Are Sensory Neurons in the Cortex Committed to Original Trigger Features?

Sensory cortices are inherently dynamic and exhibit plasticity in response to a variety of stimuli. Few studies have revealed that depending upon the nature of stimuli, excitation of the corresponding sensory region also evokes a response from other neighboring connected areas. It is even more striking, when somatosensory areas undergo reorganization as a result of an intentional disturbance and further explored as a paradigm to understand neuroplasticity. In addition, it has also been proved that drugs too can be used as a model to explore potential plasticity in sensory systems. To this aim, through electrophysiology in cats, we explored that visual neurons, throughout the cortical column, have a tendency to alter their inherent properties even when presented a non-visual stimulus. Furthermore, it was explored in mice, how the application of drugs (serotonin and ketamine) modulates potential plasticity within the visual system. Indeed, we found a shift in orientation tuning of neurons indicated by Gaussian tuning fits in both scenarios. These results together suggest that sensory cortices are capable of adapting to intense experiences by going through a recalibration of corresponding or neighboring sensory area(s) to redirect the sensory function and exhibit remarkable extent of neuroplasticity within the brain

Synchrony between orientation-selective neurons is modulated during adaptation-induced plasticity in cat visual cortex

<p>Abstract</p> <p>Background</p> <p>Visual neurons respond essentially to luminance variations occurring within their receptive fields. In primary visual cortex, each neuron is a filter for stimulus features such as orientation, motion direction and velocity, with the appropriate combination of features eliciting maximal firing rate. Temporal correlation of spike trains was proposed as a potential code for linking the neuronal responses evoked by various features of a same object. In the present study, synchrony strength was measured between cells following an adaptation protocol (prolonged exposure to a non-preferred stimulus) which induce plasticity of neurons' orientation preference.</p> <p>Results</p> <p>Multi-unit activity from area 17 of anesthetized adult cats was recorded. Single cells were sorted out and (1) orientation tuning curves were measured before and following 12 min adaptation and 60 min after adaptation (2) pairwise synchrony was measured by an index that was normalized in relation to the cells' firing rate. We first observed that the prolonged presentation of a non-preferred stimulus produces attractive (58%) and repulsive (42%) shifts of cell's tuning curves. It follows that the adaptation-induced plasticity leads to changes in preferred orientation difference, i.e. increase or decrease in tuning properties between neurons. We report here that, after adaptation, the neuron pairs that shared closer tuning properties display a significant increase of synchronization. Recovery from adaptation was accompanied by a return to the initial synchrony level.</p> <p>Conclusion</p> <p>We conclude that synchrony reflects the similarity in neurons' response properties, and varies accordingly when these properties change.</p

Visual Cells Remember Earlier Applied Target: Plasticity of Orientation Selectivity

BACKGROUND: A canonical proposition states that, in mature brain, neurons responsive to sensory stimuli are tuned to specific properties installed shortly after birth. It is amply demonstrated that that neurons in adult visual cortex of cats are orientation-selective that is they respond with the highest firing rates to preferred oriented stimuli. METHODOLOGY/PRINCIPAL FINDINGS: In anesthetized cats, prepared in a conventional fashion for single cell recordings, the present investigation shows that presenting a stimulus uninterruptedly at a non-preferred orientation for twelve minutes induces changes in orientation preference. Across all conditions orientation tuning curves were investigated using a trial by trial method. Contrary to what has been previously reported with shorter adaptation duration, twelve minutes of adaptation induces mostly attractive shifts, i.e. toward the adapter. After a recovery period allowing neurons to restore their original orientation tuning curves, we carried out a second adaptation which produced three major results: (1) more frequent attractive shifts, (2) an increase of their magnitude, and (3) an additional enhancement of responses at the new or acquired preferred orientation. Additionally, we also show that the direction of shifts depends on the duration of the adaptation: shorter adaptation in most cases produces repulsive shifts, whereas adaptation exceeding nine minutes results in attractive shifts, in the same unit. Consequently, shifts in preferred orientation depend on the duration of adaptation. CONCLUSION/SIGNIFICANCE: The supplementary response improvements indicate that neurons in area 17 keep a memory trace of the previous stimulus properties, thereby upgrading cellular performance. It also highlights the dynamic nature of basic neuronal properties in adult cortex since repeated adaptations modified both the orientation tuning selectivity and the response strength to the preferred orientation. These enhanced neuronal responses suggest that the range of neuronal plasticity available to the visual system is broader than anticipated

Cortical Plasticity under Ketamine: From Synapse to Map

Sensory systems need to process signals in a highly dynamic way to efficiently respond to variations in the animal’s environment. For instance, several studies showed that the visual system is subject to neuroplasticity since the neurons’ firing changes according to stimulus properties. This dynamic information processing might be supported by a network reorganization. Since antidepressants influence neurotransmission, they can be used to explore synaptic plasticity sustaining cortical map reorganization. To this goal, we investigated in the primary visual cortex (V1 of mouse and cat), the impact of ketamine on neuroplasticity through changes in neuronal orientation selectivity and the functional connectivity between V1 cells, using cross correlation analyses. We found that ketamine affects cortical orientation selectivity and alters the functional connectivity within an assembly. These data clearly highlight the role of the antidepressant drugs in inducing or modeling short-term plasticity in V1 which suggests that cortical processing is optimized and adapted to the properties of the stimulus

Typical example of shift in orientation preference and response modulations.

