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advice on postdoc (fwd)

Dear list members,

I just received this query from Dr. Schnitzler in Tuebingen, Germany.  It
concerns the question of where good signal-processing research on auditory
neurobiology is being carried out where one of his recent graduates might
go as a post-doc (with his own money).  I'm not so familiar with this
area;  so I thought I'd post it to the list.

You could answer directly to Dr. Schnitzler or, if you prefer, I could
forward your replies to him.



---------- Forwarded message ----------
Date: Wed, 30 Apr 1997 18:22:13 +0200
From: Hans-Ulrich Schnitzler <hans-ulrich.schnitzler@uni-tuebingen.de>
To: bregman@hebb.psych.mcgill.ca
Subject: advice on postdoc

Dear Dr. Bregman

please excuse the informal approach. When looking for a postdoc
position for one of my students I contacted Mark Konishi for help. He
recommend that I address you with the following question:

Rolf Mueller, a PhD student in my lab is looking for a postdoc
position in the states. In spring 1998 he will finish his dissertation
in which he studies the possible role of acoustical flow field
information in transfer flight and obstacle avoidance in CF bats. He
is particularly interested in signal processing especially in
simulating the function of sensory systems numerically. He is one of
the best students I ever had and very gifted for this type of
work. Additionally he works very hard and learns very fast. I am
absolutely sure that it will be no problem for him to get a two year
postdoc stipend for the states.

We are looking for a top lab where he can improve his knowledge in
signal processing and computer simulations. If possible he would
prefer a lab which works on auditory information processing but he is
also open for other fields (see the included personal profile of Rolf
Mueller). Unfortunately I do not know the top labs working in the
wanted fields. Do you have any suggestions where he could apply?

Please find a short account on Rolf Mueller, his work and his
interests, below.

I would appreciate it very much if you could be so kind as to advise
us on this issue. Sincerely, H.-U. Schnitzler

Prof. Hans-Ulrich Schnitzler
Dept. Animal Physiology, University of Tuebingen,
Morgenstelle 28, D-72076 Tuebingen, Germany
Phone +49 70 71 29 75 34 5;  Fax +49 70 71 29 26 18

Rolf Mueller - Personal profile

I have studied biology at Tuebingen University since 1990; I
specialized in neurobiology, supplemented by electronics and
genetics. My Master- and PhD-thesis are detailed below. My thesis work
is supported by a grant from Studienstiftung des Deutschen Volkes as
were my undergraduate studies. I am also participating in the graduate
program neurobiology at Tuebingen University.

Master thesis (completed December 1995):

Human listening experiments on the psychophysics of ranging paradigms.

Scientific content (abstract):
The ranging capabilities of the bats' sonar receiver have been studied
extensively in behavioral 2-AFC - experiments employing the playback
of artificial echoes to the animals' echolocation pulses. Such
procedures are thought to generate the impression of phantom targets
located at different distances. These tests made use of two paradigms:
range difference (RD), the discrimination between different echo
delays, and jitter versus non-jitter (JNJ), the detection of a
regularly alternating, 'jittering' delay against a reference of
constant delay values. The perceptional equivalence of these paradigms
hypothesized by some workers has been doubted by others and a third
paradigm, jitter-versus-jitter (JJ), which would require the
discrimination of different jitter amplitudes, has been proposed for
the purpose of clarifying this unresolved issue.
In order to obtain a first glimpse at the psychophysical properties of
the stimuli delivered in the ranging experiments, passive listening
tasks with sequences of repeated artificial 'pulse-echo-pairs' where
designed for human subjects according to all three mentioned rules and
thresholds were measured in an adaptive 3-AFC procedure.
It could be demonstrated, that the subjects' performance depended on a
variety of stimulus parameters (e.g., repetition rate, reference value
of the pulse-echo-delay and the characteristics of the individual
pulses). The nature of these relationships depended on the tested
paradigm, hinting at possible differences in the involved
mechanisms. Each paradigm seems to constitute an individual perceptual
task, with a somewhat closer relationship existing between JNJ and
One major finding was, that the JNJ-performance depended on the
pulse-echo-delay of the reference stimulus, which weakens the view,
that a grossly similar effect found in bats can be regarded as proof
for a specific internal representation of the stimuli along a range
axis in these animals. In several experiments qualitative, individual
differences among the subjects surfaced, which might be interpreted as
a hint towards the existence of individual strategies. This would
imply, that the solution of these tasks allows for different approaches.
Experimenting with human subjects in order to elucidate the
psychophysical properties of echolocation tasks, has it's limitations,
however: The subjects of this study were carrying out passive hearing tasks,
whereas ranging bats are making use of an active sensory
system. Furthermore, modifications to the stimuli were necessary for
adapting to the requirements of the human subjects. Therefore, no
attempt is made to purport that problems associated with experiments
in bats have been solved by this study once for all. Rather, these
results should be regarded as hypotheses, which may be considered worth for
further testing in bats.

Methodological qualifications
The experimental setup was entirely put together by me (on the basis
of a preexisting function library for the DSP, the stimuli were
generated on). It included: Implementation of the adaptive staircase procedure,
the user interface, interfacing to a DSP-board and generating the
stimuli there, as well as controlling the properties of the delivered
stimuli. Furthermore, I did have a closer look at adaptive psychophysical
methods and ran some Monte Carlo simulations of adaptive staircaises.

Posters at:
10th International Bat Research Conference, Boston, 1995
and 24th Goettingen Neurobiology Conference, 1996
Paper: in prep.

