Integration of local inputs in visual cortex

In mammalian visual cortex, local connections are ubiquitous, extensivelylinking adjacent neurons of all types. In this study, optical maps ofintrinsic signals and responses from single neurons were obtained from thesame region of cat visual cortex while the effectiveness of the localcortical circuitry was altered by focally disinhibiting neurons within acolumn of known orientation preference. Maps of intrinsic signals indicatedthat local connections provide strong and functional subthreshold inputs toneighboring columns of other orientation preferences, altering the observedorientation preference to that of the disinhibited column. However,measuring the suprathreshold response using single-cell recordings revealedonly mild changes of preferred orientation over the affected region.Because strongly tuned subthreshold inputs from cortex only marginallyaffect the tuning of a cortical cell's output, it is concluded that localcortical inputs are integrated weakly compared to geniculate inputs. Suchcircuitry potentially allows for the normalization of responses across awide range of input activity through local averaging.