Influence of Age and Memory on Head-Free Localization of Auditory and Visual TargetsMarina S. Dobreva1, Babak Razavi2,3,4, William E. O’Neill,1,3 and Gary D. Paige1,2,3,4

Depts of 1Neurobiology & Anatomy, and 2Biomedical Engineering, 3Centers for Navigation & Communication Sciences, and 4Visual Science,

University of Rochester School of Medicine & Dentistry, Rochester, New York, USA

Introduction Results (continued)Results1. Auditory LocalizationSpatial orientation and localization rely upon

synergistic and concordant sensori-motorintegration across several modalities. Forexample, localization of objects that emit asound and an image must match spatialrepresentations (and coordinate frames)between vision, in which space istopographically mapped directly onto the retina,and audition, in which location is computedcentrally using spatially-dependent cues.Despite dramatic differences in sensory inputand processing, vision and audition mustultimately combine spatial maps in order tointegrate a singular representation of externalspace1. Further, since the eyes move in thehead, the entire visual input also moves relativeto the head. Accurate tracking of eye positionmust occur in this process in order to maintainspace constancy2.

One common orienting behavior is the

shift gaze (eye in space, or eye + head), and inparticular the line of sight, toward objects ofinterest3-5. While the relative contribution of eyeand head to visually-guided gaze is well-documented, less is known about its auditory-driven counterpart3-13. For instance, factors thatmay influence the accuracy of auditory-evokedgaze, such as spatial memory and aging,remain largely uncharted.

In this study, we quantified gaze accuracy inlocalizing ongoing and transient (memorized)auditory and visual targets in young and elderlyhuman subjects. Spatial performance wasassessed during: 1) gaze pointing in darkness(eyes and head), and 2) head-laser pointing,using a head-mounted laser to emphasize thehead component while the eyes remainedfixated on the head-centered pointer. Thevestibulo-ocular reflex (VOR) was alsomeasured

Gaze Pointing

Young• Gaze proved near accurate

when localizing ongoing sounds (SG ong=1.01) and declined slightly withmemorized targets (SG trans=0.97, P<0.01 re ongoing).

• Typically, head undershoot (SG ong=0.90; SG trans=0.85) was compensated by eye position.

Elderly• In contrast to young adults,

gaze accuracy in the elderly demonstrated overshoot

3. Vestibulo-Ocular Reflex (VOR)

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Summary and Conclusions

YOUNG

HUMAN SUBJECTS• Young: N=8 (4 M, 4 F)

Elderly: N=7 (3 M, 4 F).• All free of neurological or sensory

abnormalities.• Normal hearing & vision.

APPARATUS AND STIMULUS• Test Chamber: dark, echo-attenuated

room.• Head Orientation: unrestrained.• Target: 3” (7.7 cm) diam. speaker with

a central red LED, hidden behind a cylindrical screen of black speaker cloth at 2 m distance under robotic control; range of ±65° horizontal (Az) × ±25°vertical (El).

• Target Distribution: 65 auditory and 47 visual randomly distributed locations across the horizontal meridian, including a subset of multi-sampled (5x) locations in 10° intervals (±40° auditory; ±30° visual).

• Auditory Stimulus: broadband (0.1 – 20 kHz) Gaussian noise at 75 dB SPL (RMS), in 150 ms bursts

One common orienting behavior is thecoordinated movement of the head and eyes to

measured.

EXPERIMENTAL PARADIGM• Eyes, head, and head-fixed laser (if on,

eyes fixated on laser) were aligned with a central fixation spot at the onset of each trial.

• The fixation spot was then extinguished with target onset (if ongoing) or offset (if transient) and localization began immediately.

• Subjects aligned head, eyes, and head-mounted laser (if on) with perceived target location. If the head-fixed laser was switched off, subjects pointed their head (nose) toward the perceived target location.

RESPONSE MEASURES• Subjects were instructed to localize

quickly but accurately. • No feedback was given on

performance. • Response endpoint was registered with

a key press.• Eye and head position were measured

using the EL-MAR eye tracker and Polhemus FastTrack, respectively.

