These subdivisions conform to similar subdivisions in other mammals fig. Because of the ubiquity of these fields, they were likely to be present in the ancestor of all rodents and all mammals. Although nomenclature varies it is that of the original authors , putative homologous areas have been color-coded for ease of comparison.
Rostral is left; medial is up. All drawings are to scale. In recent comparative studies of myeloarchitecture in our laboratory, we measured the area of a number of well-defined cortical fields in several species of rodents [Campi et al. These studies provided an opportunity to make several interesting comparisons within the order Rodentia fig. First, we compared diurnal rodents both species of squirrels as well as Nile grass rats with nocturnal Norway rats rodents.
Second, we compared arboreal tree squirrel and terrestrial ground squirrel squirrels. Finally, we compared wild-caught animals squirrels and R. Measures of the brain, the body, the dorsolateral cortical sheet, and individual cortical fields revealed several interesting findings. First, wild-caught rats had a relatively larger brain as a fraction of body weight than did the laboratory rats, and tree squirrels had a relatively larger brain than did all rodents as a fraction of body weight; fig.
Squirrels had a larger encephalization quotient EQ; the measured brain mass compared to the predicted brain mass based on body mass than did rat groups, and tree squirrels had a higher EQ than did ground squirrels fig. For measures of individual cortical fields, both species of squirrels had a larger percentage of cortex devoted to area 17 V1 as well as other visual areas including area 18, OT occipitotemporal area or V3 , and TP temporal posterior area fig.
Within the squirrel group more cortex was devoted to visual areas in the arboreal tree squirrel while more cortex was devoted to somatosensory areas S1, S2, and PV in the terrestrial and burrowing ground squirrel. Photomicrographs of 1 section from each species are shown; any individual section does not show all cortical field boundaries. In all of these sections S1 and V1 can be readily identified. The auditory core is most clearly observed in the sections representing rats but can be identified in the other species when other sections from a series are examined.
Conventions are as in previous figures [taken from Campi and Krubitzer, ; Campi et al. Each colored bar represents a different rodent. In a , the y-axis is brain weight as a percentage of body weight. In b , the y-axis is the measured brain mass compared to the predicted brain mass based on body mass. In c—e , each colored bar represents a different rodent for a specific area of cortex, and the y-axis shows the percentage of dorsolateral cortex area that individual areas occupy.
There are large differences within and across rodent groups associated with lifestyle and rearing conditions. Error bars represent the standard error of the mean. An interesting and important observation was that differences in cortical field size emerged in the same species of rat R. Additional differences were also noted at a cellular level between these groups, and the significance of this finding will be discussed at the end of the section on the visual system. Our comparisons [Campi and Krubitzer, ; Campi et al. However, the differences in cortical field size between species may also arise due to simple allometry or a linear scaling of cortical fields with brain size.
In fact, meta-analyses of data from several orders of mammals demonstrate that differences in the size of visual areas between species are due to allometry and that primary fields co-vary with brain size and not with niche specialization [e. Kaskan et al. This is surprising because, along a number of dimensions, the neocortex relates strongly to sensory specializations and lifestyle see comparisons of sensory cortex in this review.
Such discrepancies between conclusions reached from allometric studies and other studies may be methodological.
For example, in previous allometric analyses, the data were collected by multiple laboratories using a variety of techniques. Many critical measures necessary to perform an accurate allometric analysis, such as the size of the cortical sheet area or volume , brain weight or volume , and individual cortical fields area or volume , were not provided in some of the studies that formed the backbone of the meta-analysis [Jones and Burton, ; Krubitzer and Kaas, ; Felleman and Van Essen, ; Krubitzer et al. In addition, conclusions regarding specific changes in cortical area allocation were based on regression slopes derived from data that combined several mammalian orders, thereby obscuring specializations in lineage, and were skewed by including more data from highly visual lineages such as primates.
Finally, species that showed extreme specialization such as platypuses and echidnas were eliminated from the analysis. To resolve this issue, we performed our own analysis using the rodent species in which sizes of cortical fields were compared tree squirrels, ground squirrels, laboratory rats, wild-caught rats, and Nile grass rats and examined the relationship between cortical sheet size and cortical field size to see if changes in brain size alone could account for the differences in cortical field size that we observe in these rodents [Campi et al.
This analysis controls for potential lineage differences because all species are rodents, and it controls for other confounds mentioned above because the same histological methods and criteria were used to define a cortical field. We calculated the regression line for our data log transformed and found that, across rat groups, all 3 primary sensory fields had different positive slopes, meaning that as rat brains increase in size all of these fields increase in size but at different rates.
