The case for feedforward multisensory convergence during early cortical processing

The prevailing hierarchical model of sensory processing in the brain holds that different modalities of sensory information emanating from a single object are analyzed extensively during passage through their respective unisensory processing streams before they are combined in higher-order 'multisensory' regions of the cortex. Because of this view, multisensory interactions that have been found at early, putatively 'unisensory' cortical processing stages during hemodynamic imaging studies have been assumed to reflect feedback modulations that occur subsequent to multisensory processing in the higher-order multisensory areas. In this paper, we consider findings that challenge an exclusively feedback interpretation of early multisensory integration effects. First, high-density electrical mapping studies in humans have shown that multisensory convergence and integration effects can occur so early in the time course of sensory processing that purely feedback mediation becomes extremely unlikely. Second, direct neural recordings in monkeys show that, in some cases, convergent inputs at early cortical stages have physiological profiles characteristic of feedforward rather than feedback inputs. Third, damage to higher-order integrative regions in humans often spares the ability to integrate across sensory modalities. Finally, recent anatomic tracer studies have reported direct anatomical connections between primary visual and auditory cortex. These findings make it clear that multisensory convergence at early stages of cortical processing results from feedforward as well as feedback and lateral connections, thus using the full range of anatomical connections available in brain circuitry.