Visual attention: how brain makes most of visible world

THE visual system has limited capacity and cannot process everything that falls onto the retina. Instead, the brain
relies on attention to bring salient details into focus and filter out background clutter. Two recent studies by
researchers at the Salk Institute for Biological Studies, one study employing computational modelling techniques
and the other experimental techniques, have helped to unravel the mechanisms underlying attention.
In everyday viewing a visual detail that is the target of our attention is generally surrounded by a lot of stimuli that
are momentarily irrelevant to behaviour, says John H. Reynolds, Ph.D., an associate professor in the Systems
Neurobiology Laboratory at the Salk Institute, who led the study published in the March 26, 2009 issue of the
journal Neuron. Attention dynamically routes relevant information to decision-making areas in the brain and
suppresses the surrounding clutter.
But just how the brain achieves this feat has been the topic of much debate. In an earlier issue of Neuron,
Reynolds and David J. Heeger, Ph.D., a professor in the Department of Psychology and the Centre for Neural
Science at NYU, put forth a new theoretical model of attention. Their model suggests that attention co-opts the
same neural circuitry used by the visual system to adjust its sensitivity, which allows us to perceive the world
irrespective of huge changes in contrast and illumination over the day.
The central role of attention in perception has been known since the dawn of experimental psychology. An
enormous amount of research has been done on the subject, but ostensibly conflicting experimental data have
bewildered researchers for years, says Reynolds. Our model brought what seemed like a hodgepodge of
observations together within a simple framework, and our latest study tested and confirmed predictions of the
theory.
The strength of visual input fluctuates over orders of magnitude. The visual system reacts automatically to these
changes by adjusting its sensitivity, becoming more sensitive in response to faint inputs, and reducing sensitivity
to strong inputs. For example, when we walk into a darkened lecture hall on a sunny day at first we see little, but
over time our visual system adapts, increasing its sensitivity to match the environment.
A subtler version of this is the so-called contrast gain control. Spend a few minutes staring at an Ansel Adams
photograph.
You will find that your visual system will adapt to low-contrast parts of the image, revealing subtleties that were
invisible at first, explains Reynolds. SD
Heeger proposed a simple but powerful model of the cortical circuitry that helps mediate this form of automatic
gain control. We believe that this circuitry has been co-opted through evolution, enabling the brain to exploit the
same circuitry to adjust its sensitivity endogenously, says Reynolds. It doesnt just adjust sensitivity in response
to changes in input strength, it also enables the brain to emphasize task-relevant information and suppress
neuronal signals driven by task-irrelevant clutter.
Neurons in the visual cortex view the world through their receptive fields, the small portion of the visual field
individual neurons actually see or respond to. Whenever a stimulus falls within the receptive field, the cell
produces a volley of electrical spikes, known as action potentials that convey information about the stimulus in
the receptive field.
But the strength and fidelity of these signals also depends on other factors. Scientists generally agree that
neurons typically respond more strongly when attention is directed to the stimulus in their receptive fields. In
addition, the response of individual neurons can be strongly influenced by whats happening within the
immediate surroundings of the receptive field, a phenomenon known as contextual modulation.
The surround has the ability to suppress the neurons response, explains first author Kristy Sundberg, Ph.D., a
former graduate student in Reynolds lab and now a postdoctoral researcher at Yale University. It keeps us from
responding all the time if theres something thats big and uniform and not particularly interesting or useful. This
raised the possibility that the receptive field surround might provide a way to suppress the responses of
task-irrelevant distracters.
To get to the bottom of this, Sundberg set up a series of experiments in which she placed one stimulus in the
receptive field and another in the surround. As predicted by Reynolds and Heegers theory, she found that
directing attention to the centre stimulus immunized the neuron from the suppressive effects of the stimulus in the
surround. When she instead directed attention to a stimulus in the surround, it suppressed the neurons response
to the task-irrelevant stimulus in the centre.
The attentional system exploits the centre-surround organization of the receptive field to keep neurons that
transmit task-relevant information from being suppressed by distracters in the environment, while at the same
time suppressing the responses of neurons that respond to irrelevant clutter, says Sundberg. The brain uses
the receptive field surround actively to separate the wheat from the chaff.
Jude F. Mitchell, Ph.D., a postdoctoral researcher in Reynolds lab also contributed to the study. SD