Head-Laser Pointing

2. Visual LocalizationGaze Pointing

(SG ong=1.10, SG trans=1.14; both P<0.001).

• This involved less headundershoot (SG ong=0.99; SG trans=0.96) than in young subjects (both P<0.001).

• Eye contribution was similar for ongoing but increasedfor transient sounds (SG trans=0.18, P<0.001 re young).

Young• When guided by a head-fixed

laser (and eyes remained centered), gaze (or head only) pointing in young subjects demonstrated slight overshoot (SGong=1.04, P<0.001), which diminished for memorized targets (SG trans=1.00, P<0.001 re ongoing).

Elderly• Relative to young adults,

gaze accuracy in the elderly demonstrated overshoot(SG ong=1.11, SG trans=1.14; both P<0.001).

Young• Gaze proved accurate when

localizing ongoing or transient (memorized) visual targets (SG=~1.0).

• Head undershoot (SG 0 88 SG 0 87)

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Vestibulo-Ocular Reflex (VOR) Gain0.4 0.6 0.8 1.0 1.2 1.4

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• Average VOR gain (pooled) proved lower in elderly (0.87) than in young (0.91) subjects (t-test, P<0.02).

• The difference cannot fully account for gaze overshoot in the elderly.

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The authors thank John Housel, Brian Flynn, Emily Clark, Mike Rozanski, Art Dee, Bob Schor, Martin Gira, and Scott Seidmanfor their assistance. This study was supported by NIH grants RO1-AG16319, P30-DC05409 (Center for Navigation andCommunication Sciences), P30-EY01319 (Center for Visual Science), T32-EY07125 (author BR; Center for Visual Science)and T32-GM07356 (author BR; MSTP).

YOUNG

• Head-free spatial localization using gaze (eye + head) or head-laser(eyes centered in head) pointing toward ongoing auditory or visualtargets proved accurate (spatial gain, SG=~1.0) in young subjects.

• When the head and eyes were unrestrained in darkness (gazepointing), the head generally undershot but was well compensated byeye position.

ELDERLY

• Target localization in the elderly demonstrated gaze overshoot(SG>1) across modalities in contrast to young controls.

• This reflected augmented head movements compared to youngsubjects.

• Based on the VOR, a reduction in vestibular feedback during headmovement cannot explain the gaze overshoot. An equivalentreduction in cervical feedback may add to the effect.

(10 ms rise-fall time) repeating at 5 Hz; ongoing or transient (5 bursts).

• Visual Stimulus: red LED (0.2° back-projected onto the screen), 150 ms flashes repeating at 5 Hz; ongoing or transient (5 flashes).

• Trials were characterized by:• Stimulus Modality: auditory

or visual.• Stimulus Duration: ongoing

or transient (memorized).• Localization Context:

1) gaze pointing in darkness using both the eyes and head. 2) head-laser pointing with the eyes centered (re head) by fixating a head-mounted laser.

DATA ANALYSIS• Data were sorted by stimulus modality

(auditory or visual), stimulus duration (ongoing or transient), pointing method (gaze vs head-laser), and age (young vs elderly).

• Spatial gains (SG) were obtained from regressions of response vs target position.

• Multi-linear regression analysis was used to statistically compare differences in SG between conditions.

• Accuracy, the error between the response and target position, was plotted as a function of target position.

Eye eye-in-headHead head-in-spaceGaze eye-in-space=

eye-in-head + head-in-space

Visual Localization

Head-Laser Pointing

ong=0.88; SG trans=0.87) was well compensated by eye position.

Elderly• Relative to young adults,

gaze accuracy in the elderly showed overshoot(SG ong=1.04, SG trans=1.06; both P<0.001).

• The head demonstrated spatial distortion (central overshoot and peripheral undershoot) that was incompletely compensated by eye position, unlike in young subjects.

Young• Gaze with head-laser

pointing proved accurate for ongoing visual stimuli (SGong=1.0) but showed slight overshoot for memorized targets (SG trans=1.03, P<0.01 re ongoing).

Elderly• Gaze pointing in the elderly

proved similar to young controls for ongoing visual targets, but demonstrated overshoot for memorized targets (SG trans=1.11; P<0.001 re young).

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