Functional Organization of Human Visual Cortical Areas

Telencephalon: Neocortex

The neocortex is an ultracomplex, six-layered structure that develops from the dorsal palliai sector of the telencephalic hemispheres (Figs. 2.24, 2.25, 11.1). All mammals, including monotremes and marsupials, possess a neocortex, but in reptiles, i.e. the ancestors of mammals, only a three-layered neocortical primordium is present [509, 511]. The term neocortex refers to its late phylogenetic appearance, in comparison to the “palaeocortical” olfactory cortex and the “archicortical” hippocampal cortex, both of which are present in all amniotes [509].

Neural networks for internal reading and visual imagery of reading: a PET study

Regional cerebral blood flow (rCBF) measurements with positron emission tomography (PET) were made on 10 volunteers in rest condition as well as while the subjects, with closed eyes, (i) internally listed the letters of the alphabet and cited the first verse of the Hungarian national anthem, (ii) visualised the capital letters of the alphabet, and (iii) visualised the capital letters of the first verse of the Hungarian national anthem. Significant changes in rCBF indicated various networks of cortical neuronal populations active during the tasks. Internal listing, as compared to the rest condition, activated the left precentral gyrus. Visualising the letters of the alphabet, when compared to the rest condition, activated a cortical network comprising fields along the banks of the left and right intraparietal sulci, the left medial frontal, precentral and occipital sulci, and the right superior frontal gyrus. Visualising the letters of the anthem, when compared to the rest condition, activated a cortical network comprising fields along the banks of the left and right intraparietal sulci, the left medial and inferior frontal gyri, and the right anterior cingulate gyrus. Contrasting the two visualisation tasks revealed task specific activation in the left lateral occipital gyrus (alphabet vs. anthem visualisation) and in the left anterior cingulate gyrus (anthem vs. alphabet visualisation). The data indicate that visual imagery of letters of the alphabet or a text engages a widespread network of cortical fields in the visual association cortices and the frontal cortex, without the engagement of the primary (V1) and secondary (V2) visual cortical areas. This finding supports the hypothesis that neuronal populations engaged by visual imagery and visual perception only partially overlap. The networks, activated in the visualisation tasks, have a core which is identical in the different visualisation tasks. The core network is complemented in a task-specific manner by the recruitment of additional cortical neuronal populations.

Visual stimulus-dependent changes in interhemispheric EEG coherence in humans

We analyzed the coherence of electroencephalographic (EEG) signals recorded symmetrically from the two hemispheres, while subjects (n = 9) were viewing visual stimuli. Considering the many common features of the callosal connectivity in mammals, we expected that, as in our animal studies, interhemispheric coherence (ICoh) would increase only with bilateral iso-oriented gratings located close to the vertical meridian of the visual field, or extending across it. Indeed, a single grating that extended across the vertical meridian significantly increased the EEG ICoh in normal adult subjects. These ICoh responses were obtained from occipital and parietal derivations and were restricted to the gamma frequency band. They were detectable with different EEG references and were robust across and within subjects. Other unilateral and bilateral stimuli, including identical gratings that were effective in anesthetized animals, did not affect ICoh in humans. This fact suggests the existence of regulatory influences, possibly of a top-down kind, on the pattern of callosal activation in conscious human subjects. In addition to establishing the validity of EEG coherence analysis for assaying cortico-cortical connectivity, this study extends to the human brain the finding that visual stimuli cause interhemispheric synchronization, particularly in frequencies of the gamma band. It also indicates that the synchronization is carried out by cortico-cortical connection and suggests similarities in the organization of visual callosal connections in animals and in man.

The role of diaschisis in stroke recovery

BACKGROUND AND PURPOSE

Recovery from hemiparesis after stroke has been shown to involve reorganization in motor and premotor cortical areas. However, whether poststroke recovery also depends on changes in remote brain structures, ie, diaschisis, is as yet unresolved. To address this question, we studied regional cerebral blood flow in 7 patients (mean+/-SD age, 54+/-8 years) after their first hemiparetic stroke.

METHODS

We analyzed imaging data voxel by voxel using a principal component analysis by which coherent changes in functional networks could be disclosed. Performance was assessed by a motor score and by the finger movement rate during the regional cerebral blood flow measurements.

RESULTS

The patients had recovered (P<0. 001) from severe hemiparesis after on average 6 months and were able to perform sequential finger movements with the recovered hand. Regional cerebral blood flow at rest differentiated patients and controls (P<0.05) by a network that was affected by the stroke lesion. During blindfolded performance of sequential finger movements, patients were differentiated from controls (P<0.05) by a recovery-related network and a movement-control network. These networks were spatially incongruent, involving motor, sensory, and visual cortex of both cerebral hemispheres, the basal ganglia, thalamus, and cerebellum. The lesion-affected and recovery-related networks overlapped in the contralesional thalamus and extrastriate occipital cortex.

CONCLUSIONS

Motor recovery after hemiparetic brain infarction is subserved by brain structures in locations remote from the stroke lesion. The topographic overlap of the lesion-affected and recovery-related networks suggests that diaschisis may play a critical role in stroke recovery.