february
Date: Thursday, Feb. 2nd
Time: 4pm
Location: HNB small conference room.
Speaker: Susmita Chatterjee
Paper: Ishikane H, Gaugi H, Honda S, Tachibana. Synchronized retinal oscillations encode essential information for escape behavior in frogs. Nat Neurosci. 2005 Aug 8(8):1087-95.
Synchronized oscillatory activity is generated among visual neurons in a manner that depends on certain key features of visual stimulation. Although this activity may be important for perceptual integration, its functional significance has yet to be explained. Here we find a very strong correlation between synchronized oscillatory activity in a class of frog retinal ganglion cells (dimming detectors) and a well-known escape response, as shown by behavioral tests and multi-electrode recordings from isolated retinas. Escape behavior elicited by an expanding dark spot was suppressed and potentiated by intraocular injection of GABA(A) receptor and GABA(C) receptor antagonists, respectively. Changes in escape behavior correlated with antagonist-evoked changes in synchronized oscillatory activity but not with changes in the discharge rate of dimming detectors. These antagonists did not affect the expanding dark spot-induced responses in retinal ganglion cells other than dimming detectors. Thus, synchronized oscillations in the retina are likely to encode escape-related information in frogs.
Date: Thursday, Feb. 9th
Time: 4pm
Location: HNB small conference room.
Speaker: Junkwan Lee
Paper: Tu Z., Chen X., Yuille A.L., Zhu S.C. Image Parsing: Unifying Segmentation, Detection, and Recognition. Int'l J. of Computer Vision, Marr Prize Issue, 2005.
We propose a general framework for parsing images into regions and objects. In this framework, the detection and recognition of objects proceed simultaneously with image segmentation in a competitive and cooperative manner. We illustrate our approach on natural images of complex city scenes where the objects of primary interest are faces and text. This method makes use of bottom-up proposals combined with top-down generative models using the Data Driven Markov Chain Monte Carlo (DDMCMC) algorithm which is guaranteed to converge to the optimal estimate asymptotically. More precisely, we define generative models for faces, text, and generic regions- e.g. shading, texture, and clutter. These models are activated by bottom-up proposals. The proposals for faces and text are learnt using a probabilistic version of AdaBoost. The DDMCMC combines reversible jump and diffusion dynamics to enable the generative models to explain the input images in a competitive and cooperative manner. Our experiments illustrate the advantages and importance of combining bottom-up and top-down models and of performing segmentation and object detection/recognition simultaneously.
Longer version of the paper with math
Date: Thursday, Feb. 16th
Time: 4pm
Location: HNB small conference room.
Speaker: Alan Horsager
Presentation:
THE PERCEPTUAL EFFECT OF
RETINAL ELECTRICAL STIMULATION USING PULSE TRAINS
A.Horsager1,2,3*; S.H.Greenwald4; M.S.Humayun1,3; M.J.McMahon4; I.Fine1,2
1. Dept. of Ophthalmology, 2. Zilkha Neurogenetic Inst., 3. Doheny Retina Inst.,
Purpose: Retinitis
pigmentosa (RP) and age-related macular degeneration are diseases involving
progressive degeneration of photoreceptors. However, the bipolar and ganglion
cells remain relatively spared, though neurally disorganized (Marc, 2004).
Recently, a collaborative effort between Second Sight LLC and Doheny Eye
Institute has implanted six advanced RP patients with a 4x4 array of epiretinal
electrodes. Achieving functional perception using electrical stimulation will
require relatively high frequency pulse trains that produce continuous
percepts. We report here psychophysical data
from three patients examining the perceptual effect of pulse train
stimulation.
Methods: Stimuli were pulse trains consisting of 0.075 or 0.975 ms
cathodic-first charge balanced biphasic pulses. The inter-phase
interval between
the two pulses varied between 0.075 and 0.975 ms. The frequency of the
pulse
trains varied between 5 and 225 Hz. Current amplitude thresholds were
measured
using a yes-no paradigm where 50% of the trials were catch trials
containing no
stimulus. Pulse train amplitude was varied using a three down one up
staircase.
Results: An increase in pulse frequency leads to a decrease in the
current
needed to reach perceptual threshold. However the amount of total
charge needed to reach threshold increases linearly as a function
of frequency. This decrease in efficiency as a function of frequency is
more
pronounced for long than for short pulses, and does not depend on the
inter-phase interval. Conclusions: One possibility is that short pulses
result
in less sub-threshold deactivation of ion channels. Alternatively, this
effect
of frequency may be due to inhibition mediated by bipolar/amacrine
cells.
Inhibition from bipolar and amacrine cells might be weaker for short
pulses
since these cells tend to be preferentially stimulated by longer pulses
(Jensen, 2005).
Support Contributed By: Second Sight Medical Products, LLC
Date: Thursday, Feb. 23rd
Time: 4pm
Location: DRB conference room (note the location change).
Speaker: Jeff Wurfel
Paper: Zaksas D, Pasternak T. Area MT neurons respond to visual motion distant from their receptive field J Neurophysiol. 2005, Dec 94(6), 4156-67.
Neurons in cortical area MT have localized receptive fields (RF)
representing the contralateral hemifield and play an important role in
processing visual motion. We recorded the activity of these neurons
during a behavioral task in which two monkeys were required to
discriminate and remember visual motion presented in the ipsilateral
hemifield. During the task, the monkeys viewed two stimuli, sample and
test, separated by a brief delay and reported whether they contained
motion in the same or in opposite directions. Fifty to 70% of MT
neurons were activated by the motion stimuli presented in the
ipsilateral hemifield at locations far removed from their classical
receptive fields. These responses were in the form of excitation or
suppression and were delayed relative to conventional MT responses.
Both excitatory and suppressive responses were direction selective, but
the nature and the time course of their directionality differed from
the conventional excitatory responses recorded with stimuli in the RF.
Direction selectivity of the excitatory remote response was transient
and early, whereas the suppressive response developed later and
persisted after stimulus offset. The presence or absence of these
unusual responses on error trials, as well as their magnitude, was
affected by the behavioral significance of stimuli used in the task. We
hypothesize that these responses represent top-down signals from brain
region(s) accessing information about stimuli in the entire visual
field and about the behavioral state of the animal. The recruitment of
neurons in the opposite hemisphere during processing of behaviorally
relevant visual signals reveals a mechanism by which sensory processing
can be affected by cognitive task demands.