Merav Ahissar & Shaul
Hochstein talk with ScienceWatch.com and answer a
few questions about this month's Emerging Research Front
Paper in the field of
Article: View from the top: Hierarchies and reverse
hierarchies in the visual system
Journal: NEURON, 36 (5): 791-804 DEC 5 2002
Addresses: Hebrew Univ Jerusalem, Dept Neurobiol, IL-91904
Hebrew Univ Jerusalem, Dept Neurobiol, IL-91904 Jerusalem,
Hebrew Univ Jerusalem, Neural Computat Ctr, IL-91904
(addresses have been truncated.)
Why do you think your paper is highly
It introduces a coherent conceptual framework to behavioral,
electrophysiological, and imaging data of perception and perceptual
learning. In addition, it tackles the exciting issue of what brain activity
Does it describe a new discovery, methodology, or
synthesis of knowledge?
It is a new conceptual framework, which we named Reverse Hierarchy Theory
or RHT, for short.
Would you summarize the significance of your paper in
Its main novelty is in its well defined dissociation between stimulus
processing and perceptual experiences. According to RHT, while stimulus
processing is a sequence of hierarchical processes, driven in bottom-up
fashion from the sense organs (eye, ear) via a number of brain regions to
higher cortical levels, perception follows a reverse order. Conscious
perception reflects the cortical interpretation of these initial implicit
processes, which follows a reverse hierarchy, top-down driven order.
This simple dissociation explains a broad set of seemingly inconsistent
data. For example—what do we see first—the whole or the
details? On the one hand, details are processed first. On the other hand,
we are quicker at getting the gist of scenes than we are at perceiving
their components; we are quicker at perceiving whole words than the letters
that comprise them, we know the average size or direction of motion of a
group of objects rather than the sizes or motions that compose the average.
RHT asserts that only the gist is represented at high levels of the
cortical hierarchy—which is where conscious perception
begins—whereas perceiving the components requires re-entry to
lower-processing stages. This re-entry is essential for gaining conscious
access to details and components. However, attaining it successfully
requires a search backwards along the hierarchy. This backward search takes
time and specific conditions.
How did you become involved in this research and were
any particular problems encountered along the way?
For over 10 years, we had been studying perception and perceptual learning
of simple visual tasks among university students and saw huge variability
that could not be explained by any existing conceptual framework.
Where do you see your research leading in the
"...it tackles the exciting issue of
what brain activity enters
We are focusing on other senses and additional aspects of cognitive skills.
We believe that this framework may explain cognitive impairments in
specific populations. Furthermore, we believe that this basic distinction
applies in many domains, even beyond brain research.
Do you foresee any social or political implications for
We hope it will impact on concepts in education. For example, when teaching
math and sciences—what would be the most effective interplay between
building bottom-up processing procedures by using many exercises and
guiding top-down "insights" by guided conceptual explanation?
We believe that the same dissociation can also explain organizational and
general sociological processes. That is, the distinction between the
bottom-up processes that occur whenever people act in a group, versus
people's attempt to act upon them based on conscious top-down
Department of Psychology and Center for Neural Computations
Hebrew University, Jerusalem
Department of Neurobiology
Institute for Life Sciences and Center for Neural Computation
Hebrew University, Jerusalem