for aligning the "straight-ahead" of perceived sensory-motor maps, particularly during early childhood when normal ocular alignment is achieved, but also resolving challenges to normal spatial perception throughout life.

Age-related decline in Kv3.1b expression in the mouse auditory brainstem correlates with functional deficits in the medial olivocochlear efferent system.

Kv3.1b channel protein is widely distributed in the mammalian auditory brainstem, but studies have focused mainly on regions critical for temporal processing, including the medial nucleus of the trapezoid body (MNTB) and anteroventral cochlear nucleus (AVCN). Because temporal processing declines with age, this study was undertaken to determine if the expression of Kv3.1b likewise declines, and if changes are specific to these nuclei. Immunocytochemistry using an anti-Kv3.1b antibody was performed, and the relative optical density of cells and neuropil was determined from CBA/CaJ mice of four age groups. Declines in expression in AVCN, MNTB, and lateral superior olive (35, 26, and 23%) were found, but changes were limited to neuropil. Interestingly, cellular optical density declines were found in superior paraolivary nucleus, ventral nucleus of the trapezoid body, and lateral nucleus of the trapezoid body (24, 29, and 26%), which comprise the medial olivocochlear (MOC) feedback system. All declines occurred by middle age (15 months old). No age-related changes were found in the remaining regions of cochlear nucleus or in the inferior colliculus. Contralateral suppression of distortion-product otoacoustic emission amplitudes of age-matched littermates also declined by middle age, suggesting a correlation between Kv3.1 expression and MOC function. In search of more direct evidence for such a correlation, Kv3.1b knockout mice were examined. Knockouts show poor MOC function as compared to +/+ and +/- genotypes. Thus, Kv3.1b expression declines in MOC neurons by middle age, and these changes appear to correlate with functional declines in efferent activity in both middle-aged CBA mice and Kv3.1b knockout mice.

On the prediction of sweep rate and directional selectivity for FM sounds from two-tone interactions in the inferior colliculus.

Two-tone stimuli have traditionally been used to reveal regions of inhibition in auditory spectral receptive fields, particularly for neurons with low spontaneous rates. These techniques reveal how different frequencies excite or suppress the response to an excitatory frequency of a cell, but have often been assessed at a fixed masker-probe time interval. We used a variation of this methodology to determine whether two-tone spectrotemporal interactions can account for rate-dependent directional selectivity for frequency modulations (FM) in the mustached bat inferior colliculus (IC). First, we quantified the response to upward and downward sweeping, linear, fixed-bandwidth FM tones centered at a unit's characteristic frequency (CF) at 6 sweep durations ranging from 2 to 64 ms. Then, to examine how responses to instantaneous frequencies contained within the sweeps might interact in time, we varied the frequency and relative onset of a brief (4 ms) "conditioner" tone paired with a fixed 4-ms CF probe tone. We constructed "conditioned response areas" (CRA) depicting regions of suppression and facilitation of the probe tone caused by the conditioning tone. We classified the CRAs as predominantly excitatory (40.9%), inhibitory (22.7%), or mixed (36.4%). To generate FM response predictions, the CRAs were multiplied with spectrograms of the same sweeps used to assess response to FM. The predictions of FM rate and directionality were accurate by our criteria in approximately 20% of units. Conversely, the CRAs from the remaining units failed to predict FM responses as accurately, suggesting that most IC units respond differently to FM sweeps than they do to tone-pairs matched to the instantaneous frequencies contained in those sweeps. The implications of these results for models of FM directionality are discussed.

Thalamic projections to the auditory cortex in the rufous horseshoe bat (Rhinolophus rouxi). II. Dorsal fields.

In this study, we analyzed the thalamic connections to the parietal or dorsal auditory cortical fields of the horseshoe bat, Rhinolophus rouxi. The data of the present study were collected as part of a combined investigation of physiologic properties, neuroarchitecture, and chemoarchitecture as well as connectivity of cortical fields in Rhinolophus, in order to establish a neuroanatomically and functionally coherent view of the auditory cortex. Horseradish peroxidase or wheat-germ-agglutinated horseradish peroxidase deposits were made into cortical fields after mapping response properties. The dorsal fields of the auditory cortex span nearly the entire parietal region and comprise more than half of the non-primary auditory cortex. In contrast to the temporal fields of the auditory cortex, which receive input mainly from the ventral medial geniculate body (or "main sensory nucleus"), the dorsal fields of the auditory cortex receive strong input from the "associated nuclei" of the medial geniculate body, especially from the anterior dorsal nucleus of the medial geniculate body. The anterior dorsal nucleus is as significant for the dorsal fields of the auditory cortex as the ventral nucleus of the medial geniculate body is for the temporal fields of the auditory cortex. Additionally, the multisensory nuclei of the medial geniculate body provide a large share of the total input to the nonprimary fields of the auditory cortex. Comparing the organization of thalamic auditory cortical afferents in Rhinolophus with other species demonstrates the strong organizational similarity of this bat's auditory cortex with that of other mammals, including primates, and provides further evidence that the bat is a relevant and valuable model for studying mammalian auditory function.

