Dissociating mechanisms of spatial suppression and summation in human visual cortical regions MT/V5: a transcranial Direct Current Stimulation (tDCS) study and clinical implications.

Visual motion perception requires balancing spatial integration, which supports discrimination of weak or noisy signals, with spatial suppression, which limits the influence of large, high-contrast motion patterns. This balance shifts with stimulus properties: increasing size improves performance at low contrast (spatial summation) but worsens it at high contrast (spatial suppression). We tested whether MT/V5 contributes to these processes by applying transcranial direct current stimulation (tDCS) while measuring motion-direction discrimination for small and large gratings at low and high contrast. Cathodal tDCS selectively improved discrimination of large high-contrast stimuli, indicating reduced spatial suppression and supporting a role for MT/V5 in limiting perception of strong motion signals. At low contrast, cathodal stimulation impaired discrimination regardless of size, consistent with a reduced amplification of weak inputs. These effects strengthened gradually after stimulation, and were most evident following cathodal stimulation relative to sham. Analysis comparing cathodal, sham and anodal stimulation did not support a categorical polarity-specific interpretation, but indicated a polarity-sensitive pattern driven by cathodal modulation. These findings provide causal evidence that modulating MT/V5 alters both suppressive surround mechanisms at high contrast and sensitivity to weak motion signals at low contrast, implicating MT/V5 in the balance between integrative and segregative processing during motion perception.