Differentiating the Significance of Attention-related Neurons
Casey Green, greenc421 @mail.utexas.edu, University of Texas Austin
Although the physical properties of inhibitory interneurons and pyramidal
neurons have been distinguished in several ways, there has been no direct evidence for
the role each plays and how they interact, particularly with respect to their modulation
during the processing of attention-induced visual stimuli. This is a large barrier to
improving our understanding of encoding and processing visual stimuli and how the brain
handles attentive tasks in the face of these visual stimuli. This is the target of Mitchell,
Sundberg and Reynolds’ study, in which they explore the roles that narrow- and broadspiking neurons play in attention. By observing particular neurons in area V4 of the
cortex in monkeys during conditioned, high-attention tasks, they were able to distinguish
patterns of the two categories of neurons that helped deduce their function. To their
surprise, narrow-spiking neurons, which are local interneurons that do not communicate
with other areas of the cortex, exhibited the highest amount of activity, suggesting an
important role in processing attention-induced visual stimuli.
Mitchell et al began by isolating neurons that exhibited clear spike waveforms,
separating them based on potential width and other known properties. They observed
these neurons and verified their physical properties and activities, comparing them with
previous studies of cortical neurons to verify their similarities. As expected, the isolated
narrow-spiking inhibitory neurons produced higher firing rates, and narrower action
potential widths. Also congruent with previous studies, the isolated broad-spiking
excitatory neurons produced shallower waveforms, lower response, and made up
approximately 73% of the total neurons being observed, the average distribution in the
cortex. Once isolated and verified, these distinct set of neurons were tested on several
permutations of visual stimuli.
It was found that inhibitory neurons exhibited a firing rate two times that of the
broad-spiking excitatory neurons in both the presence and absence of visual stimuli.
Broad-spiking neurons also exhibited an increase, but was much more variable; some
produced significant increases, while others significant decreases. The narrow-spiking
neurons firing rate was much more consistent, producing considerable increases in all
such neurons being observed. The Fano factor was more significant for narrow- than
broad-spiking neurons, also inferring a considerable difference in the two. This suggests
that the qualities of the firing rate in the narrow-spiking neurons were dependent on
visual stimuli. There was also evidence that, when attention was directly on the stimulus
in the receptor field, there was higher reliability and consistency and reduced variability
of narrow-spiking neuronal response. This was present for broad-spiking neurons but
was much stronger in inhibitory neurons, suggesting the attention-dependent
pronouncement of narrow-spiking neurons as well. So, not only did the narrow-spiking
neurons exhibit an increase in firing strength for the attention task, but were also more
reliable and responsive.
From this, Mitchell et al conclude that these broad- and narrow-spiking neurons
are functionally distinct. Although it is very likely that they include many subcategories
of neurons within their broader functional categories, they seem to generally serve two
distinct purposes. Specifically, due to the various properties observed, such as
distribution, action potential width, and stimulus-driven activity, they conclude that most
of the broad-spiking neurons must have been pyramidal neurons, while the narrowspiking neurons were inhibitory interneurons. This conclusion, coupled with their
observations, is surprising, because it was previously assumed that increased firing rate
was due to signal transmission from one cortical area to the next. Since this job is
performed by pyramidal neurons, one would expect that the broad-spiking neurons would
exhibit the more pronounced and consistent increase in firing rates than the interneurons
did in this case, in the presence of stimuli. Thus, it appears that these narrow-spiking
interneurons play a significant, localized function in attention.
It is widely accepted that specific neurons send encoded “unary” signals
throughout the brain in parallel to perform certain tasks, such as responding to stimuli.
This study supports this claim in that it appears that specific neurons perform specialized,
complementary tasks in the presence of visual stimuli. Thus, this study may provide a
baseline for modeling the neuronal translation process from visual stimuli to the cortex.
Although still much more must be learned about the subcategories of neurons, and the
specific specialized functions they perform, an abstract model of processing visual
stimuli is feasible. As we learn more about the mapping function from the retina to the
different layers of the cortex, and the properties of the signals sent through these
mappings, more can be added to the model. The encoding of these broad- and narrowspikes seems to be the biggest obstacle, but identifying the category of neurons that
exhibit the most important role in processing attention-bearing stimuli is certainly a good
starting point.