PhD thesis (to be completed spring 1998):

The concept of the acoustical flow field and obstacle avoidance in CF-bats

Scientific content (abstract):
Bats are capable of obstacle avoidance based solely on echolocation
information. The most likely candidate for the underlying perceptual
process would in general be a combination of ranging by means of
time-of-flight measurements and binaural direction-of-arrival
determination. While it is beyond reasonable doubt, that these cues
are involved in bats' space perception, some species may exploit other signal
parameters conveying information about the spatial layout of their
environment, which could be more readily evaluated given certain
pulse designs and spatial tasks to be performed.

In looking for alternative solutions to this problem, I choose the
group of so-called CF-bats (CF = constant frequency) as a model
system. These species can be observed to travel at high flight speeds
(3-7 m/s) through obstacle rich forest habitats, maintaining an
approximately constant height above ground. This situation may thus be
thought of as a 2-D obstacle avoidance task. The signal design of the
animals' echolocation pulses (narrow-band, hence the name, and
comparatively long duration) does not lend itself easily as a
substrate for range and angle estimation in the manner depicted
above. The presence of multiple targets might be aggravating this
difficulty by causing a resolution and a correspondence problem among
the individual echoes received at each ear.
I therefore explored the informational content of the proportional
changes in center frequency (due to Doppler shifts) and amplitude (due
to geometric attenuation, absorption and compound directivity of
emitter and receiver) of the echoes in a rough, abstract-functional
analogy to the time-to-contact variable of the optic flow field. It
can be shown, that a combination of these two putative sensory
variables allows in principal for a metrical reconstruction of the
target position relative to the animals flight vector within a
hemi-plane, i.e. symmetrical solutions to the right and to the left
are found.
A closer examination of this inverse problem reveals, that proportional
frequency and amplitude changes by no means constitute an optimal
foundation for a geometrical reconstruction of obstacle position: The
cosine mapping angle to Doppler shift has minimal slope straight
ahead, thus adversely affecting the precision of position estimation,
where obstacle avoidance presumably requires most of it. Furthermore,
depending on the assumed compound directivity function of receiver and
emitter, positional ambiguities might be introduced. It should be
noted, however, that optimality is not a necessary precondition of
physiological plausibility and the error in positional judgment
associated with a given precision of estimating the sensory variables'
magnitude can be found to decrease as a function of distance to
target, thereby accommodating the task's demands. Along the same line
of thought, it may be argued, that the ambiguities eventually
introduced by the directivity are confined to single positional
estimates and could be pruned by looking at successive measurements.

I arrived at the tentative conjecture, that an adequate obstacle
avoidance maneuver, as observed in the animals, based on echolocation
is most likely to require a measure for the situation's gravity
(i.e. distance of obstacle to flight path) as well as urgency
(i.e. distance to obstacle along flight path). The need for the latter
may be attributed to the fact, that target strength could differ
considerably among obstacles, thus affecting the extent of the
echolocation system's field of view. Since detection distances are presumably
rather short in general and eclipses during the inter-pulse-intervals
restrict access to information about the target further, some
representation equivalent to a knowledge of the target position
relative to the flight path, seems highly desirable.

Having demonstrated, that a metrical reconstruction is principally
feasible (albeit not necessarily performed by the animals), there is
good reason to believe, that both demands can be met by evaluation of
our postulated sensory variables.

In order to investigate, whether the information-bearing signal
parameters are sufficiently well preserved in the auditory system's
primal sketch, synthetic echo trains endowed with the proportional
changes in frequency and amplitude pertinent to a given target
position were generated and passed through a gamma tone filter bank
adjusted to the neurophysiologically documented characteristics of
CF-bat's. Then a reconstruction of target position is
undertaken, with devoting special attention to the robustness against
noise. Specifically, I evaluated how the auditory systems temporal
integration might be able to subserve noise-reduction.

The pending two parts of the project are to deal with multi-target
situations and look at how the properties of my scene reconstruction
hypothesis might relate to the requirements of interfacing with the
execution of avoidance maneuvers.

Methodological qualifications
I have been employing a fair range of numerical methods (solving linear and
non-linear systems of equations, numerical integration, optimization,
etc.) during my work. I tried to acquaint myself with the theory of
signal processing, i.e. linear system theory (laplace and
z-transform), digital filtering and spectral
estimation. Implementations were done mostly using matlab
(symbolic maths was carried out with maple), but some stand-alone
c-code has been written also. All work was done on a unix-platform
(linux for i386).

Poster at:
25th Goettingen Neurobiology Conference, 1997

Presentation accepted for 33rd ASA Meeting, State College,

Postdoc profile:

The research I would like to participate in should address
signal processing algorithms as models for the function of the brain's
sensory systems. It could involve mimicking these functions in some
artificial system (numerical simulation or maybe even a physical
realization). The coupling of perception and action falls within the
scope of my interests, as well.

I have a keen interest in improving my theoretical background knowledge.

I would like to run computer simulations, psychophysical
experiments would also interest me. I would be comfortable doing computer
simulations without psychophysics on human observers, but not
vice-versa, since it is my aim to have my research on sensory systems'
function resting on a profound theoretical background.

Audition as a model sensory modality would be preferred, but I could
imagine participating in any research fitting under the above
specifications, since I would like to be working more on the side of
abstract function principles.
Biomimetic sonar is still on my wish list, but not of any priority.