| The response selectivity of visual cortical neurons for visual stimulus orientations,called orientation selectivity,is fundamental for perceptual identification of objects’ orientations and forms.The underlying mechanism in the formation of orientation selectivity has not been elucidated.Since that Hubel and Wiesel put forward the theory about the receptive field(RF)construction of neurons in the visual cortex,a considerable number of authors support the idea that the orientation selectivity of neurons in the visual cortex is formed by a gradual convergence of excitatory signal input from lateral geniculate nucleus(LGN).Some evidences indicate that intracortical inhibition also exerts an important role in the formation of neuronal orientation selectivity in the V1 cortex.Recent studies have found that top-down influence from higher-level visual cortex or cortical regions may modulate the response selectivity of V1 neurons for stimulus orientations,but the reports from different authors are inconsistent.The mechanism that mediates top-down influence remains unclear.Whether top-down influence on the orientation selectivity of V1 neurons closely underlies the behavioral ability in stimulus orientation identification is also unknown.To answer these questions,this study used a noninvasive and effect-reversible transcranial direct current stimulation(t DCS)to regulate the top-down influence of area7(A7)in cats,and examined the top-down influence on the response selectivity of single-unit,multi-units and local field potential(LFP)of V1 cortex to different stimulus orientations as well as the perceptual ability of cats in stimulus orientation identification.The main results were described in the following:(1)The multi-units response to different stimulus orientations in the V1 cortex of4 cats were recorded before and after the top-down influence of A7 was modulated.spike sorting of action potentials from multi-units response was performed based on Matlab and Wave＿clus toolbox.The results showed that the recorded multi-units could be isolated into two types of single-unit,including wide spike unit(WS-unit)and normal spike unit(NS-unit),and poorly-isolated unit(PI-unit).By double-gaussian function fitting of orientation-response tuning curves for different types of units and polar-axis analyzing,it is found that the PI-unit had a better tuning response to stimulus orientations,a stronger orientation selectivity and a higher sensitivity to top-down influence than NS-unit and WS-unit.The top-down influence on NS-unit was secondary to PI-unit,and WS-unit was least affected by top-down influence.These results indicated that the top-down influence of A7 on different types of units was essentially consistent although varied in effect size.To investigate into the mechanism of top-down influence of A7 on response-orientation selectivity of V1 neurons,we directly compared the properties of response-orientation tuning curves of multi-units before and after the top-down influence of A7 was modulated by t DCS.The results showed that suppression of top-down influence after cathode(c)t DCS at A7 caused a reduction in visually evoked response especially the maximum visually evoked response but not spontaneous activity of multi-units,which decreased the response signal-to-noise ratio and orientation selectivity of multi-units.(2)Using staircase method of behavioral measurement and extracellular singleunit recording technique,this study examined both behavioral performance in stimulus orientation identification and neuronal response sensitivity to stimulus orientations in the V1 cortex of cat before and after top-down influence of area 7(A7)was modulated by noninvasive t DCS.The results showed that cathode(c)t DCS but not sham(s)t DCS in A7 significantly increased the behavioral threshold in identifying stimulus orientation difference,which effect recovered after the t DCS effect vanished.Consistently,c-t DCS but not s-t DCS in A7 significantly decreased the response selectivity bias of V1 neurons for stimulus orientations,which effect could recover after withdrawal of the t DCS effect.Further analysis showed that c-t DCS induced reduction of V1 neurons in response selectivity was not resulted from alterations of neuronal preferred orientation,nor of spontaneous activity.Instead,c-t DCS in A7 significantly lowered the visually evoked response,especially the maximum response of V1 neurons,which caused a decrease in response selectivity and signal-to-noise ratio.These results indicate that top-down influence of A7 may enhance behavioral identification of stimulus orientations by increasing neuronal visually evoked response and response selectivity in the V1 cortex.(3)The local field potential(LFP)in different locations of V1 cortex was recorded before and after the top-down influence of A7 modulated using t DCS,and the top-down effects on the orientation selectivity of different neural oscillation rhythms and crossfrequency phase-amplitude coupling(PAC)of low to high rhythms were analyzed systematically.The results showed that c-t DCS induced suppression of top-down influence of A7 decreased the evoked but not spontaneous response power especially the maximum response power of different neural oscillation rhythms in response to visual stimulus orientations,which caused a significant reduction in their response signal-to-noise ratio and orientation selectivity.Specifically,the orientation selectivity reduction was larger for low-frequency especially Theta rhythm(decreased by 31.85%)but smaller for high-frequency Gamma especially High-Gamma rhythm(decreased by23.77%).Further analysis of Theta-Gamma PAC in response to visual stimulus orientations before and after suppression of top-down influence of A7 showed that topdown influence inhibition decreased the evoked but not spontaneous strength especially the maximum strength(MI)of Theta-Gamma PAC in response to visual stimulus orientations,which caused a significant reduction in the PAC’s signal-to-noise ratio and orientation selectivity(reduced by 31.2%).These results suggest that top-down influence of A7 may chiefly affect the low-rhythm especially Theta oscillation in the V1 cortex,which,in turn,phasically modulate the power of high-rhythm gamma oscillation and thus mediate V1 neurons’ activity and their response selectivity for stimulus orientations.Taken together the research results described above,we draw conclusion as follows:(1)Top-down influence of A7 can enhance the response selectivity of different types of units in the V1 cortex to visual stimulus orientations although the effect size of top-down influence varies with types of units.The top-down influence of A7 can increase the visually evoked but not spontaneous response of multi-units especially the maximum visually evoked response,which improves neuronal response signal-to-noise ratio and orientation selectivity.(2)Top-down influence of A7 may improve the behavioral ability in identification of visual stimulus orientations by increasing the response selectivity of V1 neurons to visual stimulus orientations.(3)Top-down influence of A7 may chiefly affect the low-rhythm especially Theta rhythm oscillation in the V1 cortex,which mediate the neuronal activity and orientation selectivity by phasically modulating the power of high-rhythm gamma oscillation. |
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