The University of Chicago
Department of Psychology
5848 South University Avenue
Chicago, IL, 60637
Office Phone: (773) 702-8841
Office: Green Hall, 314
Email me
David Bradley
Position
Associate Professor of Psychology
Courses
Sensation & Perception
Physiology of Vision
Research Interest
Whenever we look at
something, the image we see has to be converted from light to some other form
the brain can work with. This new form involves tiny electrical discharges,
called action potentials, emitted by each of millions of neurons in the
visual cortex. The rich information of the image is thought to be encoded in
terms of when and where these discharges occur. This massively parallel
distribution of information is known as a population code. Figure 1 shows the
idea. We are trying to understand the brain's rules for encoding and decoding
visual information in this way. One could reasonably argue that those rules
equate to perception itself.
Our work involves a combination of multichannel recording, psychophysics and
mathematical (particularly statistical) modeling. In general, our goal is to
relate certain kinds of perception—like the direction of a moving object—to
specific discharge patterns in populations of cortical neurons. Our hope is to
discover basic principles of neural coding; for example, how images are
perceptually segmented, and how direction and speed are decoded from
populations of velocity-tuned neurons.
Central to our
thinking are the multidimensional spaces mapped out by cortical areas. Figure 2
shows such a space. Area MSTd neurons are sensitive to optic flow—complex image
motion components such as expansion, contraction, rotation and linear flow.
These are continuous with each other through spiral intermediates, as shown in
Figure 2. Sliding around on a spherical surface in this space corresponds to
seeing different combinations of flow components, which are represented on the
cardinal axes (this is actually a 3D projection of a 4D space). Thus, MSTd can
be thought of as a map of 'spiral space.'
Humans rely on optic flow cues to sense their direction of self-movement, or heading. The transformations needed to convert image flow to heading are probably carried out in MSTd. Therefore, we would like to understand how the distribution of neural discharges in the MSTd tuning space encode our direction heading.
Neurons in
another area, called MT, are sensitive to the direction and speed of visual
motion, as well as the depth of objects in the visual field. Thus, they map a
3D direction-speed-depth space. We can think of the activity distribution in MT
as density in a 3D space, or by collapsing one of the dimensions, as height
over a 2D space (Fig. 3).
We think the distribution of activity in MT encodes such things as the coherence and separateness of image segments, as well as the direction and speed of moving objects. We are trying to understand how these different types of information are simultaneously expressed and read-out by other regions of the brain.
We are also
doing experiments in prosthetic vision. Here, our goal is to develop efficient
algorithms for using large arrays of stimulating microelectrodes. Drawing from
our work on population coding, we want to find optimal stimulation
protocols—which neurons to activate, and when—so that vision can be restored in
blind individuals with a minimum number of implanted electrodes. We are working
in collaboration with Dr. Philip Troyk of the Illinois Institute of Technology,
who is developing an ultraminiature 256-channel implantable cortical stimulator
(Fig. 4), as well as members of the National Institute of Neural Disorders and
Stroke, the National Institute of Mental Health, the Huntingon Memorial
Institute and the Pritzker School of Medicine.
Papers
Bradley DC, Hosseini P, Clark AM, Scott BB, Mascaro M and Berg J. A distributed neural correlate of visual motion segmentation. Submitted.
Bradley DC, Goyal MS and Scott BB. Pattern velocity computation by primate MT neurons. Submitted.
Purushothaman G and Bradley DC. A computational analysis of perceptual learning and performance of a fine visual discrimination task. Submitted.
Bradley DC., Troyk P.R., Berg J., Bak M., Cogan S., Erickson R., Kufta C., Mascaro M., McCreery D., Schmidt E., Towle V.and Xu H. Visuotopic mapping through a multichannel stimulating implant in primate V1. J. Neurophysiol. 93:1659, 2005. pdf.
Purushothaman G and Bradley DC. Neural population code for fine perceptual decisions in area MT. N. Neurosci., 8(1):99, 2005. pdf.
Bradley DC, Mascaro M and Sunthakumar S. A relational database for trial-based behavioral experiments. J Neurosci Methods., 141:75, 2005. pdf.
