Abstract
How top-down influence affects neuronal activity and information encoding in the primary visual cortex (V1) remains elusive. This study examined changes of neuronal excitability and contrast sensitivity in cat V1 cortex after top-down influence of area 7 (A7) was modulated by transcranial direct current stimulation (tDCS). The neuronal excitability in V1 cortex was evaluated by visually evoked field potentials (VEPs), and contrast sensitivity (CS) was assessed by the inverse of threshold contrast of neurons in response to visual stimuli at different performance accuracy. We found that the amplitude of VEPs in V1 cortex lowered after top-down influence suppression with cathode-tDCS in A7, whereas VEPs in V1 did not change after sham-tDCS in A7 and nonvisual cortical area 5 (A5) or cathode-tDCS in A5 and lesioned A7. Moreover, the mean CS of V1 neurons decreased after cathode-tDCS but not sham-tDCS in A7, which could recover after tDCS effect vanished. Comparisons of neuronal contrast-response functions showed that cathode-tDCS increased the stimulus contrast required to generate the half-maximum response, with a weakly-correlated reduction in maximum response but not baseline response. Therefore, top-down influence of A7 enhanced neuronal excitability in V1 cortex and improved neuronal contrast sensitivity by both contrast gain and response gain.
Highlights
How top-down influence affects neuronal activity and information encoding in the primary visual cortex (V1) remains elusive
The components of visually evoked field potentials (VEPs) recorded in the V1 cortex, including an initial negative wave N1, a positive wave P1 and a late negative wave N2, were clearly identified both before and after sham (s)- and cathode (c)-transcranial direct current stimulation (tDCS) in area 7 (A7) (Fig. 1A,B), which were similar to observations in previous studies[23,24], but showed lower amplitude in N1 wave than in P1 and N2 waves
Statistical analyses showed that the mean latency of wave N1, P1 and N2 measured before and at different time point after the end of s- and c-tDCS in A7 had no significant difference (main effect of measurement time point: N1: F(10,506) = 0.591, p = 0.822; P1: F(10,506) = 0.855, p = 0.356; N2: F(10,506) = 0.539, p = 0.863); there was no interaction between measurement time point and tDCS: N1: F(10,506) = 0.860, p = 0.571; P1: F(10,506) = 0.894, p = 0.539; N2: F(10,506) = 0.360, p = 0.963)
Summary
How top-down influence affects neuronal activity and information encoding in the primary visual cortex (V1) remains elusive. A recent study report that inactivation of A7 with local GABA application or nitrogen freezing significantly decreases the response amplitude of orientation maps in the V1 c ortex[12] These evidences indicate that A7 has an evident top-down influence on the neuronal activity in V1 cortex. These studies suggest that neurons in the V1 cortex may play a critical role in the encoding of visual stimulus contrast It is unclear how higher-order visual cortical areas affect stimulus contrast encoding in the V1 cortex, a few of studies have shown that attention can impact contrast detection threshold and neuronal contrast response functions in the higher-level visual a reas[4,19]
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