Visual stimuli activate auditory cortex in the deaf.

Previous brain imaging studies have demonstrated responses to tactile and auditory stimuli in visual cortex of blind subjects, suggesting that removal of one sensory modality leads to neural reorganization of the remaining modalities. To investigate whether similar 'cross-modal' plasticity occurs in human auditory cortex, we used functional magnetic resonance imaging (fMRI) to measure visually evoked activity in auditory areas of both early-deafened and hearing individuals. Here we find that deaf subjects exhibit activation in a region of the right auditory cortex, corresponding to Brodmann's areas 42 and 22, as well as in area 41 (primary auditory cortex), demonstrating that early deafness results in the processing of visual stimuli in auditory cortex.

An accessible pharmacodynamic transcriptional biomarker for notch target engagement.

γ-Secretase mediates amyloid production in Alzheimer's disease (AD) and oncogenic activity of Notch. γ-Secretase inhibitors (GSIs) are thus of interest for AD and oncology. A peripheral biomarker of Notch activity would aid determination of the therapeutic window and dosing regimen for GSIs, given toxicities associated with chronic Notch inhibition. This study examined the effects of GSI MK-0752 on blood and hair follicle transcriptomes in healthy volunteers. The effects of a structurally diverse GSI on rhesus blood and hair follicles were also compared. Significant dose-related effects of MK-0752 on transcription were observed in hair follicles, but not blood. The GSI biomarker identified in follicles exhibited 100% accuracy in a clinical test cohort, and was regulated in rhesus by a structurally diverse GSI. This study identified a translatable, accessible pharmacodynamic biomarker of GSI target engagement and provides proof of concept of hair follicle RNA as a translatable biomarker source.

Major glutamatergic projection from subplate into visual cortex during development.

Subplate neurons, the first neurons of the cerebral cortex to differentiate and mature, are thought to be essential for the formation of connections between thalamus and cortex, such as the system of ocular dominance columns within layer 4 of visual cortex. To learn more about the requirement for subplate neurons in the formation of thalamocortical connections, we have sought to identify the neurotransmitters and peptides expressed by the specific class of subplate neurons that sends axonal projections into the overlying visual cortex. To label retrogradely subplate neurons, fluorescent latex microspheres were injected into primary visual cortex of postnatal day 28 ferrets, just prior to the onset of ocular dominance column formation. Subsequently, neurons were immunostained with antibodies against glutamate, glutamic acid decarboxylase (GAD-67), parvalbumin, neuropeptide Y (NPY), somatostatin (SRIF), or nitric oxide synthase (NOS). Retrograde labeling results indicate that the majority of subplate neurons projecting into the cortical plate reside in the upper half of the subplate. Combined immunostaining and microsphere labeling reveal that about half of cortically projecting subplate neurons are glutamatergic; most microsphere-labeled subplate neurons do not stain for GAD-67, parvalbumin, NPY, SRIF, or NOS. These observations suggest that subplate neurons can provide a significant glutamatergic synaptic input to the cortical plate, including the neurons of layer 4. If so, excitation from the axons of subplate neurons may be required in addition to that from lateral geniculate nucleus neurons for the activity-dependent synaptic interactions that lead to the formation of ocular dominance columns during development.

Visual contrast sensitivity in deaf versus hearing populations: exploring the perceptual consequences of auditory deprivation and experience with a visual language.

Early deafness in humans provides a unique opportunity to examine the perceptual consequences of altered sensory experience. In particular, visual perception in the deaf may be altered as a result of their auditory deprivation and/or because the deaf rely heavily upon a visual language (American Sign Language, or ASL, in the US). Recently, we found that deaf, but not hearing, subjects exhibit a right visual field/left hemisphere advantage on a low-level direction of motion task, a finding that has been attributed to the deaf's experience with ASL [Psychol. Sci. 10 (1999) 256; Brain Res. 405 (1987) 268]. In order to determine whether this visual field asymmetry generalizes to other low-level visual functions, in this study we measured contrast sensitivity in deaf and hearing subjects to moving stimuli over a range of speeds (0.125-64 degrees /s). We hypothesized that if ASL use drives differences between hearing and deaf subjects, such differences may occur over a restricted range of speeds most commonly found in ASL. In addition, we tested a third group, hearing native signers who learned ASL early from their deaf parents, to further assess whether potential differences between groups results from ASL use. These experiments reveal no overall differences in contrast sensitivity, nor differences in visual field asymmetries, across subject groups at any speed tested. Thus, differences previously observed between deaf and hearing subjects for discriminating the direction of moving stimuli do not generalize to contrast sensitivity for moving stimuli, a result that has implications for the neural level at which plastic changes occur in the visual system of deaf subjects.

Establishment of patterned thalamocortical connections does not require nitric oxide synthase.

Subplate neurons are early-generated neurons that project into the overlying neocortex and are required for the formation of ocular dominance columns. A subset of subplate neurons express nitric oxide synthase (NOS) and produce nitric oxide (NO), a neuronal messenger thought to be involved in adult hippocampal synaptic plasticity and also in the establishment of certain specific connections during visual system development. Here, we examine whether the NOS-containing subplate neurons are involved in ocular dominance column formation in the ferret visual system. Ocular dominance columns form in ferrets between postnatal day 35 (P35) and P60. NOS expression in the visual subplate is low at birth, increases to a maximum at the onset of ocular dominance column formation, and falls thereafter. Nevertheless, blockade of NOS with daily injections of nitroarginine from P14 to P56 fails to prevent the formation of ocular dominance columns, although NOS activity is reduced by >98%. To test further a requirement for NOS in the patterning of connections during CNS development, we examined the cortical barrels in the somatosensory system of mice carrying targeted disruptions of NOS that also received injections of nitroarginine; cortical barrels formed normally in these animals. In addition, barrel field plasticity induced by whisker ablation at birth was normal in nitroarginine-injected NOS knock-out mice. Thus, despite the dynamic regulation of NOS in subplate neurons, NO is unlikely to be essential for the patterning of thalamocortical connections either in visual or somatosensory systems.

Subplate neuron ablation alters neurotrophin expression and ocular dominance column formation.

Ocular dominance column formation in visual cortex depends on both the presence of subplate neurons and the endogenous expression of neurotrophins. Here we show that deletion of subplate neurons, which supply glutamatergic inputs to visual cortex, leads to a paradoxical increase in brain-derived neurotrophic factor mRNA in the same region of visual cortex in which ocular dominance columns are absent. Subplate neuron ablation also increases glutamic acid decarboxylase-67 levels, indicating an alteration in cortical inhibition. These observations imply a role for this special class of neurons in modulating activity-dependent competition by regulating levels of neurotrophins and excitability within a developing cortical circuit.