The effects of early bilateral deafening on calretinin expression in the dorsal cochlear nucleus of aged CBA/CaJ mice.

The aim of this study was to test the hypothesis that calretinin (CR) levels in the aged mouse auditory brainstem depend upon hearing ability. Old animals with good hearing, and thus higher sound-evoked activity levels, were predicted to have higher levels of CR immunoreactivity than old animals with hearing loss. CR immunoreactivity was analyzed in the deep layer (layer III) of the dorsal cochlear nucleus (DCN) in CBA/CaJ mice that were bilaterally deafened at 3 months of age with kanamycin, and then aged until 24 months. This manipulation partially mimics the lack of sound-evoked auditory activity experienced by old C57BL/6J mice, who are deaf at 24 months of age (but show residual hearing at 15 months) and have lower levels of CR immunoreactivity than old CBA mice with normal hearing [Hear. Res. 158 (2001) 131]. Cell counts revealed that the density of CR+ cells in DCN layer III of the deafened CBA mice was statistically different from old intact CBA mice raised under identical conditions. Old deafened CBAs showed a decline of 47% in the mean density of CR+ cells compared to old hearing CBAs, thus supporting the hypothesis. Interestingly, while there tended to be fewer CR+ cells in the old deaf C57s as compared to young C57s and young and old CBAs with normal hearing, the difference was not statistically significant. It is possible that the residual hearing of C57 mice at 15 months may provide sufficient auditory input to maintain CR at levels higher than CBA mice that are deafened completely at 3 months of age, and are profoundly deaf for a much longer time (21 months).

Directional selectivity for FM sweeps in the suprageniculate nucleus of the mustached bat medial geniculate body.

Mustached bats emit echolocation and communication calls containing both constant frequency (CF) and frequency-modulated (FM) components. Previously we found that 86% of neurons in the ventral division of the external nucleus of the inferior colliculus (ICXv) were directionally selective for linear FM sweeps and that selectivity was dependent on sweep rate. The ICXv projects to the suprageniculate nucleus (Sg) of the medial geniculate body. In this study, we isolated 37 single units in the Sg and measured their responses to best excitatory frequency (BEF) tones and linear 12-kHz upward and downward FM sweeps centered on the BEF. Sweeps were presented at durations of 30, 12, and 4 ms, yielding modulation rates of 400, 1,000, and 3,000 kHz/s. Spike count versus level functions were obtained at each modulation rate and compared with BEF controls. Sg units responded well to both tones and FM sweeps. BEFs clustered at 58 kHz, corresponding to the dominant CF component of the sonar signal. Spike count functions for both tones and sweeps were predominantly non-monotonic. FM directional selectivity was significant in 53-78% of the units, depending on modulation rate and level. Units were classified as up-selective (52%), down-selective (24%), or bi-directional (non-selective, 16%); a few units (8%) showed preferences that were either rate- or level-dependent. Most units showed consistent directional preferences at all SPLs and modulation rates tested, but typically showed stronger selectivity at lower sweep rates. Directional preferences were attributable to suppression of activity by sweeps in the non-preferred direction (~80% of units) and/or facilitation by sweeps in the preferred direction (~20-30%). Latencies for BEF tones ranged from 4.9 to 25.7 ms. Latencies for FM sweeps typically varied linearly with sweep duration. Most FM latency-duration functions had slopes ranging from 0.4 to 0.6, suggesting that the responses were triggered by the BEF. Latencies for BEF tones and FM sweeps were significantly correlated in most Sg units, i.e., the response to FM was temporally related to the occurrence of the BEF in the FM sweep. FM latency declined relative to BEF latency as modulation rate increased, suggesting that at higher rates response is triggered by frequencies in the sweep preceding the BEF. We conclude that Sg and ICXv units have similar, though not identical, response properties. Sg units are predominantly upsweep selective and could respond to either or both the CF and FM components in biosonar signals in a number of echolocation scenarios, as well as to a variety of communication sounds.