In frontalized mammals it has been demonstrated that adaptation produces shift of the peak of the orientation tuning curve of neuron following frequent or lengthier presentation of a non-preferred stimulus. Depending on the duration of adaptation the shift is attractive (toward the adapter) or repulsive (away from the adapter). Mouse exhibits a salt-and-pepper cortical organization of orientation maps, hence this species may respond differently to adaptation. To examine this question, we determined the effect of twelve minutes of adaptation to one particular orientation on neuronal orientation tuning curves in V1 of anesthetized mice. Multi-unit activity of neurons in V1 was recorded in a conventional fashion. Cells were stimulated with sine-wave drifting gratings whose orientation tilted in steps. Results revealed that similarly to cats and monkeys, majority of cells shifted their optimal orientation in the direction of the adapter while a small proportion exhibited a repulsive shift. Moreover, initially untuned cells showing poor tuning curves reacted to adaptation by displaying sharp orientation selectivity. It seems that modification of the cellular property following adaptation is a general phenomenon observed in all mammals in spite of the different organization pattern of the visual cortex. This study is of pertinence to comprehend the mechanistic pathways of brain plasticity.

Orientation-selective neurons shift their preferred orientation after being adapted to a nonpreferred orientation. These shifts of the peaks of tuning curves may be in the attractive or repulsive direction in relation to the adapter orientation. In anaesthetized cats we recorded evoked electrical responses from the visual cortex in a conventional fashion. The recorded spikes in cortex may present two typical waveforms: regular spikes or fast spikes. However, there is no evidence whether the shapes of spikes are related to the attractive or repulsive shifts of orientation tuning curves of cells. Our results show that after adaptation the recorded cells with both attractive and repulsive shifts display one or the other shape of spike. However, the magnitude of shifts is systematically higher for regular spikes, which is attributed to putative pyramidal cells, while tuning curves for fast spikes have smaller magnitudes and are evoked by putative interneurons.

Neurons in V1 display orientation selectivity by responding optimally to a preferred orientation edge when it is presented within their receptive fields. Orientation plasticity in striate cortex occurs either by ocular deprivation or by imposition of a non-preferred stimulus for several minutes. Adaptation of neurons to a non-optimal orientation induces shifts of tuning curves towards the adapting orientation (attractive shift) or away from it (repulsive shift). Here, we investigated the effects of the neurotransmitter serotonin and antidepressant fluoxetine (a selective serotonin reuptake inhibitor) on the modulation of adaptation-induced orientation plasticity. We show that serotonin and fluoxetine promote mostly attractive shifts. Attractive shifts augmented in magnitude towards adapter, whereas repulsive neurons reversed their behavior in the direction of the forced orientation. Furthermore, neurons which retained their original preferred orientation expressed plasticity by shifting their tuning curves after drug administration mostly towards adapter. Our data suggest a pre-eminent role of fluoxetine by inducing and facilitating short-term plasticity in V1.

Visual neurons coordinate their responses in relation to the stimulus; however, the complex interplay between a stimulus and the functional dynamics of an assembly still eludes neuroscientists. To this aim, we recorded cell assemblies from multi-electrodes in the primary visual cortex of anaesthetized cats in response to randomly presented sine-wave drifting gratings whose orientation tilted in 22.5° steps. Cross-correlograms divulged the functional connections at all the tested orientations. We show that a cell-assembly discriminates between orientations by recruiting a 'salient' functional network at every presented orientation, wherein, the connections and their strengths (peak-probabilities in the cross-correlogram) change from one orientation to another. Within these assemblies, closely tuned neurons exhibited increased connectivity and connection-strengths than differently tuned neurons. Minimal connectivity between untuned neurons suggests the significance of neuronal selectivity in assemblies. This study reflects upon the dynamics of functional connectivity, and brings to the fore the importance of a 'signature' functional network in an assembly that is strictly related to a specific stimulus. Apparently, it points to the fact that an assembly is the major 'functional unit' of information processing in cortical circuits, rather than the individual neurons.

Cortical organization rests upon the fundamental principle that neurons sharing similar properties are co-located. In the visual cortex, neurons are organized into orientation columns. In a column, most neurons respond optimally to the same axis of an oriented edge, that is, the preferred orientation. This orientation selectivity is believed to be absolute in adulthood. However, in a fully mature brain, it has been established that neurons change their selectivity following sensory experience or visual adaptation. Here, we show that after applying an adapter away from the tested cells, neurons whose receptive fields were located remotely from the adapted site also exhibit a novel selectivity in spite of the fact that they were not adapted. These results indicate a robust reconfiguration and remapping of the orientation domains with respect to each other thus removing the possibility of an orientation hole in the new hypercolumn. These data suggest that orientation columns transcend anatomy, and are almost strictly functionally dynamic.

Object orientations in the visual field are columned into specific orientation domains in the primary visual cortex [area 17 (A17) and area 18 (A18)] of cats. At the single-cell level, adapting A17 neurons to a non-preferred orientation (adaptor) shifts their preferred orientation either towards the adaptor (attractive shift) or away from it (repulsive shift). As A17 and A18 are reciprocally connected, we sought to determine how changes in preferred orientations in A18 neurons are correlated with changes recorded in A17 anesthetised cats. To this end, we simultaneously traced populations of neurons in A17 and A18, using intrinsic optical imaging, before and after long (12 min) and short (3 min) adaptations. The comparison of A17 and A18 maps pre-adaptation and post-adaptation showed that variance in shift amplitudes is greater in A18 than A17 for short adaptations. Our results indicate a rapid reconfiguration of functional maps that may spread to many cortical areas.

V1 is fundamentally grouped into columns that descend from layers II-III to V-VI. Neurons inherent to visual cortex are capable of adapting to changes in the incoming stimuli that drive the cortical plasticity. A principle feature called orientation selectivity can be altered by the presentation of non-optimal stimulus called 'adapter'. When triggered, LGN cells impinge upon layer IV and further relay the information to deeper layers via layers II-III. Using different adaptation protocols, neuronal plasticity can be investigated. Superficial neurons in area V1 are well acknowledged to exhibit attraction and repulsion by shifting their tuning peaks when challenged by a non-optimal stimulus called 'adapter'. Layers V-VI neurons in spite of partnering layers II-III neurons in cortical computation have not been explored simultaneously toward adaptation. We believe that adaptation not only affects cells specific to a layer but modifies the entire column. In this study, through simultaneous multiunit recordings in anesthetized cats using a multichannel depth electrode, we show for the first time how layers V-VI neurons (1000-1200 μm) along with layers II-III neurons (300-500 μm) exhibit plasticity in response to adaptation. Our results demonstrate that superficial and deeper layer neurons react synonymously toward adapter by exhibiting similar behavioral properties. The neurons displayed similar amplitude of shift and maintained equivalent sharpness of Gaussian tuning peaks before and the following adaptation. It appears that a similar mechanism, belonging to all layers, is responsible for the analog outcome of the neurons' experience with adapter.

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