Recruitment of Local Inhibitory Networks by Horizontal Connections in Layer 2/3 of Ferret Visual Cortex. J. Neurophysiol. 89: 501-512, 2003. To investigate how neurons in cortical layer 2/3 integrate horizontal inputs arising from widely distributed sites, we combined intracellular recording and voltage-sensitive dye imaging to visualize the spatiotemporal dynamics of neuronal activity evoked by electrical stimulation of multiple sites in visual cortex. Individual stimuli evoked characteristic patterns of optical activity, while delivering stimuli at multiple sites generated interacting patterns in the regions of overlap. We observed that neurons in overlapping regions received convergent horizontal activation that generated nonlinear responses due to the emergence of large inhibitory potentials. The results indicate that co-activation of multiple sets of horizontal connections recruit strong inhibition from local inhibitory networks, causing marked deviations from simple linear integration.

The neural integration of afferent inputs evoked by spatiotemporally distributed sensory stimuli is a critical step in the formation of coherent and continuous perceptual representations. Integration mechanisms in various systems include linear and nonlinear summation of sensory responses. One well-known example in the rat barrel system is the suppressive interaction between responses to the consecutive deflection of neighboring whiskers. The mechanism underlying cross-whisker suppression has long been postulated to rely on intracortical postsynaptic inhibition, although this hypothesis has been challenged by recent reports. Here we show, using intracellular and extracellular recordings in vivo, that cross-whisker suppression occurs in the absence of cortical activity. Instead, suppression arises from local circuit operations at multiple levels of the subcortical afferent pathway and is amplified by the nonlinear transformation of synaptic input into spike output in both the thalamus and cortex. Because these cellular processes are common to neural circuits subserving visual and auditory modalities, we propose that the suppressive mechanisms elucidated here are a general property of thalamocortical sensory systems.

The function and nature of inhibition of neurons in the visual cortex have been the focus of both experimental and theoretical investigations1, 2, 3, 4, 5, 6, 7. There are two ways in which inhibition can suppress synaptic excitation2,8. In hyperpolarizing inhibition, negative and positive currents sum linearly to produce a net change in membrane potential. In contrast, shunting inhibition acts nonlinearly by causing an increase in membrane conductance; this divides the amplitude of the excitatory response. Visually evoked changes in membrane conductance have been reported to be nonsignificant or weak, supporting the hyperpolarization mode of inhibition3,9, 10, 11, 12. Here we present a new approach to studying inhibition that is based on in vivo whole-cell voltage clamping. This technique allows the continuous measurement of conductance dynamics during visual activation. We show, in neurons of cat primary visual cortex, that the response to optimally orientated flashed bars can increase the somatic input conductance to more than three times that of the resting state. The short latency of the visually evoked peak of conductance, and its apparent reversal potential suggest a dominant contribution from gamma-aminobutyric acid ((GABA)A) receptor-mediated synapses. We propose that nonlinear shunting inhibition may act during the initial stage of visual cortical processing, setting the balance between opponent 'On' and 'Off' responses in different locations of the visual receptive field.

Slow-wave sleep is characterized by alternating periods of activity and silence in corticothalamic networks. Both activity and silence are stable network states, but the mechanisms of their alternation remain unknown. We show, using simultaneous multisite intracellular recordings in cats, that slow rhythm involves all neocortical neurons and that both activity and silence started almost synchronously in cells located up to 12 mm apart. Activity appeared predominantly at the area 5/7 border and spread in both anterior and posterior directions. The activity started earlier in fast-spiking cells and intrinsically bursting cells than in regular-spiking neurons. These results provide direct evidence for two mechanisms of active state generation: spread of activity from a local focus and synchronization of weaker activity, originating at multiple locations. Surprisingly, onsets of silent states were synchronized even more precisely than the onsets of activity, showing no latency bias for location or cell type. This most intriguing finding exposes a major gap in understanding the nature of state alternation. We suggest that it is the synchronous termination of activity and occurrence of silent states of the neuronal network that makes the EEG picture during slow-wave sleep so characteristic. Synchronous onset of silence in distant neurons cannot rely exclusively on properties of individual cells and synapses, such as adaptation of neuronal firing or synaptic depression; instead, it implies the existence of a network mechanism. Revealing this yet unknown large-scale mechanism, which switches network activity to silence, will aid our understanding of the origin of brain rhythms in normal function and pathology.