Download In cognitive neuroscience, the prefrontal cortex represents a kind of

Bart Moore
Cognitive Neuroscience
Essay 2
Integration and Segregation in Prefrontal Cortex
In cognitive neuroscience, the prefrontal cortex (PFC) represents a kind of
final frontier as debate about the anatomical and functional organization is played out.
Current developments are reminiscent of questions raised by Gall’s phrenology: to
what degree is function segregated in the brain, and what functions are distributed
where? We can imagine that if there is widespread segregation of function in PFC, it
could be along lines drawn by the nature of the information being processed, or the
function being subserved, or some mix of the two. Fortunately, modern scientists
posses a plethora of advanced investigative anatomical and physiological techniques
that have been brought to bear on the issue of segregation of function in the prefrontal
cortices, and this essay will examine two primary articles: one supporting the theory
that PFC possesses anatomical and functional subdivisions consistent with a
segregation of information streams, and another paper supporting the theory that PFC
primarily serves to integrate information from other cortical areas.
Support for Segregation of Function in PFC
In extrastriate cortex, it is widely recognized that there are two separate
processing streams, loosely described as the ‘what’ and ‘where’ processing streams.
While regions in the superior parietal cortices selectively respond to visuo-spatial
stimuli, the inferior parietal and temporal regions are more selective for objects.
O’Scalaidhe, et al. (1997) report a study involving electrophysiological recordings
collected from macaque monkey PFC and propose that the ventral ‘what’ stream of
the visual cortex extends forward to PFC.
The authors found 46 ‘face selective’
cells in the inferior PFC in a highly circumscribed region supporting theories
proposing a segregation of prefrontal function by the type of sensory information.
The authors examined hundreds of cells in all areas of PFC and found face selective
cells in only in ventral areas during a passive viewing condition. Two types of faceselective cells were observed: onset and offset. Onset cells began firing when face
stimuli were presented and persisted in firing for some time thereafter. Offset cells
began firing when the face stimuli were removed and continued firing for several
seconds.
The authors found that the vast majority of cells were found in areas of PFC
which receive input from the temporal lobe visual areas, certainly supporting the idea
of a PFC extension to the ventral ‘what’ stream, and the idea of compartmentalization
of function in PFC. To their credit, the authors injected dyes at the termination of
cellular recordings in two monkeys and found projections to that site originating in
the superior temporal sulcus and inferior temporal gyrus. These are both regions
which have previously been implicated in visual face processing.
Support for Integration in the PFC
Perhaps, though, simply using the types of stimuli suitable for probing
response properties of neurons in sensory areas is not adequate for investigation in
PFC. Indeed, if one is to examine PFC for evidence of information integration,
shouldn’t a suitable experimental paradigm be used? Fortunately, Rao et al. have
conducted a similar experiment in macaque PFC which engaged both ‘what’ and
‘where’ working memory. They recorded from 195 neurons in ‘lateral’ PFC, but
unfortunately do not provide much more information regarding the specific location
of recording sites. The monkey was instructed to fixate on a fixation spot while a
pictorial stimulus was presented. After a delay, two objects were presented at two of
four positions flanking the test spot. One of the objects matched the initial object,
and after they were removed, the monkey was instructed to make a saccade to the
remembered location of the object matching the initial stimulus. Thus there were two
periods in which memory was required: after the initial presentation of the primary
stimulus, and after the presentation of the flanking ‘test’ stimuli. Furthermore, the
type of information required to be retained during those two periods differed: during
the first period, the monkey had to remember what picture was presented, while
during the second delay the monkey was required to remember where the object was
presented.
Of the 195 neurons recorded, 123 showed significant activity during the
‘what’ delay, the ‘where’ delay, or both. Of those 123 PFC neurons, 7% showed
delay activity that was significantly tuned to the ‘what’ delay period, while 41%
showed activity that was selectively tuned to the ‘where’ delay period. These results
alone would not be surprising, and could support a segregation of what and where
visual processing streams even if their location distribution sets overlapped. What is
remarkable is that over half (64/123) neurons showed significantly increased activity
during both the ‘what’ delay and the ‘where’ delay, implicating them in both object
and spatial working memory. The authors claim that these ‘what/where’ cells may
contribute to the integration of information regarding visual object identity and spatial
location. Interestingly, the ‘what/where’ cells were found in both ventral and dorsal
frontal areas. The ‘what’ and ‘where’ cells likewise showed no anatomical clustering
or segregation. These results are in contrast to those of O’Scalaidhe, et al. (1997),
who report a strong clustering of face-specific cells in the inferior PFC.
These two studies provide some fodder for each camp in the debate about
localization of function in the prefrontal lobe. The quality of the data presented is
hard to contest, and thus some reconciliation between the strong stances of the two
sets of authors must take place. In truth, the prefrontal lobe is comprised of billions
of neurons and seems somewhat different from the other lobes in that it is involved in
executive control and the genesis of action, both of which require convergent input
from different sensory streams to be useful. Attempting to wholly carve the frontal
lobe into discreet functional areas analogous to those in the occipital lobe might prove
futile, but there do appear to be some specialized areas such as Brocha’s area and the
frontal eye fields, and even perhaps an area specialized for complex objects like
faces, as suggested by the data presented by O’Scalaidhe et al.
Future Directions
Using fMRI to investigate the distribution of information processing in prefrontal
cortex would appear to be a viable way to carry the debate to a higher level, and no
doubt can address some questions about the gross architecture. fMRI lacks the
resolution, however to detect subtle concentrations of neurons such as those presented
by O’Scalaidhe, et al. The same is true for extracranial electrodes, and probably local
field electrodes as well. Are we doomed to fish with our electrode poles forever in
the prefrontal cortex in order to map its functional organization? Let us hope not!
One of the most promising avenues of research with regard to this debate is
anatomical. Indeed, one of the strongest pieces of evidence presented by O’Scalaidhe
et al. is that dyes injected into the site of face-specific frontal cells outlined
(presumably feedforward) projections from temporal lobe. Unraveling the major
anatomical projections into the PFC will be highly informative for physiologists and
should help guide the ‘poles’ of single-cell electrophysiologists.
In specific one possible avenue to be explored would be to conduct an experiment
complimentary to that of O’Scalaidhe. Instead of looking for neurons that responded
to complex objects like faces, however, one could look for neurons selective for
visuo-spatial information. It would be interesting if such a distribution of neurons
were found clustered in a ventral prefrontal area, and would suggest that the theory of
continuation of the ‘what’ and ‘where’ visual processing streams is indeed present in
the posterior frontal lobe. This notion could possibly be confirmed by again injecting
dyes into the sites of putative spatially selective cells and examining the brain for
traces of the dye in ventral parietal areas.