Corticothalamic Loops Research Articles

Early integration of retinal and extra-retinal information: recent results and hypotheses.

This review aims at highlighting and discussing some recent neurophysiological and psychological findings and hypotheses about early cortical integration of visual and extra-retinal information which is involved in the stability of visual perception in spite of the change of body orientation and distance to objects and eye movements. Within this framework, the possible role of some cortical and cortico-thalamic loops between earlier and later stages of visual processing is also emphasized. A complete study of multimodal processing is beyond the scope of this review.

The Functional Neuroanatomy of Awareness: With a Focus on the Role of Various Anatomical Systems in the Control of Intermodal Attention

This review considers a number of recent theories on the neural basis of consciousness, with particular attention to the theories of Bogen, Crick, Llinás, Newman, and Changeux. These theories allot different roles to various key brain areas, in particular the reticular and intralaminar nuclei of the thalamus and the cortex. Crick's hypothesis is that awareness is a function of reverberating corticothalamic loops and that the spotlight ofintramodalattention is controlled by the reticular nucleus of the thalamus. He also proposed different mechanisms for attention and intention (“will”). The current review presents a new hypothesis, based on elements from these hypotheses, includingintermodalattention and olfaction and pain, which may pose problems for Crick's original theory. This work reviews the possible role in awareness and intermodal attention and intention of the cholinergic system in the basal forebrain and the tegmentum; the reticular, the intralaminar, and the dorsomedial thalamic nuclei; the raphe and locus coeruleus; the reticular formation; the ventral striatum and extended amygdala; insula cortex, and other selected cortical areas. Both clinical and basic research data are covered. The conclusion is reached that the brain may work by largely nonlinear parallel processing and much intramodal shifts of attention may be effected by intracortical, or multiple corticothalamic mechanisms (small local “flashlights” rather than one major “searchlight”). But this is constrained by the functional anatomy of the circuits concerned and waking “awareness” is modulated by the many “nonspecific” systems (cholinergic from the basal forebrain, noradrenergic from the locus coeruleus, dopaminergic from the substantia nigra and ventral tegmentum, and serotoninergic from the raphe). But the principal agents for intermodal attention shifts, the “searchlight,” may be two key nuclei of the cholinergic system in the mesencephalon. Clinical loss of consciousness results from damage to these nuclei but not from damage to the cholinergic nucleus basalis of the basal forebrain.

Cortical and thalamic activity in the late phases of somatosensory evoked potentials

Evoked potentials and associated neuronal activity, induced by repetitive stimulation of the sciatic nerve with brief electrical pulses, were recorded simultaneously from the somatosensory cortex, the ventroposterolateral nucleus and the centre median nucleus in cats, under gallamine. Because the early components of the response, extending from the stimulus to 500 msec, are of much higher amplitude than the late components, extending beyond the first 500 msec, up to 2500 msec after the stimulus, the latter may be ‘masked’ when both sections, early and late, are recorded at the same magnification. For this reason the late sections of the evoked potentials and associated neuronal activity were studied separately, at higher gain. Averaging and autocorrelation studies of the late part of the evoked potential and of the associated neuronal discharge, show that their highest amplitude of oscillation and their highest degree of periodicity are reached at specific frequencies of stimulation, in the cortex as well as in the thalamus. Cross-correlation studies of the relations between gross and neuronal activities within the cortex or within the thalamus, as well as studies of the relations between cortical and thalamic activities, show that the highest degree of correlation occurs when the frequencies of stimulation are within the same range. This suggests that the development of the late oscillations of the evoked potential and of the associated neuronal discharge is based on the activity of cortical, thalamic and corticothalamic loops which respond most effectively at frequencies of stimulations related to their resonance characteristics.