The cortical column: a structure without a function

This year, the field of neuroscience celebrates the 50th anniversary of Mountcastle's discovery of the cortical column. In this review, we summarize half a century of research and come to the disappointing realization that the column may have no function. Originally, it was described as a discrete structure, spanning the layers of the somatosensory cortex, which contains cells responsive to only a single modality, such as deep joint receptors or cutaneous receptors. Subsequently, examples of columns have been uncovered in numerous cortical areas, expanding the original concept to embrace a variety of different structures and principles. A ‘column’ now refers to cells in any vertical cluster that share the same tuning for any given receptive field attribute. In striate cortex, for example, cells with the same eye preference are grouped into ocular dominance columns. Unaccountably, ocular dominance columns are present in some species, but not others. In principle, it should be possible to determine their function by searching for species differences in visual performance that correlate with their presence or absence. Unfortunately, this approach has been to no avail; no visual faculty has emerged that appears to require ocular dominance columns. Moreover, recent evidence has shown that the expression of ocular dominance columns can be highly variable among members of the same species, or even in different portions of the visual cortex in the same individual. These observations deal a fatal blow to the idea that ocular dominance columns serve a purpose. More broadly, the term ‘column’ also denotes the periodic termination of anatomical projections within or between cortical areas. In many instances, periodic projections have a consistent relationship with some architectural feature, such as the cytochrome oxidase patches in V1 or the stripes in V2. These tissue compartments appear to divide cells with different receptive field properties into distinct processing streams. However, it is unclear what advantage, if any, is conveyed by this form of columnar segregation. Although the column is an attractive concept, it has failed as a unifying principle for understanding cortical function. Unravelling the organization of the cerebral cortex will require a painstaking description of the circuits, projections and response properties peculiar to cells in each of its various areas.

Modulation of Dorsal Spinocerebellar Responses to Limb Movement. II. Effect of Sensory Input

Dorsal spinocerebellar tract (DSCT) neurons receive converging sensory inputs from muscle, skin, and joint receptors and their cerebellar projection is a product of the spinal sensory processing of movement-related information. We concluded earlier that DSCT activity relates to global rather than to local parameters of hindlimb postures and movement, specifically to a kinematic representation of the limb endpoint. The waveforms of principal components (PCs) derived from an ensemble of DSCT movement responses were found to correlate with either the waveform of the limb axis length or orientation trajectories. It was not clear, however, whether these global representations resulted from neural processing or from biomechanical factors. In this study, we perturbed the limb biomechanical factors by decoupling limb geometry from endpoint position during passively applied limb trajectories patterned after a step cycle. We used two types of perturbations: mechanical constraints that limited joint rotations and electrical stimulation of hindlimb muscles. We found that about half of the 89 cells studied showed statistically different response patterns during the perturbations. We compared the PCs of the altered responses with the PCs of the control responses, and found two basic results. With the joint constraints, >85% of the total variance in both control and changed responses was accounted for by the same five PCs that were also observed in the earlier study. The differences between altered and control responses could be fully accounted for by changes in the PC weighting, suggesting a modulation of global response components rather than an explicit representation of local parameters. With the muscle stimulation, only the first and third PCs were the same for the control and altered responses. The second PC was modified, and additional PCs were also required to account for the altered responses. This suggests that the stimulus parameters were specifically represented in the responses. The changes induced by both types of perturbation affected primarily the weighting or waveform of the second PC, which relates to the limb axis length trajectory. The results are consistent with the suggestion that information about limb orientation and length may be separately modulated.

Inhibition of canine H reflexes during locomotor-like rotation about the knee arises from muscle mechanoreceptors in quadriceps

1. H reflexes were elicited in the small muscles of the foot in the canine and human during passive locomotor-like rotation of the shank about the ipsilateral knee. The movement-induced effect was similar in the two species. In the anesthetized dog, the reflex gain was reduced by 36 +/- 8.4% (mean +/- SE) on average, compared with appropriate stationary controls. Reflexes in the human were reduced during movement to 45 +/- 3.5% of their stationary control values. 2. H reflexes were elicited in the anesthetized dog during passive locomotor-like rotation about the knee and were compared with reflexes obtained with the limb stationary. Populations of mechanoreceptors were then systematically removed to ascertain which group or groups provided the sensory input that leads to the decrease in reflex gain during movement. We hypothesized that the majority of the reflex attenuation could be attributed to muscle mechanoreceptors. 3. Reflexes continued to be significantly reduced (P < 0.05) during passive movement about the knee until the muscle mechanoreceptors of the quadriceps muscle group were deactivated. The removal of input from joint receptors or cutaneous receptors did not eliminate the gain reduction induced by the passive movement. 4. It is concluded that muscle mechanoreceptors of the quadriceps muscle group provide an inhibitory input to the H reflex pathway of the dog plantar muscle when the knee is passively moved in a locomotor-like fashion. This source of inhibition likely also contributes to the soleus H reflex gain reduction in humans.