Troyk P.R., Bak M, Berg J, Bradley DC, Cogan S, Erickson R, Kufta C, McCreery D, Schmidt E and Towle V. A model for intracortical visual prosthesis research. Artificial Organs, 27(11):1005, 2003.
Grunewald A, Bradley DC and Andersen RA. Neural Correlates of Structure-from-Motion Perception in Macaque V1 and MT. J.Neurosci,.22(14): 6195, 2002. pdf.
Shenoy KV, Bradley DC and Andersen RA. Influence of gaze rotation on the visual response of primate MSTd neurons. J. Neurophysiol., 81:6, 1999. pdf.
Bradley DC and Andersen RA. Center-surround antagonism based on depth cues in area MT receptive fields. J. Neurosci., 18:7552, 1998. pdf.
Bradley DC, Chang G. C. and Andersen RA. Encoding of 3D structure-from-motion by primate area MT neurons. Nature, 392:714, 1998. pdf.
Bradley DC, Maxwell M, Andersen RA, Banks M and Shenoy KV. Neural mechanisms of heading perception in primate visual cortex. Science, 273:1544, 1996. pdf.
Bradley DC, Qian N and Andersen RA. Integration of motion and stereopsis in cortical area MT of the macaque. Nature 373:609, 1995. pdf.
Bradley DC, Steil GM and Bergman RN. OOPSEG: a data smoothing program for quantitation and isolation of random measurement error. Comp. Prog. Meth. Biomed. 46:67, 1995
Bradley DC, Poulin RA and Bergman RN. Dynamics of hepatic and peripheral effects of insulin suggest common rate-limiting step in vivo. Diabetes 42:296, 1993
Bradley DC, Steil GM and Bergman RN. Quantitation of measurement error with "optimal segments:" A basis for adaptive time course smoothing. Am. J. Physiol. E246:902, 1993
Bradley DC and Bergman RN. Reestablishment of fasting metabolic conditions during islet suppression in the dog. Am. J. Physiol. 262:E532, 1992
Bradley DC and Bergman RN. Hepatic glucagon sensitivity in normal dogs. Am. J. Physiol. 262:E539, 1992
Bradley DC and Kaslow HR. Radiometric assays for glycerol, glucose and glycogen. Anal. Bioch. 180:11, 1989
REVIEWS
Born R and Bradley DC. Structure and function of area MT. Ann. Rev. Neurosci., 28, September 2005. pdf.
Bradley DC. Object motion: A world view. Current Biology, 2004. 14: R892-894. pdf.
Bradley DC and Wallisch P. Hide, remember, seek, Nature Neurosci, 2003. 6(1):p. 11-12. pdf.
Bradley DC. Moving through the landscape. Science, 2002. 295: p. 2385-2386. pdf.
Bradley DC. MT signals: Better with time. Nature Neurosci., 2001. 4(4):p346-348. pdf.
Bradley DC. Motion perception, psychological and neural aspects; International Encyclopedia of Social and Behavioral Sciences, Elsevier Science Ltd, 2001
Bradley DC. Early visual cortex: Smarter than you think. Current Biol. 11:R95-R98, 2001. pdf.
Andersen RA and Bradley DC. Neural mechanisms of 3D structure from motion perception in the primate. Trends in Cognitive Neurosciences. 2:222, 1998.
Andersen RA, Snyder LH, Bradley DC and Xing J. Multimodal representation of space in the posterior parietal cortex and its use in planning movements. Ann. Rev. Neurosci., 20:303, 1997. pdf.
Andersen RA. Shenoy KV. Snyder LH. Bradley DC. Crowell JA. The contributions of vestibular signals to the representations of space in the posterior parietal cortex. Annals of the New York Academy of Sciences. 871:282, 1999. pdf.
Andersen RA, Bradley DC and Shenoy KV. (1996) Neural mechanisms of heading perception and structure-from-motion. Chapter in Function & Dysfunction in the Nervous System, Cold Spring Harbor Laboratory 61st Symposium on Quantitative Biology.
Bergman RN, Bradley DC and Ader M. On insulin action in vivo: the single gateway hypothesis. Adv. Exp. Med. Biol. 334:181, 1993
Bergman RN, Steil GM, Bradley DC and Watanabe RM. Modeling insulin action in
vivo. Annu. Rev. Physiol. 54:861, 1992