The steepest slope is observed for S1 fig. That is, S1 increases in size at a faster rate than V1 and A1 as brains get larger.
The Barrel Cortex of Rodents
On the other hand, in wild-caught diurnal ground and tree squirrels, V1 has the steepest slope [Campi and Krubitzer, fig. These results point to a conclusion similar to that obtained from our more straightforward percentage measurements in squirrel and rat groups that primary sensory areas differentially change size. Thus, in addition to scaling of cortical fields with brain size, the general organization of the neocortex in terms of sensory domain allocation the amount of cortex devoted to a particular sensory system and cortical field size reflects the types of sensory and behavioral specializations associated with the lifestyle of a particular rodent.
Our review on the organization of cortical fields and the cellular composition of cortical fields substantiates this idea by demonstrating large species differences within the rodent order at all levels of organization. It should be noted that, in the rat group, S1 has the steepest slope compared with the squirrel groups in which V1 has the steepest slope. V1 individual animal points are depicted by triangles and the slope is a solid black line; S1 individual animal points are depicted by squares and the slope is a solid gray line, and A1 individual animal points are depicted by circles and the slope is a dashed line.
Symbols are colored by species. Visual cortex organization has been described in several species of rodents using a variety of techniques including electrophysiological recordings and optical imaging [Hall et al. In all rodents examined, V1, which is coextensive with architectonic area 17, contains a complete representation of the contralateral visual hemifield, with the upper quadrant represented caudolaterally and the lower quadrant represented rostromedially. The location of V1 is indicated in blue in the small brains at the top of each figure. V1 organization is similar in all rodents depicted; the upper visual field is represented caudally and the lower visual field is represented rostrally.
Just lateral to V1 is a mirror representation, i. V2, in the mouse, rat, squirrel, and hamster. In rats this region contains 2 fields, i. AL and LM. Medial to V1 is 1 or 2 separate visual areas in mice, rats, and hamsters. Central vision is highlighted by gray shading. The horizontal meridian is represented by a blue line. The vertical meridian is represented by a pink line. Arrows in ML and L of the squirrel indicate that neurons in these regions are direction selective. Rostral is to the left; medial is up.
In a—c maps are to the same scale. The squirrel d is shown at one half the scale of a—c. The relative brain-to-map scale is consistent across species, so brains in a—c are presented at the same scale. For this and the following illustrations, the maps may be redrawn for either a single case within a study or 2 cases combined; some of these are summary maps provided by the authors of a paper, and some of these are summary maps generated from more than 1 study.
Individual studies are listed around all summarized maps.
While the general location and retinotopic organization of V1 is similar across rodents and across all mammals [Van Hooser, ; Kaas, ], important interspecies differences exist and are often correlated with aspects of lifestyle. For example, the relative size of V1 varies in different species, with arboreal diurnal squirrels having a larger proportion of the cortical sheet devoted to V1 and other visual fields see above compared to terrestrial squirrels and nocturnal rats.
Passar bra ihop
There are also clear differences in the extent of cortex representing the overlap of the visual fields of both eyes fig. For example, the binocular segment of V1 is much larger in squirrels than in murine rodents and is larger in the tree squirrel than in the ground squirrel. It is possible that the degree of binocular overlap and its corresponding representation is associated more directly with terrain niche than with diel pattern due to visual adaptations such as enhanced depth perception, which would be helpful for rapid navigation and branch-to-branch leaps associated with the habits of the tree squirrel [Koprowski, ].
Studies of single units in V1 in squirrels and murine rodents demonstrate that receptive fields for neurons in V1 in mice are larger than in squirrels [Hall et al.
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While neurons in V1 in rats, mice, and squirrels contain cells that are orientation and direction selective, there are differences in the proportions of these cells [for comparisons between rodents see table 2 in Heimel et al. Rats have more direction- and orientation-selective cells than either squirrels or mice, and squirrels have more orientation-selective cells than mice. Neurons in V1 in squirrels show color opponency and have higher spatial and temporal tuning than in mice and rats.
These differences have been suggested to optimize aspects of vision in diurnal versus nocturnal visual environments.
The Functional Microarchitecture of the Mouse Barrel Cortex
A similar pattern of V1 response properties is also seen in other nocturnal and diurnal mammals, particularly in primates [for review see Heimel et al. Despite the presence of orientation-selective cells in V1, an orderly orientation map like that observed in primates has not been observed in any rodent [Van Hooser et al. The black box in a shows the area of magnification for b and c. The monocular mono and binocular bino segments are labeled.