<p>A: The first 12 min adaptation displaces the preferred orientation of the cell by 15.5° toward the adapting stimulus. The head arrow indicates the non-preferred adapting stimulus. Following a recovery period of 60 min, the cell recovered its control preferred orientation at 9.0°. Adaptation II produces an identical attractive shift of 15.1°. B, C and D: Histograms shows the response modulations at the control preferred orientation, the new preferred orientation after adaptations and the baseline level (θ = 90°), respectively. At the control preferred orientation, the mean firing rate of cell decrease after adaptation I (<i>t</i>-test, p<0.001) and returned to control level in 60 min. In parallel, the mean firing rate increase by 27% at the new preferred orientation (<i>t</i>-test, p<0.0001). Following recovery, the firing rate further increases: 48% in comparison to adaptation I (<i>t</i>-test, p<0.0001). Baseline level remains unchanged across conditions. E and F: Peri-stimulus time histograms (PSTH) are illustrated for the neuron responding to orientations in C and D, respectively. Blue curves; control condition, red curves; adaptation I, black curves; adaptation II.</p

(A) Scatter plot showing the amplitude of shifts in preferred orientation after adaptation as a function of the absolute difference between the control preferred orientation and the adapting orientation

Positive values (black dots) designate attractive shifts (n = 42) and negative values (grey dots) designate repulsive shifts (n = 30). The dashed lines in black and grey indicate the mean amplitude for attractive (17.3°) and repulsive (13.5°) shifts, respectively. (B) Scatter plot displaying the signal-to-noise (S/N) ratio of neuronal spikes' waveforms in the control condition as a function of the absolute shift amplitude (black dots indicate attractive shifts, whereas grey dots indicate repulsive shifts). Data are equally distributed around the S/N ratio mean values for both attractive (black dashed line) and repulsive shifts (grey dashed line). This distribution shows that shifts in orientation preference are unrelated to the S/N ratio (r < 0.1 regardless the direction of the shift). (C) Histograms showing the modulation of mean firing rate between , and conditions (error bars are SEM). Left: following the adaptation, a significant decrease of the firing rate is observed for the initial preferred orientation; paired sample two-tailed -test, < 0.001. Middle: in parallel, a significant increase of the response is observed for the newly acquired preferred orientation (attractive and repulsive shifts pooled together); paired sample two-tailed -test, < 0.01. Right: there are no significant changes in the response of far flank orientations (baseline); paired sample two-tailed -test, > 0.1. In all cases, recoveries are shown 60 minutes after the adaptation ended.<p><b>Copyright information:</b></p><p>Taken from "Synchrony between orientation-selective neurons is modulated during adaptation-induced plasticity in cat visual cortex"</p><p>http://www.biomedcentral.com/1471-2202/9/60</p><p>BMC Neuroscience 2008;9():60-60.</p><p>Published online 3 Jul 2008</p><p>PMCID:PMC2481260.</p><p></p

Examples of orientation tuning shifts and response improvements following repeated adaptations.

<p>Left column: orientation tuning curves showing averaged responses (pooled data). Middle and right columns: orientation tuning curves changes on trial by trial basis (n = 25 presentations) after adaptation I and II, respectively. A: The first adaptation induced a significant attractive shift of 15.5° (example used in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0003689#pone-0003689-g001" target="_blank">fig. 1, red curves</a>, <i>t</i>-test, p<0.0001). After complete recovery, adaptation II produces once again a significant attractive shift. Comparing to the first adaptation, a strong increase of the response is observed at the new preferred stimulus (amber curves, <i>t</i>-test, p<0.0001). B: In this example, the cell fails changing its preferred orientation following adaptation I; only response depression is observed (red curves). Adaptation II produces a significant shift of 28.0° (amber curves, <i>t</i>-test, p<0.0001). C: Cell displays only weak repulsive shifts of 4.0° after adaptations but its response increase by 27% at the new preferred orientation (compare red to amber curves, <i>t</i>-test, p<0.001).</p

Schematic representation of the experimental adaptation protocol.

<p>Responses to sine-wave drifting grating in 9 different orientations, covering multi-units receptive fields, were measured for 25 trials of 4.1 s presented in random order. Adaptation I: orientation tuning curves were plotted prior to and after a 12 min of continuously adaptation to a non-preferred stimulus (22.5°–67.5° off the preferred orientation). Following a recovery period of 60–90 min, orientation tuning curves were replotted. Adaptation II: the same adapting protocol was applied a second time on receptive fields and tuning curves were once again plotted. In additional experiments the duration of adaptation was increased in steps of 3, 6, 9 and 12 min.</p